Seismologists discover Calaveras-Hayward fault link, introducing possibility of much larger earthquake

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Campus seismologists have discovered that the Hayward Fault is connected to the Calaveras Fault, meaning that an earthquake could rupture on both faults and cause a more destructive earthquake than once thought.

Researchers proved that the Hayward Fault, which stretches through largely populated areas in the East Bay as far south as Fremont and as far north as San Pablo Bay at Richmond, actually touches the Calaveras Fault, which runs east of San Jose. There is an estimated 14.3 percent likelihood of a 6.7 magnitude or greater earthquake along the Hayward Fault in the next 30 years and a 7.4 percent chance on the Calaveras Fault, according to the U.S. Geological Survey.

“The smooth connection between the two faults means that an earthquake could quite easily break both faults at the same time, making for a substantially bigger and more destructive event,” said Roland Burgmann, campus professor of earth and planetary science and co-author of the study. “Deeper in the Earth, we find small earthquakes that clearly define where the connecting fault is.”

Analyzing satellite radar data helped reveal the existence of a slowly slipping fault connecting the southern Hayward Fault to the Calaveras Fault at the surface, said Burgmann, who also explained that the size of an earthquake directly scales to the dimensions of the fault that slips during the event.

Estelle Chaussard, a campus postdoctoral fellow in the Berkeley Seismological Laboratory and lead researcher, said there is the possibility of a much larger earthquake in the East Bay. Instead of a 6.9 magnitude earthquake, Chaussard predicted there could be an earthquake much larger and with much more ground motion.

Chaussard explained that California Memorial Stadium, which crosses the Hayward Fault, is slowly offset by a phenomenon known as “creep,” which is the continuous motion of a fault without any significant earthquake that releases some of the stress accumulated on the fault.

To confirm the connection between the two faults at the surface, Chaussard’s team analyzed micro-earthquakes, which they used to develop a 3-D model that illustrates the faults’ junction on the surface.

Chaussard’s team, utilizing a technique called InSAR, or Interferometric Synthetic Aperture Radar, used a satellite to send microwaves to Earth’s surface and back. From these, researchers were able to construct images called interferograms, which, when combined with many other interferograms, extract the motion over time of ground displacements of a few millimeters per year over large areas.

Because Chaussard’s discoveries are fairly recent, she does not expect the East Bay cities to take any action in strengthening seismic protection in buildings. Despite this, she said, her research will continue to focus on the creep in the region to possibly identify locations of future earthquakes.

Robert Tooke is the lead research and ideas reporter. Contact him at [email protected] and follow him on Twitter @robertono_t.