The 1933 Long Beach Earthquake (California, USA): Ground Motions and Rupture Scenario

Understanding ground motion patterns from historical earthquakes remains a key challenge in seismology. Prior research has established that fault geometry and local geology strongly influence shaking intensity. However, the specific mechanisms behind localized damage zones remain unclear. The 1933 Long Beach earthquake is a notable event due to its magnitude and impact on urban infrastructure. Existing studies have focused on fault identification and general shaking patterns. This paper aims to refine the understanding of how fault rupture and site conditions interact. By analyzing historical data, the study addresses gaps in how energy is distributed during earthquakes. The role of basin amplification and directivity effects is not yet fully resolved. This work contributes to a more detailed model of earthquake behavior in complex geological settings.

The study aims to analyze the ground motions from the 1933 Long Beach earthquake to better understand the mechanisms behind localized damage patterns. The researchers sought to determine how fault rupture and site conditions interact to produce observed shaking intensities. A key question is whether the damage in Compton can be attributed to fault characteristics or local geology. The study also explores how energy is channeled through the Los Angeles Basin. By simulating rupture scenarios, the researchers aim to test different models of ground motion. The goal is to identify which factors—such as directivity or basin effects—are most influential. The analysis is informed by historical damage reports and geological data. This approach allows for a more precise understanding of earthquake behavior in urban areas.

The researchers used a broadband simulation approach to model the 1933 earthquake's ground motions. Historical damage data was combined with geological surveys to infer shaking patterns. The Newport-Inglewood fault was identified as the likely source of the earthquake. A 25-km-long fault rupture was simulated to match observed intensity distributions. Non-linear site response was considered in soft sediment regions. The simulation included three-dimensional basin effects to account for energy channeling. Damage concentration in Compton was compared to model predictions. The results were validated against historical accounts of vertical ground motions.

The simulation results suggest that a 25-km-long fault rupture best explains the observed shaking intensity. Damage in Compton is attributed to a combination of local site amplification and directivity effects. The model indicates that energy was channeled toward the deepest part of the Los Angeles Basin. Non-linear site response in soft sediments exceeded simple model predictions. The Newport-Inglewood fault is strongly associated with the earthquake's shaking pattern. The study confirms that basin geometry plays a significant role in amplifying ground motion. Vertical ground motions in Compton reached up to 1 g according to historical accounts. The results support the idea that multiple factors interact to influence damage distribution.

The study concludes that the 1933 Long Beach earthquake's damage pattern can be explained by a combination of factors. Local site amplification, source-controlled directivity, and basin effects all contributed to the observed shaking. The Newport-Inglewood fault is the most likely source of the earthquake. The 25-km-long rupture scenario best matches historical damage reports. Non-linear site response in soft sediments was stronger than expected. The deepest part of the Los Angeles Basin experienced amplified shaking. These findings suggest that basin geometry and fault orientation are critical in predicting damage zones. The results support the need for detailed site-specific modeling in urban seismic risk assessments.