Comprehensive detection of injection-induced seismicity in ... · Meng, X., and Z. Peng (2014), Seismicity rate changes in the Salton Sea Geothermal Field and the San Jacinto Fault

Comprehensive detection of injection-induced seismicity in the Salton Sea Geothermal Field Report for SCEC Award #15077

Submitted March 15, 2016

Investigators: Zhigang Peng (Georgia Tech)

I.   Project Overview .................................................................................................................................... i  A.   Abstract ............................................................................................................................................ i  B.   SCEC Annual Science Highlights ..................................................................................................... i  C.  Exemplary Figure ............................................................................................................................. i  D.  SCEC Science Priorities ................................................................................................................... i  E.   Intellectual Merit .............................................................................................................................. ii  F.   Broader Impacts .............................................................................................................................. ii  G.  Project Publications ......................................................................................................................... ii  

II.   Technical Report ................................................................................................................................... 1  A. Detecting remotely triggered seismicity in the Salton Sea region ... Error! Bookmark not defined. A. Student Support and Involvement ................................................................................................... 2 C. References ...................................................................................................................................... 3

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I. Project Overview

A. Abstract In the box below, describe the project objectives, methodology, and results obtained and their signifi-cance. If this work is a continuation of a multi-year SCEC-funded project, please include major research findings for all previous years in the abstract. (Maximum 250 words.) We systematically detect microseismicity in the Salton Sea Geothermal Field (SSGF) using both the EN borehole network and other permanent/temporary seismic network. Our last year’s effort has been focusing on matched filter detection around the occurrence time of 5 distant earthquakes that may have triggered microseismicity in this region before. By using many relocated events as templates, we scan through 5 days before and after each distant mainshock to detect additional seismicity. Out of all events examined, the 2009/08/03 Mw6.9 Baja California earthquake has triggered microseismicity during its surface waves. While the 2008 Mw6.0 Nevada earthquake and the 2010 Mw8.8 Chile earthquake did not show clear evidence of instantenous triggering during the surface waves, we found clear increase of seismicity within the next day following the distant mainshock. Our results suggest that Salton Sea region is critically stressed and capable of being dynamically triggered by large distant earthquakes.

B. SCEC Annual Science Highlights Each year, the Science Planning Committee reviews and summarizes SCEC research accomplishments, and presents the results to the SCEC community and funding agencies. Rank (in order of preference) the sections in which you would like your project results to appear. Choose up to 3 working groups from be-low and re-order them according to your preference ranking.

Seismology Computational Science Fault and Rupture Mechanics (FARM)

C. Exemplary Figure Select one figure from your project report that best exemplifies the significance of the results. The figure may be used in the SCEC Annual Science Highlights and chosen for the cover of the Annual Meeting Proceedings Volume. In the box below, enter the figure number from the project report, figure caption and figure credits. Figure 3. (a) Raw, (b) 2-16 Hz envelope, and (c) spectrogram of vertical seismogram at station RED for the 2009 Mw6.9 Baja California earthquake. (d) Template (red) and newly detected (black) events in Salton Sea around the 2009 Baja California mainshock. The black line is the cumulative number, and the solid dashed line mark the trend right before the mainshock. After Casey et al. (SCEC, 2015).

D. SCEC Science Priorities In the box below, please list (in rank order) the SCEC priorities this project has achieved. See https://www.scec.org/research/priorities for list of SCEC research priorities. For example: 6a, 6b, 6c

2a, 2b, 2f

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E. Intellectual Merit How does the project contribute to the overall intellectual merit of SCEC? For example: How does the research contribute to advancing knowledge and understanding in the field and, more specifically, SCEC research objectives? To what extent has the activity developed creative and original concepts?

This project contributes to a better understanding of induced seismicity by geothermal power plant operation in the Salton Sea area, as well as improved detections of seismicity in southern California. It also helps to better under-stand physical mechanisms of remote triggering.

F. Broader Impacts How does the project contribute to the broader impacts of SCEC as a whole? For example: How well has the activity promoted or supported teaching, training, and learning at your institution or across SCEC? If your project included a SCEC intern, what was his/her contribution? How has your project broadened the participation of underrepresented groups? To what extent has the project enhanced the infrastructure for research and education (e.g., facilities, instrumentation, networks, and partnerships)? What are some possible benefits of the activity to society?

