Bull Earthquake EngDOI 10.1007/s10518-008-9083-4

ORIGINAL RESEARCH PAPER

Ground motions observed during the 15 August 2007Pisco, Peru, earthquake

Hernando Tavera Isabel Bernal Fleur O. Strasser Maria C. Arango-Gaviria John E. Alarcn Julian J. Bommer

Received: 7 April 2008 / Accepted: 29 July 2008 Springer Science+Business Media B.V. 2008

Abstract A Mw 7.9 earthquake event occurred on 15 August 2007 off the coast of centralPeru, 60 km west of the city of Pisco. This event is associated with subduction processes atthe interface of the Nazca and South American plates, and was characterised by a complexsource mechanism involving rupture on two main asperities, with unilateral rupture propaga-tion to the southeast. The rupture process is clearly reflected in the ground motions recordedduring this event, which include two separate episodes of strong shaking. The event triggered18 accelerographic stations; the recordings are examined in terms of their characteristics andcompared to the predictions of ground-motion prediction equations for subduction environ-ments, using the maximum-likelihood-based method of Scherbaum et al. (Bull Seismol SocAm 94(6):21642185, 2004). Additionally, macroseismic observations and damage patternsare examined and discussed in the light of local construction practices, drawing on fieldobservations gathered during the post-earthquake reconnaissance missions.

Keywords Pisco earthquake Peru-Chile Trench Subduction ground-motions Ground-motion prediction Duration Source complexity

1 The 15 August 2007 Pisco earthquake

Seismic activity in Peru is related to the subduction of the Nazca plate under the SouthAmerican plate at a mean rate of about 78 cm/year (DeMets et al. 1990; Norabuena et al.

H. Tavera I. BernalDireccin de Sismologa, Instituto Geofsico del Per, Calle Badajoz 169, Urb Mayorazgo IV Etapa,Ate, Lima, Peru

F. O. Strasser (B) M. C. Arango-Gaviria J. J. BommerCivil and Environmental Engineering, Imperial College London, South Kensington Campus,London SW7 2AZ, UKe-mail: fleur.strasser@imperial.ac.uk

J. E. AlarcnOve Arup & Partners, 13 Fitzroy Street, London W1T 4BQ, UK

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1999). The events associated with the subduction process are of two types: shallow interfaceevents occurring along the contact between the Nazca and the South American plates, anddeeper events associated with the internal deformation of the Nazca plate, hereafter referredto as intraslab events. The interface events tend to occur offshore along the coastline, at depthsof less than 60 km (Dorbath et al. 1990; Tavera et al. 2006). The occurrence of such events isquite frequent, with up to 60 events of magnitude ML 4.5 or above registered per year. Dueto their shallow depth, these events are generally felt in the epicentral area, and the larger ofthese events (Mw > 7.0) have caused widespread damage in the past. For instance, the 23June 2001 southern Peru earthquake (Mw 8.2) affected an area of 370 by 70 km extendingfrom Atico (Arequipa Region) to Ilo (Moquega Region). Intraslab events, such as the 25 Sep-tember 2005 Northern Peru event (Mw 7.2), occur at intermediate depths (60350 km) and aretherefore only rarely associated with strong shaking. Finally, small-magnitude (Mw < 6.5)shallow crustal events associated with tectonic structures accommodating the convergenceprocess occur inland, but less frequently than subduction events.

On 15 August 2007 at 23:40:58.0 UTC (18:40:58.0 local time), a large interface eventoccurred off the coast of central Peru, whose epicentre was located about 60 km west of thecity of Pisco, and about 145 km SSE of the capital city of Lima. This event and associatedaftershocks ruptured an area parallel to the coastline about 170 km long and 130 km wide.As shown in Fig. 1, this area corresponds to a previously identified seismic gap between therupture areas of the Mw 7.5 1974 Lima event and the Mw 7.7 1996 Nazca event (Tavera andBernal 2005). The focus of the event was located at 13.49 S, 76.85 W and a depth of 18 kmby Tavera and Bernal (2008) based on seismic network data. The moment magnitude of theevent was found to be Mw 7.9, which is consistent with the magnitude estimates obtainedthrough moment tensor inversion by the National Earthquake Information Center (NEIC) ofthe United States Geological Survey (USGS) and Harvard University. The source parametersof the mainshock determined by these agencies are summarised in Table 1.

The focal mechanism was determined by Tavera and Bernal (2008) from P-wave first-motion arrivals recorded on stations of the Peruvian Seismic Network operated by theNational Geophysical Institute of Peru (IGP), as well as stations from seismic networksrun by partner agencies in Argentina, Bolivia, Brazil, Chile, Colombia and Ecuador, inaddition to data from the Global Seismic Network (GSN). As illustrated in Fig. 1, thismechanism is similar to the focal mechanism suggested for large (Mw 7.5) eventsthat have previously occurred in southern and central Peru in 1940, 1942, 1966, 1974,1996 and 2001. These focal mechanisms reflect compressive stresses trending NESW,with nodal planes trending NWSE. The focal mechanism solution for the Pisco event is(1 = 311, 1 = 14, 1 = 119;2 = 161, 2 = 78, 2 = 83), where the first setof angles corresponds to the focal plane, and the second to the auxiliary plane. This solu-tion indicates that the mechanism was predominantly reverse, with a small component ofright-lateral strike-slip. Combined with the estimated hypocentral depth of 18 km, this focalmechanism is consistent with an interface event on the contact surface between the Nazcaand South American Plates.

As illustrated in Fig. 2, the mainshock was preceded by a 4.1 ML foreshock at 19:18 on 11August, whose epicentre was located 77 km northwest of Pisco, and followed by a series ofaftershocks. In the 7-day-period following the mainshock (1521 August), the seismic sta-tions at Guadalupe (GUA) and Zamaca (ZAM), located at epicentral distances of 125 and180 km, respectively, recorded approximately 3,060 aftershocks. Only 18 of these events werefelt in Pisco, Caete and Ica with MMI values of III or more. During the same period, 355aftershocks with magnitude ML 3.0 and hypocentral depths less than 50 km were recorded.As discussed above, the area covered by these aftershocks coincides with a previously

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Bull Earthquake Eng

Fig. 1 Epicentres, rupture areas and focal mechanisms of large earthquakes (Mw 7.5) that occurred inCentral Peru from 1940 to 2007. The focal mechanism for the Pisco event is taken from Tavera and Bernal(2008). The Pisco event and its aftershock sequence fully ruptured the seismic gap that had previously beenidentified between the rupture areas of the 1974 Lima and 1996 Nazca events. The focal mechanism solutionsfor all events are similar and reflect the action of compressive stresses trending NESW, with nodal planestrending NWSE

Table 1 Source parameters of the 15 August 2007 Pisco mainshock, as determined by the National Geo-physical Institute of Peru (IGP), the United States Geological Survey (USGS/NEIC) and Harvard University(HRV)

Reference Origin time Epicentrelatitude

Epicentrelongitude

Focaldepth (km)

Seismicmoment M0

Mw MS ML

IGP 23:40:58 UTC 13.49 S 76.85 W 18 8.81E+20 Nm 7.9 7.0USGS/NEIC 23:41:59 UTC 13.36 S 76.51 W 39 1.84E+21 Nm 8.0 HRV 23:41:59 UTC 13.76 S 76.97 W 33 1.11E+21 Nm 8.0 7.5

identified seismic gap. The spatial distribution of aftershocks is characterised by cluster-ing in three regions: the first cluster (G1) is located around the epicentre, while the sec-ond cluster (G2) is located at the level of the Paracas peninsula, and the third cluster (G3)extends southwards from offshore the Paracas peninsula to the Bahia Independencia area.The distribution of aftershocks is consistent with unilateral propagation of the rupture in asouth-easterly direction over a distance of about 150 km.

