From: tcsmith@cdmg-hq.UUCP (Ted Smith)
Newsgroups: ca.earthquakes
Subject: Seismology Tests in N. CA
Date: Mon, 31 Aug 92 13:18:13 PDT
X-Maildoor: WaflineMail 1.00r

The following press release from the U.S. Geological Survey Western
Region Public Affairs Office [(415) 329-4000] appeared this morning:

USGS TO CONDUCT INSTRUMENT CALIBRATION EXERCISE IN NORTHERN CALIFORNIA
QUAKE AREA

In an effort to learn more about recent earthquakes that have occurred
in Humboldt County, and to calibrate the instruments recording
aftershocks there, scientists from the U.S. Geological Survey,
Department of the Interior, will conduct an instrument calibration
exercise in several areas of western Humboldt County, Calif., the night
of Sept. 16, 1992.

The project is designed to learn more about how seismic waves travel
through the geologic structures of the area, and to calibrate
instruments that are recording aftershocks of the April 25, magnitude-7
earthquake that was centered near Petrolia.

The project involves detonating moderate-size charges of ammonium
nitrate in holes drilled 150-200 feet beneath the ground surface.

The resulting explosions will generate seismic waves equivalent to a
very small, barely noticible 2 to 2.5-magintude earthquake.  These waves
will travel through or reflect off rock layers beneath the ground
surface and be recorded on nearly 250 portable seismographs that will be
placed in the area.

The advantage in recording the induced seismic waves, over the naturally
occurring aftershocks that continue to rock the area is that scientists
will know exactly when and where the induced wave was generated.

"We need this type of controlled process to precisely calibrate our
instruments," said John Van Schaak, a USGS instrument specialist in
Menlo Park, Calif.

The seismic waves created by the explosions will be analyzed by USGS
scientists at Menlo Park and by other seismologists and geologists at
Humboldt State University in Arcata, Calif., Oregon State University in
Corvallis, Ore., and the University of Utah in Salt Lake City, Utah.
Digital tapes of recorded waves will be processed by computers to
produce graphic images of the various strata and faults.

USGS scientists in Menlo Park said the explosions will not create any
hazard for residents of the area or cause additional stress on the
faults.  "We have conducted similar projects in other seismically active
areas of the country including the San Francisco Bay area following the
Loma Prieta earthquake," said USGS seismologist Allan Lindh, "without
causing any disturbance to the land, the residents, of the fault
system."

Dr. Lindh explained that the explosions are so small in energy released
and so shallow compared to the 5-mile-plus depths where earthquakes are
generated, that the detonations can not possibly affect movement on the
faults.  "These detonations will truly be like mosiquito bites on an
elephant," said Lindh.

The charges will be detonated shortly before midnight to minimize
instrument interference from "cultural noise" such as that generated by
passing trucks, roaming cattle, etc.

"It's highly unlikely that anyone who is sleeping will be disturbed by
the shots," said Lindh, "and very likely that anyone who stays up
expecting to hear or feel something will be very disappointed."

Van Schaack said the ammonium nitrate should burn completely during the
explosions and does not present either an environmental or safety
hazard.  "All the shot sites are at least one mile from the nearest
structure," Van Schaak said.  "After the shots are completed we will cap
the drill holes about three feet below the surface and replace the
soil."

All four drill sites for the Sept. 16 seismic shots are located on
private land -- one near the Kalamath River in the northern part of
Humboldt County; one east of Arcata; one southwest of Petrolia; and one
northwest of Petrolia.

Van Schaack said the USGS appreciates the cooperation they have
experienced in dealing with landowners in the area.  "These folks don't
receive any type of compensation for letting us drill holes on their
land," Van Schaack said, "so we really appreciate their generosity in
providing sites for us to conduct these scientific investigations."

Van Schaack said he and other USGS scientists also will appreciate the
public's cooperation in protecting the nearly 250 portable seismographs
from being disturbed prior to and following the seismic exercise.  "In
some places, especially urban locations, we've lost a few instruments to
theft and vandalism," Van Schaack said, "but we're hoping the residents
of Humboldt County will treat them with respect."

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 * SLMR 2.1a #T163 * Your Motherboard wears combat boots.
                                                              
Newsgroups: ca.earthquakes