ANALYSIS OF THE 1973-74 ACORES EARTHQUAKES

byArtur Ravara1 ond R. Teixeira Duarte=

In October 1973 - February 1974 the Pico and Faial Islands (Açores) were hitby a seismic swarm reloted to volvanic activity. Damage observed on rural dwel-lings is reported with brief comments on problems relating to fatigue. in designingagainst a seismic swarm. The recorded accelerograms are onalyzed in order to iden-tify the main characteristics of the ground mation. Response spectro ond powerspectrol density ofoccelerotlon are presentsd ond the nem eplcentrcl chcrccter ofthe records is highlighted.

OBSERVE O DAMAGE

The Açores Islands. situated in the mid Atlantic rlft, have severa I! times beenhit by earthquakes of tectonic or volcanic origino In October 1973 - February 1974 aselsmíc swarm centered on Pico Island (Fig. 1) caused several hundred earth-quokes. wíth Richter magnitude ranging up to 5. and maximum Mercalli intensityup to 7.8

28°W

~GRACIOSA;:t.. TERCEIRAFAIAL ~

~~~ S.JORGEHO~ / PICO

38°N----·-4---------+~--~------------+_-----------S.MATEUS/Sta. LUZIA

300W·CORVO

G FLORES

~S. MIGUEL

Sta. MARIA~

FIG. 1 - Açores Islands

I) Head of Structures Department. Laboratorio Nacional de Engenharia Civil, Av. do Brasil Lisboa5. Portugal.

2) Civil Engineer. Laboratorio Nacional Engenharia Civil. Av. do Brasil, Lisboa 5. Portugal.

135

Those earthquakes were mainly caused bV volcanic activity. but some may 0150

have been of tectonic origino

The buildings on Pico lskind are mainly ru ro I dwelllings. often of poor masonryconstruction. Reported damage were 176 houses collopsed, 592 needing repairs and1261 showing minor damage. The St. Moteus church was havily domoqed, but theschools, when of recent constructlon. presented onlv little damage.

The effects of the earthquakes were 0150 severely felt on nearby Fainal lslcnd.where. in Horta. some tive hundred housos were damaged. The concentration ofdcrncqe in Conceição and Flamengos buroughs indicates some soil amplificationphenomena.

It must be noted that a seismic sworrn is a very different sltuotlon from theusual one in earthquake engineering where ruin is usually caused in a low-cyclef.atigue oontext. The long time duration of a swarm. and the sometimes hour-Iongground vibration may give rise to «true» fatigue problems.

ANALYSIS OF RECORDED GROUND MOTION

The progressive character of the seismic swarm made it possible to set up afield monitoring stctlon, which recorded the major shocks. These events occurredon 18 th and 23th November and on 11th Decernber. The first two were recorded inS. Mateus. the last in Santa Luzia, The N.S. component of the 11th December earth-quake was not recorded.

The accelerograms were processed at LNEC according to standard techniques,and are presented in figo 2. Maximum cccelerotíons. veloclties and olsplocementsare tcbukrtedln Table I.

EventMaximum

Acceleration(cm/sec.')

MaximumDlsplacement

(em)

Table I - Maximum Values of Ground Motion

MaximumVeloeity

(em/sec.)

! Vert 1 29,3 I 0.91 I 0.084

NO~:~ber I~~!~:==~::~-. -==-!..~;-=l~---~:;!~=--------- ---1---'- '--- --- ---

23th 1_Ver_t _1 194 __1

5.35 __ _ 0_.2-,.-3__1i NS 1 273 ,12.8 0.56

November i-EW--I 212--j---10,i-- --0.64------ ..----1----'--- --1--- -- - - --

11th . Vert 1 221 i 12.6 .__ 0,96Oecember I EW I 101 I 4.86 0.49

The response spectra are presented in figo 3 to 10 and some power spectraldensity of uccelerctlon in figo 11 to 14. Owing to the near epicentral character ofthe ground motion these spectro hove a substancially high frequency content(~ 5Hz).

136

REFERENCES

Anuário Sismológico de Portugal 1973, Serviço Meteorológico Nacional. Usbon.

Machado, F.; Trepa, N.V.; Serin, C. and Nunes, J.C.: 1975, Crise Sísmica do Pico (Açores) Nov.1973 Comunicações dos Serviços Geológicos de Portugal.

137

zo

~ -1001

w -200.....J

w +200

~ +100 ! 23/11 EWO~.At\~+i\.L!I-\-It+lI-+1, I~,~

-100 i-2001 I+200T

1+100

o ~~Jl.-WtI-+II+H\-I

-100

-200

+100~~~lA~ 11/12 EWO . IrvvWVVVJ\t~t'ifVJ\~~~~~

-100

18/11 VERT.

18/11 NS

23/11 N S

11/12 VERT.

O~~ ._~---~-~-~-~-~-~-----.

10 TIME (5)5

FIG. 2 - ACCELEROGRAMS OF 18 NOV, 23 NOV ANO 11OEC.

138

zo~o:::W....JWUU<1:

zo~<1:o:::W....J

,w·UU<1:

18/11 VERT.

e = O. 5 • lO"/.

o 2 3 4· 5 TCs)

FIG.3 - RE5PONSE SPECTRA

18/11 EW

C=O,5,1O%zof-<1:

~ ~w

~ \o 2 3 4 5 T(s)

FIG. 5 - RESPONSE SPECTRA

18/11 N S

C=O.5.1O%

2 3 4 5 T(s)

FIG. 7 - RESPONSE SPECTRA

23/11 VERI -lC=O.5.1O~. :zo

~a::w....JWUU«

o 1 2 3 4 5 T(s)

FIG. 4 - RESPONSE SPECTRA

zo~<1:a::w....JWUU«

23/11 E W

C=O.5.1O%

234

FIG.6 - RESPONSE SPECTRA

zo~<1:a::w....JWUU<1:

23/11 NS

C=O.5.1O%

o 234 5 T(s)-... FIG. 8 - RESPONSE SPECTRA

5 T(s)

139

zo~a::w....JWUU4:

11/12 VERT.

C=0.5. toei.

o 2 3 4 5 T(:;)

FIG. 9 - RESPONSE SPECTRA

-M~N

Eu'--

18/11 EW

:iWo00....JU"l

«o::~oWo...lf)

20 30 f{Hz)

FIG.ll - POWER SPECTRUM

-M~N

Eu'--

18/11 N S

zWo00....J

«o::~uWo...(/)

20 30 f(Hz)

FIG.13 - POWER SPECTRUM

140

zQ~o::W....JWUU4:

11/12 EW

~: 0.5.10·/.

o.-..0..J N«-C>'--

2 3 4 5 T{s)FIG.l0 - RESPONSE SPECTRA

-M~N

Eu- 23/11 E W

:iWooU'>..JN

4:o::..-uwo...(/)

20 30 f(Hz)

FIG.12 - POWER SPECTRUM

-M~N

E~ZWo00-lU'>«o::~uWc,(/)

23/11 NS

20 30 f(Hz)

FIG. 14 - POWER ·SPECTRUM