A major fault in California — which has been relatively quiet for the last 500 years, but is capable of producing a magnitude 7.8 earthquake — has started moving as a result of a sequence of damaging earthquakes that started near the city of Ridgecrest in California.
According to geophysicists from Caltech and NASA's Jet Propulsion Laboratory — both in Pasadena, California — who analyzed the largest earthquake sequence in Southern California in two decades, the findings provide new evidence that large earthquakes can occur in a more complex fashion than commonly assumed. The study, published in Science, documents a series of ruptures in a web of interconnected faults, with rupturing faults triggering other faults.
"This was a real test of our modern seismic monitoring system. It ended up being one of the best-documented earthquake sequences in history and sheds light on how these types of events occur," says Zachary Ross, assistant professor of geophysics at Caltech and lead author of the study.
The nearly 20-year hiatus in major seismic activity in southern California ended on July 4, 2019 with a sequence of intersecting earthquakes near Ridgecrest. The Ridgecrest earthquake sequence included a magnitude-6.4 foreshock on July 4, followed by a magnitude-7.1 mainshock nearly 34 hours later, and more than 100,000 aftershocks. The sequence rattled most of Southern California, but the strongest shaking occurred about 200 kilometers north of Los Angeles.
The rupture of the mainshock terminated only a few kilometers from the major regional Garlock Fault, which is a major east-west fault running from the San Andreas Fault to Death Valley. The paper describes the fault as a "260-km-long left-lateral strike-slip fault capable of producing Mw ~7.8 earthquakes."
The strain placed on the Garlock Fault by July's earthquake activity triggered it to start slowly moving, a process called fault creep. According to the researchers, the fault has slipped 0.8 inches or 2 centimeters since July.
The repeated occurrence of multi-fault ruptures poses a formidable challenge in quantifying regional seismic hazards, says the research team.
"The 2019 Ridgecrest sequence brought to an end the long earthquake silence in California. These events occurred within an immature fault zone and activated many orthogonal structures with lengths ranging from 1 km to more than 10 km. The largest events each ruptured multiple faults, a characteristic that has been repeatedly observed for large crustal earthquakes in recent years. Such scenarios are difficult to forecast for seismic hazard assessment. The rupture of the Ridgecrest mainshock terminated only a few kilometers from the Garlock fault, yet only a seismic creep was triggered at the closest section of the fault. Far to the southwest, a sizable swarm ensued, whereas triggered seismic activity on the entire eastern portion of the Garlock fault was negligible," the findings state.
It further says, "At such close proximity to the mainshock rupture, the stress changes imparted by the mainshock are substantial. The last major earthquake occurred at about 400 to 500 years ago. Future investigations that integrate the observed phenomena on the Garlock fault with geologic and geodetic observations will be important for understanding its contribution to seismic hazard in the Eastern California shear zone."
The team drew on data gathered by orbiting radar satellites and ground-based seismometers to piece together a picture of an earthquake rupture that is far more complex than found in models of many previous large seismic events.
Major earthquakes are commonly thought to be caused by the rupture of a single long fault, such as the more than 800-mile-long San Andreas Fault, with a maximum possible magnitude that is dictated primarily by the length of the fault. However, after the magnitude-7.3 earthquake that struck Landers, California, in 1992 — which involved the rupture of several different faults — seismologists began rethinking that model.
According to the researchers, Ridgecrest sequence is yet another example of how massive earthquakes can be generated by a web-like network of smaller interconnected faults that, when they rupture, trigger one another like falling dominoes. The sequence involved about 20 previously undiscovered faults, crisscrossing in a geometrically complex and geologically young fault zone, explains the team.
The scientists found that the magnitude-6.4 quake simultaneously broke faults at right angles to each other. They found this surprising because standard models of rock friction view this as unlikely. According to Ross, it is 'remarkable' that scientists can now resolve this level of detail.
"Over the past three decades, an increasing number of well-documented earthquakes have ruptured multiple faults, highlighting the importance of incorporating this geometric complexity into models of seismic hazard. The earthquake sequence activated a complex fault network, further illustrating the need to understand how multiple faults can rupture in a single earthquake," says the study.
The event, says Ross, demonstrates how little scientists still understand about earthquakes. "It's going to force people to think hard about how we quantify seismic hazard and whether our approach to defining faults needs to change. We cannot just assume that the largest faults dominate the seismic hazard if many smaller faults can link up to create these major quakes," says Ross.