ALBERT

Description: Error in picking arrival times of seismic phases by human analysts has concerned the seismological community since the inception of earthquake location via the Geiger (least-squares) method. Arrival-time picking error and the inaccuracy of the location model both contribute to errors in hypocentral estimates, but in such a way that they are usually not separable. Model error can be attenuated greatly by joint tomography inversions for 3D velocity and revised hypocenters. This article attempts to define picking error in a more rigorous way than the subjective judgments of human analysts concerning their accuracy and the estimates made from tomography. A precise way to estimate interevent arrival times at stations is possible through cross-correlation of waveforms. This method obtains an accurate interevent time against which interevent time from human picking can be compared, and a statistical treatment of picking error emerges from a large set of cross-correlations. Estimates of picking error can be compared with final data error estimates from joint tomography inversions and to analysts' own estimates of picking error. This article uses data from the Southern Great Basin Digital Seismic Network (SGBDSN) for the years 2000-2007. From waveform cross-correlations with this data, the standard deviation of picking error for good quality signals is 0.020 sec for P and 0.028 sec for S. The analysts' own estimates of picking error considerably exceed the values determined from cross-correlation of very good waveforms, by roughly a factor of 2 to 3 routinely. Joint structure/hypocenters tomography on a local scale (model extent 〈100 km), as gleaned from the literature, reduces the standard deviation of travel-time errors to roughly 0.08 sec. The standard deviation of residuals for a tomography study using the SGBDSN data was 0.06 sec, larger by a factor of roughly three than the picking error estimate from cross-correlation of waveforms.

Description: The Southern Great Basin Digital Seismic Network (SGBDSN) has been designed for monitoring high-frequency (1-40-Hz) local events at and near Yucca Mountain, Nevada, the designated site for the national high-level nuclear-waste repository. We find that the network is also effective as a large-aperture teleseismic array for monitoring events in and close to North Korea, the recent location of an underground nuclear test that occurred on 9 October 2006, 01:35:28 coordinated universal time (UTC), National Earthquake Information Center (NEIC) m (sub b) 4.3. We explain this by (1) low ambient noise, (2) energy-efficient propagation paths (the nuclear explosion and deep earthquakes on nearly the same ray path show dominant frequencies between 0.9 and 2.5 Hz), and (3) coherent signals across the SGBDSN. The network, when used as an array, provides a particularly good beam signal-to-noise ratio (SNR) for the nuclear explosion. Estimated beam SNR is 20 dB at frequencies between 0.9-2.5 Hz. Between January 1996 and December 2006, 58% (11 of 19) of the events with 7.1〉m (sub b) 〉3.3 located within 300 km of the North Korean nuclear explosion by NEIC are considered large enough to be confidently picked by an SGBDSN analyst. The first-arrival of the North Korean event itself is apparent on 25 of the 29 SGBDSN unfiltered recordings. The direct P phase is confidently identified using slowness and back azimuth estimated with cross-correlation and frequency-wavenumber methods. Static time corrections for beamforming are estimated using 11 deep earthquakes within 300 km of the nuclear explosion. With the statistical regression model named two-way layout, we estimate the event first-arrival time offset and the mean station effects. The resulting relative time delays are used to calibrate the nuclear explosion single-array location to the NEIC location, using the delays derived from 11 earthquakes. With magnitude substituted for time, the same method is used to estimate magnitude corrections. The SGBDSN magnitude estimate is 4.3 using the Veith-Clawson body-wave magnitude formula.

Description: In 2003, a magma intrusion event occurred at 25-30 km in the lower crust under the northwestern corner of Lake Tahoe, as evidenced by both an earthquake swarm and a geodetic displacement. This study examines the seismicity associated with that event and subsequent seismicity in the upper crust. HYPODD relocations showed that the deep swarm of approximately 1600 microearthquakes at the intrusion site was concentrated on a planar area with a strike of N42 degrees W, dipping at 39 degrees to the northeast. The largest microearthquake in this swarm was M 2.2, and an anomalously high b-value of 2.0 is seen in the recurrence-versus-magnitude plot. The swarm progressed over this plane in a somewhat irregular pattern for a period of roughly 5 months. Focal mechanisms of the deep-swarm events are highly variable and do not reflect the known regional stress field. Two months after the deep-swarm activity started, a shallow swarm of approximately 1100 microearthquakes began at 10-12-km depths in the shallow crust almost immediately above the deep swarm and continued through 2005. This swarm had a maximum M of 2.4 and a relatively high b-value of 1.5. Based on HYPODD relocations, hypocenters in this swarm are concentrated in a narrow pipelike volume, and event depths progressed steadily upward over the more than 2 yr of observation. Focal mechanisms in this shallow swarm are more consistent with the regional stress field than those of the deep swarm. Within one focal depth horizontally of the deep swarm, post-intrusion seismic activity increased significantly compared to prior years. Stress triggering from the deep magma intrusion, although based on sub-bar stress changes in the shallow crust, is a feasible explanation of the observed increase.