SMALL EARTHQUAKES IN NE KANSAS AND THEIR POSSIBLE CAUSES

During the past few years the geophysics group at Kansas State University (KSU) has installed and operated two transportable seismic arrays and a permanent seismic station in Kansas. One of the purposes of the experiments is to detect small earthquakes in the area, and another is to study the velocity and anisotropic structure of the crust and the mantle, aiming at finding a possible explanation for the occurrence of intraplate earthquakes in this area, which has experienced a few MMI 7 earthquakes since 1867.

The first transportable experiment was conducted between June, 2000 and February, 2001. Eight broadband seismic stations were deployed along a 450-km-long, EW profile between Palco and Atchison, Kansas. The second transportable experiment took place between April and October, 2001, during which eight short-period stations were deployed along the periphery of an approximate circle with a radius of about 20 km, centered at Manhattan, KS. The permanent station is located on the KSU Konza Prairie Biological Research Station. So far it is the second permanent broadband seismic station in Kansas.

While the data sets are still being analyzed, we have identified tens of small earthquakes, most of which have occurred in NE Kansas. In addition, by stacking P-to-S converted seismic waves from the the base of the crust, we have found that the crust beneath the Midcontinent rift (MCR) and the areas within about 120 km on each side of the rift axis is thickened by up to 12 km relative to the adjacent areas. The thickening was likely the result of the lateral compression during the closure of the MCR about 1.1 billion years ago. Based on previous data and our new crustal thickness measurement, we hypothesize that the long-lasting uplift of the Nemaha Ridge is the result of the uplift of the Moho toward isostatic balance. Such an uplift is possibly the ultimate cause of earthquakes in NE Kansas.