Subj:	 Researchers Testing Calif. Faults
Date:	10/20/99 5:37:19 AM Pacific Daylight Time
From:	hblondel@tampabay.rr.com (New Millennium)

Researchers Testing Calif. Faults 

Oct. 20, 1999; 7:45 a.m. EDT

LOS ANGELES (AP) –– With memories of the 7.1-magnitude Hector earthquake
still fresh, seismologists in Southern California are shaking up the
ground themselves to create a portrait of faults and hazardous areas in
the region's restless underbelly. 

A series of 93 small underground detonations, beginning today and
continuing through Sunday, is being conducted as sort of an ultrasound
procedure, aimed at giving scientists a better look at the region's
fault lines. 

Researchers said the timing of the tests, beginning days after
Saturday's Hector quake in the Mojave Desert, was a coincidence. 

"It's been planned for this particular week for two years," said Mark
Benthien, a spokesman for the Southern California Earthquake Center,
which is conducting the study with the U.S. Geological Survey. 

Researchers said the blasts will not trigger earthquakes and, if felt at
all, would resemble the rumble of a passing truck. Similar blasting was
done in 1994. 

The explosives will be set off in boreholes 60 feet deep. The holes run
in a line from Pacific Palisades inland to the San Fernando Valley, then
north across the Mojave Desert to the Tehachapi Mountains. 

Some 1,400 portable seismographs will collect data that should reveal
hidden faults and other geologic structures important to understanding
how future quakes will affect homes and other buildings. 

The so-called Hector Mine earthquake ripped a miles-long rupture across
the desert floor but was remote enough to spare urban areas from damage. 

It generated a number of moderate aftershocks, including one of
magnitude 4.6 Tuesday morning and another of 4.0 Tuesday night. 

Seismologists said three aftershocks of 5.0 or greater occurred shortly
after the main shock and up to four more of similar size were likely
within the week. 

"In addition to the classic aftershock sequence, the earthquake has
apparently triggered small earthquakes as far south as the
California-Mexico border," the USGS said in a statement. "Scientists are
monitoring these events carefully, as some of them have occurred close
to the southern end of the San Andreas fault." 

The number of aftershocks was a minor concern to the researchers. If one
occurred at the same moment as a blast it could wash out that data,
Benthien said. 

Blasts at sites near homes will use about 50 pounds of explosives. The
largest blasts, at remote sites, will use up to 2,000 pounds. 

Even that amount of explosives cannot cause an earthquake, Benthien
said. 

Construction in the region routinely involves far bigger blasts, such as
a dam project near Riverside that has used as much as 60,000 pounds of
explosives at once without causing a quake.