GILROY
– Monday’s Paso Robles quake may serve as a reminder to
Gilroyans that disaster can strike close to home suddenly and
harshly, as it has done before. Scientists and public officials
encourage residents to use the 6.5-magnitude earthquake that was
felt in Gilroy as a reminder that everyone must be
prepared should a natural disaster strike.
GILROY – Monday’s Paso Robles quake may serve as a reminder to Gilroyans that disaster can strike close to home suddenly and harshly, as it has done before. Scientists and public officials encourage residents to use the 6.5-magnitude earthquake that was felt in Gilroy as a reminder that everyone must be prepared should a natural disaster strike.
“I think that the past is a fingerprint to the future, and we know that this is the boundary between the two giant tectonic plates: the North-American and Pacific plates,” said Charles P. Watson, geologist and president of Seismo-Watch, a corporation collecting earthquake news and information. “And since those plates are in constant motion, we would expect earthquakes in the future.”
Monday’s earthquake, which killed two women when an unreinforced landmark clocktower collapsed in downtown Paso Robles, was the first to result in fatalities since the 6.7-magnitude Northridge earthquake of 1994.
The most recent earthquake of significant magnitude to shake Gilroy occurred on May 15, 2002. The 5.2-magnitude tremor took place three miles southwest of Gilroy, according to the U.S. Geological Survey. No major damage was reported, although shaking was felt in San Francisco to the North and Big Sur to the South.
The biggest earthquake in recent history struck northern California on Oct. 17, 1989, jarring Gilroy and causing widespread damage and power outages. The major 7.1-magnitude quake killed 63 people and injured 3,757 across the Bay Area. The temblor left downtown’s historic Old City Hall shuttered and closed for major repairs. It has since been retrofitted and now hosts a pair of restaurants.
Gilroy sits in the Calaveras fault zone, west of the well-known San Andreas fault, which contains a number of different strands of faults that could produce a sizable (stronger than 5.5-magnitude) earthquake. The most active of those is the Calaveras fault, which runs along the east side of the valley, from outside Hollister, past Gilroy and Morgan Hill, through the Coyote Reservoir and up through the Calaveras Reservoir at the base of Mt. Hamilton.
“The quakes that most people feel in the Gilroy area occur on the Calaveras, San Andreas and Hayward faults,” said David Oppenheimer, a seismologist with the USGS in Menlo Park. “Anywhere you see hills in coastal California, those hills are dynamic, they’re going up, and they’re going up in earthquakes.”
The Calaveras and San Andreas fault are known as strike-slip faults, producing a sideways motion.
“If the fault were to rupture the pavement, you would see one move one way and one another,” Watson said.
Monday’s earthquake, like the Northridge quake, was a thrusting movement, in which one plate moves up and over another.
The Calaveras fault is the largest fault in Santa Clara County, and longer faults are usually responsible for larger earthquakes, he said.
“There’s a lot of work that needs to be done on predicting certain magnitudes of certain faults,” Watson said.
Some of the other nearby faults are also capable of magnitude 5.0 to 6.0 earthquakes.
“I would expect to see lower- to mid-6’s from the Calaveras fault zone, really,” Watson said. “It’s capable of larger ones, but my research shows it’s probably not likely. That’s its history.”
According to the USGS, sizable quakes have made Gilroy quiver since the 18th century, several along the Calaveras.
“There were some dandy ones back around the turn of the century,” Watson said.
Besides the Great 1906 earthquake, which measured 7.9, according to USGS, a magnitude 5.8 quake was centered in Morgan Hill on July 6, 1899. Two years earlier, a 6.3 earthquake centered in Gilroy shook the area.
Some other recent substantial temblors include one of magnitude 5.8 in 1979 and the Morgan Hill earthquake on April 24, 1984. That 6.2-magnitude quake was centered 8 miles north/northwest of Morgan Hill on the Calaveras fault.
Twenty-seven people were injured, more than 500 homes and 43 commercial buildings were severely damaged, costing $8 million in damages, Watson said.
That segment of the Calaveras fault had not broken since 1911, when a July 1 quake measuring 6.6 caused heavy damage throughout the South Bay, he said.
Watson encourages Gilroyans to use the Paso Robles earthquake as a motivation to prepare for the next local shaker.
“Use this as a wake-up call to check emergency supplies, talk to family members about an emergency plan, what to do in case something happens,” he said. “Take this time to get ready because you never know when an earthquake is going to happen.”
Jeff Clet, chief of the Gilroy Fire Department, said there are set procedures for dealing with a natural disaster, such as an earthquake, specified in an emergency plan.
First, dispatchers broadcast to all emergency personnel that the area has just experienced an earthquake.
Fire personnel then survey designated areas, looking at pre-determined targets, such as high-hazard buildings, Clet said.
Road conditions are also examined to make sure they are passable.
If the disaster is serious enough, the city will establish an Emergency Operations Center, located in the basement of the Gilroy Police Department. All off-duty emergency personnel would be called on duty.
In the case of a significant disaster, such as the Paso Robles quake, GFD would need assistance from a larger department, such as San Jose, to conduct searches and rescues in the case of collapsed buildings.
“Then we’d really have to call somebody to help us,” Clet said.
GFD is currently in the process of creating a Community Emergency Response Teams program, which would train residents to help their neighborhood to respond to disasters.
