Underground Blasts to Test Quake Theory
On this fourth anniversary of the Northridge earthquake, it can safely be predicted that the shaking is not yet over.
And not just because of aftershocks.
This fall, if all goes well, scientists will set off 60 ammonium nitrate explosions beneath the Westside, the San Fernando Valley and the Mojave Desert over at least two nights to test one of the most controversial theories in the wealth of advanced research shaken loose by the Northridge quake.
Some scientists believe that a bowl-shaped dip in the bedrock more than two miles underground focused shock waves from the earthquake like a satellite dish focuses TV signals, and then beamed their magnified force to neighborhoods on the Earth’s surface far from the epicenter.
That might explain why Sherman Oaks and even Santa Monica--17 miles from Northridge--suffered much more damage from the earthquake than some communities much closer to the epicenter.
By setting off the proposed underground blasts in residential neighborhoods and on public lands--and measuring the resulting shock waves on approximately 1,000 strategically placed seismometers--the researchers hope to determine whether the theory is valid.
Keeping in mind that the Northridge fault was unknown before it caused the most costly natural disaster in U.S. history at more than $40 billion, the researchers will also use data from the explosions to search for hidden faults.
An experiment on this scale is rare for seismologists. But many of the old restrictions--those that ruled the profession before Jan. 17, 1994--no longer apply, especially locally.
The scientific legacy of the Northridge earthquake is reflected in important new geological findings, increased funding and a stronger relationship between researchers and emergency services.
It’s an exciting time to be a seismologist in Los Angeles.
“It has made the job harder,” said Paul Davis, a professor at UCLA who helped form the new bedrock theory and design the upcoming explosion test. “But it has justified experiments,” he continued with a laugh, “that are fun for us.”
One of the first surprises for researchers from the Northridge shaker was that an earthquake with a magnitude of 6.7--considered “moderate” among seismologists--could cause such widespread devastation.
“It really surprised us that this earthquake, which wasn’t that big, could do so much damage,” said James Mori, the scientist in charge of the United States Geological Survey’s office of earthquakes, volcanoes and engineering.
“To put it in perspective, every year around the world there are about 100 earthquakes between magnitude 6 and 7. These are not uncommon.”
The shaking that downed buildings also shook loose funding for scientific studies.
“The way these things sort of go is that you need a significant disaster to spur progress,” Mori said.
“It’s an opportunity to go to the federal agencies and say, ‘We just had this $30-billion loss in Southern California. If you put a little more into research and development, maybe you can reduce that the next time an earthquake comes around.’ ”
Before the Northridge earthquake, predicting damage seemed to be an easier task for seismologists.
“You could look at a soil map and guess what was going to happen,” said David Jackson, a UCLA professor and the science director of the Southern California Earthquake Center, which sponsors research projects.
“Places with loose soil got a lot of shaking, places with intermediate grain size [soil] got moderate shaking and places on hard rock got little shaking. Life seemed very understandable.”
Soon after the Northridge quake, scientists realized it wasn’t so simple. Structures built on loose soil sediments did indeed generally incur more damage than those on hard rock. But there seemed to be another force at work.
For example, because the sediment in Santa Monica is no looser than in several other areas closer to the earthquake, the severe damage in that seaside community was too severe to be explained by soil properties alone.
This realization that the earthquake had seemingly targeted certain areas came early to researchers at UCLA’s department of Earth and space sciences.
“Most of our faculty lives in either Sherman Oaks or Santa Monica,” said Davis. “This was tailor-made for us.”
But usually, it takes months or years to set up a major seismic experiment, largely because no single academic or government institution owns the numerous seismometers needed. Normally, institutions loan them to each other on a prearranged schedule.
But Davis got lucky. “A national pool of seismographs just happened to be here for a test in Santa Monica harbor that had been canceled” for reasons unrelated to the earthquake, he said. “The seismographs became available to us.”
About three months after the quake, almost 100 of the instruments were placed in backyards and gardens over a wide area.
“We advertised in the campus paper, asking, ‘How would you like to have a seismograph in your backyard?’ We were flooded with responses. Everyone was so interested in this.”
Over three weeks of testing, the instruments indicated that some neighborhoods were indeed getting aftershocks that had been somehow amplified, he said.
“It was a real puzzle,” Davis said.
The situation was not entirely new to scientists.
“There were times when damage seemed to hit some particular neighborhood and everyone wondered why,” Jackson said. “People had ideas about it, but there wasn’t enough data available to be able to get the goods.
“This time we had very good data on just where the strong shock waves hit.”
Studying local underground maps produced by oil companies in their search for fossil fuels, the researchers noticed at the edge of the Santa Monica Mountains, on the Santa Monica side, a bowl-shaped dip in the bedrock, about 1.8 miles wide and stretching from about a half-mile north of San Vicente Boulevard down to Santa Monica City Hall.
“We started thinking that this sort of structure had the ability to do the focusing,” said Davis. They became convinced it explained the damage in Santa Monica.
“We’re not as sure about Sherman Oaks,” Davis said. “The focusing effect could have played a role, but there are factors there too probably involving the soil.”