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Bull Earthquake Eng

Fig. 2 Spatial distribution of the larger aftershocks (ML 3.0) of the Pisco event that occurred from 15 to21 August 2007. The epicentre of the mainshock is denoted by a white star. The epicentre of the 11 August4.1 ML foreshock is indicated by a small black star; the aftershock area of this foreshock is shown as a greyrectangle. As discussed in the text, three clusters of aftershocks (G1, G2 and G3) have been identified for themainshock. The approximate extent of the rupture area inferred from the aftershock distribution is shown asa dashed line. The arrow indicates the direction of rupture propagation

Furthermore, the aftershock clusters roughly coincide with the locations of the regionsof large slip (asperities) in the teleseismic source inversion of Ji and Zeng (2007), as shownin Fig. 3. This model was derived using the GSN broadband waveforms downloaded fromthe NEIC data centre, which included 14 teleseismic broadband P-waveforms, 8 broadbandSH-waveforms, and 26 long-period surface waves selected based upon data quality and azi-muthal distribution. The slip history was constrained using the finite-fault inverse algorithmof Ji et al. (2002), based on the USGS hypocentral coordinates and quick moment tensorsolution. The maximum slip is of the order of 800 cm and occurs on an asperity located inthe southwestern corner of the fault rupture area, in the Bahia Independencia region offshoreParacas peninsula. Source models by other authors (Yagi 2007; Konca 2007; Valle et al.2007) exhibit a similar pattern of complex rupture involving unilateral propagation to thesoutheast from an asperity located in the vicinity of the hypocentre to a second asperitylocated underneath the Bahia Independencia area.

The average rupture velocities of 1.31.5 km/s implied by these source models are signifi-cantly lower than the average rupture velocities of 2.53.5 km/s commonly found for subduc-tion earthquakes (Pelayo and Wiens 1992; Mai 2004). Table 2 summarises average rupturevelocities for a number of interface events that have occurred along the Peru-Chile trench

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Bull Earthquake Eng

-78o -77o -76o -75o-16o

-15o

-14o

-13o

-12o

LonE

LatN

IGP Mainshock Epicentre

USGS/NEIC Mainshock Epicentre

Aftershocks ML _> 4.5 (IGP)

Extent of aftershock area (IGP)

Ji and Zeng (2007) fault plane

Slip in Ji and Zeng (2007) modelMaximum slip ~ 800 cm

LIMA

Chincha Alta

Pisco

Ica

Direction of rupture

propagation

San Vicentede Canete

100 km

Fig. 3 Source process of the Pisco mainshock, from the teleseismic inversion of Ji and Zeng (2007). Largeraftershocks are also plotted; their locations coincide with regions of higher slip

that are comparable in size and depth to the Pisco event. This table shows that the rupturevelocity observed for the Pisco event constitutes an anomaly even at a regional level. Sladenet al. (2007) point out that while rupture velocities for this event are limited to values below2 km/s based on the duration of the P-wave signal, it is difficult to clearly resolve betweendifferent low rupture velocities, in view of the complex geometry of the subduction interface.

The complexity and long duration of the source process is reflected in the instrumentalobservations of ground motions, which clearly show the arrival of two separate wave trainscorresponding to the two main rupture episodes. The principal focus of the current paper isthe analysis of the ground motions observed during the Pisco event, including both instru-mental recordings of strong ground-motion in Lima and Ica, and macroseismic intensityobservations, used here to infer the level of shaking in areas where instrumental observationsare not available.

2 Instrumental recordings of strong ground-motion

The Pisco sequence was registered on a total of 18 accelerographs belonging to the networksoperated by IGP (National Seismic Network, seven contributing stations), the Japan-PeruCentre for Seismic Research and Disaster Mitigation (CISMID, five contributing stations),the South American Regional Seismological Centre (CERESIS, three contributing stations),the Catholic University of Peru (PUCP, one station), and the Peruvian state water company

123

Bull Earthquake Eng

Tabl

e2

Ave

rage

rupt

ure

velo

citie

sof

inte

rfac

esu

bduc

tion

even

tsal

ong

the

Chi

le-P

eru

tren

chde

term

ined

from

seis

mic

wav

efor

min

vers

ion

Eve

ntSo

urce

mod

elre

fere

nce

Epi

cent

rela

titud

eE

pice

ntre

long

itude

Foca

lde

pth

(km

)Se

ism

icm

omen

tM0

Mw

Ave

rage

rupt

ure

velo

city

(km

/s)

Lim

a,Pe

ru,0

3/10

/197

4H

artz

ella

ndL

ange

r(1

993)

12.3

9S

77.6

7W

121.

18E+2

1N

m8.

03.

0V

alpa

rais

o,C

hile

,03/

03/1

985

Cho

yan

dD

ewey

(198

8)33

.12

S71

.62

W40

3.49

E+2

0N

m7.

83.

5M

endo

zaet

al.(

1994

)33

.13

S71

.61

W40

1.96

E+2

1N

m8.

23.

0A

ntof

agas

ta,C

hile

,30/

07/1

995

Del

ouis

etal

.(19

97)

23.4

3S

70.4

8W

361.

22E+2

0N

m8.

02.

8C

arlo

etal

.(19

99)

23.3

4S

70.2

9W

261.

60E+2

1N

m8.

13.

2a

Naz

ca,P

eru,

12/1

1/19

96Sp

ence

etal

.(19

99)

14.4

9S

75.6

3W

211.

38E+2

1N

m8.

13.

0Sa

licho

net

al.(

2003

)14

.49

S75

.63

W21

6.57

E+2

0N

m7.

82.

7A

requ

ipa,

Peru

,23/

06/2

001

Kik

uchi

and

Yam

anak

a(2

001)

16.1

5S

73.4

0W

302.

20E+2

1N

m8.

21.

9b

Bile

kan

dR

uff

(200

2)

33

2.30

E+2

0N

m8.

42.

2b

Tave

raet

al.(

2006

)16

.31

S73

.82

W29

1.30

E+2

1N

m8.

12.

0Pi

sco,

Peru

,15/

08/2

007

Jian

dZ

eng

(200

7)13

.36

S76

.52

W30

1.84

E+2

1N

m8.

01.

4K

onca

(200

7)13

.35

S76

.51

W39

1.84

E+2

1N

m8.

01.

5c

Yag

i(20

07)

13.3

5S

76.5

1W

392.

00E+2

1N

m8.

11.

3V

all

eet

al.(

2007

)

32

8.10

E+2

0N

m7.

91.

3

All

even

tslis

ted

are

com

para

ble

insi

zean

dde

pth

toth

ePi

sco

even

ta

Ave

rage

rupt

ure

velo

city

of3.

0to

3.2

km/s

whe

nun

cert

aint

ies

are

cons

ider

edb

Ave

rage

rupt

ure

velo

city

infe

rred

from

sour

cedu

ratio

nan

dfa

ultd

imen

sion

sc

Slad

enet

al.(

2007

)no

teth

atw

hile

rupt

ure

velo

citie

sar

elim

ited

tova

lues

belo

w2

km/s

base

don

the

dura

tion

ofth

eP-

wav

esi

gnal

,it

isdi

fficu

ltto

clea

rly

reso

lve

betw

een

diff

eren

tlow

rupt

ure

velo

citie

s

123

Bull Earthquake Eng

-78o -77o -76o -75o -74o -73o-16o

-15o

-14o

-13o

-12o

-11o

-10o

Lon E

LatN

Ji & Zeng (2007)fault plane

Extent of aftershockarea (IGP)

IGP MainshockEpicentre

USGS/NEICMainshock Epicentre

Aftershocks ML _> 4.5

SEDAPAL

IGP

CISMID

CERESIS

PUCP

ANC

NNA

MAY

CDL-CIPCAL

PUCPRINCER

ANR

MOLLMO

CSMANC E1

E2

GUA PCN

ICA2CPR

100 km

Fig. 4 Location of strong-motion recording stations. The inset shows in more detail the accelerographicstations located in the Lima metropolitan area

(SEDAPAL, two stations). The location of these stations is shown in Fig. 4. Only two of thestations, Parcona (PCN) and Ica (ICA2), are located in the city of Ica, about 120 km SE ofthe epicentral region, while the others are located within the Lima urban area. Station GUA(Guadalupe, Ica Region) did not record the mainshock due to instrumental problems, butprovided records for several aftershocks. For station LMO (Universidad Agraria, La Molina,Lima), only the value of PGA is known.

