Earthquake hazards increased after the 2019 Ridgecrest event

By Christian Fogherty, science author (@ ChristianFoger1)

Seismologists have used a recently developed model to predict aftershocks after the last Ridgecrest earthquake sequence. Their findings revealed a short-term increase in the risk of earthquakes in the region.

Quote: Fogerty, C., 2020 Earthquake risk increased after the 2019 Ridgecrest event, Tumblr, http://doi.org/10.32858/temblor.089

Image of surface deformation caused by the Ridgecrest earthquake sequence in July 2019. Credit: USGS

Most residents of Southern California are aware of the earthquake threat posed by the San Andreas error. But a network of small malfunctions in this region also poses a great risk of earthquakes, not only because it can generate large earthquakes, but because vibration along one error can lead to others returning to life.

On July 4, 2019, a 6.4-magnitude earthquake rocked the Searles Valley area around Ridgecrest, California. Thirty-four hours later, a stronger 7.1 degree shock occurred. In a research recently published in seismic research messages, scientists used a newly developed model to predict the aftershocks after this event and assess the likelihood that these earthquakes will lead to greater vibration [Milner et al., 2020].

The map of Southern California focused on the Ridgecrest area and surrounding faults in the Mojave Desert. Credit: USGS

Earthquakes cause aftershocks

When stress – accumulated along the surface of the rift – is eliminated in one place during the earthquake, it is redistributed throughout the region. This redistribution process rips other earthquakes, generally the smallest, with the same close errors and errors. Medium to large earthquakes can cause hundreds of thousands of these aftershocks. Scientists look at aftershocks to learn more about earthquakes in a region in real time. When earthquakes are placed in a prediction model, scientists can calculate the probability of rupture of a nearby defect during a given period.

One of the newly developed model of earthquake prediction takes into account the concept that errors are more likely to rupture in response to near seismic activity, providing a more comprehensive picture of the potential for aftershocks. This new model, known as a epidemiological orgasm sequence, is borrowed from epidemiology publications. It is similar to the models used to learn more about the spread of SARS-CoV-2, the virus that causes COVID 19.

“This improves the ability to accurately predict the sequence of aftershocks, and also provides better restrictions on where these aftershocks are likely to occur,” says David Shelley, USGS research geophysicist.

Ridgecrest dependencies

Immediately after the Searles Valley earthquake, Kevin Milner, a computer scientist at the epicenter of the Southern California Earthquake and one of the main developers of the new model, started work, and eventually produced 100,000 projections by 4 pm. Later that day – each has a possibility. After the major shock of 7.1 occurred, another researcher updated the new data prediction model.

This type of real-time modeling gives decision makers invaluable information for preparing for an imminent potential event. With expectations created in the hours after the event, the California Earthquake Advisory Committee informed the governor of potential hazards later that day.

The commission was particularly concerned about the possibility of a rupture along the Garlock error, a long slip error near the Ridgecrest region capable of causing a major earthquake. The prediction model initially gave a 0.65% chance of a Jarlock rupture greater than its -7.0 strength in the 30 days following the main shock, but as more aftershocks occurred and the model was updated within the next few days to better represent the rupture surfaces, the possibilities jumped up to 5.1 %. These high probabilities of a continued earthquake are characteristic of the days immediately following the earthquake.

Garlock’s fault runs along the foothills of the Sierra Nevada mountains at the northern edge of the Mojave Desert. In this photo, Garlock’s fault meets San Andreas’s fault at the base of the San Gabriel Mountains in the distance on the left. Credit: NASA / JPL / NIMA

Future expectations

Over time, as the sequences wither, the chances of aftershocks decrease. About 10 months after the event, the odds of Garlock’s rupture have almost dropped to the pre-Ridgecrest sequence. Milner notes that “there is definitely still a chance for a large Garlock event depending on the model.” But he says, over the next ten years, it is slightly higher than the long-term model expects.

Garlock error forecast is torn over a period of one week. Black lines represent a long-term background risk, blue lines represent a risk to these errors over time, and red lines created by the new model add Ridgecrest events to these two lines, thus showing the most recent risks with all events included. These two graphs are separated by 7 months, clearly demonstrating Garlock settles into his background, or a time-independent risk. Credit: Kevin Milner

As for the San Andreas error to the west, the chances of a Ridgecrest aftershock lead to a major rupture of this error have increased immediately after the earthquake, although it is not much higher than the risk of daily background. Unlike Garlock’s probability jump of about 3 orders of magnitude in the first week, the risk of San Andreas error increased by about 1 by volume. In other words, the Great Quake has become nearly 1,000 times more likely along the Garlock and 10 times the fault of San Andreas. And with every new event, new possibilities come. With the predictions of aftershocks, the rate of events is degraded, but the magnitude does not change. There is still a chance for a strong event to play another sequence, etc. As Milner said, “If Garlock goes, then we’ll start getting nervous about San Andreas.”

Map of a complex network of malfunctions in Southern California and the possibilities of a 7 + Ridgecrest aftershock along its length for the next ten years. Credit: Kevin Milner

At this time, the Ridgecrest sequence is generally believed to die. A crawl movement is observed along the jarlock, but only at shallow depths where seismic activity is unlikely to occur (Ross et al., 2019). Even with this peace of mind, the possibilities found from this and other prediction models demonstrate that the time immediately after an earthquake is critical. “Although these odds seem low, the consequences of an earthquake on Jarlock or especially San Andreas will be very high. So, when an earthquake occurs, it is important that people prepare for additional earthquakes,” notes Jane Hardick, USGS geophysicist and member of the working group that I developed the model.

Predicting operational aftershocks

For seismologists, one of the biggest targets on the technological horizon is predicting an operational earthquake. The term “operation” refers to prediction models that are automatically updated after an earthquake. Currently, there are a few researchers who are able to operate the model used in this study, due to the deliberate effort on the part of Milner and his colleagues to expand access to the model code and technical documents. The fully automated version is still quite a few obstacles.

“It is definitely possible to activate this model, and we are looking forward to proceeding this path, but this will require investment,” Milner said. With the right computing resources, this goal can be achieved, and it can provide an unprecedented amount of detail and accuracy of probability. But getting here does not only require resource allocation; it also requires a change in the general way of reporting seismic risks to the public. “In conjunction with all the work to activate these expectations, we will also have to really think about the best ways to present information in a beneficial way that encourages appropriate responses from the public,” Milner said.

Check the risk of earthquake in Tempelor

In-depth reading

Milner, K. R., Field, E. H., Savran, W. H., Page, M. T., & Jordan, T. H. (2020). Operational earthquake prediction during 2019 Ridgecrest, California, seismic sequence with UCERF3-ETAS model. Earthquake Research Letters 91 (3), doi.org/10.1785/0220190294

Ross, Z, Edine, B. Jia, Z. , Stephenson, Awl, Chung, M. , Wang, X. , Beech, z. , Simmons, M. , Fielding, EJ, Yoon, S. Hackson, E. Moore, AW, Liu, Z., and Jung. J (2019). Cross-hierarchical cross fracture in the Ridgecrest earthquake sequence 2019. Science 366 (6463), doi.org/10.1126/science.aaz0109

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