Earthquake Prediction on Shaky Ground?
Beil, Laura, Science News
Earthquake predictions on shaky ground?
The U.S. Geological Survey (USGS) last week slightly increased its estimated probability of a major earthquake occurring in southern California within 30 years, while two new studies suggests that forecasting such an event may be even trickier than geologists think.
Representing the work of a dozen scientists over the past year, the USGS document estimates a 60 percent probability of an earthquake of at least 7.5 magnitude along the San Andreas fault near Los Angeles. A 1980 study had put the probability at 50 percent. The report also gives a 50 percent likelihood for a similar earthquake in the San Francisco Bay area during the next 30 years.
The USGS scientists calculated their estimates from the dates and sizes of previous ruptures along the San Andreas fault system. They noted that major quakes occur northeast of Los Angeles on average every 130 years, with the most recent in 1857. But in the July 8 SCIENCE, Gordon Jacoby and Paul Sheppard of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y., and Kerry Sieh of California Institute of Technology in Pasadena say the fault moves with such irregularity that earthquakes there remain too uncertain to forecast from historical records.
After measuring rings from 70 trees, Jacoby and his colleagues believe an earthquake occurred along the Mojave segment of the fault in 1812--the year of a large rupture known as the San Juan Capistrano quake (SN: 4/18/87, p.255). Previously, scientists had blamed this earthquake on a coastal fault, not on the San Andreas, since historical accounts describe coastal damage. However, the researchers found that nine trees growing within 20 meters of the fault suffered a severe shock in 1812 characteristic of a large, growth-stunting earthquake. Assuming trees are accurate historians, they say, only 45 years separate the two most recent major quakes. …