The values of peak ground acceleration (PGA) and peak ground velocity (PGV) recordedat these stations during the mainshock are summarised in Table 3, along with the source-to-site distances. For the 12 stations for which mainshock acceleration traces have been madeavailable by the networks (2 in the Ica area and 10 in Lima), the traces were processedindividually as follows: after correcting for baseline shifts, the records were filtered usinglow-cut filter frequencies determined by considering the signal-to-noise ratio between therecord and a model of the noise obtained from the pre-event memory of the digital records.Since this type of model does not account for signal-generated noise (Boore and Bommer2005), the results were checked through visual inspection of the velocity and displacementtraces obtained from the filtered acceleration record. Visual inspection of these traces wasalso the basis for the selection of the low-cut filter frequency when no pre-event memorywas available. PGA, PGV and pseudo-acceleration response spectra values for 5% dampingwere obtained from the processed records.

123

Bull Earthquake Eng

Tabl

e3

Peak

grou

ndac

cele

ratio

n(P

GA

),pe

akgr

ound

velo

city

(PG

V),

Ari

asin

tens

ity(I

A)

and

sign

ific

ant

dura

tion

(D5

95)

valu

esre

cord

edfo

rth

e15

Aug

ust

2008

Pisc

om

ains

hock

Stat

ion

Stat

ion

code

Inst

r.aN

etw

ork

Loc

atio

nbR

epic

(km

)R

jbd

(km

)R

rup

d(k

m)

PGA

e(c

m/s

2)

PGV

e(c

m/s

)IA

e(m

/s)

D5

95f

(s)

Anc

onA

NC

DIG

P11

.776

0S

192.

413

1.0

139.

454

.75

2.59

0.20

998.9

977

.150

0W

58.7

33.

540.

189

99.4

178

ma.

m.s

.l.27

.98

2.56

0.07

510

6.39

Asa

mbl

eaN

acio

nald

eR

ecto

res

AN

RD

CE

RE

SIS

12.1

230

S15

1.9

88.7

99.3

64.9

15.

100.

399

101.

7776

.976

0W

85.9

18.

300.

464

99.1

115

0m

a.m

.s.l.

73.7

66.

950.

238

101.

01C

alla

oC

AL

DC

ISM

ID12

.060

0S

161.

510

5.4

112.

595

.86

10.7

50.

595

109.

1877

.150

0W

100.

9012

.45

0.56

810

7.69

39m

a.m

.s.l.

31.7

04.

210.

095

106.

42C

DL

-CIP

CD

L-C

IPD

CIS

MID

12.0

920

S15

6.2

96.1

105.

158

.41

4.36

0.14

710

0.49

77.0

490

W54

.49

8.54

0.17

999.2

432

.21

4.67

0.07

810

1.33

Cer

esis

CE

RD

CE

RE

SIS

12.1

030

S15

4.3

91.9

102.

157

.88

5.68

0.23

410

1.48

76.9

980

W58

.18

8.37

0.23

310

0.23

149

ma.

m.s

.l.36.9

75.

140.

121

104.

00C

ism

idC

SMD

CIS

MID

12.0

133

S16

4.9

103.

311

2.6

45.0

83.

000.

215

101.

8977

.050

2W

73.7

76.

410.

330

97.8

513

0m

a.m

.s.l.

32.5

83.

770.

078

103.

03E

stan

que-

1E

1A

SED

APA

L12

.033

0S

161.

896

.710

7.8

59.9

8

76.9

750

W54

.88

279

ma.

m.s

.l.30

.38

Est

anqu

e-2

E2

ASE

DA

PAL

12.0

550

S15

9.1

92.8

104.

512

.74

76

.944

0W

20.5

827

6m

a.m

.s.l.

11.7

6L

aM

olin

aU

nive

rsid

adA

grar

iaL

MO

DIG

P12

.085

0S

155.

890

.310

1.9

21.2

0

76.9

480

W25

.30

275

ma.

m.s

.l.14

.20

May

oraz

goM

AY

DIG

P12

.055

0S

159.

192

.810

4.5

59.6

93.

650.

169

103.

4276

.944

0W

54.3

54.

670.

187

100.

1428

6m

a.m

.s.l.

31.0

83.

910.

076

102.

30

123

Bull Earthquake Eng

Tabl

e3

cont

inue

d

Stat

ion

Stat

ion

code

Inst

r.aN

etw

ork

Loc

atio

nbR

epic

(km

)R

jbd

(km

)R

rup

d(k

m)

PGA

e(c

m/s

2)

PGV

e(c

m/s

)IA

e(m

/s)

D5

95f

(s)

Mol

ina

MO

LD

CIS

MID

12.1

000

S15

3.8

85.5

98.3

68.8

34.

090.

364

101.

5176

.890

0W

78.1

04.

350.

439

99.7

414

5m

a.m

.s.l.

56.2

13.

880.

249

100.

61

aa

NN

AD

IGP

11.9

870

S16

6.3

93.8

107.

318.4

62.

310.

027

104.

0376

.839

0W

22.1

63.

680.

035

100.

8057

5m

a.m

.s.l.

21.4

83.

700.

028

102.

28U

nive

rsid

adC

atol

ica

delP

er

PUC

PD

PUC

P12

.074

0S

158.

910

010

8.4

66.6

07.

460.

226

94.7

477

.080

0W

59.6

65.

020.

200

100.

8687

ma.

m.s

.l.39.7

14.

670.

109

102.

36R

inco

nada

RIN

DC

ER

ESI

S12

.083

5S

155.

788

.810

0.9

113.

404.

280.

830

102.

6076

.920

8W

109.

106.

381.

004

101.

8587

ma.

m.s

.l.58.0

3.90

0.26

810

2.78

Cer

roPr

ieto

Gua

dalu

peC

PRD

IGP

13.9

950

S12

8.1

0.0

37.7

75.7

840

W50

2m

a.m

.s.l.

Gua

dalu

peG

UA

DIG

P13

.995

0S

128.

10.

037.8

75.7

840

W55

5m

a.m

.s.l.

Ica

2IC

A2

AC

ISM

ID14

.090

0S

137.

80.

036.9

334.

1062

.27

3.69

385.4

675

.730

0W

271.

6039

.06

3.03

986.7

240

9m

a.m

.s.l.

192.

9015

.05

1.49

887.8

1Pa

rcon

aPC

ND

IGP

14.0

420

S13

8.6

0.0

39.8

457.

5028

.03

3.39

480

.63

75.6

990

W48

8.40

23.5

83.

139

80.6

557

5m

a.m

.s.l.

300.

