Kris L. Pankow

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.

The SEA99 Ground-Motion Predictive Relations for Extensional Tectonic Regimes: Revisions and a New Peak Ground Velocity Relation

The values of σ 3 listed in table 1 of Pankow and Pechmann (2004) are too large by a factor of √2 because they were …

Rupture directivity of the 3 November 2002 denali fault earthquake determined from surface waves

The M w 7.9 earthquake that struck central Alaska on 3 November 2002 was preceded 11 days earlier by an Mw 6.7 strike-slip foreshock on 23 October 2002. Both events were predominantly strike-slip and ruptured structures associated with the Denali fault system. Previous studies have shown that the mainshock began with failure on a relatively small northeast-striking reverse fault, before breaking out for 300 km of right-lateral strike-slip rupture. Aftershock patterns suggest that the fore- shock ruptured a region west of the mainshock, which began near the eastern extent of the foreshock sequence and proceeded east-southeast. To constrain and to quantify source duration and directivity effects, we examine surface-wave displacement seis- mograms and use an empirical Greens function (EGF) to isolate and explore main- shock rupture kinematics. Our particular interest lies in large-amplitude focussing caused by directivity. We observe Love and Rayleigh wave amplification of two orders of magnitude in the period range from 10 to 33 sec. These remarkable directivity-enhanced surface waves triggered small earthquakes more than 3000 km from the mainshock rupture.

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.

Constraints on the Kurile slab from shear wave residual sphere analysis

We analyze residual spheres for teleseismic S wave arrival times from 10 intermediate and deep focus earthquakes in the Kurile subduction zone. Long-period WWSSN and CSN recordings of transverse component S and ScS arrivals provide good azimuthal coverage for each event, with arrival time estimates that compare well with short-period observations. In order to isolate the near-source structure, substantial lower mantle and receiver path effects are suppressed both by using empirical station/path statics derived from S, sS, ScS, and sScS arrivals for a total of 28 northwestern Pacific events and by constructing differential residual spheres. Ray tracing through three-dimensional aspherical shear velocity models supports the inferred large magnitude of the deep mantle and near receiver effects, although existing models do not appear to provide accurate corrections. The events are relocated in the PREM structure, and a modest spatial smoothing of the residuals is applied. The resulting residual spheres are modeled using a finite difference travel time method that accounts for three-dimensional ray path perturbations. Our starting models are derived from slab structures determined by previous P wave residual sphere analyses. We explore the compatibility of the P and S wave observations to constrain the Vs/Vp velocity ratio of the slab and to test whether the S wave arrival times place additional constraints on the slab geometry. The modeling supports the previous results favoring penetration of the Kurile slab to at least 800 km depth for most of the length of the arc, with some evidence that the northern portion of the slab broadens by as much as a factor of 3 upon entering the lower mantle. The preferred slab shear velocity models have relatively low (2–3%) shear velocity contrasts below 670 km depth, which can be accounted for by slab temperature reductions of 300°–400°.

Continuous infrasonic recording within the Utah regional seismic network

In May 2006, the University of Utah Seismograph Stations, as part of a larger consortium, installed an infrasound array east of the Salt Lake Valley (NOQ). Infrasound can be thought of as seismology in the air focusing on subaudible acoustic signals in the frequency band from 0.01 to 20 Hz. Advantages of installing an infrasound array in northern Utah include a proximity to solid earth sources (mine blasts and earthquakes) and the possibility of incorporating the data into a regional seismic network. The primary application of infrasound is in identification of explosions detonated at or near the solid earth‐atmosphere boundary. However, to fully utilize the infrasonic data, better understanding of infrasonic source characteristics and propagation is needed. Goals of the infrasound installation at NOQ include (1) use of local ground truth information to assess the role of infrasound in source identification; (2) classifying properties of the infrasound signal by source type; (3) assessing the time varying...