3 TremorScope: Imaging the Deep Workings of the San Andreas Fault

Roland Burgmann, Richard Allen, Pascal Audet, Douglas Dreger, Robert Nadeau, Barbara Romanowicz,Taka’aki Taira, Margaret Hellweg

3.1 Introduction

Until recently, active fault zones were thought to de-form via seismic earthquake slip in the upper, brittlesection of the crust, and by steady, aseismic shear be-low. However, in the last few years, this view has beenshaken by seismological observations of seismic tremordeep in the roots of active fault zones. First recognizedon subduction zones in Japan and the Pacific Northwest,tremor has also been found to be very active on a shortsection of the San Andreas to the southeast of one of themost densely monitored fault segments in the world, nearParkfield (Nadeau and Dolenc, 2005). This deep (∼20-30km) zone of activity is located right below the nucleationzone of the great 1857 Fort Tejon earthquake. Thus, un-derstanding the temporally and spatially complex fault-ing process in this zone may help us better understandthe conditions that lead to such large ruptures.

3.2 The Project Plan

The tremor source region is south-east of existingseismic networks around Parkfield, along the San An-dreas Fault. We are adding eight seismic stations, theTremorScope (TS) network, in this area to complementexisting instrumentation.

Figure 2.6: Installation of seismic vault at TremorScopestation THIS.

Six of eight planned sites for the TS network have beenpermitted and the first two two surface stations installed.Figure 2.6 shows installation of the seismometer vaultat station THIS. Surface installations have a broadbandseismometer, an accelerometer and a digitizer. The bore-hole sites, with a hole about 300 m deep will have an ac-celerometer at the surface. Downhole will be a cemented,three-component set of gimballed, 2 Hz geophones. Threeboreholes will be equipped with a Guralp downhole sen-sor package, consisting of a three-component broadbandseismometer, a three-component accelerometer and a dig-itizer. At all locations, data will be logged onsite andforwarded to Berkeley for real-time processing. The datawill be used in real-time earthquake monitoring (see Op-erational Section 8), as well as for tremor studies. Thefirst data are in. Figure 2.7 shows triggered tremor fromthe M8.6 earthquake of April 11, 2012, in the recording ofstation TSCN. This tremor was also apparent at HRSNstations.

3.3 Perspectives

Data from the TremorScope project will improve earth-quake monitoring in the region south of Parkfield. In-sights from the project will also contribute to under-standing tremor and slip in other regions of the worldwhere such phenomena have been observed, but are notnearly as accessible. Should a great San Andreas earth-quake occur during this experiment, the network wouldalso provide unprecedented and exciting insights into theseismic rupture process.

3.4 Acknowledgements

This work is funded by grant 2754 from the Gordonand Betty Moore Foundation.

3.5 ReferencesNadeau, R., and D. Dolenc (2005), Nonvolcanic tremors

deep beneath the San Andreas fault, Science, 307, 389,doi:10.1126/science.1107142.

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Figure 2.7: Triggered tremor in the surface waves of the April 11, 2012, M8.6 earthquake which occurred off the WestCoast of Northern Sumatra. Tremor is apparent in the records of HRSN stations CCRB and MMNB as well as atTremorScope station TSCN.

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