Seismologists have long studied sequences of large earthquakes occurring closely in time and location, known as doublet quakes, which can significantly increase the devastation of seismic events. In Southern California, researchers with the U.S. Geological Survey (USGS) have modeled a hypothetical doublet scenario involving a magnitude 7.25 rupture of 52 miles along the Newport-Inglewood fault—running through Orange County, Long Beach, and South Los Angeles—followed by a magnitude 7.05 event several hours later on the nearby Palos Verdes fault.

The concept of doublets is not limited to two events; in western Afghanistan, a series of four magnitude 6.3 earthquakes occurred between October 7 and October 14, 2023. This quadruplet sequence caused extensive building collapses and resulted in approximately 2,400 deaths, as detailed by scientists presenting at the 2024 American Geophysical Union meeting.

There is no single, universally accepted definition for what constitutes a doublet quake. Generally, it involves two earthquakes of similar magnitude occurring within minutes to days of each other. Seismologist Lucy Jones prefers a more specific definition, requiring the two quakes in a sequence to be within 0.4 magnitude units of one another. The 2019 Ridgecrest earthquakes, consisting of a magnitude 6.4 event followed 34 hours later by a magnitude 7.1 quake, exemplify a debated doublet sequence, with some experts considering it such and others viewing it differently.

The occurrence of doublets is attributed to the readiness of nearby faults to rupture. A slip on one fault can trigger a neighboring fault already close to failure, or a system of interconnected faults may rupture in stages after brief delays. USGS research has also explored potential aftershock or triggered quake scenarios, including the 2008 ShakeOut report, which envisaged a major magnitude 7.8 earthquake on the southern San Andreas fault followed days later by a magnitude 6.95 quake impacting cities such as Sacramento and Modesto—raising concerns over levee integrity and flood control.

Jones is working on updating the ShakeOut scenario to include the possibility of a large San Andreas event followed by a significant quake on the Hollywood and Raymond faults, which lie beneath central Los Angeles and the San Gabriel Valley. Such sequences highlight the risk of multiple damaging tremors hitting the same region in short succession.

Recent events in Venezuela, where two strong earthquakes occurred less than a minute apart, illustrate the complexity of identifying distinct doublets. Zhongwen Zhan, director of the Caltech Seismological Laboratory, noted that continuous seismic data suggests the tremors might be a single prolonged event rather than separate quakes, a view echoed by Jones, who likened it to past experiences where main shocks and aftershocks blend together in public perception.

Experts emphasize that doublets and sequences of strong aftershocks heighten risks, particularly because buildings weakened by an initial quake may be more vulnerable to collapse during subsequent shaking. Maria Mohammed, president of the Structural Engineers Association of Southern California, cautions that damaged structures lose strength, underscoring the need for careful post-quake inspections. Historical examples include the 2011 Christchurch earthquake sequence, where buildings already compromised by earlier tremors suffered fatal failures.

Historical earthquake sequences also demonstrate the potential for moderate quakes to trigger larger events. For instance, moderate quakes in Monterey County preceded the massive 1857 magnitude 7.9 San Andreas earthquake. Similarly, the 1992 Joshua Tree earthquake initiated a series of aftershocks culminating in the 7.3 Landers earthquake, which was strong enough to be felt as far as Denver.

Doublet sequences have been recorded in California’s past, including the 1987 Imperial County events where a magnitude 6.2 quake was followed 11 hours later by a 6.6 tremor, causing injuries and millions in damages. Other notable sequences occurred near Mammoth Lakes in 1980, involving four moderate earthquakes over three days that caused property damage and injuries and raised concerns about volcanic activity in the region.

These events serve as a reminder of the complex and interconnected nature of fault systems and the importance of preparedness for multiple damaging quakes in close succession. Authorities continue to refine models and response plans to address the challenges posed by such sequences and to better inform the public about the risks.