Subsections

Anomalous Moment Tensor Solutions for The Geysers, CA

Jennifer Taggart, Summer Ohlendorf (UW - Madison), Sean Ford (LLNL), Peggy Hellweg, and Doug Dreger

Introduction

The Geysers, CA, hosts a Calpine geothermal plant that is an important source of geothermal energy in California. Seismicity in the region is high and has greatly increased in recent years, correlating with geothermal energy production by Calpine. Seismicity is generally shallow, and earthquakes in the region tend to be small ($M_{w}$$<$4.5). The bulk of the seismicity is thought to be induced by geothermal operations. Part of the Clear Lake Volcanic Field, The Geysers appear to have formed in a gap created as the Mendocino Triple Junction migrated north. An underlying pluton forms the geothermal heat source close to the surface.

Research Objectives

Moment tensor solutions are calculated in real time for most events with $M_{w}$$\geq$3.5 in Northern California (Pasyanos et al, 1996, Romanowicz et al, 1993) using the waveform moment tensor inversion code developed by Dreger (Minson and Dreger, 2008). Routine moment tensor solutions for The Geysers, computed with pure double couple (DC) and compensated linear vector dipole (CLVD) components (deviatoric solutions), often have a high non DC component. Non DC elements in the solutions to these Geysers events may be real (Dreger et al, 2000, Dreger et al, 2008, Ford et al, 2008), especially for the larger events ($M_{w}$4+). A program for full moment tensor inversions, including the isotropic (ISO) component, is available, and large events with a significant non DC component warrant a closer look with this code.

Mechanisms appear to cluster and vary. Some events, mostly in the southeast corner of the region, have solutions with high DC components that appear to reflect strike slip faulting. Others, scattered through the northern part of the region, may have normal faulting. In addition, the moment magnitudes routinely computed with the moment tensor program frequently have a higher $M_{w}$ than $M_{L}$, indicating some difference in the radiated energy from these events that may correspond to a real difference in the events (see map, Figure 2.29).

Figure 2.29: The first approach yielded six full moment tensor solutions with significant ISO components, while the second approach yielded one high DC solution and one solution with a significant ISO component. Green's function source depth for Group I is 2 km; for Group II, 5 km. Dark gray dots indicate 2008 background seismicity for The Geysers, while light gray beach balls represent best fit DC solutions from the BSL moment tensor catalog in that region (1990 through January 2009). Black beach balls represent full moment tensors from the two groups.
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Full Moment Tensors at The Geysers

We are reviewing moment tensors for some of the Geysers events. Our first efforts have used two different procedures, discussed here. For six events, Group I, chosen for their occurrence during the USARRAY installation in Northern California (see table), we first gathered all available low-noise data within 5 degrees of each event and then removed all stations with a best-fit variance reduction of 40% or below. Our best moment tensor solutions used data from between 20 and 60 stations. The large number of stations used provided robust solutions (Ohlendorf, 2008). As Geysers seismicity tends to be shallow, a source depth of 2 km was used. Group II comprised two events with only 46% and 18% respective DC components in the Berkeley Moment Tensor Catalog. Review of Group II events followed the procedure normally used during routine event review at UCB. Final solutions had 11 and 14 stations, respectively.


Table 2.2: Top: Full moment tensors at source depth 2 for the Group I events. Bottom: Best solutions at source depth 5 for the Group II events using the deviatoric code, the full moment tensor code with the ISO option turned off, and the full moment tensor from the full moment tensor code.
Date Location Type Stations DC/CLVD/ISO (percent) VR (percent)
5/12/2006 38.814 -122.814 Full 65 53/1/46 71.1
10/20/2006 38.867 -122.787 Full 62 44/21/35 69.5
04/30/2007 38.820 -122.823 Full 27 37/15/50 65.6
07/20/2007 38.810 -122.800 Full 24 13/21/66 59.1
12/1/2007 38.733 -122.933 Full 20 18/31/50 65.1
11/17/2005 38.814 -122.782 Full 21 51/25/24 54.7
1/4/2009 38.782 -122.773 Deviatoric 13 61/39/0 80.7
1/4/2009 38.782 -122.773 Full (ISO off) 11 82/18/0 81.6
1/4/2009 38.782 -122.773 Full 11 83/11/5 81.7
5/30/2008 38.776 -122.764 Deviatoric 14 39/61/0 77.9
5/30/2008 38.776 -122.764 Full (ISO off) 14 64/36/0 74.3
5/30/2008 38.776 -122.764 Full 14 20/44/36 75.6


All six Group I events had best fitting moment tensor solutions with significant non DC components (total $_{\%ISO+\%CLVD}$ 47-87%). Interestingly, the deviatoric solution from the full moment tensor code immediately yielded a significantly higher %DC for both Group II events, which was not expected. The full moment tensor solution further resolved the first event as having 83% DC (using a source depth of 5 km). The full moment tensor solution for the latter event looked primarily explosive, with only 20% DC.

Perspectives

The deviatoric solutions found with the full moment tensor code for both Group II events yielded significantly higher DC components than the best deviatoric solutions for those events determined using the deviatoric code. We intend to investigate this further.

In Figure 2.29, Group I and II events are located in the same area, the southern region of The Geysers, where Berkeley's best DC solutions appear to reflect strike slip mechanisms.

The Berkeley Moment Tensor catalog (http://seismo.berkeley.edu/~mike/solutions.new) contains numerous Geysers events, which will be reviewed with the full moment tensor code to determine whether they have significant ISO components. Computing full moment tensors for these events, especially for the larger events, may yield more information about the causes of seismicity at The Geysers.

Acknowledgements

Earthquake monitoring and reporting activities at the BSL are supported by the CISN funding of the California Governor's Office of Emergency Services under contract 6023-5 and the US Geological Survey project 07HQAG0013.

References

Ohlendorf, S., and D. Dreger, Evidence for Dilational Processes Accompanying Earthquakes at The Geysers Geothermal Field, California, The Third Annual Geoscience Graduate Symposium, Department of Geology and Geophysics, UW-Madison, Madison, WI, May 7-8, 2009 (poster).

Dreger, D. S., H. Tkalcic, and M. Johnston, Dilational processes accompanying earthquakes in the Long Valley Caldera, Science, 288, 122-25, 2000.

Dreger, D., S.R. Ford, and W.R. Walter, Source analysis of the Crandall Canyon, Utah mine collapse, Science, 321, 217, 2008.

Ford, S. R., D. S. Dreger and W. R. Walter, Source Characterization of the August 6, 2007 Crandall Canyon Mine Seismic Event in Central Utah, Seism. Res. Lett., 114, B01306, doi:10.1029/2008JB005743.

Minson, S. and D. Dreger, Stable Inversions for Complete Moment Tensors, Geophys. Journ. Int., 174(2), 585-592, doi:10.1111/j.1365-246X.2008.03797.x.

Pasyanos, M.E., D.S. Dreger, and B. Romanowicz, Toward Real-Time Estimation of Regional Moment Tensors, Bull. Seism. Soc. Am., 86, 1255-1269, 1996.

Romanowicz, B. D. Dreger, M . Pasyanos, and R. Uhrhammer, Monitoring of Strain Release in Central and Northern California Using Broadband Data, Geophys. Res. Lett., 20, 1643-1646, 1993.

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