Earthquake swarm and b-value characterization of the Yellowstone volcano-tectonic system

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high-quality data of three different periods, 1973–1981, 1984–1994, and 1995–2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of ∼2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

Probabilistic Earthquake Relocation in Three-Dimensional Velocity Models for the Yellowstone National Park Region, Wyoming

Recorded seismicity for the Yellowstone National Park region, comprising 25,267 earthquakes from November 1972 to December 2002, has been relocated using three-dimensional velocity models and probabilistic earthquake location. In addition, new coda magnitudes for earthquakes between 1984 and 2002 were computed by using an improved coda magnitude equation. Three-dimensional velocity models for earthquake location were computed by inverting subsets of high quality data of three different periods, 1973-1981, 1984-1994, and 1995-2002, for hypocenter locations and seismic velocities. Earthquakes were relocated by using a nonlinear, probabilistic solution to the earthquake location problem. Fully nonlinear location uncertainties included in the probabilistic solution allow a better and more reliable classification of earthquake locations into four quality classes. Earthquake locations show an improvement in location accuracy with time, which we attribute to improved network geometry and more precise timing of arrival times. No large systematic shifts of the relocated earthquake locations are observed, except a systematic shift of _2 km to greater depth. The new relocated earthquake locations show tighter clustering of epicenters and focal depths when compared with original earthquake locations. The most intense seismicity in terms of number of earthquakes and cumulative seismic moment release in the Yellowstone National Park region occurs northwest of the Yellowstone caldera between Hebgen Lake and the northern rim of the caldera. Seismicity within the Yellowstone caldera is diffuse, and shallow individual clusters of earthquakes can be associated with major hydrothermal areas.

Probabilistic earthquake relocation in three-dimensional velocity models for the Yellowstone National Park region, Wyoming

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.

Local Magnitude Determinations for Intermountain Seismic Belt Earthquakes from Broadband Digital Data

Earthquake probabilities for the Wasatch front region in Utah, Idaho, and Wyoming

We have calibrated new coda-magnitude (MC) equations for local earthquakes digitally recorded since 1981 in the Utah (UT) region and since 1984 in the Yellowstone National Park (YP) region by the University of Utah Seismograph Stations (UUSS) network. The primary motivation for this study was the recognition of systematic time-dependent differences between MC and local magnitude (ML), ranging up to 0.4 and 0.9 units in the UT and YP regions, respectively. The new MC equations, calibrated against MLs determined from paper and synthetic Wood-Anderson records, are: MC = -2.25 + 2.32 log τ + 0.0023Δ in the UT region, MC = -2.60 + 2.44 log τ + 0.0040Δ in the YP region, where Δ is epicentral distance in kilometers and τ is signal duration in seconds on a short-period vertical-component record, measured from the P-wave onset to the time that the average absolute value of the ground velocity drops below 0.01724 microns/sec. To determine the constants in the MC equations, we used an orthogonal regression method rather than linear regression. The latter produces biased results because the errors in log τ and Δ are not negligible compared to the errors in ML. The signal duration definition used in previous UUSS MC calculations was the time from the P-wave onset to when the signal drops below the pre-event noise level. This definition, coupled with changes in noise levels and instrument gains, was largely responsible for the time

Correction of Systematic Time-Dependent Coda Magnitude Errors in the Utah and Yellowstone National Park Region Earthquake Catalogs, 1981-2001

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Correction of Systematic Time-Dependent Coda Magnitude Errors in the Utah and Yellowstone National Park Region Earthquake Catalogs, 1981-2001