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
Shan
xi G
rabe
nBohai 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
Shan
xi G
rabe
nBohai 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
Shan
xi G
rabe
nBohai 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
Shan
xi G
rabe
nBohai 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
Shan
xi G
rabe
nBohai 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 Gra
ben
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