2015

.37

1.92

281.4

6

The

uppe

rpar

toft

heta

ble

corr

espo

nds

tost

atio

nsin

Lim

aPr

ovin

ce,a

ndth

elo

wer

part

tost

atio

nsin

Ica

Prov

ince

.Val

ues

forP

GV

,IA

and

D5

95ar

eon

lylis

ted

fort

hose

reco

rds

who

setr

aces

wer

eav

aila

ble

toth

eau

thor

sa

Type

ofin

stru

men

t:D

,dig

ital;

A,a

nalo

gue

bC

oord

inat

esan

dal

titud

eof

stat

ion

abov

em

ean

sea

leve

l(a.

m.s

.l.)

cC

alcu

late

dw

ithre

spec

tto

IGP

epic

entr

allo

catio

nd

Cal

cula

ted

with

resp

ectt

oJi

and

Zen

g(2

007)

faul

tpla

nege

omet

rye

From

top

tobo

ttom

:NS,

EW

and

vert

ical

com

pone

ntof

grou

ndm

otio

nf

Sign

ific

antd

urat

ion:

inte

rval

betw

een

5%an

d95

%of

the

Ari

asin

tens

ity

123

Bull Earthquake Eng

The complex rupture process of the Pisco earthquake, characterised by two wave frontscorresponding to the rupture of the two main asperities, is clearly reflected in the accelero-grams recorded in the forms of two wave trains hereafter called R1 and R2. As a consequenceof the location of the asperities at either end of the rupture surface and the low rupture-propagation velocity, these two wave trains are separated by a phase of weaker motionswhich lasts about 6070 s. This is shown in Figs. 5 and 6 for accelerograms recorded in Icaand Lima, respectively.

0 50 100 150 200

-400

-200

0

200

400

Time (s)

EW

Station ICA2, Ica city - Rrup = 37 km - SOIL

0 50 100 150 200

-400

-200

0

200

400

Acc

eler

atio

n(c

m/s

2 )

Time (s)

NS

0 50 100 150 200

-400

-200

0

200

400

Time (s)

VE

RT

0 50 100 150 200

-400

-200

0

200

400

Time (s)

EW

Station PCN (Parcona), Ica city - Rrup = 40 km - SOIL

0 50 100 150 200

-400

-200

0

200

400

Acc

eler

atio

n(c

m/s

2 )

Time (s)

NS

0 50 100 150 200

-400

-200

0

200

400

Time (s)

VE

RT

PGAR1 = 271.6 cm/s2

PGAR1 = 334.1 cm/s2

PGAR1 = 192.9 cm/s2

PGAR1 = 488.4 cm/s2

PGAR1 = 457.1 cm/s2

PGAR1 = 298.8 cm/s2

PGAR2 = 173.7 cm/s2

PGAR2 = 225.1 cm/s2

PGAR2 = 106.0 cm/s2

PGAR2 = 144.6 cm/s2

PGAR2 = 149.5 cm/s2

PGAR2 = 147.4 cm/s2

Fig. 5 Accelerograms recorded in Ica Province at stations ICA2 (top) and PCN (bottom). Both stations arelocated directly above the fault rupture plane (Rjb = 0 km) (see Table 4 for details of the site conditions)

123

Bull Earthquake Eng

0 50 100 150 200

-100

-50

0

50

100

Time (s)

EW

Station NNA (Nana), Lima - Rrup = 107 km - ROCK

0 50 100 150 200

-100

-50

0

50

100

Acc

eler

atio

n(c

m/s

2 )

Time (s)

NS

0 50 100 150 200

-100

-50

0

50

100

Time (s)

VE

RT

0 50 100 150 200

-100

-50

0

50

100

Time (s)

EW

Station RIN (La Rinconada), Lima - Rrup = 101 km - SOIL

0 50 100 150 200

-100

-50

0

50

100

Acc

eler

atio

n(c

m/s

2 )

Time (s)

NS

0 50 100 150 200

-100

-50

0

50

100

Time (s)

VE

RT

PGAR1 = 17.1 cm/s2

PGAR1 = 18.5 cm/s2

PGAR1 = 14.5 cm/s2

PGAR1 = 91.3 cm/s2

PGAR1 = 82.6 cm/s2

PGAR1 = 54.3 cm/s2

PGAR2 = 22.2 cm/s2

PGAR2 = 17.1 cm/s2

PGAR2 = 21.5 cm/s2

PGAR2 = 109.1 cm/s2

PGAR2 = 113.4 cm/s2

PGAR2 = 58.0 cm/s2

Fig. 6 Comparison of accelerograms recorded in Lima on rock (station NNA, top) and on soil (station RIN,bottom)

Figures 7 and 8 show the response spectra obtained at the Lima stations, for the EW andNS components of motion respectively. Figure 9 shows the same information for the stationslocated in the district of Ica. In these figures, in addition to the response spectra computedfor the full record, the response spectra calculated considering each of the two wave trainsseparately are included. For the Lima records, the ground-motion amplitudes are consis-tently higher for the second wave train (R2) than for the first (R1), at all response periods.As a result, the response spectra for the whole record coincide with those obtained consider-ing R2 alone. Conversely, the response spectra calculated for the Ica records coincide with

123

Bull Earthquake Eng

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CSM-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CDL-CIP-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

MOL-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CAL-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )NNA-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

ANC-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

MAY-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

RIN-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )ANR-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CER-EW

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 ) Full RecordR1R21977 Code

PUCP-EW

Fig. 7 Response spectra recorded at stations in Lima Province and Callao Region, east-west component. R1and R2 are the two wave trains corresponding to the rupture of the two main asperities. The code spectrashown are computed for the relevant site class, as determined in Table 4

those calculated considering R1 alone, as the amplitudes associated with this wave train arehigher than those associated with R2, at all response periods considered. This pattern is alsoreflected in the build-up of the Arias intensity: for the Lima stations, 3040% of the totalArias intensity is built up during R1, while this fraction is about 60% for the Ica records.A likely explanation for this difference in behaviour is the location of the stations with

123

Bull Earthquake Eng

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CSM-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CDL-CIP-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

MOL-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CAL-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

NNA-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

ANC-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

MAY-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

RIN-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

ANR-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 )

CER-NS

0 0.5 1 1.5 20

100

200

300

400

500

600

Period (s)

SA

5%(c

m/s

2 ) Full RecordR1R21977 Code

PUCP-NS

Fig. 8 Response spectra recorded at stations in Lima Province and Callao Region, north-south component.R1 and R2 are the two wave trains corresponding to the rupture of the two main asperities. The code spectrashown are computed for the relevant site class, as determined in Table 4

respect to the source: whereas the Ica stations are located directly above the rupture surfaceand southeast of the epicentre, such that the rupture propagates towards them, the Lima sta-tions are located some 160 km to the northwest of the epicentre, with the rupture propagatingaway from them. Considering the location of the asperities on the fault plane, this meansthat the ground motions in Ica can be expected to have enhanced levels of high-frequency

123

Bull Earthquake Eng

0 0.5 1 1.5 20

500

1000

1500

Period (s)

SA

5%(c

m/s

2 )ICA2-NS

0 0.5 1 1.5 20

500

1000

1500

Period (s)

SA

5%(c

m/s

2 )

ICA2-EW

0 0.5 1 1.5 20

500

1000

1500

Period (s)

SA

5%(c

m/s

2 )

PCN-NS

0 0.5 1 1.5 20

500

1000

1500

Period (s)

SA

5%(c

m/s

2 )

Full Record R1 R2 1977 Code 2003 Code

PCN-EW

Fig. 9 Response spectra recorded at stations in the Ica Province. R1 and R2 are the two wave trains corre-sponding to the rupture of the two main asperities. The code spectra shown are computed for the relevant siteclass, as determined in Table 4

(short-period) motions, due to the simultaneous arrival of radiation from several parts of thefault plane. Conversely, for the Lima stations, it is the low-frequency (long-period) part ofthe radiation that is expected to be enhanced.