This project supported a female SCEC intern Bridget Casey (from Virginia Tech) to conduct research at Georiga Tech in Summer 2015. The remaining funding will be used to support summer research of a female graduate stu-dent Chenyu Li in summer 2016.

G. Project Publications All publications and presentations of the work funded must be entered in the SCEC Publications data-base. Log in at http://www.scec.org/user/login and select the Publications button to enter the SCEC Pubi-cations System. Please either (a) update a publication record you previously submitted or (b) add new publication record(s) as needed. If you have any problems, please email [email protected] for assistance.

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II. Technical Report The technical report should describe the project objectives, methodology, and results obtained and their significance. If this work is a continuation of a multi-year SCEC-funded project, please include major re-search findings for all previous years in the report. (Maximum 5 pages, 1-3 figures with captions, refer-ences and publications do not count against limit.) A. Detecting remotely triggered seismicity in the Salton Sea region

Through a previously funded SCEC grant with Debi Kilb in 2010, Doran et al. [2011] per-formed a comprehensive search of dynamically triggered earthquakes in the SSGF since 2000 (Figure 1). They manually inspected the high-frequency energy in band-pass filtered waveforms and spectrograms during the surface waves of both teleseismic (i.e. epicentral distances > 1000 km; Mw ≥ 7.5) and regional events (i.e. epicentral distances 100 - 1000 km; Mw ≥ 5.5) using the EN network. Out of 90 mainshocks, we have found 3 teleseismic and 7 regional events that trig-gered seismic activity in the SSGF. The peak ground velocity generated by all the triggering mainshocks exceed 0.03 cm/s, corresponding to a dynamic stress of ~2 kPa. This threshold is consistent with ones found in the Long Valley Caldera [Brodsky and Prejean, 2005] and other geothermal regions in California [Aiken and Peng, 2014]. The triggered events occur almost in-stantaneously with the arrival of large amplitude seismic waves and seem to be modulated by the passing surface waves. The triggered signals have very short P- to S-arrival intervals, sug-gesting that they likely originated from brittle failure in the shallow crust.

One potential issue of such analysis is that the identification of triggered seismicity is done by visual inspection [e.g., Aiken and Peng, 2014]. This is rather time consuming and is not easi-ly reproducible. Our group has recently developed a waveform matching method that utilizes

waveforms of existing events as templates, and scans through the continuous waveforms to detect additional smaller events [Peng and Zhao, 2009; Meng and Peng, 2014, 2016]. This procedure can also be used to detect remotely triggered seismicity [van der Elst et al., 2013; Wang et al., 2015; Yao et al., 2015].

Here we use the same method [Casey et al., 2015] to detect microseismicity around 5 target dis-tant mainshocks that were analyzed before by Doran et al. [2011]. We use waveforms recorded by the EN borehole network and generated by 6958 relocated events as templates, and scan through five days be-fore and five days after each distant mainshock. We apply a 10-25 Hz filter to the continuous waveforms, from which we subsequently cut the template wave-forms, and then use a 6-s window around the arrival of the P wave. Next, we compute the cross-correlation (CC) values for all station-channel pairs, and stack them after shifting the time windows back to the template origin times. Finally, we select detections that are above 12 times the median absolute deviation of a daily stacked CC trace. We also estimate the magnitude of the newly detected event by computing the median amplitude ratio between detected and template events. Figure 2 shows an example of newly

Figure 1. Map of the Salton Sea Geother-mal Field. The red triangles mark the loca-tions of the EN network stations; the purple circles are the relocated earthquake events, from 2007/12/31 to 2014/01/11, in the catalog compiled by Dr. X. Chen.

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detected event with the inferred magnitude of -0.44. The mean CC value is 0.59, well above the 12 times MAD threshold of 0.098, sug-gesting that this is a genuine detection.

