Allen L. Husker

Tomography and thermal state of the Cocos plate subduction beneath Mexico City

[1]xa0The geometry and thermal state of the subducting Cocos plate beneath Mexico City has been enigmatic because of the absence of a deep Wadati-Benioff zone. We present a tomographic image of the slab based on inversion of 8869 teleseismic P wave travel times measured on a portable broadband seismic network. The images combined with receiver function analysis show that the slab runs flat from the coast to near Mexico City, where it dives into the mantle just before the Trans-Mexican Volcanic Belt with a dip of ∼75°. It continues down to a depth of ∼500 km at a distance of 400 km from the trench, where the tomography reveals that the dipping portion ends. As well as standard block tomography, we invert the travel time residuals for the parameters of a thermal slab model and find a slab thickness of 40 km that is consistent with the (15 Ma) age of the Cocos plate. The combination of a young hot plate and truncation at depth can explain the lack of deep seismicity due to high temperatures and lower negative buoyancy compared with an older, thicker, nontruncated plate.

Seismicity in Idaho and Montana Triggered by the Denali Fault Earthquake: A Window into the Geologic Context for Seismic Triggering

We present a case study of dynamically triggered seismicity in Idaho and western Montana from the 2002 MW 7.9 Denali fault earthquake to investigate the relationship between measured geological discriminants and propensity for trig- gering. We first establish triggering. We find events that are not reported in the Advanced National Seismic System catalog in Idaho and Yellowstone following the Denali fault earthquake by filtering broadband waveforms. An ML 4.6 earthquake is discovered near New Meadows, Idaho, during the passage of the Rayleigh waves and another earthquake probably located near Yellowstone. We find that central western Idaho and Yellowstone have statistically significant seismicity increases by applying a b test to the cataloged events in the 48 hr after the Denali fault earthquake in Idaho and surrounding regions. We also find that Pine, Idaho, may have triggered events, but the measurement is not robust because of uncertainty in the background seismicity rate. Both central western Idaho and Yellowstone triggered previously during the 1992 Landers earthquake. We then try to determine the local geological conditions necessary for triggered seismicity to occur. Geothermal regions with high total dissolved solids and background seismicity appear to be favorable conditions to trigger earthquakes in the study.

Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe

The Los Angeles Basin Passive Seismic Experiment (LABPSE) involved the installation of an array of 18 seismic stations along a line crossing the Los Angeles basin from the foothills of the San Gabriel Mountains through the Puente Hills to the coast. At 3-5 km spacing between stations the array has much higher resolution than the permanent network of stations in southern California. This resolution was found to be important for analyzing the factors that govern the amplitude variation across the basin. We inverted spectra of P- and S-body-wave seismograms from local earthquakes (ML 2.1-4.8) for site effects, attenuation, and corner frequency factor using a standard model that assumes geometric spreading varying as inverse distance, exponential attenuation, and an x 2 source model. The S-wave attenuation was sep- arable into basin and bedrock contributions. In addition to the body-wave analysis, S-wave coda were analyzed for coda Q and coda-determined site effects. We find S- wave Q (QS) in bedrock is higher than in the basin. High-frequency QS is higher than low-frequency QS. Coda Q (Qc) is higher than QS. P-wave Q (QP) was not separable into basement and bedrock values, so we determined an average value only. The corner frequencies for P and S waves were found to be nearly the same. The standard model fit over 97% of the S-wave data, but data from six clustered events incident along the basin edge within a restricted range of incidence and azimuth angles gen- erated anomalous amplitudes of up to a factor of 5 higher than predicted. We test whether such basin-edge focusing might be modeled by catastrophe theory. After ruling out site, attenuation, and radiation effects, we conclude a caustic modeled as a diffraction catastrophe could explain both the frequency and spatial dependence of the anomalous variation.