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Recent and Long-Term Behavior of the Brawley Fault Zone, Imperial Valley, California: An Escalation in Slip Rate?

Meltzner, Aron J. and Rockwell, Thomas K. and Owen, Lewis A. (2006) Recent and Long-Term Behavior of the Brawley Fault Zone, Imperial Valley, California: An Escalation in Slip Rate? Bulletin of the Seismological Society of America, 96 (6). pp. 2304-2328. ISSN 0037-1106. http://resolver.caltech.edu/CaltechAUTHORS:20140801-105556060

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Abstract

The Brawley fault zone (bfz) and the Brawley Seismic Zone constitute the principal transfer zone accommodating strain between the San Andreas and Imperial faults in southernmost California. The bfz ruptured along with the Imperial fault in the 1940 M_w 6.9 and the 1979 M_w 6.4 earthquakes, although in each case only minor slip apparently occurred on the bfz; several other episodes of slip and creep have been documented on the bfz historically. Until this study, it has been unclear whether the past few decades reflect average behavior of the fault. Two trenches were opened and a series of auger holes were bored across three strands of the bfz at Harris Road to compare the amount of slip observed historically with the displacements observed in the paleoseismic record. Evidence is presented, across the westernmost strand of the bfz and across the entire bfz at Harris Road, to show that both the average vertical slip rate observed in modern times (since 1970) and the vertical creep rate (excluding coseismic slip) observed during the 1970s are significantly higher than the long-term average. Across the westernmost strand, the long- term vertical rate is 1.2 (+1.5/−0.5) mm/yr, and the average rate since about a.d. 1710 is determined to be no greater than 2.0 mm/yr; in contrast, the average vertical rate between 1970 and 2004 across that strand was at least 4.3 mm/yr, and the 1970s vertical aseismic creep rate was 10 mm/yr. Likewise, across the entire bfz, the long- term vertical rate is 2.8 (+4.1/−1.4) mm/yr, whereas the rate between 1970 and 2004 was at least 7.2 mm/yr, and the 1970s aseismic creep rate was 10 mm/yr. The long-term strike-slip rate cannot be determined across any strands of the bfz but may be significant. In contrast to the commonly accepted higher sedimentation rates inferred for the entire Imperial Valley, we find that the average sedimentation rate on the downthrown side of the bfz adjacent to Mesquite Basin, in the millennium preceding the onset of agricultural influences, was at most 3.5 mm/yr. Finally, a creep event occurred on the bfz during our study in 2002 and is documented herein.


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http://dx.doi.org/10.1785/0120050233DOIArticle
http://bssa.geoscienceworld.org/content/96/6/2304PublisherArticle
Additional Information:© 2006 Seismological Society of America. Manuscript received 14 November 2005. We thank the many people who have been directly and indirectly involved with this project. First, we express our gratitude to the Imperial Irrigation District for granting us access to the site. Foremost, we thank Majdi Barjous, Chris Madden, Chris Lynch, Danielle Verdugo, Faten Khatib, and Chris Walls for field assistance, including help in hand-excavating and deepening parts of the trench, help in etching contacts and photographing the walls, and help in surveying, and also for offering valuable feedback and sanity during the hot summer days in the Imperial Valley. We are indebted to Mark Osterkamp for his friendly conversations and his eagerness to help us, for sharing his knowledge of the land, and for bringing to our attention the existence of several early surveyors’ maps. We also thank Charles Lovett of the Imperial County Surveyor’s Office for offering his assistance and providing us with copies of those early maps. We thank Daniel Ragona and Jeri Young for thoughtful suggestions and feedback, and Clarence Allen for providing unpublished material and helpful advice. We are grateful to Michael Rymer for his assistance in searching through boxes of unpublished leveling data, and to Gordon Seitz for speedy results in the analysis of our radiocarbon samples. Finally, we thank Sally McGill and Heidi Stenner for excellent reviews that led to substantial improvements of this article. This project was funded by U.S. Geological Survey grant 03HQGR0017.
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USGS03HQGR0017
Record Number:CaltechAUTHORS:20140801-105556060
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20140801-105556060
Official Citation:Meltzner, A. J., Rockwell, T. K., & Owen, L. A. (2006). Recent and Long-Term Behavior of the Brawley Fault Zone, Imperial Valley, California: An Escalation in Slip Rate? Bulletin of the Seismological Society of America, 96(6), 2304-2328. doi: 10.1785/0120050233
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:47791
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:01 Aug 2014 21:41
Last Modified:01 Aug 2014 21:41

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