Figure 3(a-c) shows the raw, band-pass-filtered and spectrogram recorded at station RED during the 2009 Mw6.9 Baja California earthquake. Several high-frequency micro-earthquakes occurred during the teleseismic wave trains, indicating that they are likely trig-gered by the Baja mainshock. Figure 3d shows matched filter detection results about half day before and one day after the mainshock. A swarm-like sequence in the SSGF occurred about half day before the mainshock, but the activity was low right be-fore. Right after the mainshock, there was a clear increase of local seismicity that lasted for about 18 hours. We also detected two magnitude>4 events. However, we did not observe a clear aftershock sequence following these two events. Hence, they are likely false detections.

Figure 4 shows additional two examples of detections around the 2008 Mw6.0 Nevada earthquake and the 2010 Mw8.8 Chile earth-quake. We found clear increase of seismic ac-tivity right after the distant mainshocks. In addition, there were further increases of seismic activ-ity about 16-18 hours after the mainshock. Whether they represented any delayed triggering or simply a reflection of regular swarm activities in the SSGF remains to be analyzed further. B. Student Support and Involvement

This work described above was primarily performed by SCEC intern Bridget Casey in sum-mer 2015, under the guidance of GT graduate student Xiaofeng Meng. Meng graduated in summer 2015 and is now a postdoc at UW. We plan to involve a second-year graduate student Chenyu Li to wrap up this work in summer 2016.

Figure 2. A waveform comparison of the template (red) and newly detected event (gray). The red dashed line marked the origin time of the template event. Other information are marked in the header.

Figure 3. (a) Raw, (b) 2-16 Hz envelope, and (c) spectrogram of vertical seismogram at station RED for the 2009 Mw6.9 Baja California earth-quake. (d) Template (red) and newly detected (black) events in Salton Sea around the 2009 Baja California mainshock.

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C. References (with publications/meeting abstracts supported by the grant marked in bold)

Aiken, C., and Z. Peng (2014), Dynamic triggering of microearthquakes in three geothermal/volcanic regions of California, J Geophys Res., 119, 6992-7009, doi:10.1002/2014JB011218.

Brodsky, E. E., and S. G. Prejean (2005), New constraints on mechanisms of remotely triggered seismicity at Long Valley Caldera, J Geophys Res-Sol Ea, 110(B4).

Casey, B., X. Meng, D. Yao, X. Chen and Z. Peng (2015), Systematic detection of remotely triggered seismicity in Salton Sea with a waveform matching method, abstract submitted to the annual Southern California Earthquake Center meeting, Palm Springs, CA.

Doran, A., X. Meng, Z. Peng, C. Wu, and D. Kilb (2010), Dynamic triggering of earthquakes in the Salton Trough of Southern California, Seismol. Res. Lett., 82(2), 290.

Meng, X., and Z. Peng (2014), Seismicity rate changes in the Salton Sea Geothermal Field and the San Jacinto Fault Zone after the 2010 M-w 7.2 El Mayor-Cucapah earthquake, Geophysical Journal International, 197(3), 1750-1762. Meng, X. and Z. Peng (2016), Increasing lengths of aftershock zones with depths of moderate-size earthquakes on the San Jacinto Fault suggests triggering of deep slip in the middle crust, Geophys. J. Int., 204(1), 250-261. Peng, Z., and P. Zhao (2009), Migration of early aftershocks following the 2004 Parkfield earth-quake, Nature Geosci., 2, 877–881, doi: 10.1038/ngeo697. Wang, W., X. Meng, Z. Peng, Q.-F. Chen, and N. Liu (2015), Increasing background seismicity and dynamic triggering behaviors with nearby mining activities around Fangshan Pluton in Bei-jing, China, J. Geophys. Res. Solid Earth, 120, 5624–5638, doi:10.1002/2015JB012235. Yao*, D. Z. Peng and X. Meng* (2015), Systematical search for remotely triggered earthquakes

Figure 4. Template (red) and newly detected (black) events in Salton Sea around the (a)2008 Mw6.0 Nevada and (b) 2010 Mw8.8 Chile mainshocks. Other symbols and notations are the same as in Fig-ure 3.

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in Tibetan Plateau following the 2004 M 9.0 Sumatra and 2005 M 8.6 Nias earthquakes, Ge-ophys. J. Int., 201(2), 543-551, doi: 10.1093/gji/ggv037.