Gene A. Ichinose

Source Parameters of Eastern California and Western Nevada Earthquakes from Regional Moment Tensor Inversion

The moment tensors and centroid depths are estimated for eastern California and western Nevada earthquakes from regionally recorded long-period seismograms using the moment-tensor inversion method. We compiled the moment tensor solutions into a catalog complete for earthquakes with M W > 4 since 1990. The earthquakes in this study are mostly located within the aftershock zones of Eureka Valley, Double Spring Flat, Coso, Ridgecrest, Fish Lake Valley, and Scottys Junction, with the remaining earthquakes distributed across the region. We validated the moment tensor solutions by a comparison with Harvard centroid moment tensor (CMT) and P -wave first-motion focal mechanism solutions. The mean difference in strike, dip, and rake between our moment tensors and either Harvard CMT solutions or first-motion focal mechanisms were less than approximately 15° with a standard deviation less than 10°. We also examine the solution mean and variance by inverting resampled datasets using the delete- j Jackknife resampling method. Based on two well-recorded sample events, with more than 11 and 14 associated recording stations, the P - and T -axis trend and plunge can be determined to within ±10° when at least three or four well-distributed stations are used in the inversion. We separated the T -axis trends based on the approximate boundaries for three tectonic regions: Sierra Nevada, Walker Lane-eastern California shear zone, and Basin and Range. Relative to the mean T -axis trend for the Sierra Nevada, the mean T -axis trend for the Walker Lane region is rotated clockwise by 25° and the mean T -axis trend for the Basin and Range is rotated clockwise by 40°. We separated the earthquakes into strike-slip and normal-slip focal mechanisms based on a simple rake angle criterion. About 70% of modern earthquakes and 73% of the 11 large historical earthquakes since 1860 are in the strike-slip category. The fraction of seismic moment released as strike-slip earthquakes is approximately 50% for modern earthquakes and 75% for historical earthquakes. The difference is well within the variability expected from short catalogs. These results emphasize the importance of the component of right-lateral shear within the region and are qualitatively consistent with recent geodetic results. Manuscript received 11 January 2002.

The potential hazard from tsunami and seiche waves generated by large earthquakes within Lake Tahoe, California-Nevada

We investigate the potential of local earthquakes to generate tsunamis and seiches within Lake Tahoe. We calculated the long wavelength oscillations generated by 3 hypothetical M w > 7 earthquake scenarios for faults with normal slip directly under and outside the lake basin. The scenarios involving fault slip under the lake are the North Tahoe-Incline Village and West Tahoe-Dollar Point scenarios. The Genoa scenario involves a fault that crops out 10 km east of the lake. Faulting beneath the lake generates a tsunami followed by a seiche that continues for hours with waves as large as 3 to 10 m. The seiche potentially threatens low lying lakeside communities and lifelines. We also compare the spectral characteristics of synthetic tide gauge records with wind swell observations. The fundamental mode calculated for a seiche is consistent with the wind swell observations.

Crustal thickness variations beneath the peninsular ranges, southern California

We investigate the crustal thickness under the Peninsular Ranges using P-to-S converted phases of teleseismic body waves recorded on a temporary broadband seismometer array and isolated by the receiver function method. Ps minus P times at sites west of a compositional boundary that separates the Peninsular Ranges batholith into east and west zones indicate a relatively flat, deep Moho. Ps minus P times at sites east of the compositional boundary decrease eastward, Moho depth estimates (made from the Ps delays and crustal velocities from seismic tomography) indicate a relatively constant 36 to 41 km thick crust in the western zone. In the eastern zone the crust thins rapidly from 35 km thick at the compositional boundary to 25 km at the edge of the Salton trough, a lateral distance of 30 km. The lack of correlation between topography and Moho depths suggests compensation via lateral density variations in the lower crust or upper mantle. We propose that the compositional boundary decouples the eastern and western portions of the batholith, and that the eastern portion has thinned in response to regional Miocene extension, or Salton trough rifting, or both.

