Scientists Think Each Big Quake Sets Up Another

Scientists investigating the cat’s cradle of faults underlying Southern California believe the Northridge earthquake--the costliest in U.S. history--measurably increased the quake potential in areas of metropolitan Los Angeles in a pattern that seems to have shaped major temblors here since 1933.

A new analysis of nine serious earthquakes over the past 61 years suggests that each relayed enough subtle stress along the region’s intricate fault lines to trigger a subsequent serious earthquake years later, like a falling domino knocking over the next block in line.

The stress generated by the 1971 Sylmar earthquake, for example, may have triggered the 1987 Whittier Narrows earthquake and advanced the onset of January’s devastating Northridge temblor by several decades, a team of earthquake experts reported todayin the journal Science.

In a sense, the precariously balanced faults underlying the region can feel one another tremble across space and time, even though the amount of force involved may be no more than that generated by a person pressing his palms together.

No one knows exactly what triggers any one earthquake or what dictates its timing. Scientists have been groping for decades toward an understanding of the signals that precede a major seismic shock.

The new insight into the aftermath of the Northridge earthquake arises from an analysis of the shifting tensions generated by local quakes of magnitude 6 or more, done by scientists from the U.S. Geological Survey in Menlo Park, the Woods Hole Oceanographic Institute in Massachusetts and the Institut de Physique du Globe in Strasbourg, France.

The researchers emphasized that their work cannot be used to predict when the next major earthquake will occur in Southern California, but that it can serve as a reliable indicator of where the earthquake potential in the Los Angeles region has risen and where it has dropped.

Other earthquake experts said Thursday they were intrigued by the work but remained skeptical.

“It is interesting, but it is not proven,” said USGS seismologist Lucile M. Jones in Pasadena. “The work is important, but it is as much for the questions that it raises as for what it answers. If the (Sylmar) quake really triggered Northridge, then what made it wait for 23 years?”

In its study, the team calculated that the cumulative effect of all the major earthquakes in the region since 1933 has been to lessen the stress throughout a zone extending from the San Fernando Valley south to the coast and along the San Andreas Fault between the Tejon Pass and Palmdale.

However, stresses have increased in a region encompassing the central Los Angeles Basin and areas west of Northridge.

“We all know that earthquakes drop stress, but they also increase the stress at other points. These stress changes are small and cannot produce an earthquake by themselves,” said USGS earthquake expert Ross S. Stein, who led the team that calculated the stress changes.

“These stress changes can advance an earthquake by several decades or even trigger one,” he said. “We do sense that these earthquakes over very long distances feel each other. We think because they interact and feel each other, they slow each other down or speed each other up.”

The group calculated that, as a result of the Jan. 17 quake, the geologic vise collapsing the region by about half an acre every year is today squeezing harder on faults under a wide area of the Los Angeles Basin: the San Fernando Valley, Thousand Oaks and Agoura Hills, Simi Valley, Glendale, and parts of Downtown and the Westside.

The team also calculated that, as a consequence of the Northridge quake, pressure has eased slightly in Malibu, Topanga Canyon, along the northern part of Santa Monica Bay and through portions of the Santa Clarita Valley.

“We need to look at those areas now that have faults whose stress has increased as a result of past earthquakes,” said Thomas L. Henyey, a USC earthquake expert and director of the Southern California Earthquake Center. “Those may be areas we may want to key on, but you are dealing with large patches of ground and many, many faults. You cannot say which of these faults will be affected.”

The team calculated the distribution of small pressure changes radiating from the epicenter of each major earthquake, revealing a distinctive pattern of higher stress levels that resembles the outline of a butterfly. When the researchers calculated the pattern for the 1971 Sylmar quake, they found that subsequent major earthquakes seemed to cluster in lobes of the butterfly “wings.”

The pattern of increased stress in the aftermath of the Northridge earthquake also reveals the characteristic butterfly shape, and now some seismologists believe that they are detecting increased numbers of barely perceptible micro-quakes in the lobes of the wings.

While Stein’s study has taken a regional approach, a second group of USGS experts has been analyzing the changing stress on individual faults. Since Jan. 17, they have identified half a dozen quakes with a magnitude greater than 5 that were triggered in locations “where they would have been encouraged by the Northridge shock,” said USGS geophysicist Robert W. Simpson.

“When you are looking at the potential interaction of faults, the stresses are pretty small,” Simpson said. “We are accumulating a pile of evidence that is suggesting these small, static stress changes do make a difference.”

Stress Patterns

The earthquake potential in some areas of metropolitan Los Angeles rose slightly as a consequence of the Jan. 17 Northridge earthquake, according to a new analysis of how earthquakes affect each other. Some scientists believe that stresses from the 1971 Sylmar earthquake, for example, helped trigger the 1987 Whittier Narrows quake and the temblor in Northridge earlier this year.

Sources: Science magazine, U.S. Geological Survey