The quality of hypocenter locations is governed by many factors and should be considered when assessing earthquake hazards, comparing seismicity in various areas, associating earthquakes with mapped faults, and in other applications of these data. The accuracy (difference between actual and calculated locations) is limited by systematic errors such as inadequate velocity models, possible consistent misidentification of phases, and systematic station delays (dependent on the local geology beneath the site). Accuracy can be evaluated using data from explosions where the origin time and location is known. Precision (a measure of the ability to recalculate the same location, which determines the reliability of seismicity patterns) is affected by random timing errors and the use of a variety of station subsets. Precision can be estimated for each earthquake using standard statistical methods in the location routine, HYPOINVERSE (Klein, 1978).
For each earthquake in the catalog, there are six parameters listed that are indicative of the quality of the solution: NO, GAP, DMN, RMS, ERH, and ERZ; see Format for Earthquake Summary Data: July 1962 – Present. The RMS residual listed for each solution is a weighted root mean square of the differences between the observed and calculated arrival times. The RMS residual reflects both random timing errors and systematic errors due to such factors as inaccurate velocity models, misinterpretation of phases, and consistent but uncorrected station delays. Generally, if the hypocenter is well surrounded by stations (as determined by a small value for GAP), the number of arrival times used (NO) is greater than 5, and the velocity model is well known, then RMS is a measure of the correctness of the velocity model and of phase identification and timing. ERH and ERZ are the standard horizontal and vertical errors in kilometers, respectively. These parameters are simplified errors from a three-dimensional error ellipsoid (Klein, 1978). To a first approximation, the 95-percent confidence intervals in horizontal location and depth are ±2.2 ERH and ±2.0 ERZ, respectively. The error estimates are a function of a user specified estimate of random reading error, the RMS residual, the array geometry used to locate the earthquake, and the velocity model. The error calculations assume that the only source of error is random errors in the arrival times. Thus, the results are a measure of the precision of each earthquake hypocenter.
Surface explosions from many quarries, particularly in north central Utah along the Wasatch Front, are routinely recorded. Known locations of several blasts from the Keigley quarry, the Bingham Canyon Copper mine, and the Devil’s Slide quarry have been compared with our instrumental locations. The comparisons suggest a map location accuracy of approximately ± 1 km for well recorded earthquakes. Depth determinations are judged to be reliable to within ± 2.0 km if DMN (the distance to the closest recording station) is approximately equal to or less than the depth. An asterisk next to the depth is used in the earthquake catalog to indicate events with poor depth control, i.e., earthquakes that do not have a recording station within 10 km of the epicenter or within a distance to the epicenter of twice the depth.
The magnitude above which the catalogs are complete varies considerably with time and location. Since the beginning of 2000, the Utah region catalog is estimated to be systematically complete above magnitude 1.5 in north central and central Utah, above magnitude 1.7 in southwestern Utah (Pankow et al., 2004), and above magnitude 3.0 in southeastern Utah and the eastern Uinta Basin. Since the beginning of 1995, the Yellowstone region catalog is estimated to be systematically complete above magnitude 1.5 (Husen et al., 2004). Magnitudes are judged to be accurate to within ± 0.3 magnitude units.
Blasting
Blasting for mining, road and dam construction, seismic exploration, and military ammunition disposal is common in the region spanned by the UUSS network—except in Yellowstone National Park. Most of the blasts are too small for an epicenter calculation and are automatically excluded from the catalog. Nevertheless, locatable blasts are recorded regularly in the region. Events identified as blasts by contacting individual blasting operations and/or correlation with known blasting areas and the time of day of frequent blasting have been removed from the earthquake catalog. Data from blasts are kept, however, for analysis in special studies. Note that some unrecognized blasts may still be included in the catalog.
References
Husen, S., S. Weimer, and R. B. Smith (2004). Remotely triggered seismicity in the Yellowstone National Park region by the 2002 MW 7.9 Denali fault earthquake, Alaska, Bull. Seism. Soc. Am. 94 (6B), S317-S331, doi:10.1785/0120040617.
Keller, G. R., R. B. Smith, and L. R. Braile (1975). Crustal structure along the Great Basin Colorado Plateau transition from seismic refraction studies, J. Geophys. Res. 80 (8), 1093-1098, doi:10.1029/JB080i008p01093.
Klein, F. W. (1978). Hypocenter location program HYPOINVERSE, U. S. Geol. Surv., Open-File Rept. 78-694, 102 pp, doi:10.3133/ofr78694.
Pankow, K.L., W.J. Arabasz, J.C. Pechmann, and S.J. Nava (2004). Triggered seismicity in Utah from the 3 November, 2002, Denali fault earthquake, Bull. Seism. Soc. Am. 94 (6B), S332-S347, doi:10.1785/0120040609.