The Ridgecrest earthquake sequence that started a year ago at the southern end of the Walker Lane provided clear reminders that (i) potentially damaging large earthquakes occur in Southern California on timescale of decades and (ii) up close views reveal important features not accounted for by current understanding of accumulation and release of seismic energy. The recent June 4, 2020, M5.5 aftershock at the southern end of the 2019 Ridgecrest rupture, and the June 24, 2020, M5.8 earthquake near Lone Pine (also likely part of the Ridgecrest sequence) illustrate the complex nature of earthquakes interactions in space and time. The Ridgecrest sequence combined with the 1992 Landers (M7.3) and 1999 Hector Mine (M7.1) earthquakes span much of the distance between the San Andreas Fault to the south and the Owens Valley Fault that ruptured in the 1872 Lone Pine earthquake (M7.4-7.9) to the north. The June 24, 2020, M5.8 Lone Pine earthquake continues this trend.

 

The Ridgecrest sequence began with the July 4 M6.4 earthquake (plus foreshocks) that ruptured two orthogonal faults—one with a SW-NE orientation and left-lateral slip, and a shorter SW-NE-oriented rupture with right-lateral slip. The M7.1 mainshock on July 5 re-ruptured the right-lateral fault and dramatically extended the surface rupture close to the Garlock Fault to the south and close to the Coso geothermal field to the north. Much of the earthquake rupture is within the Naval Air Weapons Station China Lake. This necessitated permission and close coordination with the base authorities, facilitated by the US Geological Survey and California Geological Survey, and escorts of field crews due to security and safety issues.

 

Shortly after the July 4, 2019, M6.4 earthquake, SCEC coordinated with colleagues in government, academic and industry organizations rapid research and communication activities, including assessing probabilities of future large earthquakes in the area. Following the M7.1 Ridgecrest earthquake on July 5 2019, teams of researchers from many organizations conducted rapid geologic, geotechnical, geodetic and seismological field investigations to capture surface evidence and geophysical data that become obscure and decay fast with time. The rich data sets generated by the Ridgecrest sequence have led to dozens of published papers so far (and more in preparation) that contribute to improved understanding of earthquake processes. Some outstanding issues highlighted by the sequence include the dynamic generation of multi-scale near-orthogonal and sub-parallel ruptures and lines of seismicity, arrest and re-initiation mechanisms responsible for the 34 hours delay between the 6.4 and 7.1 events, and interactions of the Ridgecrest sequence with the Garlock fault to the south and the Coso area and Owens Valley to the north.

 

The collection of articles presented in this Ridgecrest-anniversary newsletter provides highlights from geology, geodesy, seismology, geotechnical and forecasting studies associated with the Ridgecrest sequence, with a focus on community activities. A selective list of communications is also provided. These articles and other papers on the sequence have useful insights on rapid post-earthquake reconnaissance, and detailed observations of structures and processes ranging from the surface to the upper mantle. Many fundamental unresolved issues remain, but the studies also clearly illustrate that the field of earthquake science is at a stage of rapid development, owing to the combination of detailed in-situ data and advanced analysis techniques.

 

Sincerely,

 

Yehuda Ben-Zion, SCEC Director