Calaveras-Hayward fault link means potentially much more powerful quakes
The UC Berkeley team used nineteen years of satellite data to map ground deformation using interferometric synthetic aperture radar (InSAR) and measure creep along the southern end of the Hayward Fault, and found, surprisingly, that the creep didn’t stop south of Fremont, the presumed southern end of the fault, but continued as far as the Calaveras Fault.
“We found that it continued on another 15 kilometers and that the trace merged with the trace of the Calaveras Fault,” Chaussard said. In addition, seismic data show that micro-earthquakes on these faults 3-5 kilometers underground also merge. “With this evidence from surface creep and seismicity, we can argue for a direct junction on the surface and at depth for the two faults.”
Both are strike-slip faults — the western side moves northward relative to the eastern side. The researchers found that the underground portion of the Hayward Fault meets the Calaveras Fault ten kilometers farther north than where the creeping surface traces of both faults meet. This geometry implies that the Hayward Fault dips at an angle where it meets the Calaveras Fault.
InSAR revolutionizes mapping
Chaussard said that the nineteen years of InSAR data, which came from the European Space Agency’s ERS and Envisat satellites between 1992 to 2011, were critical to connecting the two faults.
Creep, or the surface movement along a fault, is evidenced by offset curbs, streets and home foundations. It is normally determined by measuring the location of points on opposite sides of a fault every few years, but that is hard to do along an entire fault or in difficult terrain. InSAR provides data over large areas even in vegetated terrains and outside of urban areas, and with repeated measurements over many years InSAR can detect deformation with a precision of 2 millimeters per year.
“With InSAR, we have access to much larger spatial coverage,” said Chaussard, who has been expanding the uses of InSAR to measure water resources and now ground deformation that occurs between earthquakes. “Instead of having a few points, we have over 200,000 points in the Bay Area. And we have access to areas we couldn’t go to on the ground.”
She noted that while creep relieves stress on a fault gradually, eventually the surface movement must catch up with the long-term underground fault movement. The Hayward Fault moves at about ten millimeters per year underground, but it creeps at only three to eight millimeters per year. Earthquakes occur when the surface suddenly catches up with a fault’s underground long-term movement.
“Creep is delaying the accumulation of stress needed to get to an earthquake, but it does not cancel the earthquake,” Chaussard said.
Other co-authors are seismologists Robert Nadeau, Taka’aki Taira and Ingrid Johanson, as well as graduate student Chris Johnson, all of UC Berkeley; and Heresh Fattahi of the University of Miami in Florida. The work was supported by NASA and the USGS.
— Read more in E. Chaussard et al., “Potential for larger earthquakes in the East San Francisco Bay Area due to the direct connection between the Hayward and Calaveras Faults,” Geophysical Research Letters (accepted article; forthcoming 2015) (DOI: 10.1002/2015GL063575)