If the focus theory is proven, it doesn’t necessarily mean that Santa Monica or Sherman Oaks are in any more danger than anyplace else, except from quakes with the same epicenter as the Northridge temblor.
“The focusing effect moves as the source of the earthquake changes,” Davis said, “just like rays of light focused by a magnifying lens move if you move the light source.
“If the epicenter had been a few kilometers in a different direction, the focusing effect would have not hit the same area.”
The planned test explosions--officially called the Los Angeles Region Seismic Experiment--will cost an estimated $500,000. The Southern California Earthquake Center and National Science Foundation have agreed to fund two-thirds of the project. Davis said negotiations are underway with the United States Geological Survey to foot the rest of the bill.
The test is not unprecedented. This will a sequel to a similar 1994 test conducted on a line that stretched north from Azusa through the San Gabriel Valley and up near Boron.
The results of the first test--which was planned and scheduled before the Northridge earthquake--were somewhat less than had been hoped. The scientists conducting the first experiment said they felt their explosions had not been big enough to allow clear discernment of underground structures.
At a 1996 earthquake briefing, Mori described the images from the test as “sometimes exciting,” but disappointingly not clear enough to reveal more about fault structures.
As currently planned, the southern tip of the line of explosions in the upcoming test will be on the coast in Malibu, following a path north through Topanga State Park and entering the San Fernando Valley near Braemar Country Club in Tarzana.
The test area moves through Reseda, onto the campus of Cal State Northridge, through Granada Hills and into the Santa Susana Mountains. From there it goes through Newhall and other parts of Santa Clarita before moving north into the Los Angeles National Forest and Mojave Desert.
The science of such an experiment, Davis said, is the easy part.
“It’s the permitting that’s hard,” he said.
“On public lands [in rural areas], we need to have a wildlife expert and artifacts expert look at every place we want to use,” to meet state and federal regulations that outlaw disturbing animals and American Indian burial areas, Davis said.
Surprisingly, it’s easier to get approvals for explosions in populated areas, he said. The explosives are placed on areas such as golf courses, athletic fields and empty lots to distance them from private homes. The researchers need the owners’ permission but have never had to deal with city officials or other local government agencies unless the land is government-owned, he said.
In addition, the explosion team warns neighbors for blocks around. “We do a lot of knocking on doors,” Davis said.
To neighbors, he said, the blast 30 feet underground will feel like a weak earthquake. The only damage the blast leaves on the surface is a small crater, which the researchers fill in.
They will be asking homeowners to allow them to place seismographs about the size of desktop computers in their yards.
No one is paid for permission to detonate an explosion or place an instrument on their land, but Davis said researchers have seldom run into resistance.
“We explain why this is important,” he said, “and we send everyone a full report of what we find. People like to participate. It’s a community thing.”
A positive outcome of the Northridge earthquake for scientists was the improvement in equipment available to measure earthquakes.
“It allowed us to upgrade the entire seismic network,” said Mori, noting that a new agency, sponsored by Caltech, the USGS and the state, is working toward producing damage prediction maps minutes after a major earthquake.
Instead of waiting hours for scattered reports to indicate where the worst of the quake damage can be found, the new system will feed to the Internet and police, fire and other emergency services an instant map of where the shaking was strongest, painting the hardest-hit zones in red.
David Wald, who heads the map project at USGS, showed a sample of how the map would have depicted Los Angeles after the Northridge earthquake.
“If we had all this technology available back then,” said Wald, “we could have known all this in five minutes.”
Times staff writer Ken Reich contributed to this story.
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Focusing the Earthquake, a Theory
Some scientists believe that the reason Santa Monica suffered more damage than some San Fernando Valley communities closer to the epicenter of the Northridge earthquake was that a bowl-shaped depression in bedrock focused shock waves much like a magnifying glass would.
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Cross section of the Valley and Santa Monica Mountains, facing east.
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1. When earthquake hit, seismic waves traveled in all directions.
2. Waves traveled through the bedrock with a high seismic velocity.
3. At the intersection of the Santa Monica fault and a curve in the bedrock, the direction of the seismic waves was altered. The fault ad curve acted as a convex lens to focus the waves on the city of Santa Monica.
Source: Professor Paul M. Davis, UCLA Department of Earth & Space Sciences
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Quake Update
An updated recap on the Jan. 17, 1994 Northridge earthquake:
Deaths: 72
Injuries: 11,846 (People in Los Angeles, Ventura and Orange counties who received hospital treatment for quake-related injuries.)
Structures damaged: 114,039
Assessed damage costs: More than $40 billion
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Aftershocks / As of Sept. 30, 1997
Total: 14,532
3.0 - 3.9: 413
4.0 - 4.9: 52
5.0 - greater: 11
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Source: FEMA
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The Recovery
Although the Northridge earthquake may already be a fading memory for some, aftershocks continue, and so does the recovery. Of the $7.8 billion in federal aid obligated after the magnitude 6.7 quake, which struck four years ago today, nearly $1.8 billion has yet to be spent. The slowest to rebuild? Hospitals and schools, some of which are only now beginning repairs, according to FEMA’s John Treanor.
Quake Update / Los Angeles Times
