IS INCREASED SEISMICITY IN OKLAHOMA/KANSAS WHY M>4 EARTHQUAKES IN THE NEW MADRID/WABASH SEISMIC ZONE HAVE CEASED?

A large population and the potential for strong earthquakes makes the New Madrid/Wabash region (NMWB) of considerable concern as a seismic hazard. In the NMWB the rate of M≥4 from 1970 through 2008 averaged 2 every 3 years, but no M≥4 earthquakes have occurred within the last 12 years. This is twice the length of previous intervals without M≥4 earthquakes in the NMWB. Since 2008, there have been 96 M≥4 earthquakes in a 4.0^o of latitude by 3.5^o of longitude area of Oklahoma and Kansas (OKKS) equivalent in area to the NMWB. From 1970 through 2008 there were only 3 M≥4 earthquakes in OKKS. A significant body of evidence supports an underground fluid-injection induced origin for the increased post-2008 OKKS seismicity. Is the current lull in M≥4 NMWB events a random fluctuation or is it a consequence of the OKKS increased seismicity? Using data from the ANSS earthquake catalog for the last 50 years, we computed seismic energy release for the NMWB and for the OKKS. Although the rate of smaller earthquakes (M2.5 to M3.5) has increased by ~50% in the NMWB, energy release in OKKS far exceeds NMWB since 2008. A mechanism for the transfer of seismicity from NMWB to OKKS is not clear, but our empirical observation regarding M≥4 earthquakes suggests that increase in strain rate in one part of a rigid plate interior can diminish strain in a contiguous part even if the increased strain rate is induced by shallow crustal fluids. The rate of M≥4 earthquakes in OKKS has slowed down significantly since 2019 (2 M≥4 EQs as of July 1, 2020). We propose a testable hypothesis. If the rate of M≥4 EQs stays low in the OKKS zone, the NMWB zone will resume M≥4’s in the not-too-distant future, perhaps one to two years.