Figures 79 also display the elastic design accelerations prescribed by the 1977 and 2003Peruvian seismic codes. The 2003 code spectrum is only shown for the Ica stations, sinceit is more conservative than the 1977 code spectrum, which lies well above the observedground motions for all Lima stations (Figs. 7 and 8). For the Ica stations (Fig. 9), while theground motions that would have resulted from R2 occurring in isolation generally fall belowthe 1977 spectrum, the more conservative 2003 design spectrum is required to match theamplitudes of the ground motion associated with R1.

By definition, the elastic response spectra for the full records will correspond to the enve-lope of the response spectra that would be obtained if R1 and R2 occurred separately. In thepresent case, the difference in the responses to R1 and R2 is sufficiently large for one of theseground motion phases to completely dominate the elastic response spectrum, as discussedpreviously. In reality, however, structures will be subjected to the cumulative effects of R1and R2, and the seismic demand they experience may be better represented using inelas-tic response spectra. Figure 10 shows the constant-ductility inelastic response spectra of theeast-west components of the accelerograms recorded at station Callao (CAL) in Lima and

123

Bull Earthquake Eng

0 0.5 1 1.5 20

50

100

150

200

250

Period (s)

SA

5%(c

m/s

2 )

Full Record R1 R2

= 2

0 0.5 1 1.5 20

50

100

150

200

250

Period (s)

SA

5%(c

m/s

2 )

Station CAL, Callao, Lima - EW Component

= 4

0 0.5 1 1.5 20

50

100

150

200

250

Period (s)

SA

5%(c

m/s

2 )

= 6

0 0.5 1 1.5 20

200

400

600

800

1000

Period (s)

SA

5%(c

m/s

2 )

= 2

0 0.5 1 1.5 20

200

400

600

800

1000

Period (s)

SA

5%(c

m/s

2 )

Station PCN, Parcona, Ica - EW Component

= 4

0 0.5 1 1.5 20

200

400

600

800

1000

Period (s)

SA

5%(c

m/s

2 )

= 6

Fig. 10 Constant-ductility inelastic response spectra for the east-west component of the accelerogramsrecorded at station CAL in the Lima area, and station PCN in Ica. R1 and R2 are the two wave trainscorresponding to the rupture of the two main asperities. PCN is located on stiff soil (NEHRP site class C),whereas CAL is located on soft soil (NEHRP D/E)

station Parcona (PCN) in Ica, for ductility values varying from 2 to 6. These plots show thatthe trends observed for the elastic spectra persist for inelastic response, with R1 motionscontrolling the response in Ica and R2 motions controlling the response in Lima.

In addition to the source rupture process, the geological conditions at the recording siteshave a strong influence on the amplitude of the ground motions recorded. The landscapeof Peru has been shaped by the Nazca-South American plate convergence and the Andeanorogenesis. The main geomorphic units are therefore oriented parallel to the coastline andthe Andean mountain chain, along a predominantly NNWSSE axis. These units include,from west to east (Tavera and Buforn 2001): (1) the coastal plain, no more than 40 km inwidth and limited to the east by the Cordillera batholith; (2) the volcanic and plutonic rocksof the western Cordillera, of Mesozoic and Cenozoic age; (3) below 9 S, the high Mesozoicplateaus of the Altiplano; (4) the wide eastern Cordillera anticline, Precambrian to Paleozoicin age; (5) the sub-Andean region, representing the eastern active boundary of the AndeanCordillera, and bordered by the Amazonian plain to the east.

The region affected by the Pisco earthquake covers predominantly the first two of theseunits. In particular, the Lima basin borders the western Cordillera foothills, which consistmainly of intrusive rocks from the coastal batholith that are deeply cut by the Rmac andChilln rivers and numerous ravines, resulting in rugged topography. Additionally, a moregentle topography can be observed, which consists of hills formed of volcanic rocks and

123

Bull Earthquake Eng

underlying a sedimentary cover of interlayered shales and granitic volcanic intrusions. Ero-sion processes have resulted in the accumulation of colluvial deposits at the bottom of theravines, as well as the formation of an alluvial plain through transport and deposition ofmaterials by the rivers and their tributaries. Metropolitan Lima is located in the basins ofthe Rmac and Chilln rivers; the surface geology consists mainly of sedimentary depositswith varying levels of compaction. The Ica region covers the western flank of the AndesCordillera and the coastal plain, which is formed of Quaternary alluvial and wind-bornedeposits. The coastal plain is intersected by the intrusive outcrops of the lower Coastal Cor-dillera. The city of Ica is located in the central part of the region, in the sedimentary basin ofthe river Ica. As a result, site conditions typically include sedimentary and alluvial soils.

Table 4 summarises the site conditions at the recording stations of the accelerograms exam-ined in this study. The site conditions assigned to the various sites were based on informationcollected from a number of sources, including descriptions of the surface geology (EERI2007; Bernal and Tavera 2007a, b), the site category (rock, soil or firm ground) assigned byRodriguez-Marek et al. (2007), VS profiles obtained from spectral analysis of surface waves(SASW) measurements at the two Ica stations (Rosenblad and Bay 2008), tentative VS pro-files inferred by Bernal and Tavera (2007a, b) based on an infinite flat-layered half-spacemodel, as well as the natural period of the site as mapped by Aguilar and Alva (2007) in theirmicrozonation of Lima. The predominant period of each record, T0,REC, defined as the periodcorresponding to the highest H/V ratio in terms of response spectra, following the approachadopted by Zhao et al. (2006a), was also computed for comparison. Finally, the spectral shapeof the records was considered by normalising the response spectra by their PGA value (forall records) and by dividing the spectra recorded at soil stations by the spectrum obtained onrock at NNA station (Lima records only).

For compatibility with the equations used in the comparison exercise discussed later inthe text, the stations were assigned site classes following the New Zealand (NZ) site clas-sification scheme used by McVerry et al. (2006) and the scheme based on natural periodintroduced by Zhao et al. (2006a), hereafter called JP classification, in addition to the morewidely adopted NEHRP site classification. In order to ensure consistency between the siteclassifications assigned in the various schemes, all the information available for each sta-tion was considered, but the various pieces of information were not given equal weight, inview of the limitations associated with some of the methods employed. For instance, theVS,30 values inferred from the Bernal and Tavera (2007a, b) inversions, have been foundto be biased towards low values, and would lead to site classifications that are inconsistentwith the geological and geotechnical descriptions of the site. Furthermore, the non-uniquecharacter of solutions obtained from inverse analysis needs to be borne in mind. As a result,these VS profiles have been used predominantly in the assignment of the NZ site classes todistinguish between shallow and deep soil sites (NZ class C and D, respectively). Similarly,while the natural period (T0,CISMID) obtained from the microzonation map of Aguilar andAlva (2007) is generally the preferred input for assigning the JP site classes, it has beenfound in some cases that the mapped natural period appears to be inconsistent with otherdescriptors, which could be due to local deviations of the geological profile from the charac-teristic profile mapped. Examples of the latter include stations CDL-CIP, CER and PUCP, forwhich the spectral shape and the comparatively long T0,REC indicate a softer soil (JP classIII) than would be expected from the mapped value of T0,CISMID. Again, the predominantperiod calculated directly from the records (T0,REC) was used as a guide in assigning the siteclasses, rather than a direct input to the JP classification scheme, since the values obtainedcould be biased due to non-linearity effects. Table 4 shows that most of the stations locatedon NEHRP sites C and C/D (ANC, CSM, MAY, MOL, RIN) have a natural period of about

123

Bull Earthquake Eng

Tabl

e4

Site

cond

ition

sat

the

stat

ions

used

for

the

com

para

tive

anal

ysis

ofgr

ound

mot

ions

Stat

ion

code

Geo

logi

cala

ndge

otec

hnic

alin

form

atio

nav

aila

ble

Site

clas

sad

opte

din

anal

ysis

Surf

ace

geol

ogya

VS,

30b

(m/s

)SC

RM

cT

0,R

EC

d(s

)T

0,C

ISM

IDe

(s)

NE

HR

PfN

Zg

JPh

CO

DE

i

AN

CA

lluvi

algr

avel

(soi

l)B

280

Soil

0.30

0.2

0.3

C/D

CII

II0.

