Detailing a Shallow Crustal Earthquake Swarm beneath the Mogul, Nevada with PASSCAL RAMP Instrumentation
Detailing a Shallow Crustal Earthquake Swarm beneath the Mogul, Nevada with PASSCAL RAMP Instrumentation, Figure 2 Figure 2.
Double-difference relocated earthquakes of the Mogul, Nevada earth- quake swarm. The mainshock had an Mw of 5.0, but a local magnitude Ml 4.7. The mainshock hypocenter was approximately 3 km depth. Hypocenters do not coincide with any known fault or geologic structure. Downtown Reno, Nevada is near the east edge of the image.
An anomalous swarm of shallow earthquakes began on 28 February 2008 beneath the suburban communities of Mogul and Somersett, Nevada, about 12 km west of downtown Reno. Initially the swarm consisted of a few earthquakes per week, but accelerated during March 2008 to include several felt events per day. Residents routinely reported feeling earthquakes of Ml 1.5 or smaller. Earthquake depths as shallow as 2 km were confirmed by a short-period station in the epicentral region. The relatively slow onset and sustained activity of the swarm provided time to request seismic instruments from the PASSCAL Rapid Array Mobilization Program (RAMP) pool. Four RAMP stations with broadband and strong-motion sensors were deployed in the epicentral area by 9 April 2008. The swarm began to accelerate in earnest in early April, with four Ml 3 or greater earthquakes on 15 April, two Ml 4 events on 24 April, and what proved to be the mainshock, an Mw 5.0 earthquake at 06:40 on 26 April 2008. This earthquake produced component ground accelerations in excess of 0.8 g at two stations, and horizontal vector accelerations in excess of 1.1 g at one station [Anderson et al., 2009]. These ground motions caused widespread non-structural and content damage to residences in the area.
<p>Because of the social impact of the swarm and its potential to continue or even grow, six RAMP instruments deployed in central Nevada were relocated in early May to the epicentral area. An innovation in this deployment is that the RAMP stations were integrated by IP radio telemetry, beginning in late April 2008 into the permanent seismic monitoring network of the region operated by the University of Nevada Reno (UNR) (Figure 1). Antelope processing software at UNR allowed RAMP stations to be integrated seamlessly into the main network. This provided UNR personnel with highest-quality real-time information as the swarm evolved.
</p><p>The Mogul swarm continued at a high level of daily activity through June 2008, and did not return to an activity rate similar to its first week until mid-August. Double-difference hypocentral relocations (Figure 2) identify a NW-trending vertical structure, consistent with the mainshock and focal mechanisms, with a smaller NE-trending spur toward the north end of the rupture zone. The Mogul swarm did not occur on any known or suspected fault and geologic investigation has not identified any surface expression or structure. Continuous GPS data gathered with the event resolved motions up to 2.5 cm at the surface, but indicate a geodetic moment release approximately twice as large as would be expected from the seismicity.
</p><p>The Mogul sequence was the subject of a one-hour documentary by the National Geographic organization Naked Science series and aired by national cable-TV providers. Information is available at http://channel.nationalgeographic.com/series/ naked-science/4232/Videos#tab-Overview.
</p><p>References
</p><p>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.
</p><p>Acknowledgements: The permanent network in western Nevada and easternmost California was supported by the United States Geological Survey under Cooperative Agreement 07HQAG0015.</p>