Could the M7.1 Ridgecrest, CA Earthquake Sequence Trigger a Large Earthquake Nearby?

Science Center Objects

Release Date: SEPTEMBER 30, 2019

Two of the first questions that come to mind for anyone who just felt an earthquake are, “Will there be another one?” and “Will it be larger?”.

gray-scale map with black dots and colored lines

All earthquakes magnitude 2.5 and greater in the Ridgecrest sequence area July 4 to August 15 are shown as black dots. The M6.4 earthquake on July 4 and the M7.1 mainshock on July 5 are shown as red stars. The major nearby fault zones are labeled (Public domain.)

The Ridgecrest, California earthquake sequence sparked similar questions: Did the M6.4 earthquake on July 4, 2019 trigger the M7.1 on July 5, 2019? Could the sequence trigger earthquakes on other, nearby faults? The aftershocks from these two earthquakes are adjacent to the Owens Valley fault to the northwest, and the Garlock fault to the southeast. Could these aftershocks trigger a large earthquake on one of those or other nearby faults?

The first question is relatively easy to answer: when faults are close together and the earthquakes occur in rapid succession, they can be called triggered. Because the M6.4 and the M7.1 earthquakes were on two adjacent faults and occurred within 34 hours of one another, we can say with high confidence that the July 4th event triggered the July 5th event. The second question is about potential earthquakes in the future. We cannot predict earthquakes, but we can estimate the chances of an earthquake based on what has happened in past earthquake sequences.

What is the chance of another large earthquake?

The USGS aftershock forecast estimates the chances of a larger earthquake, as well as the number of smaller aftershocks we can expect. As more aftershocks are included in the analysis, the aftershock forecast can be fine-tuned and updated on a regular basis. As of this writing (on September 12, 2019) the chances of an M7+ aftershock are less than 1 in 400, and for an M6+ are 2% for the time period up to October 12.

Where might another large earthquake occur?

map with gray dots

All earthquakes magnitude 2 or greater in the region of the 1992 Joshua Tree-Landers-Big Bear sequence from April 22 to July 15 are shown as gray dots. The mainshocks for each of the 3 main sequences are shown as red stars. (Public domain.)

Whereas the aftershock forecast is tuned more to a general estimate of the pace and magnitude of earthquakes for a particular sequence, another type of earthquake forecast focuses more on the big picture in California, addressing the possibility of a large earthquake on one of California’s known faults. To determine the chances of a triggered “larger aftershock” due to the Ridgecrest sequence, we turn to this other analysis. We consider the possibility of both triggered events and earthquakes that rupture more than one fault. Because large earthquakes cause aftershocks, earthquakes tend to occur in groups.  We have seen this in past earthquake sequences in California, for example in 1992 during the Joshua Tree-Landers-Big Bear sequence of earthquakes.  This sequence started on April 22 with an M6.1 near Joshua Tree National Park. Then the aftershock sequence migrated northward over the next couple of months, triggering the M7.3 Landers earthquake, which was followed 3 hours later by the M6.3 Big Bear earthquake 32 km (20 mi) to the west.

For the Ridgecrest earthquake sequence, the closest large active faults are the Garlock, Owens Valley, Panamint Valley and Blackwater faults. The Garlock fault extends from the San Andreas Fault in the west to Death Valley in the east, and hasn’t had a major earthquake in about 500 years. The Owens Valley fault runs roughly northwest-southeast starting near the northern tip of the Ridgecrest fault, and it last ruptured in a large earthquake (about M7.8) in 1872. The Panamint Valley fault parallels the Ridgecrest fault to the east and also had a large earthquake about 500 years ago.  The Blackwater fault lays just south of the recent M7.1 rupture and like the faults that ruptured in Ridgecrest, has had perhaps only one earthquake in the last 10,000 years.

