Although the new system would be somewhat irrelevant in predicting earth movement in Malibu, new research efforts are being concentrated on “the bird’s nest of fault structures” that occur along the Santa Monica Mountains, which are poorly understood.
By Hans Laetz/Special to The Malibu Times
Federal engineers, taking the first baby steps toward actually predicting earthquakes, have unveiled a computerized Internet forecast that calculates the chances of aftershocks.
And since one in 10 aftershocks are actually bigger than the first quake, this new U.S. Geologic Survey map will occasionally, but certainly not always, predict the possibility of a pending big quake, said excited USGS scientists in Pasadena last week.
But the scientists admit that the earth motion prediction maps may be somewhat irrelevant in a location like Malibu, where geologic conditions vary practically street-by-street, making earth movement predictions dicey.
And although the system cannot predict all major surprises, it does give the general public access to the same information that scientists have been using to predict generalized aftershock activity, said Lucy Jones, the chief USGS quake specialist at Pasadena.
“This will tell you when it is safe to go put back up your china after a major quake,” Jones said.
One out of every two noticeable earthquakes is preceded by a small foreshock, but nearly all large quakes have aftershocks of some sort, Jones said. The new maps crunch new earthquake data into the knowledge bank of aftershock happenings, and then automatically calculate which areas are at greater risk for an aftershock.
Not yet plugged into the computer, however, is specific information about earthquake faults and surface movement characteristics. Once those criteria are added in a few months, the computerized quake predictions will be much more valuable, Jones acknowledged.
Nevertheless, this is the first time that any official quake prediction system has ever been installed by the U.S. government.
“This combines what we know about the long-term probability of earthquakes with the short-term clustering models of aftershocks,” said Tom Jordan, director of the Southern California Earthquake Center.
Last week, the maps showed slightly elevated quake risk in the Parkfield, Bakersfield and Mount Lassen areas, all three of which had small quakes earlier and a consequent elevated quake risk as a result.
“If you were in Gorman, your chance of an earthquake right now is about the same as your risk of being in a car accident, which is about one in 2,500,” said Jones, the USGS Scientist in charge for the Earthquake Hazards Program. In the rest of the state, the average quake probability varies from 1 in 10,000 to 1 in 100,000, depending on proximity to known faults.
The Gorman area’s chances were elevated because a 5.2 magnitude quake had struck the area several days before, Jones said, and aftershocks were statistically more likely.
And while scientists hail the new maps for being the first earthquake prediction system ever deployed, they caution that it can only predict about half of the quakes that strike California, because foreshocks only occur half the time.
The maps use the Modified Mercalli Movement Scale, which charts actual ground movement. This is the same scale used in “shake maps” on television and the Internet, and is different from the familiar Richter scale of intensity, which measures a quake’s energy.
Local mountain and underground structures can funnel earthquake energy long distances. Some of the biggest, damaging movements in recent California quakes happened as far as 50 miles from the actual epicenter, where intensity magnitudes are assigned.
And USGS experts admit the earthquake motion predictor is unable to zoom into particular areas where localized conditions, such as waterlogged sand or bedrock, can drastically affect how much earth movement will be felt on the surface.
“The highest resolution is one point every five kilometers,” Jones said, meaning an area of 9.7 square miles, which can contain many different geologic conditions, will all appear the same and without those distinctions being noted.
Scientists said a hot spot of severe possible destruction, such as the Marina District of San Francisco, where homes constructed on fill material collapsed in the 1989 Loma Prieta quake, would not yet show up as high risk on the Internet risk map because of those two shortcomings.
Jordan said the Malibu and Santa Monica Mountains areas have such varied geologic structure that the predictions of localized earth movement from the maps can only be used in a general way.
In the 1994 Northridge quake, Malibu neighborhoods were badly shaken, but not nearly as much as Santa Monica or Fillmore, where localized conditions caused very destructive earth movements. The new quake movement predictor put online last week would not discern differences of that type, Jordan said.
Of particular interest to Malibu, Jordan said new research efforts are being concentrated on what he called “the bird’s nest of fault structures” that occur along the Santa Monica Mountains. These faults, including the Malibu coastal and Anacapa-Dume faults, are poorly understood.
Last week, a USGS study surfaced that predicted a 60 percent chance of a magnitude 6.5 quake, and much smaller possibility of a 7.5 magnitude disaster, along the Anacapa-Dume fault off the Malibu coast.
Jordan said the current state research project into tsunami probability along the Malibu coast will be more precise once the earth’s forces are better understood in this specific area.
The new aftershock alert program was devised by Jones and Matt Gerstenberger, who earned his doctorate at a Swiss technical university in the five-year effort it took to perfect the system.
The Internet maps use the same familiar color scale that the USGS shake maps do, with a rainbow stretching from calm blue to do-not-put-the-china-out red.