Mg 6.7 coast of N. California -

The Big Quake Question: What Comes After Four?

A 4.9 magnitude earthquake centered in San Bernardino County rattled a large section of Southern California on Thursday, the third significant temblor to hit the state in less than a week.
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While the quake did not cause major injuries or damage, it shook nerves across the region just two days after a 7.2 quake off the Northern California coast prompted a tsunami warning and four days after many residents were jolted awake by a 5.2 quake centered near Anza.

Then around 11 p.m. Thursday, a quake with a preliminary magnitude of 6.4 rattled the ocean floor off Northern California, 125 miles west of Eureka. There were no immediate reports of damage or injuries.

It was not strong enough to generate a tsunami warning, a spokeswoman for the
U.S. Geological Survey said.

It was, however, probably an aftershock from Tuesday's quake in the area, she said.

Seismologists said that they found no immediate connection between the other quakes. But they were studying whether the Thursday afternoon quake, north of Yucaipa, could be linked to Sunday's Anza quake because they occurred 25 miles apart.

Officials said Southern California usually experiences quakes of this magnitude several times a year, but acknowledged that it's rare for them to occur so close together.

"It is unusual. But we've seen it before," said Caltech seismologist Kate Hutton, noting that quakes often come in clusters over periods of years — a phenomenon that scientists cannot fully explain.

The series of earthquakes was enough to revive anxious chatter Thursday of the coming Big One, a massive quake along the San Andreas fault. Hutton and other experts said they can understand the concern.

"I can empathize why people feel that," added Lucy Jones, the scientist in charge of the U.S. Geological Survey's Southern California office. "We don't handle randomness well. We like to make patterns. The chances are we expect two 'fives' in a week once every 10 years. It's been very quiet. During the '80s, we had earthquakes every day from 1987 to 1994. People are out of habit. They've been lulled down."

The last time the state experienced a similar earthquake cluster was 1986, when the Bishop area was hit by a series of quakes of up to 6.1 in magnitude. Experts said the biggest concern is that smaller quakes could trigger large quakes. Thursday's quake occurred along an as-yet-undetermined "splinter fault" near the San Andreas.

Seismologists said there was a 1-in-20 chance that Thursday's quake was a foreshock — a quake that precedes another quake of magnitude 5 or greater. Such quakes usually occur within hours of each other, but can occur as far apart as five days.

"There's a small chance that this was a foreshock, but it's probably not," Hutton said.

Both this week's Inland Empire quakes occurred near the San Andreas fault, a wide gouge in the Earth's crust where tectonic plates grind against each other. Thursday's quake was centered 8 miles from the fault, while the Anza quake was roughly 25 miles away, along the San Jacinto fault.

The San Andreas, long considered by scientists as a likely source of a catastrophic temblor, has erupted before, causing the great quake of 1906 that devastated San Francisco.

The entire San Andreas fault system is more than 800 miles long and extends 10 miles deep. Scientists say the San Andreas and other faults are storing up energy that is released in an Earth shuddering explosion when the plates slip against one another.

Scientists speculate that earthquake clusters result when energy has been stored for long periods and is released periodically.

"The biggest earthquakes relieve stress," Jones said. "They transfer energy. It relieves stress out of the Earth. When that happens, the Earth relaxes and it stops producing so many small earthquakes."

Also this week, a magnitude 7.8 temblor hit Chile on Monday, killing at least 11 people, and a magnitude 6.8 quake struck the Aleutian Islands off Alaska on Tuesday. Both were preceded and followed by smaller quakes.

Some scientists believe one earthquake can shake loose, or trigger, another nearby or elsewhere in the world. But officials expressed doubts that the Chile or Aleutian Islands quakes were related to those in Southern California because of the distance.

For all their ability to describe the size and location of quakes, scientists acknowledge that there is still much they don't know. Some say that the San Andreas and San Jacinto faults are overdue for large earthquakes, but they cannot say when.

Thirty years ago, seismologists believed they were on the cusp of discovering how to predict earthquakes.

Today, few scientists hold out such hope.

"In terms of earthquakes, the question now is: Will they ever be predictable?" Jones said.

"We know the big picture. But why the earthquake happened today and not yesterday, or last year, or 10 years ago, we just don't know. We also don't know what makes them stop."

Many Southland residents find this uncertainty troubling.

"I think this is leading up to the Big One," said Mentone resident Cora Embry, who grabbed her young son and ran from her home when the shaking began Thursday.

"I feel a big earthquake coming. They say there is no such thing as earthquake weather, but there is."

Thursday's first temblor struck about 1:53 p.m., three miles northeast of Yucaipa, 72 miles east of downtown Los Angeles. The quake, which struck roughly eight miles below ground, triggered rock slides in the San Bernardino Mountains and injured at least one Lake Arrowhead woman when it sent a chandelier crashing onto her head.

In areas close to the epicenter, residents described a shock that almost buckled their knees, caused large panes of glass to shiver and sent furniture pounding against the floor.

While seismologists characterized the earthquake as small — it was strong enough to toss items from shelves and crack walls, but not big enough to damage buildings — residents who lived near the epicenter said it seemed larger.

Redlands resident Susan Mosher was home studying for the bar exam when her dogs began barking, and the interior living room wall began cracking.

"We've had a lot of earthquakes — this is the first one that scared me," Mosher said.

Residents throughout the Los Angeles Basin felt a quivering.

Scientists suggested that the shaking may have seemed much more severe than it was because Southern California is coming off a long period of relative calm, seismically speaking.

"We've had a very quiet decade," Jones said. "We live in earthquake country and we should remember that."

LA Times