The Role of Lithology in Fault Re-Strengthening: A Case Study of the 2011 Prague, Oklahoma Induced Earthquake Sequence

Abstract:

Faults dynamically weaken during earthquakes and strengthen between them. Thus, fault healing is an important part of the seismic cycle, which depends on in situ conditions as well as lithology. Fault re-strengthening after earthquakes can be studied at large scales via repeating earthquakes and at small scales through slide-hold-slide laboratory friction experiments. However, the ability to connect the wide range of scales is limited by uncertainty about the lithology (as well as fractures and fluids) where repeating earthquakes occur at depth. Here, we analyze events from the 2011 Prague, Oklahoma induced earthquake sequence, which included three Mw 5+ events. The Mw 5.7 mainshock initiated in crystalline basement with aftershock activity extending into the shallower, predominantly limestone, Hunton and Arbuckle groups. We identify repeating events in both the basement and the Arbuckle group by cross correlating waveforms with a 1 Hz high-pass filter using an average cross correlation value of at least 90% at three stations. Repeating earthquakes are recorded immediately after the November 6 Mw 5.7 and continue for at least two months. These events occur at the top and bottom edges of the main patch of aftershock activity, suggesting that afterslip may have been occurring in these areas surrounding mainshock larger slip patches.

To compare fault re-strengthening from repeating earthquakes to laboratory derived healing rates, we sampled cores of Arbuckle limestone and collected basement rock samples from outcrops. We measured frictional strength and healing in a triaxial deformation apparatus at confining pressures of 25-100 MPa to mimic lithostatic stress of each lithological layer and fluid pressures ranging from 10 MPa to near lithostatic pressures. We find that increasing pore pressure increases system stiffness, allowing for more healing during a given time period. We compare the healing rates measured within the lab to those from repeating earthquakes and determine how healing rates might differ within a single fault due to lithologic differences, and whether such differences in healing rates are responsible for future strength heterogeneity at the end of the next seismic cycle.