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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.