Seismologists unable to forecast ‘The Big One’

Seismologists unable to forecast ‘The Big One’

Experts disagree on the periodicity of strong quakes

By Tatiana Botton

Although some believe that a big earthquake could happen any
time from tomorrow until the next decade, most geologists doubt
there is such thing as "The Big One," a UCLA geologist said.

What some people call "The Big One" has been on many minds since
the 6.8 Northridge earthquake last January.

However, one of magnitude eight or higher has not hit the
Southern California area since 1857, when the San Andreas Fault,
located just 37 miles north of UCLA, rocked the area.

"When a lot of people are suggesting ‘The Big One,’ they are
really suggesting another one like (the 1857 earthquake) of
magnitude 8. That one rung some bells in Salt Lake City, and
stopped some clocks in San Francisco," said geology Professor David
Jackson, also a member of the Southern California Earthquake
Center, and vice chair of its board of directors.

Earthquakes of this magnitude occur on an average of 10 times
every thousand years, seismologists said.

However, seismologists and earthquake specialists don’t really
agree on the periodicity of these big earthquakes.

Some say that no patterns exist because no patterns have yet
been found. Currently, the best researchers can do is look back to
the last few decades and say that the probability of an earthquake
magnitude seven or higher is 90 percent in the next 30 years,
Jackson said.

But some other scientists think that big earthquakes only happen
once every 50 to 300 years.

"Really big earthquakes probably happen every 130 or 150 years,
and this has been deduced from digging and looking at the
geological records," said Paul Davis, chairman of the UCLA
department of earth and space sciences.

The earthquake in Hai Cheng, China in 1975, was the only
earthquake ever predicted. This was possible because of the
increasing number of foreshocks that led up to the 1975
temblor.

Although the Chinese police decided to evacuate the city after
experiencing several foreshocks, not all large earthquakes can be
accurately predicted in this manner.

Many false alarms have resulted from this method of
prediction.

Currently, there is nothing useful to accurately predict a
quake, Davis said.

Although foreshocks may not reveal anything about the future,
aftershocks can reveal something about the past.

"Aftershocks are retrospective ­ they are telling us
something about a big event that happened in the past," said Leon
Knopoff, a UCLA professor of physics and geophysics.

In the year after the Northridge earthquake, Southern California
experienced about 30,000 aftershocks.

UCLA is part of the Southern California geological system
located at the border of the Pacific and North American plates.

When the plates move, the irregular borders bend, causing
earthquakes and additional faults to appear. These faults are
evidence of geological damage to the earth.

"If we look at a time scale of a million years, it would look as
if the (plates) were shifting continuously," Knopoff said.

However such damage to the earth’s crust cannot be done without
an earthquake of magnitude 6.5 or bigger, Knopoff said.

Many studies analyze and try to understand the reason why
earthquakes occur.

In Jackson’s studies, he looks at the motion of the earth’s
plates by placing brass discs on the ground and analyzing the
motion of earth’s uppermost layer.

However, these studies only reveal the movement of the earth,
and cannot predict anything, Jackson said.

"I don’t think that we are at the engineering stage to forecast
the earthquakes" Knopoff added.

Although the San Andreas Fault is very long, the area that can
be considered most dangerous stretches between Parkfield and San
Bernadino, and transverses Palmdale.

This is the same area where the1857 earthquake occurred. If
there is another large earthquake centered at that area, it could
last 130 seconds, Davis said.

"(However), the accelerations of the buildings will be about the
same as the one at Northridge, it will just go for a longer
period," Davis said.

For the last 15 years, UCLA geologists have been studying the
infrequent earthquakes of magnitudes five or higher.

Various research results will be presented in February at an
international seismologists conference titled "Earthquake
Prediction ­ the Scientific Challenge."

There, scientists and speakers from around the globe will talk
about the problems of earthquake prediction.

"The scientific problems are not in saying that an earthquake
will happen, anybody can do that. The question is to understand the
success of a prediction, and that is a very difficult problem,"
Knopoff said.

"There is an eagerness and desire on the part of the public to
be able to have earthquake predictions ­ but there is a lot of
work that has to go on before we can get to that," he added.