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

Thesis (Ph.D.) University of Alaska Fairbanks, 2001The 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

Kinematic behavior of southern Alaska constrained by westward decreasing postglacial slip rates on the Denali Fault, Alaska

Long-term slip rates for the Denali Fault in southern Alaska are derived using ^(10)Be cosmogenic radionuclide (CRN) dating of offset glacial moraines at two sites. Correction of ^(10)Be CRN model ages for the effect of snow shielding uses historical, regional snow cover data scaled to the site altitudes. To integrate the time variation of snow cover, we included the relative changes in effective wetness over the last 11 ka, derived from lake-level records and δ^(18)O variations from Alaskan lakes. The moraine CRN model ages are normally distributed around an average of 12.1 ± 1.0 ka (n = 22, ± 1σ). The slip rate decreases westward from ~13 mm/a at 144°49′W to about 7 mm/a at 149°26′W. The data are consistent with a kinematic model in which southern Alaska translates northwestward at a rate of ~14 mm/a relative to a stable northern Alaska with no rotation. This suggests progressive slip partitioning between the Denali Fault and the active fold and thrust belt at the northern front of the Alaska range, with convergence rates increasing westward from ~4 mm/a to 11 mm/a between ~149°W and 145°W. As the two moraines sampled for this study were emplaced synchronously, our suggestion of a westward decrease in the slip rate of the Denali Fault relies largely upon the measured offsets at both sites, regardless of any potential systematic uncertainty in the CRN model ages

Aftershock Sequences Modeled with 3-D Stress Heterogeneity and Rate-State Seismicity Equations: Implications for Crustal Stress Estimation

In this paper, we present a model for studying aftershock sequences that integrates Coulomb static stress change analysis, seismicity equations based on rate-state friction nucleation of earthquakes, slip of geometrically complex faults, and fractal-like, spatially heterogeneous models of crustal stress. In addition to modeling instantaneous aftershock seismicity rate patterns with initial clustering on the Coulomb stress increase areas and an approximately 1/t diffusion back to the pre-mainshock background seismicity, the simulations capture previously unmodeled effects. These include production of a significant number of aftershocks in the traditional Coulomb stress shadow zones and temporal changes in aftershock focal mechanism statistics. The occurrence of aftershock stress shadow zones arises from two sources. The first source is spatially heterogeneous initial crustal stress, and the second is slip on geometrically rough faults, which produces localized positive Coulomb stress changes within the traditional stress shadow zones. Temporal changes in simulated aftershock focal mechanisms result in inferred stress rotations that greatly exceed the true stress rotations due to the main shock, even for a moderately strong crust (mean stress 50 MPa) when stress is spatially heterogeneous. This arises from biased sampling of the crustal stress by the synthetic aftershocks due to the non-linear dependence of seismicity rates on stress changes. The model indicates that one cannot use focal mechanism inversion rotations to conclusively demonstrate low crustal strength (≤10 MPa); therefore, studies of crustal strength following a stress perturbation may significantly underestimate the mean crustal stress state for regions with spatially heterogeneous stress