California's quake deficit fades
by R. Monastersky
As Californians recover from El Nino-driven storms, they can take comfort from the news that the ground underfoot is less fickle than it once seemed.
In 1995, a panel of seismologists concluded that Southern California had experienced too few earthquakes since 1850 to relieve all the energy that had accumulated in the crust. The situation had created a quake deficit that would have to be paid back with larger or more frequent earthquakes in the future, they said (SN: 1/21/95, p. 37).
Southern California's quake-producing faults.
Now, two teams of researchers are disputing the evidence of a quake deficit in the region, and their studies are drawing positive reviews from other seismologists. "If you add all these things up, it's pretty clear now that this deficit can go away. It doesn't have to exist," comments Thomas L. Henyey, director of the Southern California Earthquake Center (SCEC), the Los Angeles–based consortium that issued the 1995 report.
Earthquake-producing stress in Southern California comes from the ongoing collision of the North American and Pacific tectonic plates -- two large pieces of Earth's outer shell that grind past each other. The SCEC report estimated seismic hazard by looking at the rate of past earthquakes, the geology of faults, and ground movement. Written primarily by David D. Jackson of the University of California, Los Angeles, the report covered the southern 40 percent of the state.
The SCEC study concluded that magnitude 6 and larger earthquakes should have occurred once every 1.6 years -- twice their actual rate. To relieve all the energy that had accumulated in the crust, earthquakes would eventually have to strike more frequently or smaller faults would have to join together to produce a giant quake, Jackson and his colleagues reported.
A reanalysis of the SCEC study has uncovered subtle flaws that combined to overestimate the amount of energy building in the crust, reported a team of scientists at a meeting of the Seismological Society of America in Boulder, Colo., earlier this week. Edward H. Field and James F. Dolan of the University of Southern California in Los Angeles and Jackson collaborated on the new study. When they constructed an improved model, no deficit emerged.
Thomas C. Hanks and Ross S. Stein of the U.S. Geological Survey in Menlo Park, Calif., questioned the SCEC study for other reasons. The report relied on a list of magnitude 6 and larger earthquakes since 1850, but Hanks and Stein argue that the list may overlook quakes from early in that period, when California's inland population was sparse. The rate of observed earthquakes since 1903 is 40 to 50 percent higher than that since 1850, which reduces the apparent deficit, they report.
While the new studies will please many Californians, they will not dispel the concerns of Los Angeles residents. Seismologists still believe that faults around metropolitan Los Angeles have a major quake deficit that will eventually lead to larger or more frequent tremors there.
Field, E.H. 1998. An integrated seismic-hazard source model for Southern California: No deficit or M>8 earthquakes required. Meeting of the Seismological Society of America. Boulder. March.
Hanks, T.C., and R.S. Stein. 1998. Earthquake deficits, seismic moment deficits, and M>6 seismicity in Southern California since 1903. Meeting of the Seismological Society of America. Boulder. March.
Monastersky, R. 1995. Los Angeles faces a dangerous quake debt. Science News 147(Jan. 21):37.
Thomas L. Henyey
University of Southern California
Department of Earth Sciences
Los Angeles, CA 90089-0740
copyright 1998 ScienceService