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HR: 14:15h
AN: S53C-03 INVITED
TI: GPS and InSAR Monitoring of the Mogul Swarm: Evidence for Mainly Aseismic Fault Creep, with Implications for Seismic Hazard
AU: * Blewitt, G
EM: gblewitt@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AU: Bell, J
EM: jbell@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AU: Hammond, W C
EM: whammond@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AU: Kreemer, C
EM: kreemer@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AU: Plag, H
EM: hpplag@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AU: DePolo, C
EM: cdepolo@unr.edu
AF: Nevada Bureau of Mines and Geology, University of Nevada, Reno, 1664 N. Virginia St., MS 178, Reno, NV 89557, United States
AB: The relative fraction of aseismic slip that occurs in the seismogenic zone has implications for earthquake hazard, because aseismic creep tends to release stresses that have accumulated by relative plate motion. The phenomenon of aseismic fault creep is well documented in segments of the San Andreas Fault, in Japan, and more recently, in the Cascadia subduction zone where creep occurs episodically in "slow earthquakes", as detected by continuous GPS (CGPS). Another mechanism for fault creep is afterslip following large or great earthquakes, which can rival the magnitude of the displacement associated with the main shock, as was the case for the 2005 Mw 8.7 Nias Earthquake, again detected by CGPS. Here we report on the CGPS detection of aseismic fault creep that significantly exceeds the co-seismic displacement of the moderate Mw 5.0 Mogul earthquake of 26 April 2008. This was the largest event of the Mogul-Somersett earthquake swarm that lasted from approximately March-July, 2008, a few km west of Reno, Nevada, USA. We installed the GPS network in March 2008, in rapid response to the onset of the swarm. The network has an inter-station spacing of ~2 km in the near field. The GPS data indicate that aseismic afterslip occurred for several weeks after the main event, with a decaying signature. Two stations apparently straddled the previously unrecognized NNW-SSE striking fault, and detected a total displacement of ~40 mm toward each other, yet only ~15 mm occurred on 26 April. As such, the Mw 5.0 event and the subsequent afterslip is perhaps the smallest event to have ever been observed directly by several GPS stations. Our modeling of the event is also constrained by InSAR, which helps constrain spatial details of the slip distribution. Results to date already indicate that the GPS and InSAR constraints appear to be compatible. The post-seismic surface displacement field has the same general pattern of the co-seismic displacement field, consistent with models of shallow slip (a few km) on a NNW-trending right-lateral strike-slip fault. Despite an extensive search on the ground, no surface rupture was found in the area of this modeled fault. Two potentially important questions arise from these observations: (1) Is mainly aseismic slip the rule or the exception for all moderate magnitude earthquakes, or is this only typically associated with shallow earthquake swarms like the 2008 Mogul-Somersett sequence? (2) What is the implication of the answer to the first question for monitoring earthquakes using InSAR, which is limited by the time interval between repeat passes? One thing that is clear, is that rapid-response GPS will be required to address these questions, which are important for the assessment of seismic hazards.
UR: http://www.seismo.unr.edu/feature/2008/mogul.html
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 1242 Seismic cycle related deformations (6924, 7209, 7223, 7230)
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: 2008 Fall Meeting