1. CONTRASTING SEISMIC RATES BETWEEN THE NEW MADRID AND WABASH VALLEY SEISMIC ZONES: STRESS TRANSFER OR AFTERSHOCKS? Miguel Merino, Seth Stein & Emile Okal Northwestern University Mian Liu University of Missouri

2. 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

3. “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

7. during the period prior to the period instrumental events Earthquakes in North China Ordos Plateau Shanxi Graben Bohai Bay Beijing 1303 Hongtong M 8.0 Liu, Stein & Wang 2010 Weihi rift

8. during the period prior to the period instrumental events Earthquakes in North China Ordos Plateau Shanxi Graben Bohai Bay Beijing 1556 Huaxian M 8.3 Weihi rift

9. during the period prior to the period instrumental events Earthquakes in North China Ordos Plateau Shanxi Graben Bohai Bay Beijing 1668 Tancheng M 8.5 Weihi rift

10. during the period prior to the period instrumental events Earthquakes in North China Ordos Plateau Shanxi Graben Bohai Bay Beijing 1679 Sanhe M 8.0 Weihi rift

11. during the period prior to the period instrumental events Earthquakes in North China Ordos Plateau Shanxi Graben Bohai Bay Beijing 1966 Xingtai M 7.2 1976 Tangshan M 7.8 1975 Haicheng M 7.3 Weihi rift

12. Historical Instrumental Shanxi Graben Weihi rift

13. New Madrid & Wabash are similar

15. New Madrid & Wabash earthquakes both probably occur - at least in part - by reactivating faults associated with Paleozoic rifting Braile et al., 1986

16. Compare seismicity: Similar number of magnitude 5 events

17. 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

18. Li et al., 2007

19. High stressing rate could give rise to low b value Wiemer & Schorlemmer. 2007 San Andreas Fault, Parkfield

20. 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

21. 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

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

23. Although we often consider b ~1 the norm, low values are common for intraplate areas Sykes et al. 2008 Okal & Sweet 2007 b=0.7

24. Okal and Romanowicz, 1994

25. 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