CONTRASTING SEISMIC RATES BETWEEN THE NEW MADRID AND

WABASH VALLEY SEISMIC ZONES: STRESS TRANSFER OR

AFTERSHOCKS?

Miguel Merino, Seth Stein & Emile OkalNorthwestern University

Mian LiuUniversity of Missouri

Mid-continental seismicity is time-variable

Faults switch on & off: mechanisms unclear

Active for short periods & dormant for long ones

Is seismicity migrating from New Madrid to Wabash?

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 China

Large events often pop up where there was little seismicity!

OrdosPlateau

Sha

nxi G

rabe

n

Bohai Bay

Beijing

1303 HongtongM 8.0

Liu, Stein & Wang 2010

Weihi rift

Intraplate region of North China

during the periodprior to the periodinstrumental events

Earthquakes in North China

Large events often pop up where there was little seismicity!

OrdosPlateau

Sha

nxi G

rabe

n

Bohai Bay

Beijing

1556 HuaxianM 8.3

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North China

Large events often pop up where there was little seismicity!

OrdosPlateau

Sha

nxi G

rabe

n

Bohai Bay

Beijing

1668 TanchengM 8.5

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North China

Large events often pop up where there was little seismicity!

OrdosPlateau

Sha

nxi G

rabe

n

Bohai Bay

Beijing

1679 SanheM 8.0

Weihi rift

during the periodprior to the periodinstrumental events

Earthquakes in North China

Large events often pop up where there was little seismicity!

OrdosPlateau

Sha

nxi G

rabe

n

Bohai Bay

Beijing

1966 XingtaiM 7.2

1976 TangshanM 7.8

1975 HaichengM 7.3

Weihi rift

No large (M>7) events ruptured the same fault segment twice in N. China since 1303

Historical

Instrumental

Shan

xi G

rabe

n

Weihi rift

New Madrid & Wabash are similar

Both active today

Since 1811-12 M 7 events

- M 6 events only in NMSZ

- M 5 events throughout region

6Ka Wabash paleoearthquake probably similar size to New Madrid 1811-12

Obermeier, 1998

New Madrid & Wabash earthquakes both probably occur - at least in part - by reactivating

faults associated with Paleozoic rifting

Braile et al., 1986

Compare seismicity: Similar number of magnitude 5 events

log10N = a – bM

b values (slopes) – differ

New Madrid has more small events

New Madrid b = 0.95

Wabashb = 0.72

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

1) Stress Migration

Model predicts increased stress in Wabash Valley since 1811-1812 events

Li et al., 2007

High stressing rate could give rise to low b value

Creeping fault segment

Locked fault segment(asperities)

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

Rate-state friction predicts aftershock

duration 1/loading rate

Plate boundary faults quickly

reloaded by steady plate motion which overwhelms stress effect of mainshock

Faults in continents reloaded much more slowly, so

aftershocks continue much

longer

Stein & Liu, 2009

long aftershock sequences often observed in slowly

deforming continental interiors

Stein & Liu 2009

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

Entire region b = .9Excluding both seismic zones b = .83

Wabash value lower than New Madrid’s but closer to that for central U.S. excluding both zones.

Wabash appears more typical of the central U.S., and New Madrid value seems higher.

NM

W

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

New York–Philadelphia area

Sykes et al. 2008

Okal & Sweet 2007

b=0.7

Okal and Romanowicz, 1994

b ~ 1 arises for global data set averaging large magnitude range

including M >7

Conclusions

New Madrid b ~ 1, Wabash b ~0.7

Wabash similar to rest of central US, New Madrid higher

Implications:

New Madrid seismicity dominated by aftershocks of the 1811-1812 earthquakes

Wabash value need not indicate loading by stresses due to these large earthquakes