Map of the Salton Trough, regional faults, Lake Cahuilla shoreline (as of 1725), and history of paleoseismic studies.

The Salton Trough, including its extension down into Baja California, is perhaps one of the best laboratories in the world to study transtensional fault interaction, fault zone architecture and damage zones, and paleoseismology, all components of understanding earthquake physics and the earthquake process itself. The region has a rich history of paleoseismic studies (see map) that encompass the entire southern San Andreas system, including the San Andreas, San Jacinto, Elsinore and Imperial faults. This is facilitated by the frequent presence of ancient Lake Cahuilla, which has filled to 13 m elevation six times in the past 1100 years. Many of the aforementioned faults cross the shoreline and sediments of Lake Cahuilla, which allows for the sequencing of large earthquakes in the Salton Trough for the past 1100 years. Moreover, recent studies compiling more than 400 ¹⁴C dates from the Lake Cahuilla basin have yielded a rich and precise record of the lake high-stands, which allows for the dating of the large earthquakes captured in the lake sediments to be placed into a regional stratigraphic system.

The Salton Trough lies in a hyper-arid climate regime, which means that there is little to no vegetation cover. This has resulted in excellent exposures of faults and their damage zones. Recent studies after the 2010 M7.3 El Mayor-Cucapah earthquake demonstrate the role of fluids during the earthquake process, as well as demonstrate shallow high heat signatures, as recorded in the mineralogy and chemistry in the damage zone (Ostermeijer et al., 2020; Dorsey et al., 2021). Preliminary work on the damage zones along faults that host M6.5 vs >M7 earthquakes suggests a relationship between damage signature and earthquake magnitude, a topic that can be well-explored in this region.

Dr. Ashley Griffith, professor at Ohio State University, sampling the damage zone along the southern Elsinore fault. Photo by Thomas Rockwell  (June, 2021).

The 2010 El Mayor-Cucapah earthquake produced a 130 km-long oblique rupture that involved many faults in the Sierra Cucapah and southeast into the Colorado delta adjacent to the Sierra Mayor (Fletcher et al., 2014; Teran et al., 2015). This follows the 1892 M7.2 rupture of the Laguna Salada fault, which produced at least 55 km of oblique rupture with displacements of up to 6 m (Rockwell, et al., 2015). The fault system in the Sierras Cucapah and Major are one of the best areas in the world to study the 4D mechanics of oblique extension. Recurrence intervals for large earthquakes are measured in thousands of years, which means that precise dating of past events is not as critical as for fast moving faults. Detailed study of all of the elements of this fault system could provide a complete rupture model that encompasses a large volume of crust over a period of several tens of thousands of years.

Integration of the large variety of information that can be gleaned from this region, which not only has a large amount of data in hand, but is also rich in potential, may provide unique insights into the rupture process and other aspects of earthquake physics.

About the Author

Thomas Rockwell, Emeritus Professor of Geology at SDSU, where he maintains an active research program and continues to teach part time. His research focuses on the occurrence of earthquakes in time and space, as studied through geologic methods, as well as the characteristics of faults, their associated damage zones, and their evolution.

References​

• Dorsey, M.T., T.K. Rockwell, G.H. Girty, G.A. Ostermeijer, J. Browning, T.M. Mitchell, J. Fletcher, 2021, Evidence of hydrothermal fluid circulation driving elemental mass redistribution in an active fault zone. J. Structural Geology, v. 144.. doi.org/10.1016/j.jsg.2020.104269
• Fletcher, J.M., O. J. Teran, T. K. Rockwell, M. Oskin, K. W. Hudnut, K. J. Mueller, R. M. Spelz, S. O. Akciz, E. Masana, G. Faneros, A. Morellan, J. Stock, A. Elliott, P. Gold, J. Liu-Zeng, A. Gonzalez, D. Lynch, A. Hinojosa, J. Gonzalez, 2014, Assembly of a large earthquake from a complex fault system: surface rupture kinematics of the April 4, 2010 El Mayor-Cucapah Mw7.2 earthquake. Geosphere, v. 10, no. 4, p. 797-827.
• Ostermeijer, G., T. Mitchell, F. Aben, M. Dorsey, T. Rockwell, J. Fletcher, J. Browning, F. Ostermeijer, 2020, Damage Zone Heterogeneity on Seismogenic Faults in Crystalline Rock; a Field Study of the Borrego Fault, Baja California. J. Structural Geology, 137. doi.org/10.1016/j.jsg.2020.104016
• Rockwell, T.K., J. M. Fletcher and O. J. Teran, A.P Hernandez, K. J. Mueller, J.B. Salisbury, S. O. Akciz, P. Štěpančíková, 2015, Reassessment of the 1892 Laguna Salada Earthquake – fault kinematics and rupture patterns, Bulletin of the Seismological Society of America, v. 105, no. 6, doi: 10.1785/0120140274
• Teran, O.J., J.M. Fletcher, T.K. Rockwell, K.W. Hudnut, M.E. Oskin, R.M. Spelz, S.O. Akciz, 2015, Coseismic rupture fabric in transtensional shear zones. Geosphere, v. 11, no. 3, p.  899-920.  doi:10.1130 /GES01078.1.