Rare 'super shear' earthquakes could devastate California

Most people in California expect large earthquakes, but there is a faster type that can hit with greater force and over a wider area.

These rare supershear earthquakes, ruptures that move faster than waves shaking side to side, can hit cities with particularly intense ground motion.

Scientists working with California's statewide earthquake center say many of the large earthquakes California is likely to face in the coming decades could fall into this category.

They warn that current building codes and risk plans do not fully reflect what these events can actually do.

Understanding supershear earthquakes

The work was supervised by Ahmed El-Banna, a professor of geosciences at the University of Illinois Urbana-Champaign, in collaboration with colleagues at the California Statewide Earthquake Center (SCEC).

His research focuses on how energy is released through complex fault ruptures and how this energy is translated into vibration at the surface.

In a typical earthquake, rupture propagates along the fault at a slower speed than shear waves, which are seismic waves that shake the ground sideways. A superrupture outpaces these waves, so energy accumulates at the tip of the rupture and forms a sharp shock front.

This shock is why these earthquakes can be so devastating. A nearby town could be hit first by the high-speed rupture front, then by subsequent waves in what Al-Banna describes as a “double whammy.”

Many faults that can host these events are strike-slip faults, which are fractures in which blocks of the Earth's crust slide horizontally past each other rather than moving up or down.

The San Andreas Fault, which extends about 800 miles across California, is the most famous example of this type of structure.

Because the rupture velocity is very high, a supershear event can maintain the concentration of strong vibration in a long and narrow corridor along the fault.

This is different from normal earthquakes, where more intense shaking tends to weaken with distance and in a more widespread pattern.

When and where they strike

For a long time, seismologists thought that supershear earthquakes were strange outliers. A recent study found that in the past 15 years, 14 out of 39 major earthquakes worldwide showed clear signs of supershear rupture, accounting for about a third of the total.

Independent work looking at earthquakes since 2000 shows something similar. One research group found that about 14% of large earthquakes hit supershear velocities, more than double previous estimates that put the rate below 6%.

Recent disasters have shown how this happens in real life. The 7.5-magnitude Palu earthquake in Indonesia in 2018 ruptured a long segment of a strike-slip fault at ultra-shear velocities and triggered deadly landslides and tsunamis that killed thousands of people.

California and supershear earthquakes

California lies on a tangle of large faults that run close to major cities. The San Andreas, San Jacinto, and Hayward faults all crisscross areas filled with highways, pipelines, data centers, and dense housing.

Long-range forecasts show the state isn't just waiting for one rare event. Statewide forecasts estimate there is a 99 percent chance that California will experience at least one magnitude 7 earthquake within a 30-year period.

An official assessment puts the odds of a magnitude 8 or larger earthquake occurring in California within the next 30 years at about 7 percent, which is small but not negligible for an event of this size.

Supershear ruptures are more likely to occur on the types of long, straight fault segments that may host these large earthquakes.

The opinion piece emphasizes that current building codes often assume that the strongest shaking will be perpendicular to the fault.

In a supershear earthquake, the most damaging energy can remain parallel to the fault, which means communities along the fault's trace could face a much stronger blow than expected.

Prepare for super-shear earthquakes

California risk planning must keep pace with this science. The researchers confirmed that these events are expected to occur in the coming decades, regardless of how prepared the country is.

One important step is improving monitoring. Dense networks of powerful motion artifacts near major faults would provide scientists with clearer data on how ruptures initiate, accelerate, and sometimes transition to supershear.

Another step is to use advanced computer models that test how real cities respond to supershear scenarios.

These simulations help determine which neighborhoods, bridges or hospitals will experience the most severe shaking and where targeted retrofits could have the greatest impact.

Better science alone cannot fully meet the challenge. Al-Banna stressed that reducing risks will require a coordinated effort between agencies, researchers and communities.

The study is published in Earthquake Research Letters.

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