Study suggests that smaller earthquakes generate most strong ground motions

According to a new study, smaller earthquakes have caused the strongest shaking incidents in the vicinity of the epicentral areas.

The study was recently published in Seismological Research Letters by scientists from the United States Geological Survey (USGS) and the University of Southern California in Los Angeles. 

The team focused on large datasets of earthquakes ranging from M 0.5 to M 8.0 to correlate the intensity of the seismic shocks with the generated strong ground motions. The researchers calculated the expected strong motions triggered from an earthquake of a specific magnitude at certain distances taking into consideration the Gutenberg-Richter magnitude-frequency law (the probabilistic frequency of an earthquake decreases as its magnitude increases). The findings show that, while small earthquakes generally produce weaker shaking, the correlation between the two parameters is highly variable and there are many cases in which a greater shaking than expected was detected.

The study suggests that those observations of small-magnitude seismic shocks and strong shaking are indeed a source of seismic hazard. Scientists have developed a theoretical model showing that the seismic hazard curves are positively skewed with respect to magnitude. This is a combinational outcome that derives from both the aforementioned variability and the high number of smaller earthquakes that generally occur.

Moreover, the findings show that a greater earthquake will not necessarily generate stronger ground motions than a smaller one but its impact will affect a wider area. Therefore, engineering projects could be more affected by medium earthquakes that also have a greater occurrence probability than by a massive seismic shock that seldom occurs. Examples include the M5.7 earthquake that occurred in Santa Rosa, California (1969) and caused the equivalent of today's $50 million in infrastructure damage and the M 5.7 Salt Lake City earthquake in 2020 that resulted in severe damage to several buildings. “For a lot of us, if we do look back over our personal experiences, the earthquake that we had the greatest amount of damage from is not the largest magnitude earthquake that we’ve felt at all,” Dr. Sarah Minson, lead author of the study and a Research Geophysicist at the USGS, stated.

The team acknowledges that there is no need for changes in Ground Motion Prediction Equations (GMPEs) or Probabilistic Seismic Hazard Assessment (PSHA) since the variation of the expected strong motions and the variability of different potential earthquakes are taken into consideration. Therefore, eventually, seismic hazard models do not account for the magnitude of an earthquake. Nevertheless, the findings could have a significant impact on Earthquake Warning Systems (EEW) which currently function better for major earthquakes (>M7.0). Hence, many EEW systems will not provide reliable warning alerts during a medium earthquake.

In terms of human psychology regarding earthquake awareness, the authors suggest that it is more efficient for people to prepare for smaller earthquakes than focusing on the impact of a vast seismic shock.

*The chart above shows the total area expected to experience shaking above a certain Peak Ground Acceleration (PGA) threshold. Despite the total area per occurrence of a single event is wider in larger earthquakes, given the Gutenberg–Richter law, a greater area will eventually be affected due to the number of medium earthquakes.