The San Andreas Fault is, of course, the largest fault in California, representing the main boundary between the North American and Pacific tectonic plates. It’s about 130 km (80 mi) to the west of the Ridgecrest earthquakes. The last large earthquake on San Andreas section nearest to Ridgecrest was an M7.9 in 1857.

two graphs with blue and red dots

Recorded earthquakes during the first 3 days of the July 2019 Ridgecrest earthquake sequence.  Each earthquake is shown as a dot, a blue dot for earthquakes smaller than magnitude 4, and a red dot for earthquakes magnitude 4 and larger.  The horizontal axis shows the time (local Pacific time) of each earthquake, and the vertical axis shows its magnitude.  Right after the magnitude 6.4 and 7.1 earthquakes, there is a gap at small magnitudes because too many aftershocks are happening for them all to be detected. (Public domain.)

map with gray dots and red lines

The five fault zones in the vicinity of the Ridgecrest earthquake sequence: Owens Valley fault zone, Panamint Valley fault zone, Garlock fault zone, Blackwater fault zone, and San Andreas fault zone. Earthquakes of magnitude 2.5 and greater in the Ridgecrest sequence from July 4 to August 15, 2019 are shown as black dots. (Public domain.)

There will probably not be a larger earthquake as part of the Ridgecrest aftershock sequence, and the aftershock sequence will continue to decay. If there is a future large earthquake in the Ridgecrest sequence, it is most likely to happen in the general vicinity near Ridgecrest and Trona, because this is where most of the aftershocks are currently taking place. The main swath of aftershocks extends to the Garlock fault southeast of Ridgecrest, and we have also observed a separate, small cluster of aftershocks on the Garlock fault about 20 miles (12 miles) southwest of Ridgecrest. That small cluster, which has produced 3 earthquakes of magnitude 3.0 to 3.2 since the Ridgecrest earthquakes, is located where a similar number of earthquakes occurred in 1981 without any larger earthquakes.

 Before Ridgecrest, the probabilities of a damaging earthquake on the Garlock, Owens Valley, Blackwater and Panamint Valley faults was low; about 3% in the next 30 years.  The USGS estimates that the probability of a M7.5+ earthquake on these four faults is now about 6 times above the normal background probability through mid-September, but the chances will go down to 2 to 5 times the background level during the next year. The possibility for a triggered event on the more distant San Andreas Fault (80 miles away) has not changed significantly from the background probability, and remains about 20% in the next 30 years. The bottom line is that the most likely scenario is a continued decay of the Ridgecrest aftershock sequence. There’s a small chance that there will be another potentially damaging earthquake in the Ridgecrest area. The least likely scenario is a large earthquake on one of the nearby major faults labeled on the map.

USGS scientists are carefully monitoring activity throughout the region and will continue to provide information to help people stay safe and care for themselves and each other.

What can you do?

If you live in an area where there are earthquakes, you should always be prepared for one to happen. The USGS Earthquake Hazards Program website includes a wealth of resources and links for earthquake preparedness.

- written by Lisa Wald, Jeanne Hardebeck, Morgan Page, Andy Michael, Sara McBride, Nicholas van der Elst, and Ned Field, USGS, September 2019

For More Information

The Scientists Behind the Science

headshot

Jeanne Hardebeck. (Public domain.)

Jeanne Hardebeck has worked at the USGS since 2003, studying earthquake statistics and the physics of how earthquakes trigger other earthquakes. She became interested in earth sciences while growing up in the eastern Sierra, about 100 miles north of Ridgecrest. She enjoys hiking in the Sierras, bicycling, and ocean swimming.

person on dirt trail with child on their shoulders

Morgan Page. (Public domain.)

Morgan Page has worked at the USGS for ten years researching in areas where statistics and seismology intersect. She loves solving puzzles of all kinds, not just science-related.

Andy Michael

Sara McBride

person sitting on small surface ledge

Nick van der Elst sits on offset caused by the July 2019 Ridgecrest earthquake. (Public domain.)

Nicholas van der Elst has been a geophysicist with the USGS since 2015, where he works on earthquake physics and aftershock forecasting. When not in the office, he can be found enjoying the less destructive side of California geology through mountaineering, caving, and rock climbing.

Ned Field