Title

Investigation of an Unusually Shallow Earthquake Sequence in Mogul, NV from a Discrimination Perspective

Document Type

Conference Proceeding

Publication Date

2011

Abstract

Distinction between very shallow (1-4 km deep) earthquakes and underground nuclear explosions remains an important issue in discrimination research. This is because very shallow earthquakes are seldom well-recorded and documented. As a consequence, their source parameters, scaling and their local and regional propagation characteristics are not well understood. Thus, unlike for deeper events, the effectiveness of the discrimination algorithms when applied to unusually shallow earthquakes has been only rarely determined. The primary objective of this study is a detailed investigation of a sequence of earthquakes that occurred during 2008 west of Reno, Nevada, in Mogul. This sequence consisted of over 1700 earthquakes with ML of 1.0 or greater, concentrated in depth between 1 and 4 km and ranging in magnitude up to Mw 5.0 (the “main shock”). Preliminary analyses of the main shock have revealed uncharacteristically high amplitude near-field ground motions and uncharacteristically rapid attenuation with distance, which could affect the magnitude estimates and the discrimination metrics. For our investigations, we use a unique broadband and strong-motion recording database, from stations as close as 1 km from the epicenter of the main shock and most of the smaller events. We have organized our study in four tasks: First, our investigations started by building a detailed shallow event database. In addition to available public domain information on very shallow event discrimination analysis, the database includes near-field and seismic network recordings of all the ML>1.0 Mogul, Nevada earthquakes located by the Nevada Seismological Laboratory; mining explosions (ML>2.0) and crustal earthquakes (ML>2.5) in the in the Reno-Carson City, NV, area; the waveforms from the 1993 Rock Valley sequence (depth < 4 km, mb≤4.3) and waveforms from the 2007 Crandall Canyon mine collapse, (ML 3.9). The objectives of the second task are to analyze the stress drop and radiated energy of the main shock, principal foreshocks, and principal aftershocks as well as to investigate the source scaling of the Mogul sequence. We will estimate source parameters (moment, corner frequency, stress drop) for ML ≥ 3.5 Mogul events using three related methods: 1) Spectral measurements; 2) Empirical Green's Functions and 3) Measurements of Lg/Sn coda envelopes. Third, we will investigate the effect of source depth on P and S spectra and spectral ratios, signal complexity, and the mb versus Ms discriminant. Fourth, we will attempt to understand the origin and nature of the near-source attenuation and its effect on magnitude estimates and energy content-related discriminant performance. We will compare our results to other very shallow earthquake sequences, induced seismicity and nuclear explosions results.

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Citation / Publisher Attribution

2011 Monitoring Research Review: Ground-Based Nuclear Explosion Monitoring Technologies, p. 592-600

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