searching for long duration aftershocks in continental interiors

Searching for Long Duration Aftershocks in Continental Interiors

Miguel Merino, Seth SteinNorthwestern University

Mid-continental seismicity is time-variable

Faults switch on & off: mechanisms unclear

Active for short periods & dormant for long ones

Aftershocks continue for long times

What does a seismicity map tell us?

McKenna. Stein & Stein, 2007

“During the past 700 years, destructive earthquakes generally occurred in different locations, indicating a migration of seismicity with time.”

(Camelbeeck et al., 2007)

Royal Observatory of Belgium Catalog

Migrating seismicity:

NW Europe

during the periodprior to the periodinstrumental events

Earthquakes in North ChinaEarthquakes in North China

OrdosPlateau

Shan

xi Gr

aben

Bohai Bay

Beijing

1303 HongtongM 8.0

Liu, Stein & Wang 2010

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North ChinaEarthquakes in North China

OrdosPlateau

Shan

xi Gr

aben

Bohai Bay

Beijing

1556 HuaxianM 8.3

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North ChinaEarthquakes in North China

OrdosPlateau

Shan

xi Gr

aben

Bohai Bay

Beijing

1668 TanchengM 8.5

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North ChinaEarthquakes in North China

OrdosPlateau

Shan

xi Gr

aben

Bohai Bay

Beijing

1679 SanheM 8.0

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North ChinaEarthquakes in North China

OrdosPlateau

Shan

xi Gr

aben

Bohai Bay

Beijing

1966 XingtaiM 7.2

1976 TangshanM 7.8

1975 HaichengM 7.3

Weihi rift

Historical

Instrumental

Shan

xi Gra

ben

Weihi rift

Rate-state friction predicts aftershock duration

1/loading rate

Plate boundary faults quickly reloaded by steady

plate motion after large earthquake

Faults in continents reloaded much more

slowly, so aftershocks continue much longer

Current seismicity largely aftershocks rather than

implying location of future large events

Stein & Liu, 2009

General pattern of long aftershock sequences in slowly deforming continental interiors

Stein & Liu 2009

Long duration aftershock sequences resolvable from low intraplate

background

Parsons, 2009

California Seismicity 2003-2006

Many aftershock

zones are still visible

today, including 1952

Kern County earthquake aftershocks

Seismicity (1970-1974) visible in the aftershock zones of large past central Nevada seismic belt

earthquakes

Systematicdecrease in

seismicity withtime

Ryall, 1977

Aftershock sequences continue in Haicheng and Tangshan >30 years after the main shocks

M. Liu

Question:

Do zones of low-magnitude seismicity within continents reflect aftershocks continuing for long times

or loci of future earthquakes

What does a seismicity map tell us?

McKenna. Stein & Stein, 2007

Tuttle (2009)

Meers fault, OklahomaActive 1000 years ago, dead now

Obermeier, (1998)

Wabash: M~7 6 Kybp

Seismicity migrates in Central US

Is seismicity migrating from New Madrid to Wabash?

What does seismicity show?

Why b-value difference?

1) Wabash has a relatively low b value. Could indicate high fault stressing rates, consistent

with stress migration following large 1811-1812 earthquakes

2) New Madrid has a relatively high b value. Could reflect NMSZ having more small earthquakes

that are 1811-1812 aftershocks

Li et al., 2007

High stressing rate could give rise to low b value

Wiemer & Schorlemmer. 2007

San Andreas Fault, Parkfield

2) Many recent NMSZ

events appear to be 1811-12 aftershocks

- have been used to map presumed rupture

- rate & size decreasing

- largest at the ends of presumed 1811-12

ruptures

Stein & Newman, 2004

To see whether New Madrid or Wabash anomalous,compare to central U.S background seismicity

NM

W

Although we often consider b=1 the norm, low values are common for intraplate areas

Sykes et al. 2008

Okal & Sweet 2007

Okal and Romanowicz, 1994

Numerical Simulation:How long do we expect to see

aftershocks in New Madrid• ++

Aftershock catalog:- Omori’s Law for #

earthquakes per year- b value (probability of given earthquake M/yr)-Uniformly distributed

NS and normal distribution EW across a

NS fault

Aftershock catalog:- Omori’s Law for #

earthquakes per year- b value (probability of given earthquake M/yr)-Uniformly distributed

NS and normal distribution EW across a

NS fault

Background catalog:- a value/unit area

- b value-Uniformly distributed

in model region

Background catalog:- a value/unit area

- b value-Uniformly distributed

in model regionCombine

background and aftershock catalogs

for designated aftershock region

Combine background and

aftershock catalogs for designated

aftershock region

T-test to check how long

aftershocks are detectable

from the background

T-test to check how long

aftershocks are detectable

from the background

Synthetic Catalogs

Synthetic Aftershock Catalog

Synthetic Background Catalog

Synthetic Catalog T-Test

€

TValue =XAftershock−μBackgroundSdBackground / n

Test probability that observed rate of seismicity (aftershocks) is significantly different from mean (background)

Different length catalogs simulated to decide when aftershocks can no longer be resolved from background

Conclusions

New Madrid seismicity dominated by aftershocks of 1811-1812 earthquakes

Seismicity here would remain detectably different from the background for ~200-230 years

Aftershocks could still be noticeable for even longer time in less seismic areas (US east coast?)

Concentrations of small intraplate seismicity may reflect large past (“ghost”) earthquakes

Could test possibility with paleoseismology