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Exceptional Ground Motions from the April 26, 2008 Mogul
Nevada Mw 5.0 Earthquake Recorded by PASSCAL Rapid Array
Mobilization Program (RAMP) Stations
Glenn Biasi (University of Nevada Reno), John G. Anderson (University of Nevada Reno), Kenneth D. Smith (University
of Nevada Reno), Ileana Tibuleac (University of Nevada Reno), Rasool Anooshehpoor (University of Nevada Reno),
David von Seggern (University of Nevada Reno)
An unusually shallow swarm of earthquakes began 28 February 2008 beneath Mogul, Nevada, a small suburb a few km west
of Reno. Earthquake depths of as shallow as 2 km were confirmed on instrumentation immediately over the hypocentral area,
and residents routinely reported feeling events as small as Ml 1.5 or smaller. The swarm also exhibited an accelerating moment
release over a period of several weeks. The slow build-up of the swarm allowed UNR to deploy four PASSCAL RAMP instruments
in the epicentral area in early April 2008 to improve location control and map swarm evolution. Importantly, the RAMP
stations included strong-motion sensors to ensure that any larger earthquakes would be recorded on scale.
Seismic activity accelerated throughout April, culminating in an Mw 5.0 mainshock at 06:40 UTC 26 April 2008 [Anderson
et al., 2009]. Two stations immediately above the mainshock recorded component accelerations over 800 cm/s². Mean horizontal
accelerations exceed 0.6 g at three of the four stations. The strongest peak acceleration was 1,164 cm/s², or about 1.19 g, at
station MOGL, about 0.4 km from the epicenter. The peak vector velocity at MOGL was 54 cm/s, at a frequency of about 3 Hz.
These accelerations far exceed design ground motions for the residential construction of the community, but nevertheless, damage
to wood-frame buildings there was minimal. Ground motions also far exceeded predictions from the major ground motion
regression equations, apparently because of the shallow hypocentral depth of about 3 km. At the same time, attenuation of peak
ground motions was greater than predictions, suggesting that source-side attenuation was greater than for earthquakes with a
more typical depth of 5-10 km. Peak accelerations and velocities from this earthquake would place it in the top 25 strongest
ground motions ever recorded [Anderson, 2010]. These amazing recordings and new insights into shallow earthquake swarms
would have remained unknown without timely access to PASSCAL RAMP instruments.
References
Anderson, J. G., I. Tibuleac, A. Anooshehpoor, G. Biasi, K. Smith, and D. von Seggern (2009). Exceptional ground motions recorded during
the April 26, 2008 Mw 5.0 earthquake in Mogul, Nevada, Bull. Seismol. Soc. Amer., 99, 3475-3486.
Anderson, J. G. (2010). Source and site characteristics of earthquakes that have caused exceptional ground accelerations and velocities, Bull.
Seismol. Soc. Amer., 100, 1-36.
Acknowledgements: The permanent network in western Nevada and easternmost California was supported by the United States Geological
Survey under Cooperative Agreement 07HQAG0015.
Mogul, Nevada community (Google Earth; center
39.5186, -119.9262; ~1.5 km N-S photo extent), with
component ground accelerations measured from
PASSCAL RAMP stations MOGL and MOGE. April 26,
2008 Mw 5.0 mainshock epicenter is indicated by the
red circle.
2010 IRIS Core Programs Proposal | Volume II | Earthquake Source Studies | II-57