SIO scientists leading the way on quake knowledge
Last week's 5.4 magnitude earthquake in the Los Angeles basin grabbed San Diegans attention, but 10 to 20 much smaller earthquakes go unnoticed in the region everyday --- except by the scientists at Scripps Institute of Oceanography.
"They are too small to be felt, but they tell us a lot about where faults are and where the seismic action is," said Peter Shearer, a professor of geophysics at the institute.
While earthquakes cannot be precisely predicted, Scripps scientists are at the forefront of earthquake research and their findings help Californians better prepare for when the ground really starts shaking.
Many of the smaller 1 to 2 magnitude earthquakes are occurring along the San Jacinto fault zone, which runs northwest/southeast through the Anza-Borrego desert. This fault is parallel with the longer San Andres fault farther to the east.
Around San Diego County, there are several offshore faults, the small Rose Canyon fault through La Jolla and the long Elsinore fault, which lies closest to Temecula and Julian. However, these faults all have relatively low seismic activity.
The greatest earthquake hazard to San Diego is posed by the San Andres and San Jacinto faults because they have much more activity. San Andres, as the boundary between two major tectonic plates, is capable of the largest earthquake, however, the proximity of San Jacinto could possibly cause more shaking damage.
"Recently, there seem to be more activity further north and to the east, but that's not say we could have a significant earthquake in San Diego tomorrow," Shearer said.
Frank Vernon, a Scripps seismologist, installed a system of broadband and strong motion seismic sensors in the Anza region around the San Jacinto fault zone in the 1980s. The system collects extremely high-quality data that is now integrated with larger regional data systems, delivering data within 10 seconds of real time.
There are 21 Anza sensors that relay data to a sensor on Mount Soledad in La Jolla, which then sends the data to computers at Scripps. The data is so detailed it is used to develop U.S. Geological Survey maps.
It also provides the foundation for much earthquake research.
"We have incredible data sets available for everybody to use," said Dr. Debi Kilb, a seismologist and director of the Scripps Visualization Center.
Vernon has used the data to compare the characteristics of the smaller 1, 2 and 3 magnitude earthquakes with the largest earthquakes recorded by the sensors, up to a 5.3 magnitude.
"The properties of the smaller earthquakes do seem to be able to project up to at least the 5 level," Vernon said.
Earthquakes are caused when the tectonic plates that make the Earth's crust suddenly move against each other and release a wave of energy that shakes the ground. The plates are always moving, but usually at the pace fingernails grow.
Kilb studies earthquakes and their aftershocks - the smaller earthquakes following the original movement. As technology has improved, Kilb said researchers found aftershocks are more far reaching than previously expected.
For example, a magnitude 6 earthquake on a 15-mile-long fault in Los Angeles could set off aftershocks not 30 miles away as previous thought, but 300 miles away, or as far as Las Vegas and beyond.
Kilb is also investigating the perceived absence of immediate aftershocks, which have traditionally been expected to measure about one unit of magnitude less than the original earthquake. She is trying to determine if is indeed absent, or if the aftershock data is hidden by the original earthquake data, called the mainshock.
Knowing difference is important in understanding earthquakes. If the mainshock is hiding the initial aftershocks, "we might wrongly assume that no aftershocks occur at all in the first few minutes of the aftershock sequence," said Kilb, a Carmel Valley resident.
This would lead us to hypothesize that things are essentially 'frozen' for awhile before the aftershocks can begin."
Earthquakes often begin gradually and so it is difficult to pinpoint when they begin, which helps scientists identify their location.
In his varied research, Shearer has developed a more accurate method to identify the earthquake's source location, working with graduate students and researchers at California Technical Institute.
Instead of using one seismic reading, which is accurate within one or two kilometers, Shearer uses two readings, overlapping one over the other until they line up. This pinpoints the exact time the quake started, and thus the location, within hundreds or even tens of meters.
Before, maps of earthquakes looked like a diffuse cloud over broad area. After using the new method to reanalyze data from 400,000 small earthquakes since 1991, "we're now able to see linear features, which suggest faults," Shearer said.
This method revealed much of the shaking in Imperial Valley near the Salton Sea, known as the Brawley Seismic Zone, is along a series of small east-west faults, not the major northwest-southeast faults.
More research needs to be done to locate faults, Shearer said. Seismologists did not immediately know if the July 29 earthquake that rattled Los Angeles was caused by a known fault or a new one.
"There are many faults that have not been mapped," Shearer said. "A popular misconception is that we only have to worry about a big earthquake on the San Andres fault. The hazard is at least as great, if not greater, on a smaller fault closer to more densely populated areas of Los Angeles."