Marginal Tsunami Risk in O.C.
Like most scientists specializing in earthquakes, Lisa Grant, professor of environmental health, science and policy, believes that the issue of whether California will be struck by a major earthquake is less a question of ‘if’ than a question of ‘when.’
Grant, a fault line specialist, does not believe that the Southern California area is at a high risk for a tsunami caused by a large earthquake in the area. However, recent events have prompted her to revisit the possibility.
Like many local scientists, Grant views tsunamis to be a lesser concern when pitted against the likelihood of land-based earthquakes.
‘There’s a lower probability of tsunamis than earthquakes,’ she said, ‘but the probability is not insignificant.’
Southern Californians can also take a small degree of solace from the fact that really big earthquakes, like the most recent one in Southeast Asia, are caused by vertical subduction faults. Nearly all of the fault lines on the western coast of North America have tectonic plates that move horizontally against one another as opposed to one moving beneath the other.
However, even though tsunamis are not a major concern to local scientists, it does not mean that they haven’t occurred.
From her measurements of coastal uplift, Grant speculates that some time between the late 1600s and the mid-1700s, an earthquake in the Irvine area probably caused tsunamis.
Grant went on to say that in the early 1800s, the Santa Barbara Mission was damaged by a tsunami and the San Juan Capistrano Mission was destroyed.
More recently, in 1964, tsunamis originating from a magnitude 9.2 earthquake in Alaska killed 11 people in California and caused damage as far south as Huntington Beach.
While the shape of the California coast and the Channel Islands deflect waves from all directions, most scientists who consider earthquakes and tsunamis as a possible threat to Southern California believe that anything can happen.
Tsunamis are not only caused by earthquakes, but can also be caused by underwater landslides. A map of the Los Angeles basin by the United States Geological Survey showed an underwater landslide that may have occurred over 7,000 years ago, causing the massive water displacement necessary for a tsunami.
While volcanic eruptions, underwater landslides, major weather disruptions and occasional asteroid landings can potentially cause the extreme water displacement necessary for a tsunami, earthquakes are the primary impetus.
When one tectonic plate is driven below another beneath the ocean, the major uplift of the earth’s surface displaces water. Tsunamis occur as part of the ocean’s effort to regain equilibrium. This situation is like a stone being tossed in still water, but on an epic scale.
Waves caused by such a disruption travel at speeds close to 500 miles per hour. In the deepest parts of the ocean, they are barely noticeable, but as they approach shallow coastlines the energy causes devastating building-sized waves.
Because effects from land-based earthquakes are the primary concern in Southern California, Grant specializes in studying fault lines and has studied the San Joaquin Hills blind fault near Irvine. Unlike the infamous San Andreas Fault, blind faults don’t show up on surface photos, are difficult to map and are often overlooked.
For a long time, Grant had trouble generating interest in her work with the San Joaquin Fault. This changed when another blind fault caused the magnitude 6.7 Northridge earthquake in 1994.
‘To this day the Northridge earthquake is the most expensive disaster in U.S. history,’ Grant said. ‘It caused over $40 billion in damage.’
The tendency of such blind faults to be overlooked and their ability to cause such destruction is reason for increased concern. Grant sees attention paid toward the threat of terrorism as a detriment to research into real possibilities of earthquake occurrence.
According to Grant, the operative term in determining the likelihood of earthquakes is ‘forecasting’ rather than ‘prediction.’
‘Prediction may or may not be possible in the future,’ Grant said. ‘But if a weather forecaster says there’s a 50 to 60 percent chance of rain, aren’t you going to reach for your umbrella?’
To forecast earthquakes, scientists inventory faults and measure their earthquake capabilities and rate of production before calculating earthquake probability. Because earthquakes operate on geological time measured in hundreds of years, the process does not lend itself to real-time prediction.
Though these facts do little to alleviate concerns, warning systems off the coast of California are continually upgraded and modernized to give residents a jump on tsunamis in the event of an
occurrence. However, the greater likelihood of land-based earthquakes offers a greater area of concern.