10[3

60m

/s]

AN

RA

lluvi

algr

avel

(soi

l)B

205

Firm

grou

nd0.

500.

20.

3D

CII

II0.

15[2

55m

/s]

CA

LSo

ftso

ilA;S

oftc

layB

;Gra

nula

rfil

love

rfin

est

ratifi

edso

ilsC

75So

il0.

530.

50.

6D

/EE

IVII

I0.

52[1

80m

/s]

CD

L-C

IPD

ense

,stif

fgr

avel

depo

sit(

Lim

aC

ongl

omer

ate)

A;A

lluvi

algr

avel

(soi

l)B

Fi

rmgr

ound

0.82

0.1

0.2

DC

III

II0.

30[2

55m

/s]

CE

RA

lluvi

algr

avel

(soi

l)B

Fi

rmgr

ound

0.28

0.1

0.2

DC

III

II0.

45[2

55m

/s]

CSM

Den

se,s

tiff

grav

elde

posi

t(L

ima

Con

glom

erat

e)A

;Allu

vial

grav

el(s

oil)

B18

4Fi

rmgr

ound

0.05

0.2

0.3

CC

III

0.10

[523

m/s

]M

AY

Sand

and

siltB

276

Soil

0.22

0.2

0.3

CC

IIII

0.20

[523

m/s

]M

OL

Shal

low

soil

over

lyin

gde

nse

Lim

aC

ongl

omer

ateA

;San

dB38

0So

il0.

130.

20.

4C

CII

II0.

20[5

23m

/s]

NN

AR

ockB

R

ock

0.10

B

BI

I0.

22[7

60m

/s]

PUC

PA

lluvi

algr

avel

(soi

l)B

125

Firm

grou

nd0.

900.

20.

3D

DII

III

0.90

[255

m/s

]

123

Bull Earthquake Eng

Tabl

e4

cont

inue

d

Stat

ion

code

Geo

logi

cala

ndge

otec

hnic

alin

form

atio

nav

aila

ble

Site

clas

sad

opte

din

anal

ysis

Surf

ace

geol

ogya

VS,

30b

(m/s

)SC

RM

cT

0,R

EC

d(s

)T

0,C

ISM

IDe

(s)

NE

HR

PfN

Zg

JPh

CO

DE

i

RIN

Fill

cons

istin

gof

sand

,silt

and

grav

elB

200

Soil

0.32

0.2

0.3

C/D

CII

II0.

30[3

60m

/s]

ICA

2Si

ltysa

nd,s

oilA

312

Soil

0.72

D

CII

III

0.48

[312

m/s

]PC

NSo

ilA45

6So

il0.

42C

CII

II0.

54[4

56m

/s]

aD

escr

iptio

nof

surf

ace

geol

ogy

profi

le,b

ased

onth

efo

llow

ing

refe

renc

es:A

=E

ER

I(2

007)

;B=

Ber

nala

ndTa

vera

(200

7a,b

);C

=in

form

atio

npr

ovid

edby

the

stro

ng-m

otio

nne

twor

kin

the

acce

lero

gram

head

ing

bA

vera

gesh

ear-

wav

eve

loci

tyov

erth

eto

p30

m.F

orth

eIc

ast

atio

ns,t

his

isba

sed

onth

eV

Spr

ofile

sob

tain

edby

Ros

enbl

adan

dB

ay(2

008)

usin

gSA

SW.F

orth

eL

ima

stat

ions

,th

eva

lue

tabu

late

dis

ate

ntat

ive

estim

ate

ofV

S,30

base

don

the

VS

profi

lein

ferr

edby

Ber

nala

ndTa

vera

(200

7a,b

)us

ing

anin

finite

flat-

laye

red

half

-spa

cem

odel

cSi

tecl

ass

assi

gned

byR

odri

guez

-Mar

eket

al.(

2007

)d

Pred

omin

antp

erio

dca

lcul

ated

from

acce

lero

gram

byco

nsid

erin

gth

eH

/Vra

tioof

the

resp

onse

spec

tra,

follo

win

gth

eap

proa

chof

Zha

oet

al.(

2006

a).T

heto

pva

lue

corr

espo

nds

toth

eea

st-w

estc

ompo

nent

ofm

otio

n,w

hile

the

botto

mva

lue

corr

espo

nds

toth

eno

rth-

sout

hco

mpo

nent

eN

atur

alsi

tepe

riod

(T0)

infe

rred

from

the

mic

rozo

natio

nm

apof

Lim

a(A

guila

ran

dA

lva

2007

).V

alue

sar

eno

tava

ilabl

efo

rth

eN

NA

stat

ion

inL

ima,

nor

for

the

Ica

stat

ions

fSi

tecl

ass

acco

rdin

gto

the

NE

HR

P(1

997)

prov

isio

ns.T

henu

mbe

rin

brac

kets

corr

espo

nds

toth

eV

S,30

valu

eas

sum

edw

hen

expl

icitl

yre

quir

ed,f

ollo

win

gth

ere

com

men

datio

nsof

Atk

inso

nan

dB

oore

(200

3).F

orth

eIc

asi

tes,

the

VS,

30va

lues

from

the

SASW

mea

sure

men

tsca

rrie

dou

tby

Ros

enbl

adan

dB

ay(2

008)

wer

eus

edg

Site

clas

sac

cord

ing

toth

eN

ewZ

eala

ndsi

tecl

assi

ficat

ion,

whi

chis

base

don

surf

ace

geol

ogy,

geot

echn

ical

prop

ertie

san

dde

pth

tobe

droc

k(s

eeM

cVer

ryet

al.

(200

6)fo

rde

tails

)h

Site

clas

sac

cord

ing

toth

eZ

hao

etal

.(20

06a)

sche

me,

cons

ider

ing

VS,

30an

dth

ena

tura

lper

iod

ofth

esi

tei

Site

clas

sas

sum

edto

com

pute

the

desi

gnlo

ads

pres

crib

edby

the

1977

and

2003

Peru

vian

seis

mic

code

s:I=

rock

orde

nse

grav

el;I

I=de

nse

sand

orha

rdco

hesi

veso

il;II

I=lo

ose

sand

orso

ftco

hesi

veso

il

123

Bull Earthquake Eng

0.2 s, which provides a check on the consistency of the site classes assigned according to thevarious schemes.

Most of the stations are situated on alluvial gravel, sand and silt, classified as dense andstiff soils (NEHRP class C and D). For instance, the MOL station is situated on shallowsoil overlying dense stiff gravel deposits locally known as Lima Conglomerate, and stationsCDL-CIP and CSM are described as located directly on Lima Conglomerate. Only one sta-tion, NNA, is located on rock (NEHRP class B). Similarly, only one of the Lima stations,CAL, exhibits features consistent with very soft soil. This station is located close to the coastin the Callao district, in an area of reclaimed land over soft soil. Another station located onreclaimed land is RIN, which is located on loose granular fill composed of gravel, silt and finesand. The recording from this station is associated with a large H/V amplification ratio of 8at about 0.3 s. The spectral shape and predominant period of this record are generally consis-tent with a shallow layer of fill overlying a denser deposit, and therefore this site is assignedNEHRP class C/D for the purposes of analysis, although it is expected that the amplificationof the ground motions at this site will be difficult to capture using generic factors.

