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CONTRASTING SEISMIC RATES BETWEEN THE NEW MADRID AND WABASH VALLEY SEISMIC ZONES:

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. Mid-continental seismicity is time-variable

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CONTRASTING SEISMIC RATES BETWEEN THE NEW MADRID AND WABASH VALLEY SEISMIC ZONES:

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  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) Migrating seismicity: NW Europe Royal Observatory of Belgium Catalog

  4. Intraplate region of North China Earthquakes in North China during the period prior to the period instrumental events Large events often pop up where there was little seismicity! Beijing Bohai Bay Ordos Plateau Shanxi Graben 1303 Hongtong M 8.0 Weihi rift Liu, Stein & Wang 2010

  5. Earthquakes in North China during the period prior to the period instrumental events Large events often pop up where there was little seismicity! Beijing Bohai Bay Ordos Plateau Shanxi Graben Weihi rift 1556 Huaxian M 8.3

  6. Earthquakes in North China during the period prior to the period instrumental events Large events often pop up where there was little seismicity! Beijing Bohai Bay Ordos Plateau Shanxi Graben Weihi rift 1668 Tancheng M 8.5

  7. Earthquakes in North China during the period prior to the period instrumental events Large events often pop up where there was little seismicity! 1679 Sanhe M 8.0 Beijing Bohai Bay Ordos Plateau Shanxi Graben Weihi rift

  8. Earthquakes in North China during the period prior to the period instrumental events Large events often pop up where there was little seismicity! 1975 Haicheng M 7.3 Beijing Bohai Bay 1976 Tangshan M 7.8 Ordos Plateau Shanxi Graben 1966 Xingtai M 7.2 Weihi rift

  9. Historical Instrumental No large (M>7) events ruptured the same fault segment twice in N. China since 1303 Shanxi Graben Weihi rift

  10. 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

  11. 6Ka Wabash paleoearthquakeprobably similar size to New Madrid 1811-12 Obermeier, 1998

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

  13. log10N = a – bM bvalues (slopes) – differ New Madrid has more small events New Madrid b= 0.95 Wabash b = 0.72 Compare seismicity: Similar number of magnitude 5 events

  14. Why b-value difference? 1) Wabash has a relatively low bvalue. Could indicate high fault stressing rates, consistent with stress migration following large 1811-1812 earthquakes 2) New Madrid has a relatively high bvalue. Could reflect NMSZ having more small earthquakes that are 1811-1812 aftershocks

  15. 1) Stress Migration Model predicts increased stress in Wabash Valley since 1811-1812 events Li et al., 2007

  16. High stressing rate could give rise to low b value San Andreas Fault, Parkfield Creeping fault segment Locked fault segment (asperities) Wiemer & Schorlemmer. 2007

  17. 2) Many recent NMSZ events appear to be 1811-12 aftershocks Stein & Newman, 2004 • have been used to map presumed rupture • - rate & size decreasing • largest at the ends of presumed 1811-12 ruptures

  18. long aftershock sequences often observed in slowly deforming continental interiors 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 Stein & Liu 2009

  19. To see whether New Madrid or Wabash anomalous, compare to central U.S background seismicity Entire region b = .9 Excluding 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

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

  21. b ~ 1 arises for global data setaveraging large magnitude range including M >7 Okal and Romanowicz, 1994

  22. 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

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