Quakes Aren’t Tied Together, Scientists Say

The earthquakes that rattled both Northern and Southern California on consecutive days this week even startled a few scientists, although they insisted there is no reason to believe that the widely separated earthquakes were related events.

“We’re uneasy when we see earthquakes popping off all over the place,” said William Ellsworth, a seismologist and geophysicist with the U.S. Geological Survey in Menlo Park.

But he quickly added that he doesn’t think that Tuesday’s quakes near Upland caused Wednesday’s quakes near Watsonville.

“If there is a mechanism (that would cause that), we don’t understand it,” Ellsworth said.

Caltech staff seismologist Kate Hutton was a little more succinct. Asked if earthquakes can cause other temblors hundreds of miles away, she said bluntly:

“No.”

The question pops up every time earthquakes strike at about the same time at widely separated locations, partly because “we tend to see patterns in everything,” Hutton said.

There have been enough quakes in California this week to jangle even the strongest of nerves. At least 14 quakes, ranging up to 5.3, hit the area between Watsonville and Santa Cruz Wednesday morning on the heels of a series of quakes near Upland on Tuesday measuring up to 4.6. The Southern California desert community of Indio also had two small quakes Wednesday morning measuring 3.2 and 3.8.

When an earthquake strikes, it rearranges stress patterns in the Earth’s upper crust that can cause aftershocks or even other quakes nearby, but “changes (in stress) even several miles away are minuscule,” Hutton said. So she sees no way that quakes at one end of the state can cause temblors at the other.

However, adjacent areas are another matter entirely, and experts were examining evidence Wednesday that the Watsonville temblors may indeed have been caused by the devastating Loma Prieta quake that struck Oct. 17. The Loma Prieta quake caused extensive damage as far away as San Francisco, and Ellsworth said that Wednesday’s temblors seem to have extended the October rupture southward on the San Andreas Fault.

If so, Wednesday’s earthquakes could offer scientists a classic study in how some temblors can cause other nearby quakes by redistributing stress patterns in the Earth’s crust.

“It’s like a jigsaw puzzle,” Ellsworth said. “And we just had a whole bunch of new pieces dumped into the puzzle all at once.”

Earthquakes are caused by the movement of the huge slabs of the Earth’s crust, called tectonic plates. The giant plates literally float on the Earth’s molten mantle, grinding past each other as they move about the surface of the planet.

The Pacific Plate, which underlies nearly the entire Pacific Ocean, and the North American Plate, which includes nearly the entire continent, grind past each other along the West Coast. One of the most prominent manifestations of that process is the San Andreas Fault that rips through the length of California.

In some areas, the San Andreas moves almost continuously in a process that allows the plates to pass each other gradually with minimal problems. But in most areas, the fault sticks like two pieces of sandpaper being rubbed together. Occasionally, the pieces slip, releasing enormous amounts of energy through an earthquake.

The interaction of various blocks of the Earth’s crust can be extremely complex, so at this point scientists are struggling to understand the dynamics of what now appears to be a series of Northern California quakes that began with the Oct. 17 temblor that killed 67 people.

The October quake hit in the mountains between Santa Cruz and San Jose, and it struck along a section of the San Andreas that has been seismically “quiet” for some time, Ellsworth said.

However, when the two plates suddenly slipped in what scientists call the “Loma Prieta quake” last October, it redistributed stress patterns in nearby blocks of the Earth’s crust, and scientists have been trying ever since to find out just how the patterns have changed.

While admitting that the evidence is “preliminary,” Ellsworth believes they found out Wednesday just where the stress moved to.

It apparently moved south to the next section of the San Andreas, which has been relatively active over the years. Wednesday’s activity was “much more vigorous than we have seen in the past,” Ellsworth said.

The quakes ranged up to 5.3 in magnitude, and they may not ultimately be classified as aftershocks to the October temblor because they struck on a different segment of the fault.

Wednesday’s quakes, like all temblors, also changed stress patterns in that area, but it will be months before scientists figure out which areas are the most likely candidates for quakes in the near future.

But most experts would agree on at least one point: Wednesday’s quakes in Northern California have nothing to do with the temblors that shook much of Southern California in recent days.

“It’s just too far away,” said Caltech’s Hutton.

RECENT QUAKE ACTIVITY

While scientists found the succession of earthquakes this week in Northern and Southern California intriguing, they insist they are not related. Tuesday’s quakes that hit the Upland area did not cause Wednesday’s quakes near Watsonville because the distance between the two areas is just too great, they say. “When an earthquake happens, it releases strain in the immediate area,” said Caltech seismologist Kate Hutton, and that can cause aftershocks nearby. But areas that are farther away are subject to very different geophysical conditions, and one quake is unlikely to cause another temblor even 20 or 30 miles away.

Watsonville

A swarm of earthquakes struck the Bay Area beginning at 6:37 Wednesday morning. The largest of the temblors was a magnitude 5.3 quake, with at least 13 others registering 1.6 or more by nightfall. Seismologists said Wednesday’s quakes resulted from stress patterns created by the 7.1 magnitude Bay Area quake that killed 67 last October and caused damage estimated at more than $7 billion.

Upland

A daylong series of quakes jolted the Upland area on Tuesday. The quakes, all aftershocks of a 5.5-magnitude temblor on Feb. 28, measured between 4.6 and 2.6 on the magnitude scale.

Indio

Two quakes struck the Indio area at about 7:30 a.m. Wednesday. One measured 3.8 on the magnitude scale, the other 3.2. Seismologists said both were aftershocks of a 4.1 quake there earlier this month.