Seismological Constraints On Tectonics Of Southern And Central Alaska: Earthquake Locations And Source Mechanisms

Abstract

The major emphasis of this thesis is on investigations of earthquake locations and source mechanisms and what we can learn about Earth structure from them. I used a Joint Hypocenter Determination (JHD) method to improve the earthquake locations obtained after routine data processing. Over 15,000 subduction zone earthquakes in southern Alaska and over 3,600 crustal earthquakes in central Alaska with magnitudes ML ? 2 that occurred from 1988 to 2000 were relocated. I found that the relative earthquake locations can be improved with the use of the JHD relocation technique (30--60% reduction in RMS residuals). Thus, many details of the subduction zone geometry and crustal structure can be mapped. To constrain source characteristics, I use a moment tensor inversion method that simultaneously inverts for the source parameters and velocity structure. First, I apply this technique to the sequence of strong earthquakes in the Kodiak Island region, including December 6, 1999 and January 10, 2001 MW 7 events. Next, I expand this approach to moderate-sized ( ML ? 4) crustal earthquakes in central Alaska and calculate 38 moment tensors. I demonstrate that the moment tensor inversion of regional waveforms provides reliable results even when recordings from a single broadband station are used. A catalog of the moment tensors together with the focal mechanisms obtained using conventional P-wave first motion analysis is used to calculate principal stress directions in central Alaska. I find that the stress state in the crust is inhomogeneous and that the orientation of the maximum compressive stress changes from a SE-NW to SSW-NNE orientation from west to east across interior Alaska. One more topic of this thesis is the application of the array analysis to understanding characteristics of anomalous seismic phases observed in the records of the intermediate-depth Alaskan subduction zone earthquakes. I identified two secondary phases arriving with 1--3 s and 7--12 s delays after the first P-wave arrival. They are interpreted as S-to-P and P-to-S converted phases at the upper/lower surface of the subducted slab.