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Finding the fault: Scientists piece together recent seismic events

In the wake of the big earthquake, there’s a groundswell of activity among seismologists who are trying to piece together what happened and why.

The impact to San Diego of the 7.2-magnitude earthquake centered about 140 miles south in Mexico on Easter Sunday could have been much worse if not for the topography of the area, said Jose Restrepo, a structural engineering professor at Jacobs School of Engineering. Restrepo was among a group of UC San Diego researchers who spent an entire day inspecting earthquake damage in Mexicali and Calexico in order to learn from the disaster.

“We missed the brunt of the earthquake, whose energy was focused over the mountains in Ocotillo, Mexicali and Calexico,” he said. “The damage was much more severe in Calexico, where a lot of buildings have come down.”

Restrepo added that if the quake had happened on a weekday rather than the weekend, “the number of casualties would have increased exponentially,” because many more people would have been outside where impacts were greater, rather than relatively safe at home.

What most people don’t realize about major quakes like the one just experienced is they’re not “isolated” events that just happen and then it’s all over. “We’ve had over 300 aftershocks over magnitude 3,” said Debi Kilb, Ph.D., a Scripps Institution of Oceanography seismologist, just two days after the Mexicali quake that measure 7.2 on the Richter scale. “But that isn’t really a surprise: We’ve had very productive aftershock sequences in quakes from that region.”

Could San Diego experience the big one unlike any in our recorded history? “Anything’s possible,” Kilb said. “But it’s more likely we would have a large rupture on a fault relatively far from San Diego.”

San Diego County has a number of active earthquake faults generally running northwest to southeast. From east to west, the major active faults consist of the San Jacinto, Elsinore, La Nacion and Rose Canyon faults crossing La Jolla onshore, and the Coronado Bank, San Diego Trough and San Clemente faults offshore.

Ongoing field and laboratory studies suggest the following maximum likely magnitudes for local faults: San Jacinto (6.4 to 7.3), Elsinore (6.5 to 7.3), Rose Canyon (6.2 to 7.0), La Nacion (6.2 to 6.6), Coronado Bank (6.0 to 7.7), San Diego Trough (6.1 to 7.7) and San Clemente (6.6 to 7.7).

Large or small, most earthquakes are caused by the slippage of masses of crustal rock along earth fractures called faults. Faults that have horizontal movement are called strike-slip faults.

“Almost all of the faults in the San Diego region are strike-slip faults,” said Kilb, noting that the point can be illustrated by putting your two hands together. “That’s our fault plane. Push really hard. ... Then move one hand down compared to the other, that’s the slip. It will stick, nothing will happen for awhile — then stick-slip.”

Restrepo is part of the world-renowned structural engineering faculty at UCSD who perform large-scale seismic tests at the UCSD Englekirk Structural Engineering Center. At 25 by 40 feet, the world’s first outdoor shake table, it is also the largest shake table in the U.S. able to test structures weighing up to 2,200 tons and as tall as 100 feet.

Englekirk has performed seismic tests on a variety of structures, including a three-story precast concrete parking garage; a seven-story building; wind turbines; a one-story masonry brick veneer house; and a 1930s-era, three-story masonry brick veneer structure.

The shake table research has practical applications affecting “everyday life.”

“We’re like medical doctors for buildings and bridges,” Restrepo said of table tests revealing structural strengths and weaknesses, which he added allows scientists to devise ways to “protect these kinds of things.”

Quake a powerful reminder

The Easter Sunday temblor that shook both sides of the U.S.-Mexico border should come as a wake-up call to San Diegans and serve as a reminder that they live in earthquake country, one disaster relief official said.

“We see this as a teachable moment, now people are paying attention,” noted Yvette Urrea Moe, public information specialist for the San Diego County Office of Emergency Services.

Urrea Moe said there’s a lot of misinformation still out there about what people should — and should not — do during a quake.

“It surprises me a lot of people are still talking about heading for the doorways to brace themselves,” she said. “That information was last-generation. It’s changed. We tell people to seek cover under a table or desk, preferably a sturdy one, or to drop, cover and hold on.”

No matter where you are during a quake, the most important thing to remember is to cover your head to shield it from falling or flying debris. If caught outside, the advice is similar to avoiding lightning — stay in the open, away from trees or power lines. If on a roadway, avoid overpasses or bridges.

County OES has a Family Disaster Plan and Personal Survival Guide available at www.sdcounty.ca.gov/oes. The plan lists easily accessible items that should be stockpiled in the home during an emergency to sustain a family and pets for a minimum of 72 hours.

That list includes: a two-week supply of medicines and prescription drugs; 1 gallon of water per person per day; a first aid kit and book; canned or dehydrated food requiring minimal heat or water; a non-electric can opener; blankets or sleeping bags; portable radio with spare batteries; a fire extinguisher; a flashlight; a nonelectric watch or clock; large plastic trash bags; hand soap and liquid detergent; personal toiletries; toilet paper and paper towels; and newspapers to wrap garbage or waste.

“There are a lot of things people can do that are fairly simple to prepare for an earthquake,” Urrea Moe said. “It doesn’t have to cost a lot of money.”

She offered two other tips: “Have a reunion location in the event you’re separated by disaster and also an out-of-state location to contact. Also learn the location of your utility shut-off valves, like gas, in case there’s a leak.”

When given a choice between being inside or out during a quake, Urrea Moe noted: “People running outdoors can be hit by some falling debris from buildings. There could be (downed) power lines out there. It’s safer to stay inside.”

The best thing about being earthquake-safe is it prepares you for other disasters as well. “If you’re prepared for an earthquake, you’re pretty much ready for any emergency your family could face,” Urrea Moe said.

How to prepare for a quake

  • Inspect your home, identify potential hazards and evacuation routes.
  • Secure water heater and tall or heavy furniture to wall studs.
  • Move heavy items to lower shelves.
  • Install clips, latches and other locking devices on cabinet doors.
  • Provide strong support and flexible connections on gas appliances.
  • Remove or isolate and secure flammable materials.
  • Review and practice this plan.

What to do during a quake

  • Stay indoors. Move away from windows, bookcases and tall shelves. Get under a table or desk and hold onto it. If there is no desk or table to get under, brace yourself in an interior corner. Watch for falling, flying or sliding objects.
  • If outdoors, move to an open area away from buildings, trees, power poles, brick or block walls, and other objects that could fall.
  • If in a car, stop and stay in it until the shaking stops. Avoid stopping near trees and power lines or on or under overpasses or bridges.
  • If in a high-rise building, do not use the elevator to evacuate. Use the stairs.
  • If in a store, get under a table or any sturdy object. Do not run for the exit. Choose your exit carefully.

If you evacuate ...

  • Prominently post a message indicating where you can be found and the date and time you left.
  • Make arrangements for pets.

... take with you

  • Medicines and first-aid kit
  • Flashlight, radio and batteries
  • Important papers and cash
  • Food, blankets and extra clothes