A New Fault Model for the 1933 Long Beach Earthquake, Long Beach Area, Southern California

Steve Boljen, Dan Gish

Newly identified thrust faults and their corresponding thrust sheets, combined with recent micro-earthquake epicenters, better explain anomalous rupture data observed during the 1933 Long Beach Earthquake than previous models based exclusively on Newport-Inglewood Fault Zone strike-slip faulting. A high-quality 45 km2 3D seismic dataset was recorded in 2017, centered along the Seal Beach Anticline, providing direct confirmation of a much more complex system of previously unrecognized thrust faults and cross faults encompassing the east flank of Wilmington Anticline, Seal Beach Anticline, and Los Alamitos Anticline, extending onshore at least 6 miles into the Los Angeles Basin. Additionally, more than 1200 micro-earthquakes recorded by a dense seismic network during 2017 have been located several miles to either side of the NIFZ and correlate with the newly identified thrust sheets and areas of recent deformation, indicating the faults are active. Based on this new fault model, energy propagation from the 1933 Earthquake appears to have transitioned from essentially pure horizontal strike-slip displacement along the NIFZ at its southern epicenter to high vertical-component transpressional displacement upon encountering the Garden Grove Fault – Coastal Fault thrust salient at the southern end of Seal Beach Anticline near Seal Beach Naval Weapons Station.