Walter J. Arabasz

Triggered Seismicity in Utah from the 3 November 2002 Denali Fault Earthquake

Immediately after the arrival of the surface waves from the M 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 c and/or M 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 ± 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. Online Material : Catalog of earthquakes used in this study.

A methodology for probabilistic fault displacement hazard analysis (PFDHA)

We present a methodology for conducting a site-specific probabilistic analysis of fault displacement hazard. Two approaches are outlined. The first relates the occurrence of fault displacement at or near the ground surface to the occurrence of earthquakes in the same manner as is done in a standard probabilistic seismic hazard analysis (PSHA) for ground shaking. The methodology for this approach is taken directly from PSHA methodology with the ground-motion attenuation function replaced by a fault displacement attenuation function. In the second approach, the rate of displacement events and the distribution for fault displacement are derived directly from the characteristics of the faults or geologic features at the site of interest. The methodology for probabilistic fault displacement hazard analysis (PFDHA) was developed for a normal faulting environment and the probability distributions we present may have general application in similar tectonic regions. In addition, the general methodology is applicable to any region and we indicate the type of data needed to apply the methodology elsewhere.

Coal-Mining Seismicity and Ground-Shaking Hazard: A Case Study in the Trail Mountain Area, Emery County, Utah

We describe a multipart study to quantify the potential ground-shaking hazard to Joes Valley Dam, a 58-m-high earthfill dam, posed by mining-induced seismicity (mis) from future underground coal mining, which could approach as close as ∼1 km to the dam. To characterize future mis close to the dam, we studied mis located ∼3–7 km from the dam at the Trail Mountain coal mine. A 12-station local seismic network (11 stations above ground, one below, combining eight triaxial accelerometers and varied velocity sensors) was operated in the Trail Mountain area from late 2000 through mid-2001 for the dual purpose of (1) continuously monitoring and locating mis associated with longwall mining at a depth of 0.5–0.6 km and (2) recording high-quality data to develop ground-motion prediction equations for the shallow mis. (Ground-motion attenuation relationships and moment-tensor results are reported in companion articles.) Utilizing a data set of 1913 earthquakes ( M ≤ 2.2), we describe space-time-magnitude distributions of the observed mis and source-mechanism information. The mis was highly correlated with mining activity both in space and time. Most of the better-located events have depths constrained within ±0.6 km of mine level. For the preponderance (98%) of the 1913 located events, only dilatational P -wave first motions were observed, consistent with other evidence for implosive or collapse-type mechanisms associated with coal mining in this region. We assess a probable maximum magnitude of M 3.9 (84th percentile of a cumulative distribution) for potential mis close to Joes Valley Dam based on both the worldwide and regional record of coal-mining-related mis and the local geology and future mining scenarios.

Mining-Related and Tectonic Seismicity in the East Mountain Area Wasatch Plateau, Utah, U.S.A.

As part of a larger multi-institutional seismic monitoring experiment during June–August 1984 in the eastern Wasatch Plateau, Utah, data from a subarray of 20 portable seismographs were used to investigate seismicity in the East Mountain area, an area of active underground coal mining and intense microseismicity. Eight stations of the subarray were concentrated on top of East Mountain, about 600 m above mine level, at an average spacing of 2 to 3 km. The primary objective was the accurate resolution of hypocenters and focal mechanisms for seismic events originating at submine levels. Data from high-resolution seismic reflection profiles and drill-hole sonic logs yielded a detailed velocity model. This model features a strong velocity gradient in the uppermost 1 km, which has a significant effect on takeoff angles for first-arrivingP-waves from shallow seismic events. Two hundred epicenters located with a precision of ±500 m cluster within an area about 5 km in diameter and show an evident spatial association with four sites of longwall mining during the study period. A special set of foci rigorously tested for focal-depth reliability indicates submine seismicity predominating within 500 m of mine level and extending at least to 1 km, and perhaps to 2 km, below mine level. Continuous monitoring for a 61-day period (June 15–August 15) bracketed a 16-day mining shutdown (July 7–22) during which significant seismicity, comparable to that observed before the shutdown, was observed. Ten focal mechanisms for seismic events originating at or down to 2 km below mine level nearly all imply reverse faulting, consistent with previous results and the inferred tectonic stress field. Enigmatic events recorded with all dilatational first motions can be fit with double-couple normal-faulting solutions if they in fact occurabove mine level, perhaps reflecting overburden subsidence. If these events are constrained to occur at mine level, their first-motion distributions are incompatible with a double-couple source mechanism.

New agenda for committee on seismology

An aggressive agenda was shaped to serve the U.S. seismological community and relevant government agencies as seismology advances into the 21st century at the Committee on Seismologys meeting, held from October 22 to 23 in Washington, D.C., under the new chairmanship of Thomas H. Jordan. The Committee on Seismology is a member of the National Research Councils Board on Earth Sciences and Resources. The committees mission is to keep track of major trends and developments in seismology and allied fields, provide timely studies for government agencies on special subjects or problems, keep abreast of international seismological activities, advise government agencies on the operation of federally supported seismograph networks and data-dissemination facilities, and coordinate seismological-related activities within the National Research Council, particularly in the fields of engineering seismology, rock mechanics, geodesy, geodynamics, geology, and seismic verification of nuclear test ban treaties.