James C. Pechmann

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 …

Local magnitude determinations for intermountain seismic belt earthquakes from broadband digital data

The University of Utah Seismograph Stations (uuss) earthquake catalogs for the Utah and Yellowstone National Park regions contain two types of size measurements: local magnitude ( M L ) and coda magnitude ( M C ), which is calibrated against M L . From 1962 through 1993, uuss calculated M L values for southern and central Intermountain Seismic Belt earthquakes using maximum peak-to-peak (p-p) amplitudes on paper records from one to five Wood–Anderson (W-A) seismographs in Utah. For M L determinations of earthquakes since 1994, uuss has utilized synthetic W-A seismograms from U.S. National Seismic Network and uuss broadband digital telemetry stations in the region, which numbered 23 by the end of our study period on 30 June 2002. This change has greatly increased the percentage of earthquakes for which M L can be determined. It is now possible to determine M L for all M ≥3 earthquakes in the Utah and Yellowstone regions and earthquakes as small as M To maintain continuity in the magnitudes in the uuss earthquake catalogs, we determined empirical M L station corrections that minimize differences between M L s calculated from paper and synthetic W-A records. Application of these station corrections, in combination with distance corrections from Richter (1958) which have been in use at uuss since 1962, produces M L values that do not show any significant distance dependence. M L determinations for the Utah and Yellowstone regions for 1981–2002 using our station corrections and Richter’s distance corrections have provided a reliable data set for recalibrating the M C scales for these regions. Our revised M L values are consistent with available moment magnitude determinations for Intermountain Seismic Belt earthquakes. To facilitate automatic M L measurements, we analyzed the distribution of the times of maximum p-p amplitudes in synthetic W-A records. A 30-sec time window for maximum amplitudes, beginning 5 sec before the predicted Sg time, encompasses 95% of the maximum p-p amplitudes. In our judgment, this time window represents a good compromise between maximizing the chances of capturing the maximum amplitude and minimizing the risk of including other seismic events.

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.

Magnitude‐based discrimination of man‐made seismic events from naturally occurring earthquakes in Utah, USA

We investigate using the difference between local (ML) and coda/duration (MC) magnitude to discriminate manmade seismic events from naturally occurring tectonic earthquakes in and around Utah. For 6,846 well-located earthquakes in the Utah region, we find that ML-MC is on average 0.44 magnitude units smaller for mining induced seismicity (MIS) than for tectonic seismicity (TS). Our interpretation of this observation is that MIS occurs within near-surface low-velocity layers that act as a waveguide and preferentially increase coda duration relative to peak amplitude, while the vast majority of TS occurs beneath the near-surface waveguide. A second dataset of 3,723 confirmed or probable explosions in the Utah region also has significantly lower ML-MC values than TS, likely for the same reason as the MIS. These observations suggest that ML-MC is useful as a depth indicator and could discriminate small explosions and mining-induced earthquakes from deeper, naturally occurring earthquakes at local-to-regional distances.

Afterslip Enhanced Aftershock Activity During the 2017 Earthquake Sequence Near Sulphur Peak, Idaho

An energetic earthquake sequence occurred during September to October 2017 near Sulphur Peak, Idaho. The normal-faulting Mw 5.3 mainshock of 2 September 2017 was widely felt in Idaho, Utah, and Wyoming. Over 1,000 aftershocks were located within the first 2 months, 29 of which had magnitudes ≥4.0 ML. High-accuracy locations derived with data from a temporary seismic array show that the sequence occurred in the upper (<10 km) crust of the Aspen Range, east of the northern section of the range-bounding, west-dipping East Bear Lake Fault. Moment tensors for 77 of the largest events show normal and strike-slip faulting with a summed aftershock moment that is 1.8–2.4 times larger than the mainshock moment. We propose that the unusually high productivity of the 2017 Sulphur Peak sequence can be explained by aseismic afterslip, which triggered a secondary swarm south of the coseismic rupture zone beginning ~1 day after the mainshock. Plain Language Summary During the fall of 2017, an energetic sequence of earthquakes was recorded in southeastern Idaho. The mainshock had a moment magnitude of Mw 5.3, yet thousands of aftershocks were detected. We found that the unusually high productivity of this earthquake sequence can be explained by extra sliding that occurred just after the mainshock. This extra sliding happened too slowly to generate seismic waves, but it was large enough to alter the stress in the crust such that the extra aftershocks were created. Our finding suggests that in this region of Idaho, some of the strain that is built up by tectonic forces is released in slow-slip or creep events. This discovery will ultimately lead to more accurate forecasts of seismic hazard in the region.