Report on the August 2012 Brawley Earthquake Swarm in Imperial Valley, Southern California

Egill Hauksson, Joann M. Stock, Roger Bilham, Maren Boese, Xiaowei Chen, Eric J. Fielding, John E. Galetzka, Kenneth W. Hudnut, Kate Hutton, Lucile M. Jones, Hiroo Kanamori, Peter M. Shearer, Jamison H. Steidl, Jerome A. Treiman, Shengji Wei, & Wenzheng Yang

Published 2013, SCEC Contribution #1678

The 2012 Brawley earthquake swarm occurred in the Brawley Seismic Zone (BSZ) within the Imperial Valley of southern California (Figure 1). The BSZ is the northernmost extensional segment of the Pacific-North America plate boundary system. Johnson and Hill (1982) used the distribution of seismicity since the 1930s to outline the geographical extent of the BSZ, defining boundaries of the BSZ as shown in Figure 1. The north-south extent ranges from the northern end of the Imperial Fault (about 10 km north of the US Mexico international border) to the southern end of the San Andreas Fault, where it terminates in the Salton Sea.
During the seismically active period of the 1970s, the BSZ produced close to half of the earthquakes recorded in California (Johnson and Hill, 1982; Hutton et al., 2010). However, for two decades following the 1979 Imperial Valley mainshock M6.4 and its aftershock sequence, the BSZ was much less active. In general the BSZ seismicity is indicative of right-lateral strike-slip plate motion accompanied by crustal thinning as well as possible associated fluid movements in the crust (Chen and Shearer, 2011).
The 2012 Brawley swarm produced more than 600 events recorded by the USGS-Caltech Southern California Seismic Network (SCSN). Other monitoring instruments in the region, such as the Global Positioning System (GPS) network, creepmeters, and the Wildlife Liquefaction Array also recorded signals from the largest events. In addition, InSAR satellites collected images from space.

Hauksson, E., Stock, J. M., Bilham, R., Boese, M., Chen, X., Fielding, E. J., Galetzka, J. E., Hudnut, K. W., Hutton, K., Jones, L. M., Kanamori, H., Shearer, P. M., Steidl, J. H., Treiman, J. A., Wei, S., & Yang, W. (2013). Report on the August 2012 Brawley Earthquake Swarm in Imperial Valley, Southern California. Seismological Research Letters, 84(2), 177-189. doi: 10.1785/0220120169.