3 tremorscope: imaging the deep workings of the...

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3 TremorScope: Imaging the Deep Workings of the San Andreas Fault Roland B¨ urgmann, 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, brittle section of the crust, and by steady, aseismic shear be- low. However, in the last few years, this view has been shaken by seismological observations of seismic tremor deep in the roots of active fault zones. First recognized on subduction zones in Japan and the Pacific Northwest, tremor has also been found to be very active on a short section of the San Andreas to the southeast of one of the most densely monitored fault segments in the world, near Parkfield (Nadeau and Dolenc, 2005). This deep (20-30 km) zone of activity is located right below the nucleation zone 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 understand the conditions that lead to such large ruptures. 3.2 The Project Plan The tremor source region is south-east of existing seismic networks around Parkfield, along the San An- dreas Fault. We are adding eight seismic stations, the TremorScope (TS) network, in this area to complement existing instrumentation. Figure 2.6: Installation of seismic vault at TremorScope station THIS. Six of eight planned sites for the TS network have been permitted and the first two two surface stations installed. Figure 2.6 shows installation of the seismometer vault at station THIS. Surface installations have a broadband seismometer, 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. Three boreholes will be equipped with a Guralp downhole sen- sor package, consisting of a three-component broadband seismometer, a three-component accelerometer and a dig- itizer. At all locations, data will be logged onsite and forwarded to Berkeley for real-time processing. The data will be used in real-time earthquake monitoring (see Op- erational Section 8), as well as for tremor studies. The first data are in. Figure 2.7 shows triggered tremor from the M8.6 earthquake of April 11, 2012, in the recording of station TSCN. This tremor was also apparent at HRSN stations. 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 world where such phenomena have been observed, but are not nearly as accessible. Should a great San Andreas earth- quake occur during this experiment, the network would also provide unprecedented and exciting insights into the seismic rupture process. 3.4 Acknowledgements This work is funded by grant 2754 from the Gordon and Betty Moore Foundation. 3.5 References Nadeau, R., and D. Dolenc (2005), Nonvolcanic tremors deep beneath the San Andreas fault, Science, 307, 389, doi:10.1126/science.1107142. 10

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Page 1: 3 TremorScope: Imaging the Deep Workings of the …earthquakes.berkeley.edu/annual_report/ar11_12/2012rs3.pdfshaken by seismological observations of seismic tremor deep in the roots

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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Page 2: 3 TremorScope: Imaging the Deep Workings of the …earthquakes.berkeley.edu/annual_report/ar11_12/2012rs3.pdfshaken by seismological observations of seismic tremor deep in the roots

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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