ALBERT

Description: In this note, we expand on a previous study of strong ground motions in extensional tectonic regimes by Spudich et al. (1999). First, we correct the predictive relations they determined for horizontal peak ground acceleration and 5% damped pseudovelocity response for an approximately 20% overprediction of rocksite data that they noted in their article. Second, we regress data compiled in their study to determine a predictive relation for geometric mean horizontal peak ground velocity. Peak ground velocity estimates are needed to apply some commonly used methods for predicting earthquake damage and Modified Mercalli intensities. However, there are few other recently published predictive relations for peak ground velocity--and none that are specifically designed for use in extensional tectonic regimes.

Description: Immediately after the arrival of the surface waves from the M (sub w) 7.9 Denali fault earthquake on 3 November 2002, the University of Utah regional seismic network recorded an abrupt increase in local microseismicity throughout most of Utah's main seismic belt. We examined this increase in the context of the regional background seismicity using a catalog of 2651 earthquakes from 1 January 2000 to 30 June 2003. Statistical analyses of this catalog above spatially varying magnitudes of completeness ranging from 1.2 to 1.7 allow us to reject with 〉95% confidence the null hypothesis that the observed increases were due to random occurrence. The elevated seismicity was most intense during the first 24 hr (〉10 times the average prior rate) but continued above background level for 25 days (at the 95% confidence level) in most areas. We conclude that the increased seismicity was triggered by the Denali fault earthquake, which occurred more than 3000 km from the study region. High peak dynamic stresses of 0.12 to 0.35 MPa that occurred during the passage of the Love waves are consistent with the interpretation of triggering. The peak dynamic stresses were estimated by measuring peak vector velocities at 43 recording sites, 37 of which were relatively new strong-motion stations of the Advanced National Seismic System. The triggered events ranged in magnitude (M (sub c) and/or M (sub L) ) from less than 0 to 3.2 and were widely distributed across the state, primarily in seismically active regions. In contrast to many previously published observations of remotely triggered seismicity, the majority of the triggered earthquakes did not occur near Quaternary volcanic vents or in areas of magma-related geothermal activity. In several areas the triggered seismicity was spatially clustered (more than five earthquakes each separated by 〈5 km). Double-difference relative relocations for the earthquakes in three of these clusters indicate that most, but not all, of the triggered events were spatially separated from source zones of prior seismicity during 2000-2002. Focal mechanisms for the two largest triggered events have northeast- to northwest-trending tension axes, which are unusual for the region where they occurred. The temporal decay of the triggered activity was similar to that of Utah aftershock sequences and can be described by the modified Omori's law with a p-value of 0.6 to 0.7. The frequency-magnitude distribution of the triggered earthquakes is also similar to that of Utah aftershocks and, for the study area as a whole, can be described by the Gutenberg-Richter relation with a b-value of 0.81+ or -0.16. These similarities between the triggered seismicity and Utah aftershock sequences suggest the possibility that the initiation and development of both could result from the same causative mechanisms.