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Searching for Long Duration Aftershocks in Continental Interiors

Searching for Long Duration Aftershocks in Continental Interiors. Miguel Merino, Seth Stein Northwestern 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

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Searching for Long Duration Aftershocks in Continental Interiors

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  1. Searching for Long Duration Aftershocks in Continental Interiors Miguel Merino, Seth Stein Northwestern University

  2. 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

  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. General pattern of long aftershock sequences in slowly deforming continental interiors 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 Stein & Liu 2009

  11. Long duration aftershock sequences resolvable from low intraplate background Parsons, 2009

  12. California Seismicity 2003-2006 Many aftershock zones are still visible today, including 1952 Kern County earthquake aftershocks

  13. Seismicity (1970-1974) visible in the aftershock zones of large past central Nevada seismic belt earthquakes Systematic decrease in seismicity with time Ryall, 1977

  14. Aftershock sequences continue in Haicheng and Tangshan >30 years after the main shocks M. Liu

  15. 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

  16. Wabash: M~7 6 Kybp Obermeier, (1998) Tuttle (2009) Seismicity migrates in Central US Meers fault, Oklahoma Active 1000 years ago, dead now

  17. Is seismicity migrating from New Madrid to Wabash? Both active today Since 1811-12 M 7 events - M 6 events only in NMSZ - M 5 events throughout region

  18. log10N = a – bM b values (slopes) – differ New Madrid has more small events New Madrid b = 0.95 Wabash b = 0.72 What does seismicity show?

  19. 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

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

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

  22. 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

  23. 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

  24. Although we often consider b=1 the norm, low values are common for intraplate areas Sykes et al. 2008 New York City–Philadelphia area b = .7 Okal & Sweet 2007

  25. b about 1 comes from combining large magnitude ranges, including M >7 Okal and Romanowicz, 1994

  26. Numerical Simulation:How long do we expect to see aftershocks in New Madrid • ++ • Background catalog: • a value/unit area • b value • Uniformly distributed in model region Combine background and aftershock catalogs for designated aftershock region T-test to check how long aftershocks are detectable from the background • 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

  27. Synthetic Catalogs Synthetic Aftershock Catalog Synthetic Background Catalog

  28. Synthetic Catalog T-Test 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

  29. 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

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