Unstable Rocks and Earthquake Hazard – Times-Standard

Large earthquakes are relatively rare events. The challenge in seismic risk analysis is to know the strength and likelihood of earthquakes in the future. Hundreds or thousands of years may elapse between successive events in a given area. The automated records are only a century old and are not nearly sufficient for risk analysis. Newspapers, church records, diaries, and other writings extend the record backwards, but they do not provide broad, continuous or quantitative coverage. China is the only country in the world with written records dating back 2,000 years, but even in China, not all areas were inhabited and some strong earthquakes were not included in their records.

I was in graduate school when I first heard the term paleontology. Kerry Sieh, while still a college student, began searching trenches for geological evidence of earthquakes in the 1960s and 1970s. He perfected the use of the backhoe as a field geological tool during his Ph.D. San Andreas Fault Zone Studies. Kerry’s work took the chronology of earthquakes in San Andreas from the first known event in 1857 to the sixth century. His daughter attended Humboldt State University and we enjoyed regular visits and seminars for a few years from Kerry. Trench studies have become the norm in earthquake geology, and my colleagues Gary Carver, Bud Burke, Mark Hemphill-Haley and legions of HSU undergraduates and technical graduate students around the world have used it.

Backhoes are useful when a fault breaks the surface, but they are of little use in checking for past earthquakes if faults are not visible and you do not know where to dig. In many areas, thick layers of sediment mask the traces of the rift. Marine malfunctions may produce strong vibrations on the ground but they are out of reach for bulldozers.

In 1992 I heard Jim Brown give a paper at the fall meeting of the American Geophysical Union. AGU is a huge meeting and most of the conversations and posters don’t make a lasting impression on me. But I remember Brun’s speech clearly. He looked at the distribution of unstable rocks and rocks and compared them to earthquake probability maps. The hypothesis was that these rocks had remained precariously balanced for tens of thousands of years. If they were still standing then no strong vibration would occur in that period of time. Brunn, a seismologist at the University of Nevada Reno, is a geophysicist and expert in seismic waves. He authored one of the most widely cited papers on the nature and strength of vibrations induced by faulty slip in 1970. But Jim was not a single one who worked solely on computers and laboratories. He also spent a lot of time in the desert examining field faults and conducting prospective studies after the earthquake.

If you’ve bled into canyon lands in Utah, Arizona, Colorado, or New Mexico, you’ve noticed large rocks floating precariously over the landscape. Brunn realized that unstable balanced rocks (PBRs – a geological term) were not only great photographic sites, but also indicators of past earthquakes, or rather non-earthquakes.

PBRs weren’t the main focus of Brun’s studies at the time, but he put several graduate students to work developing a computer program to analyze the physics behind these rocks and estimate the level of vibration it would take to displace them. Using this methodology, he was able to place restrictions on areas that have experienced strong shaking in the past ten thousand years, often the time window to determine whether or not the bug is active.

The M7.1 Hector Mine earthquake of 1999 in the Mojave Desert proved to be a boon for Brown’s studies. He had studied the area before the earthquake and identified many rocks that stood before the earthquake and collapsed afterward. Using data from a powerful close-in motion instrument, Brun was able to calibrate his models and estimate that the earthquake’s vibration must exceed 20% of the gravitational acceleration to drop the rocks. From the age of the rocks, it has been estimated that at least 10,000 years since similar shaking occurred in the area.

Unstable rocks appeared in the news last month when Anna Rudd, a graduate student at Imperial College London, published a study on finely balanced rocks in the journal AGU Advances. Anna works on the coast of Central California near the PG&E Diablo Canyon Nuclear Power Plant. Anna and the Imperial College team use cosmic surface exposure history to get a more accurate estimate of how long the rock has held in place. Cosmic dating is a bit like a very bad sunburn. It measures the changes in the surface of the rock due to cosmic rays – the longer it is exposed, the worse the “ burning ”, so to speak. There is no shortage of seismic risk studies in the vicinity of Diablo Canyon, the only operational nuclear power plant in California. Nuclear power plants and waste repositories are sites where identifying potential seismic hazards has health impacts on society. What I found particularly interesting about the new study is that it conflicts with the current consensus based on modeling the magnitude and duration of earthquakes returning from specific faults in the region. Rudd’s group found that strong vibrations in the region are significantly less frequent than previously thought, reducing the maximum number of earthquakes over the past 10,000 years in the region by about 27%.

Diablo Canyon was originally designed to withstand a magnitude 6.75 earthquake and was later strengthened to make it through a 7.5. PG&E plans to shut down the two reactors in 2024 and 2025.

Note: Find out more about Anna Rudd and colleagues’ study at https://temblor.net/earthquake-insights/ready-to-roll-rocks-improve-seismic-hazard-models-12080/.

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