Some California bugs may have longer recurring periods than thought

Improved seismic frequency interval estimates can explain the apparent earthquake gap in California.

Written by Rachel Rochester, Ph.D., science writer (@RaRaRhapsody)

Citation: Rochester, R., 2022. Some California Errors May Have Longer Repetition Periods Temblor, Temblor, http://doi.org/10.32858/temblor.279

In California, scientists suspected that a major earthquake was likely to hit one of the state’s major faults. Ancient seismologists, looking for evidence of past earthquakes by sometimes drilling into the faults themselves, have documented eight land-rupturing earthquakes along the San Andreas and San Jacinto faults since 1800, but none in the past 100 years. The long gap led to concerns that these key-hit slip errors were ready to slip again, which could cause trouble for California.

However, in a new study published in Seismology Research Letters, a scientist suggested that the length of time experts expect between Earth-shaking earthquakes, called the “recurrence period,” may be longer than previously thought for some California faults.

An earthquake in 1906 erupted along the San Andreas Fault. Credit: JB Macelwane Archive, Saint Louis University

enter uncertainty

Faults produce many large and small earthquakes over time as stress periodically builds up and is released. When scientists assess the frequency of slips in such an event, they rely on estimates of when previous earthquakes occurred, which often come from disturbed sediment records or compensating features in the landscape.

But many factors can influence these ancient records, and calculating this uncertainty can improve estimates of earthquake frequency, according to Devin McPhillips, a USGS geologist and author of the study.

At eight major ancient study sites—six on the southern San Andreas Fault, one on the San Jacinto Fault in Southern California and one on the Hayward Fault in northern California—McPhillips has assessed the convincingness of evidence gathered from rock and sediment layers. that past earthquakes have occurred. He then calculated a factor known as “event probability,” which is the probability that the fault had been affected by a previous proposed earthquake.

So far, event probability has rarely been taken into account in estimates of recurrence intervals. In the few cases where they were combined, they were represented by two separate numbers; Scientists will present one mean recurrence period using only the most credible evidence, and another using all available evidence, regardless of quality.

MacPhillips simplified the process by visualizing the probability of the event with a single number. Its approach builds on the foundational work of previous studies. Record separately the strength of the evidence for each earthquake and the quality of the sediment record in which the evidence was collected.

“I hope more and more people will use some sort of classification system to move forward to get the probability of an event,” MacPhelps says. Measuring a semi-qualitative classification system can be difficult because these systems are subjective. MacPhillips says he’s focused on making his classification system concise, in part, so that it can be more easily replicated.

Using this method, McPhillips found that repeat periods for some sections of the San Andreas, Hayward, and San Jacinto faults were, on average, 16% longer than previously expected. Importantly, the newly calculated recurrence periods were sufficiently different from previous estimates that increase the likelihood of long seismic outages.

Some sections of the San Andreas fault may have longer recurring periods than previously thought. Credit: John Wiley, CC BY 3.0

detective work

Learning about past earthquakes involves detective work. Although the evidence for seismic activity may be strong, it is not always arguable. Evidence collected at any particular site is somewhat uncertain, says USGS geophysicist Glenn Biasi, who was not involved in the study. He says analyzing earthquake data in the geological record is like analyzing traffic based on snapshots in time.

The street next to the school will see a flurry of activity during the fifteen minutes when students are dropped off, but after parents, students and buses leave, it can look almost deserted. Analyzing just one of those footage won’t reveal the full picture of the traffic flow on the street.

As in the traffic analogy, looking at the geological record more comprehensively can reveal a more detailed picture. One benefit of maximum likelihood thinking, as MacPhillips does, says Biasi, is that it can help explain “a wave, or even an absence of data.”

When calculating recurrences, experts must consider not only uncertainties, such as those MacPhelps focused on, but also missed events, says Kate Sharer, a research geologist with the USGS who was not involved in the study. The missing events are prehistoric earthquakes that are not easily confirmed because any evidence of their occurrence has been obscured, for example, by surface erosion or burrowing organisms. Like uncertainties, missed events pose a particular problem for iteration interval calculations because including them can artificially shorten the iteration interval estimates. When paleo-seismologists attempt to analyze missed events in the estimates, they run the risk of overcorrection, thus “over-interpreting” the number of earthquakes that have occurred in the past. The frequency periods reviewed by the McPhillips study already included considerations of missed events, but had not previously addressed the uncertainty of event likelihood.

Sharer says turning the observational data into a single metric “was really important to the new update of the National Earthquake Hazard Map, which really addressed the probability of missed events, but didn’t have a way of dealing with the issue of over-interpretation”. This problem of over-interpretation is exactly what the new McPhillip scale aims to mitigate. “This is an issue that a lot of newspapers and paleontologists have grappled with, but they haven’t crossed the finish line quantitatively,” Sharer says.

The current national long-range earthquake risk map. Credit: USGS, Public Domain

reassuring results

This work confirms that current methods for estimating repetition intervals are generally effective, says MacPhillips. Although the incorporation of event probability lengthened the mean repetition period of errors considered in this study, the resulting shift was also convenient, as it confirmed that incorporating probabilities of events or missing events altered recurrence periods, but Not big enough to necessarily warrant an overhaul. previous estimates.

“One of the most important conclusions for me is that in a lot of applications … you probably don’t have to worry so much about the potential for missed events or events,” MacPhillips says. “This is reassuring, and it supports a lot of previous work,” he says.

“However, I hope that future investigations of the ancient time will determine the probability of events and missing events in order to improve estimates of the future iteration period.”

in-depth reading

Biasi, J and K. Sharer (2019). Unexpected current earthquake hiatus at paleoseismic frontier sites in California. Seismol.l Res. Lett. , 90, no. 3, 1168–1176.

Castillo, B. A., S. F. McGill, K. M. Scharer, D. Yule, D. McPhillips, J. McNeil, S. Saha, N. D. Brown, and S. Moon (2021). Prehistoric earthquakes on the forbidden strip of the San Andreas Fault, North Palm Springs, CA, Geosphere 17, no. 3, 685-710.

McPhillips, D.; (2022). Revised periods of seismic frequency in California, USA: Paleozoic sites and application of event probabilities. Seismic search letters.

Sharer, K.; , R. Weldon, c. Biasi, A. Strig, T. Fomal (2017). Earth-rupturing earthquakes on the Great Northern Bend of the San Andreas Fault, California, 800 CE to Present, J. Geoffs. Precision. 122, no. 3, 2193-2218.

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