As can be seen from the comparison of the traces recorded at stations NNA and RIN pre-sented in Fig. 6, as well as from the response spectra presented in Figs. 7 and 8, site conditionshave a significant impact on the amplitude of the ground motions, with H/V amplificationfactors of up to 10 (observed at station CAL at 1.25 s). Furthermore, at stations CAL, CSM,PUCP and RIN, the spectral peak corresponding to the stronger wave train (R2) occurs atlonger periods than the spectral peak corresponding to R1. This could be indicative of soilnon-linearity effects, or alternatively, of differences in the source spectra of the two subevents,or in the path characteristics.

A notable characteristic of the Pisco earthquake is the long duration of strong shakingresulting from the complexity of the rupture process. Based on the analysis of records atlocal and teleseismic distances, the duration of the rupture process was estimated to be around210 s (Tavera et al. 2007). This duration is almost twice the duration observed for the 23 June2001 Arequipa, southern Peru earthquake, which had a similar magnitude (Mw = 8.2).The significant durations (time of the build-up from 5% to 95% of the Arias intensity) of theaccelerograms used in the analysis are listed in Table 3. These durations are slightly longer forthe Lima records (about 100 s) than for the Ica records (about 80 s), reflecting the lengtheningof the signal as the waves travel farther away from the source.

In order to investigate the predictability of the ground motions observed during the Piscomainshock, they are compared to the predictions from a suite of equations for subduction-zone environments. The predictive models used here include a set of equations for interfacesubduction earthquakes developed using worldwide data (Atkinson and Boore 2003), as wellas regional equations developed for subduction-zone environments in Japan (Kanno et al.2006; Zhao et al. 2006b) and New Zealand (McVerry et al. 2006). The PGA equation of Ruizand Saragoni (2005), developed entirely with data recorded on the Chilean strong-motionnetwork during subduction events, is also included in this comparison. The selected equationsand their basic characteristics are listed in Table 5.

First, a qualitative comparison is carried out by plotting the observed values of PGAagainst median predictions from the various equations (Fig. 11). For a meaningful compar-ison, differences in the definitions used for the parameters of the equations, such as thehorizontal component, the magnitude scale and the source-to-site distance metric, have to beadjusted appropriately as they can otherwise result in systematic differences in the medianpredictions from the equations (Bommer et al. 2005). Except for the Ruiz and Saragoni (2005)equations, which use surface-wave magnitude MS and hypocentral distance Rhyp, the equa-tions are uniform in terms of the magnitude definition and distance metric used (moment

123

Bull Earthquake EngTa

ble

5Pr

edic

tive

equa

tions

for

grou

nd-m

otio

nsfr

omsu

bduc

tion

even

tsus

edin

the

pres

ents

tudy

Ref

eren

ceG

eogr

aphi

cco

vera

geG

roun

d-m

otio

npa

ram

eter

Hor

izon

tal

com

pone

ntde

fini

tion

Dat

abas

em

etad

ata

cove

rage

Site

clas

sific

atio

nsc

hem

e

Atk

inso

nan

dB

oore

(200

3)C

asca

dia,

Japa

n,M

exic

o,E

lSa

lvad

or,P

acifi

cno

rthw

est,

Ala

ska,

Cen

tral

Chi

lean

dPe

ruan

dth

eSo

lom

onIs

land

s

PGA

,PSA

Ran

dom

Mw

5.5

8.3

Rru

p11

550

kmN

EH

RP

site

clas

ses

base

don

VS,

30va

lue.

Onl

yfo

urcl

asse

s(B

toE

)ar

em

odel

led.

Kan

noet

al.(

2006

)Ja

pan,

supp

lem

ente

dby

377

over

seas

reco

rds

PGA

,PG

V,P

SAV

ecto

rial

lyre

solv

edM

w5.

08.

2R

rup

145

0km

Exp

licit

cons

ider

atio

nof

VS,

30th

roug

hap

plic

atio

nof

anem

piri

calc

orre

ctio

nfa

ctor

toth

ere

gres

sion

resu

ltsob

tain

edw

ithou

tthe

site

term

.M

cVer

ryet

al.(

2006

)N

ewZ

eala

nd,

supp

lem

ente

dby

66ov

erse

asre

cord

s

PGA

,PSA

Geo

met

ric

mea

nM

w5.

26.

8R

rup

304

00km

Thr

eesi

tecl

asse

sfo

llow

ing

New

Zea

land

site

clas

sific

atio

n,w

hich

isba

sed

onde

scri

ptio

nof

surf

ace

geol

ogy,

geot

echn

ical

prop

ertie

sof

the

mat

eria

lsan

dde

pth

tobe

droc

k.C

lass

esA

and

Bar

elu

mpe

din

toa

sing

leca

tego

ry,c

lass

Eis

not

mod

elle

d.Z

hao

etal

.(20

06b)

Japa

n,su

pple

men

ted

by20

8ov

erse

asre

cord

s

PGA

,PSA

Geo

met

ric

mea

nM

w5.

08.

3R

rup

030

0km

Four

site

clas

ses

dete

rmin

edba

sed

onV

S,30

valu

ean

dna

tura

lper

iod

Tof

the

site

.Pr

ovid

ean

equi

vale

nce

with

NE

HR

Pcl

assi

ficat

ion.

Rui

zan

dSa

rago

ni(2

005)

Chi

lePG

AL

arge

rPG

AM

S6.

27.

8R

hyp

363

15km

Two

site

clas

ses

(roc

kan

dha

rdso

il)ba

sed

onde

scri

ptio

nof

surf

ace

geol

ogy.

123

Bull Earthquake Eng

magnitude, Mw, and rupture distance, Rrup). As shown in Table 4, the predictive equations allconsider different site classification schemes; however, the schemes used by McVerry et al.(2006), Kanno et al. (2006) and Zhao et al. (2006b) are designed to be compatible with theNEHRP classification. For the Ruiz and Saragoni (2005) equation, generic rock is assumedto be equivalent to the NEHRP B/C boundary (VS,30 = 760 m/s), and hard soil is consideredequivalent to the NEHRP C/D boundary (VS,30 = 360 m/s).

Adjustments to account for differences in horizontal component definitions are madeusing the correlations derived by Beyer and Bommer (2006), adopting the geometric meanof the ground motion as reference definition. The Kanno et al. (2006) equation uses thevectorially-resolved component (square root of sum of squares of the two components in thetime domain), for which correlations are not available. For the PGA comparison shown inFig. 11 the vectorially-resolved component is assumed to be 27% greater than the geometricmean (Bragato and Slejko 2005). Figure 11 shows that the Ruiz and Saragoni (2005) equations

100 200

10

20

100

200

1000

Hypocentral distance, Rhyp (km)

PG

A(c

m/s

2 )

RockHard soil

Ruiz and Saragoni (2005) - MS 7.5

20 30 100 200 300

10

20

100

200

1000

Rupture distance, Rrup (km)

PG

A(c

m/s

2 )

NEHRP Class BNEHRP Class CNEHRP Class DNEHRP Class E

Atkinson and Boore (2003) - Mw 7.9

20 30 100 200 300

10

20

100

200

1000

Rupture distance, Rrup (km)

NZ Class A/BNZ Class CNZ Class D

McVerry et al. (2006) - Mw 7.9

20 30 100 200 300

10

20

100

200

1000

Rupture distance, Rrup (km)

PG

A(c

m/s

2 )

VS,30 = 760 m/sVS,30 = 523 m/sVS,30 = 255 m/sVS,30 = 180 m/s

Kanno et al. (2006) - Mw 7.9

20 30 100 200 300

10

20

100

200

1000

Rupture distance, Rrup (km)

