Image credit: Ben van der Pluijm

ANN ARBOR—A magnitude 7.1 earthquake jolted California on Friday night after a 6.4-magnitude event in the same area on Thursday. University of Michigan experts are available to comment.

Nathan Niemi

Nathan Niemi is a professor of geological sciences in the Department of Earth and Environmental Sciences. His research focuses on the tectonic processes that lead to mountain building.

“The occurrence of a magnitude 7.1 earthquake only a day after a magnitude 6.4 earthquake is unusual but not unprecedented,” Niemi said. “The U. S. Geological Survey aftershock predictions on July 4 included the possibility of an earthquake larger than magnitude 7, although the likelihood of such an earthquake was considered to be small.”

A similar sequence of events occurred in 1992 in Southern California, where the magnitude 6.5 Joshua Tree earthquake is considered a foreshock to the magnitude 7.3 Landers earthquake, although those earthquakes were two months apart, Niemi said. The aftershock sequence from yesterday’s magnitude 7.1 earthquake will last for months or years, and large (magnitude 5-6) aftershocks would not be unexpected.

“The possibility of an aftershock similar in size to yesterday’s magnitude 7.1 event cannot be discounted. However, the length and complexity of faults in this region likely precludes a significantly larger event,” Niemi said.

The magnitude 7.1 earthquake that occurred on July 5 appears to be a continuation of the earthquake sequence that began on July 4, Niemi said. Aftershocks associated with the magnitude 6.4 earthquake on July 4 defined two distinct arrays, one oriented northwest-southeast, and the other oriented southwest-northeast, suggesting the earthquake caused slip on two different fault planes.

The northwest-southeast fault that ruptured during the magnitude 6.4 earthquake appears to have ruptured again during Friday night’s magnitude 7.1 earthquake. The aftershock sequence of the magnitude 7.1 rupture stretches approximately 40 km (25 miles) in length, the majority of which is within the China Lake Naval Weapons Center facility.

Earthquakes of the magnitude of yesterday’s 7.1 event will impact other faults in the region, altering the likelihood of earthquakes on those faults. Some faults will have increased stress resulting from this event, while others will see decreased stresses, Niemi said.

The effects of these stress changes may not be immediately manifested but may result in increased seismic activity in this region in the coming months to years. These effects will be localized in the region surrounding yesterday’s event and are unlikely to extend to fault systems in the major metropolitan regions of California, he said.

“Tremendous advances in technology—including understanding of earthquakes, early warning systems and automated response systems—have been developed since the last major earthquake in California in a major metropolitan area, about 25 years ago in Los Angeles and 30 years ago in the Bay Area. These recent earthquakes serve as a reminder of the seismic risk to California and an opportunity to assess how response systems have functioned.”

“Earthquakes larger than yesterday’s have occurred, historically, in both northern and southern California, and yesterday’s earthquake is a reminder of the need to remain prepared for such events,” Niemi said.

Contact: 805-587-2237, naniemi@umich.edu

Benander Plujim

Ben van der Pluijm is an expert on geohazards, including earthquakes and volcanoes, and their impacts on society. He is a professor in the Department of Earth and Environmental Sciences.

Friday night’s magnitude 7.1 earthquake was about 10 times stronger than the July 4 event, so it was more widely felt and potentially more damaging. But both quakes occurred in a sparsely populated region.

The 7.1-magnitude earthquake occurred along the northwest-southeast East California Shear Zone that parallels the right-lateral plate boundary to the west. Displacement from the larger quake was on the order of meters over a fault length of several tens of kilometers.

More magnitude 7 earthquakes are unlikely in the area, but many smaller ones will continue, van der Pluijm said.

“Thursday’s 6.4-magnitude earthquake and its aftershocks may have triggered the 7.1, but both are ultimately the result of regional fault stresses from plate movement that are regularly released,” van der Pluijm said. “The relative displacement of the Pacific plate is about 5 centimeters per year, creating stresses that accumulate and that must be released periodically.”

Friday night’s earthquake was equivalent in energy to about 45 Hiroshima-type atomic bombs, van der Pluijm said. The July 4 earthquake was equivalent in energy to about four Hiroshima-type atomic bombs.

The use of technical terms like “main shock” and “aftershock” is semantics. The largest earthquake in a cluster is called the main shock, and earthquakes that occur hours to days before it are called foreshocks. Earthquakes that occur days to weeks after the main shock are called aftershocks.

“Do these earthquakes change the potential for the Big One? No, because continuing Pacific-North America relative plate movement in western California means that a large quake along the San Andreas Fault system must occur in the near future. These recent California earthquakes to the east do nothing to reduce that threat.”

Contact: vdpluijm@umich.edu; 734-663-9134 (home), 734-678-1397 (cell), vdpluijm55 (Skype).

Jeroen Ritsema

Jeroen Ritsema is a professor of geophysics in the Department of Earth and Environmental Sciences. His research involves the analysis of seismic waves to image Earth’s interior.

It is not unusual that a magnitude 7.1 earthquake followed a magnitude 6.4. The June 28, 1992, Landers earthquake in Southern California, just north of Palm Springs, was a similar horizontal-sliding earthquake east of the San Andreas Fault in the Mojave Desert. This 7.3 earthquake was preceded on April 23, 1992, by the magnitude 6.2 Joshua Tree earthquake. The Landers earthquake itself was followed three hours later by the magnitude 6.5 Big Bear earthquake.

“This shows that large earthquakes can change stresses and increase the likelihood of earthquakes on nearby faults,” Ritsema said. “Moreover, the Landers earthquake triggered seismicity more than 300 kilometers to the north. Seismologists think that the magnitude 5.5 Little Skull Mountain earthquake on June 29, 1992, near Yucca Mountain was triggered by the Landers earthquake.”

“All large earthquakes are followed by a sequence of smaller-magnitude aftershocks that decay in frequency with time, a phenomenon known as Omori ‘s Law. Earthquakes with magnitudes between 6.5 and 7 can occur as pairs with the large event following the smaller event, as the 2019 Ridgecrest sequence and the Landers sequence from 1992 have shown.”

The Landers (1992), Northridge (1995), Hector Mine (1999) and Ridgecrest (2019) earthquakes demonstrate that earthquake potential is widespread in Southern California, Ritsema said.

“The last major earthquake on the southern segment of San Andreas Fault occurred in 1857, with an estimated magnitude of 8,” he said. “Southern California is a tectonically active region with numerous earthquake faults. One should be prepared for earthquakes on the San Andreas Fault and on the many faults, some of which are unmapped, within the urban centers of the Los Angeles region.”

Contact: 734-615-6405, jritsema@umich.edu