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 What does a seismicity map tell us? 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 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

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

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

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

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

Historical Instrumental Shanxi Graben Weihi rift

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

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

California Seismicity Many aftershock zones are still visible today, including 1952 Kern County earthquake aftershocks

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

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

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

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

Is seismicity migrating from New Madrid to Wabash?

What does seismicity show?

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

Li et al., 2007

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

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

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

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

Okal and Romanowicz, 1994

Numerical Simulation: How long do we expect to see aftershocks in New Madrid ++ 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 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 Background catalog: - a value/unit area - b value -Uniformly distributed in model region 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

Synthetic Catalogs Synthetic Aftershock Catalog Synthetic Background Catalog

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

Conclusions New Madrid seismicity dominated by aftershocks of earthquakes Seismicity here would remain detectably different from the background for ~ 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