Earthquake source mechanisms and three-dimensional wavefield simulations in Alaska

• Main Author: Silwal, Vipul
• Other Authors: Tape, Carl, Christensen, Douglas, West, Michael, Ruppert, Natalia, Freymueller, Jeffrey
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2018
• Subjects: Seismic event location; Alaska; Earthquakes; Southcentral Alaska; Seismic waves; Seismic wave propagation; Plate tectonics; Fairbanks; Anchorage; Yakutat
• Online Access: http://hdl.handle.net/11122/9685

Dissertation (Ph.D.) University of Alaska Fairbanks, 2018 This thesis presents: (1) a set of earthquake source mechanism catalogs for Alaska and (2) a threedimensional seismic velocity model of Alaska. The improved earthquake sources are used within the velocity model for generating synthetic seismograms, which are then compared with recorded seismograms to assess the quality of the velocity model. An earthquake source mechanism can be modeled as a moment tensor, which is a 3 × 3 symmetric matrix. We estimate the moment tensor for earthquakes by comparing observed waveforms (body waves and surface waves) with synthetic waveforms computed in a layered model. The improved moment tensor solutions are obtained by utilizing both the body waves and surface waves at as many broadband stations as possible. Further improvement in the inversion technique is obtained by (1) implementation of L1 norm in waveform misfit function and (2) inclusion of first-motion polarity misfit in the misfit function. We also demonstrate a probabilistic approach for quantifying the uncertainty in a moment tensor solution. Moment tensors can be used for understanding the tectonics of a region. In the Cook Inlet and Susitna region, west of Anchorage, we determined moment tensor solutions for small-tointermediate magnitude (M ≥ 2.5) crustal earthquakes. Analyzing these small earthquakes required us to modify the misfit function to include first-motion polarity measurements, in addition to waveform differences. The study was complemented with the probabilistic hypocenter estimation of large historical earthquakes (Mw ≥ 5.8) to assess their likelihood of origin as crustal, intraslab, or subduction interface. The predominance of thrust faulting mechanisms for crustal earthquakes indicate a compressive regime within the crust of south-central Alaska. Wavefield simulations are performed in three regions of Alaska: the southern Alaska region of subduction, the eastern Alaska region with the accreting Yakutat microplate, and the interior Alaska ...