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

摘要

The primary objective of this study was a detailed investigation of an unusually shallow earthquake sequence that occurred in 2008 in Mogul, west of Reno, Nevada. The main shock (MS) of this sequence occurred at 2.8 km below the surface and satisfied criteria for GT1 qualification. The source properties of the Mogul earthquakes with ML >/= 3 in the Mogul sequence were investigated. Moment magnitude and stress drop were estimated using: 1) A spectral method, which resulted in an estimate of the MS stress drop of 8.2 MPa (82 bars), with a corner frequency of 0.8s; 2) An Empirical Green's Function method using the whole wave train at epicentral distances < 10 km, which estimated a main shock stress drop of 17.5 MPa. The source area of the Mogul earthquake was estimated to be ~ 2 km along strike and 1.5 km along dip, with the greatest energy release in the depth range of 2.0-3.2 km, with more energy released above the hypocenter (2.7 km) than below; 3) Using Lg coda spectral ratios, estimates of the apparent stress for the MS were 1.2 MPa, for a moment of 4.07 1016 Nm, with interquartile range (iqr) of 0.84 MPa. The MS stress drop was estimated at 5.6 MPa, with iqr of 4.1 MPa. The MS corner frequency was 0.72 s, with iqr 0.2 s. Unlike for the main shock, the local and moment magnitude values for the smaller earthquakes with ML > 3.0 were within 0.2 magnitude units (m.u) when estimated from Lg coda. Investigations of the P/Sg and the P/Lg discriminant at seven local and regional stations in the western Great Basin showed that the Mogul earthquakes were discriminated from commercial and nuclear explosions even for small magnitude (ML < 2) events. The best earthquake explosion separation was observed for Pg/Lg (Pg/Sg) ratios at 6-8 Hz, and for Lg spectral ratios taken in the 1 to 2 and 6 to 8 Hz frequency bands, similar to previous observations in Nevada.