Two faults in California are actually connected to form one. If they rupture together, it could result in a significantly more destructive earthquake than previously thought.
The 70-kilometer-long Hayward Fault is already known as one of the most dangerous in the country because it runs through large population areas from its northern limit on San Pablo Bay at Richmond to its southern end south of Fremont.
Seismologists have determined it is essentially a branch of the Calaveras Fault that runs east of San Jose.
Magnitude 7.3 Quake
“The maximum earthquake on a fault is proportional to its length, so by having the two directly connected, we can have a rupture propagating across from one to the other, making a larger quake,” says lead researcher Estelle Chaussard, a postdoctoral fellow in the University of California, Berkeley, Seismological Laboratory.
“People have been looking for evidence of this for a long time, but only now do we have the data to prove it.”
In an update of seismic hazards last month, the US Geological Survey estimated a 14.3 percent likelihood of a magnitude 6.7 or greater earthquake on the Hayward Fault in the next 30 years, and a 7.4 percent chance on the Calaveras Fault.
The estimates are based on the assumption that the two faults are independent systems, and that the maximum quake on the Hayward Fault would be between magnitudes 6.9 and 7.0. Given that the Hayward and Calaveras faults are connected, the energy released in a simultaneous rupture could be 2.5 times greater, or a magnitude 7.3 quake.
Creep Connects Two Faults
“A rupture from Richmond to Gilroy would produce about a 7.3 magnitude quake, but it would be even greater if the rupture extended south to Hollister, where the Calaveras Fault meets the San Andreas Fault,” Chaussard says.
There has always been ambiguity about whether the two faults are connected, Chaussard says. The Hayward Fault ends just short of the Calaveras Fault, which runs about 123 kilometers from north of Danville south to Hollister in the Salinas Valley.
For the study, published in Geophysical Research Letters, researchers used 19 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 says. 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 underground portion of the Hayward Fault meets the Calaveras Fault 10 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.
The 19 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,” says 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.”
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 10 millimeters per year underground, but it creeps at only 3 to 8 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 says.
Other researchers from UC Berkeley and from the University of Miami in Florida are coauthors of the study, which was supported by NASA and the US Geological Survey.