Seismic Source and Attenuation Studies in the Central and Eastern United States


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Virginia Tech


To better understand the ground motion and associated seismic hazard of earthquakes in the central and eastern United States (CEUS), this dissertation focuses on the source parameters and wave propagation characteristics of both tectonic earthquakes and induced earthquakes in the CEUS. The infrequent occurrence of significant earthquakes in the CEUS limits the necessary observations needed to understand earthquake processes and to reduce uncertainty in seismic-hazard maps. The well-recored aftershock sequence of the 2011 Mineral, Virginia, earthquake offers a rare opportunity to improve our understanding of earthquake processes and earthquake hazard in this populous region of the United States. Moreover, the rapid increase of seismicity in the CEUS since 2009 that has been linked to wastewater injection has raised concern regarding the potential hazard. In this dissertation, I first present a detailed study of the aftershock sequence of the 2011 Mw 5.7 Mineral, Virginia earthquake. It involves the hypocenter locations of ~3000 earthquakes, ~400 focal mechanism solutions, statistics of the aftershock sequence, and the Coulomb stress modeling that explains the triggering mechnanism of those aftershocks. Second, I examine the S-wave attenuation at critical short hypocentral distances (< 60 km) using the aftershock data. The observed S-wave amplitudes decay as a function of hypocenter distance R according to R^-1.3 - R^-1.5, which is substantially steeper than R^-1 for a homogeneous whole space. Finally, I propose and apply a stable multi-window coda spectral ratio method to estimate corner frequencies and Brune-type stress drops for the 2011 Mineral, Virginia mainshock and aftershocks, as well as induced earthquakes in Oklahoma. The goal of this comparative study is to find out whether or not there are systematical differences in source parameters between tectonic earthquakes and induced earthquakes in the CEUS. I found generally much higher stress drops for the Mineral, Virginia sequence. However, the stress drops for those induced earthquakes in Oklahoma exhibit large varation among individual earthquake sequences, with the large mainshocks having high stress drops (20-30 MPa, Brune-type) except for the 2011 Mw 5.6 Prague, Oklahoma earthquake. And spatially varying stress drops indicates strong fault heterogeneity, which in the case of induced earthquakes may be influenced by the injection of fluids into the subsurface.



Virginia aftershocks, geometrical spreading, induced earthquakes, stress drop