Moment Tensor and Rupture Model for the 1949 Olympia, Washington, Earthquake and Scaling Relations for Cascadia and Global Intraslab Earthquakes

We reanalyzed the 13 April 1949 Olympia, Washington, earthquake by using digitized records and first-motion polarities from long-period seismograms. The moment-tensor mechanism is normal faulting with a down-dip-trending T axis similar in style to other Cascadia intraslab earthquakes. The total seismic moment is 1.3 × 1026 dyne cm ( M w 6.7) and the hypocenter depth is 60 km. Additional inverse modeling for the kinematic rupture process assuming the steeply east-dipping fault plane from the moment tensor resulted in a slightly higher total moment of 1.9 × 1026 dyne cm ( M w 6.8). The earthquake ruptured to the south with at least two subevents. The combined area of asperities and seismic moment for the 1949 earthquake was compiled with those from the 1965 Seattle-Tacoma and the 2001 Nisqually earthquakes and with those from Japan and Mexico to develop a source-scaling relation separate from shallow global strike-slip earthquakes. We infer that deeper intraslab earthquakes have a significantly smaller combined area of asperities than those compiled for shallower strike-slip earthquakes with the same seismic moment. This difference in rupture area leads to a 3- to 5-fold increase in stress drop for earthquakes with seismic moments between 1024 and 1028 dyne cm.

Effects of Irregular Structure of the Mississippi Embayment on Ground-Motion Amplification

The objective of this study is to evaluate effects of the presently available 3D velocity model of the Mississippi embayment structure on the amplification of seismic waves by using simulated finite-difference seismograms. Effects of both 2D and 3D embayment basement structures were considered. The 3D model included information of the near-surface velocities that were derived from the existing 1D velocity models. The 2D crustal model was taken from Catchings (1999), which extended from Saint Louis, Missouri, to Memphis, Tennessee. Finite-difference seismograms were simulated for point sources embedded at both ends of the 2D structure. These seismograms were examined to distinguish features like peak amplitude amplifications and duration of seismograms when the seismic waves propagated from the Mississippi Embayment toward the Illinois basin and vice versa. To establish a working 3D structure model of the embayment, we compiled geologic information of the region on material properties of the shallow structure and used the 3D model developed at the Center of Earthquake Research Institute (ceri), Memphis, as the starting model. The 3D model was used to generate finite-difference seismograms along several profiles for a M w 7.2 scenario earthquake occurring on the New Madrid fault zone. An equivalent 1D model, which included the basin materials, was also used to compare the 3D versus equivalent 1D ground motions simulated using the finite-difference method. To establish the amplification factors due to the surface sediments in the 3D model, finite-difference seismograms were also computed for a 1D hard-rock reference model. These 1D and 3D responses of the Mississippi embayment were used for estimating ground-motion amplification at sites where the depth to the basement is deeper than 500 m. Our investigation suggests that the deep structure of the Mississippi Embayment has little impact on long-period ground- motion amplitudes ( T ≥ 2 sec) for large earthquakes that rupture in the central part of the basin. This supports the hypothesis that for engineering purposes ground motions simulated based on the equivalent 1D crustal model are adequate for representing ground motions from future large earthquakes ( M w > 7) occurring in the New Madrid seismic zone.

Probabilistic Tsunami Hazard Analysis for Ports and Harbors

The December 2004 Sumatra-Andaman earthquake emphasized the need for a consistent and comprehensive assessment of tsunami hazard. The authors have developed a method for Probabilistic Tsunami Hazard Analysis (PTHA) based on the traditional Probabilistic Seismic Hazard Analysis (PSHA) and therefore completely consistent with standard seismic practice. In lieu of attenuation relations, it uses the summation of finite-difference Green’s functions that have been pre-computed for individual subfaults, which enables one to rapidly construct scenario tsunami waveforms from an aggregate of subfaults that comprise a single large event. For every fault system, it is then possible to integrate over sets of thousands of events within a certain magnitude range that represents a fully probabilistic distribution. Because of the enclosed nature of ports and harbors, effects of resonance need to be addressed as well, which is why this method has been extended to not only analyze exceedance levels of maximum wave height, but also of spectral amplitudes. As in PSHA, these spectral amplitudes can be matched with the spectral response of harbors, and thus allow a comprehensive probabilistic analysis of tsunami hazard in ports and harbors.