JP Class IJP Class IIJP Class IIIJP Class IV

Zhao et al. (2006) - Mw 7.9

NEHRP Class BNEHRP Class CNEHRP Class C/DNEHRP Class DNEHRP Class D/ENEHRP Class Unknown(not used in analysis)

NEHRP SITE CLASS ASSUMED IN ANALYSIS

Fig. 11 Comparison between recorded PGA values and predictions of selected GMPE for subduction-zoneenvironments, as well as the equation of Ruiz and Saragoni (2005) based on Chilean data

123

Bull Earthquake Eng

provide reasonable predictions for the Lima class C to D sites, but significantly underpredictthe ground motions observed at the Ica stations. This is likely to be related to use of hypocen-tral distance, which poorly discriminates between sites located above the rupture plane (Ica)and those located a significant distance away (Lima). The other four models consistentlyoverpredict recorded data for the Lima NEHRP class B and C soil sites located about 100 kmfrom the source. Observations from class C/D and class D sites are generally overpredictedtoo, except for the McVerry et al. (2006) model. PGA values at short distances are grosslyunderpredicted, except by the Kanno et al. (2006) model, which tends to overpredict observa-tions from all site classes. Out of the five models considered, only twoAtkinson and Boore(2003) and McVerry et al. (2006)include soil non-linearity effects in their functional form.This would be expected to affect the quality of predictions for the softest site consideredin this study (CAL, class D/E); however, no consistent pattern of under- or over-predictioncan be observed for this station. Overall, for the PGA data available for this particular event,the level of agreement between observed and predicted ground motions seems to be drivenpredominantly by the modelling of magnitude scaling and distance attenuation.

A more detailed quantitative comparison between observed and predicted motions iscarried out using the maximum-likelihood-based method of Scherbaum et al. (2004). Thismethod allows the ranking of a set of candidate predictive equations according to theircapability to predict observed ground-motions, using a number of different goodness-of-fitmeasures. These goodness-of-fit measures are based on the distribution of the normalisedresiduals (difference between the observed and the predicted value of ground-motion dividedby the standard deviation of the predictive equation), and include the mean (MEANNR),median (MEDNR), standard deviation (STDNR) and median likelihood values (MEDLH)of the normalised residuals. A detailed explanation of these parameters can be found inScherbaum et al. (2004). The Scherbaum technique has been successfully applied in severalrecent studies to examine the suitability of existing equations for the prediction of groundmotions in different regions (e.g. Douglas et al. 2006; Bindi et al. 2006; Drouet et al. 2007;Hintersberger et al. 2007; Stafford et al. 2008).

For comparison with the set of predictive equations, the geometric mean of both hori-zontal components and the vectorially resolved component are computed for PGA and the5%-damped pseudo-spectral response acceleration at seven selected periods, ranging from0.05 to 3.0 s. Table 6 shows the results obtained in terms of the goodness-of-fit measuresMEANNR, MEDNR, STDNR and MEDLH, as well as the overall ranking of the equationin terms of capability to predict the observed ground motions, following the ranking schemeof Scherbaum et al. (2004).

The results show that the capability of the selected equations to predict the observedground motions varies significantly along the range of periods considered. Except for theAtkinson and Boore (2003) equation, which shows an unacceptable capability (rank D)at all periods considered, the equations generally perform better at short response periods(T 0.40 s), with low to intermediate prediction capabilities. The PGA equation of McVerryet al. (2006) is the only model associated with a high predictive capability. At long responseperiods (T 1.0 s), all models generally show a low or unacceptable prediction capability.This might indicate that the ground motions observed during the Pisco event have an unusuallow-frequency content when compared to other subduction-zone ground motions. The rankassigned to the models at the various response periods considered is in this case driven bythe central statistics of the distribution, MEANNR and MEDNR, which tend to deviate sig-nificantly from zero. These deviations are generally negative, which shows that the observedground motions were overall lower in amplitude than would have been expected from pre-dictions using currently available equations for subduction-zone environments. In view of

123

Bull Earthquake Eng

Tabl

e6

Com

pari

son

ofob

serv

edgr

ound

mot

ions

with

pred

ictio

nsfr

omse

lect

edeq

uatio

ns,u

sing

the

met

hod

ofSc

herb

aum

etal

.(20

04)

PGA

SA5%

at0.

05sa

SA5%

at0.

10s

SA5%

at0.

20s

SA5%

at0.

40s

SA5%

at1.

00s

SA5%

at2.

00s

SA5%

at3.

00s

Sche

rbau

met

al.(

2004

)cl

assi

ficat

ionb

Atk

inso

nan

dB

oore

(200

3)D

DD

DD

DD

D

McV

erry

etal

.(20

06)

A

CB

DD

CC

Kan

noet

al.(

2006

)C

BB

BC

DD

D

Zha

oet

al.(

2006

b)C

BB

CD

DD

D

Atk

inso

nan

dB

oore

(200

3)c

ME

AN

NR

1.0

640

.992

0.6

410

.976

1.0

350

.928

0.7

120

.420

ME

DN

R1

.392

1.0

070

.770

1.1

461

.387

1.1

691

.016

0.8

08ST

DN

R1.

211

0.97

70.

974

1.01

11.

111

0.79

80.

876

1.00

5M

ED

LH

0.13

50.

176

0.36

30.

234

0.12

00.

242

0.30

50.

418

McV

erry

etal

.(20

06)

ME

AN

NR

0.23

3

0.62

40

.327

0.6

170

.636

0.6

630

.368

ME

DN

R0.

017

0.

610

0.2

350

.987

1.0

460

.740

0.6

66ST

DN

R0.

951

0.

750

0.80

01.

141

1.08

10.

987

1.10

9M

ED

LH

0.74

1

0.53

10.

631

0.25

70.

203

0.27

30.

499

Kan

noet

al.(

2006

)M

EA

NN

R0

.655

0.4

260

.256

0.3

890

.553

1.0

481

.206

1.5

13M

ED

NR

0.6

360

.422

0.1

840

.406

0.5

721

.083

1.3

921

.658

STD

NR

0.55

10.

661

0.52

80.

571

0.56

00.

437

0.52

20.

784

ME

DL

H0.

525

0.51

40.

667

0.56

10.

517

0.27

90.

163

0.09

3Z

hao

etal

. (20

06b)

ME

AN

NR

0.5

700

.297

0.3

750

.578

0.7

250

.875

1.0

691

.150

ME

DN

R0

.666

0.3

320

.420

0.5

870

.994

0.8

041

.184

1.3

47ST

DN

R0.

617

0.60

00.

493

0.52

50.

678

0.50

70.

530

0.67

3M

ED

LH

0.50

50.

576

0.60

90.

557

0.32

00.

421

0.23

60.

177

The

uppe

rpa

rtof

the

tabl

egi

ves

anov

eral

lrat

ing

ofth

epe

rfor

man

ceof

each

equa

tion

for

the

grou

nd-m

otio

npa

ram

eter

ofin

tere

st,w

hile

the

low

erpa

rtpr

ovid

esde

tails

ofth

epa

ram

eter

sca

lcul

ated

toas

sign

this

ratin

ga

Coe

ffici

ents

are

nota

vaila

ble

for

this

peri

odfo

rth

eA

tkin

son

and

Boo

re(2

003)

and

McV

erry

etal

.(20

06)

equa

tions

.For

Atk

inso

nan

dB

oore

(200

3),t

hean

alys

isfo

r0.

04s

isus

edas

apr

oxy

bA

sses

smen

tof

capa

bilit

yof

equa

tion

topr

edic

tth

eob

serv

edgr

ound

mot

ions

,us

ing

the

follo

win

gra

nkin

gsc

hem

e:

Ran

kA

(hig

hca

pabi

lity)

:M

EA

NN

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