CONTRASTING SEISMIC RATES BETWEEN THE NEW MADRID AND WABASH VALLEY SEISMIC ZONES: STRESS TRANSFER OR AFTERSHOCKS?

The Wabash Valley, the northeastern extension of the New Madrid seismic zone, has been seismically active in the past centuries. Despite their similarities, the two zones have an intriguing difference in seismicity rates. Both have similar numbers of magnitude 5-6 earthquakes, but New Madrid has a higher b-value and hence more small earthquakes. We see two possible causes for this difference. The low Wabash value could indicate high stressing rates consistent with stress migration following the 1811-1812 New Madrid earthquakes. Alternatively, the New Madrid zone has a high b because many of the earthquakes there are aftershocks of the 1811-1812 earthquakes. We assess whether the values are “high” or “low” by comparing them to those for the entire central U.S. excluding both the New Madrid and Wabash zones. We find that the Wabash Valley b value is lower than New Madrid’s but closer to that for the central U.S. excluding both zones. We hence view the Wabash value as more typical of the central U.S., and New Madrid value as unusually high, because New Madrid seismicity is dominated by aftershocks of the 1811-1812 earthquakes. Hence the lower Wabash value need not indicate loading by stresses due to these large earthquakes. Assessing whether such loading is occurring will require assessing whether the associated strain signal is resolvable in GPS data.