Abstract

An unusually shallow swarm of earthquakes in Mogul, a suburb in west Reno, Nevada, started 28 February 2008 and continued for several months. Temporary instruments in the epicentral region constrained event depths and locations and provided onscale recordings of the strong ground motions and supplemented a network of over 21 permanent Advanced National Seismic System (ANSS) strong-motion stations deployed throughout the metropolitan Reno area. By the end of August, the swarm had produced over 200 earthquakes with magnitude (ML) greater than 2.0. The largest earthquake in the sequence occurred at 06:40 UTC on 26 April 2008 (11:40 p.m. on 25 April local time), with magnitude ML 4.7, Mw 5.0, and a hypocentral depth of about 3.1 km.

In the Mw 5.0 mainshock, mean-horizontal peak ground acceleration exceeded 0.6g at three stations. The strongest peak vector acceleration was 1164 cm/sec2, or about 1.19g, at the station MOGL (∼0.4 km from the epicenter) at a frequency of about 3 Hz. The mean-horizontal peak velocity exceeded 12 cm/sec at the four nearest stations, and the peak vector velocity at the strongest was 54 cm/sec. The duration of the strongest shaking was about 2 sec at these stations. The observed peak accelerations and peak velocities decreased more rapidly with distance than predicted, to values below the median predictions at most stations in the Reno metropolitan area. The shallow source depth is, to some degree, a likely contributing factor to these unusual observations. The motions in the residential neighborhood near the epicenter are significantly greater than the seismic forces required by the 1985 Uniform Building Code Zone 3 that was in effect when the Mogul neighborhood was developed, but damage to the wood-frame structures in the neighborhood was minimal.

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