Re: Earth Sciences 1983

On Sep 8, 5:06 am, Petra <petras...@xxxxxxxxxxx> wrote:
If you've ever wondered if earth science has changed here are some
notes from this event occurring near Challis, Idaho in 1983.

Borah Peak, ID, Oct 28, 1983 (8:06 a.m.), ~ 15 mi. W of Mackay, ID,
Magnitude: 7.3 Felt Area: 855,000 Sq. Kilometers

This earthquake included every form of interest in an earthquake. From
shifting, cracking, liquifaction, sand boils, huge fissures and great
boulders on the move, it's one to note.
The photo journal includes very interesting photos.

WHEN THE EARTH MOVES

By Jim Connor
The surface of the planet Earth is in endless motion. The rocks of the
continents are made up of lighter stuff than the molten rocks of the
mantle on which they float. Great heat and pressure within the mantle,
miles below the earth's surface, cause the continents to move relative
to each other. This motion seems very slow relative to a human
lifetime, less than inches a century but planet Earth has a lot of
time.

In places these forces tear the earth's crust apart and the molten
rock, magma, moves to the surface from volcanos. Continental masses
may collide head on, or bump and slide past each other. There are
analogies to be seen in spring break-up of ice on a river or on a lake
on a windy day. The ice cracks and breaks, wind causes great piles of
ice on the windward shore. Eons ago India and Asia collided, and great
heaps of rocks pushed up. We call them the Himalayan Mountains.

Similar forces are at work in Central Idaho.

Rock is an elastic substance. That means it resists being bent,
twisted, stretched or compressed. If enough force is applied it
suddenly breaks. When this occurs in a huge mass of rock in the earth,
an earthquake is produced.

Faults

The zone of rock breakage is called a fault. If forces in the earth
continue to act on the rock in a fault zone, it may shift again,
either up and down or sideways, or both. Such faults are called
"active faults." Some faults remain active for millions of years,
producing earthquakes of large magnitudes many years or even centuries
apart.

Faults are very common features in mountainous areas. One can easily
count 100 faults on the geologic map of the Challis 2° Quadrangle
published by the Idaho Bureau of Mines and Geology in 1979. Others are
buried and have yet to be found. It is unlikely all these faults are
"active" but many of them are. A number of small earthquakes reported
from the Stanley Basin, Bull Trout Lake and Lowman areas this summer
are the result of movement on faults in those areas.

The Mount Borah Fault

Some faults extend for many miles. The well known San Andreas fault in
California is one example. Dr. Spencer H. Wood, geologist at Boise
State University and other geologists believe a large fault extends
along the western base of the Lost River Range

from Arco to north of Challis, a distance of 90 miles. This fault has
not yet been named but the "Mount Borah Fault" seems likely. Monday
evening Dr. Wood stated, "A very similar fault exists along the
western base of the Lemhi Range in the Little Lost and Pahsimeroi
Valleys. It is probable this fault is also active."

Prior to the major earthquake on October 28, 1983, the Mount Borah
Fault was know to be active. Dr. Wood reports numerous small tremors
of magnitude 3 or less have been recorded in recent years. "Such
tremors are common to many fault zones," he said. They are too common
to be used to predict a major earthquake.

The Epicenter

Were it not for friction, the forces acting on a fault would cause
slow steady movement. Broken rock under pressure does not easily slip
past itself. As tension builds on a fault, the rock is deformed
slightly and finally at some point, the epicenter, the rock gives way.
In a large earthquake cubic miles of earth suddenly moves. This
creates shock waves far greater than any explosive man has ever
created. These shock waves may circle the earth several times. They
produce what is experienced as an earthquake. As a general rule, the
deeper the epicenter is located, the farther away is the earthquake
felt. Hence, it can be concluded that the Mount Borah epicenter was at
great depth, although its exact position is not yet known.

The Shock Waves

Several types of shock waves are generated during an earthquake. The
primary wave is a compression wave. It is the type wave that a miner
tries to create when blasting rock. It is "explosive," and moves
through rock at a speed of four miles per second. When the energy of
the primary wave escapes into the air, it produces the roaring sound
many people report hearing during an earthquake. Primary waves pass
directly through the earth and can be recorded on the other side.

Secondary waves cause up and down oscillation of the ground. These
produce much of the shaking sensation people feel. Although many
people report seeing ground waves, some geologists doubt they exist.
Secondary waves move at the rate of two miles per second.

Two other waves are generated--Love waves, named after mathematician
A. E. H. Love, and Raleigh waves, named after Lord Raleigh. Both these
are complex surface waves which add to the vibrations felt

The Seismograph

An instrument of much value in studying earthquake intensity and
behavior is the seismograph. The seismograph consists of a motionless
pendulum, a rotating drum fitted with a *** of paper, and a device
to mark the paper. The pendulum resists motion when the ground moves
in an earthquake, but the drum shakes with the ground. The marking
device makes a trace of the relative motion of the drum on the paper.
The amount of deflection from the center line on the drum is a measure
of the intensity of an earthquake.

Great accuracy in modern seismographs is obtained by mounting a mirror
on the pendulum and reflecting a thin laser beam from the mirror to
photographic paper on the drum. Modern seismographs are also fitted to
a very accurate clock so instruments around the world can be
correlated to each other.
A trained seismologist can distinguish the various type waves
generated in an earthquake.

Locating Epicenters

Because the strong primary and secondary waves travel at different
speeds, a single seismograph is sufficient to determine how far away
an earthquake occurred. There is no information from what direction
the shock waves come. From a single seismogram it can be determined an
earthquake occurred at a certain time on a circle at a fixed distance
from the seismograph.
A second seismograph placed at another location some distance away
produces a second circle. This enables geologists to define two places
where the earthquake may have occurred--the points where the two
circles intersect. A third seismograph station enables geologists to
pick the exact spot--the single spot where all three circles
intersect.

With the aid of computers and satellite communications, dozens of
seismograph stations may be used in calculating the epicenter, and the
location is sent to television stations within minutes.

Earthquake Triggers

The causes of earthquakes are fairly well understood. What triggers
them at a certain instant is, and has been, the subject of scientific
study, intelligent folklore, myths and pure bunk. It is possible, for
instance, that a cow stomped her hoof and that tiny amount of energy
was sufficient to trigger the Mount Borah quake. It is unlikely.

There are indications storm fronts with changing barometric pressures
might trigger some of them. Dr. Peter Isaacson, a geologist at the
University of Idaho, speculates heavy rains last summer might have
triggered the Mount Borah quake. "Rainfall may have lubricated the
fault zone," he said.

Dr. Nick Saum, a geologist from Golden, Colorado, believes the moon
plays a role. The same forces that produce the tides act upon the
land. Saum believes earthquakes are more likely to occur near new or
full moon when the tidal forces are strongest. There seems to be some
correlation to that, but on October 28, the moon was at the third
quarter.

Seismographs "Off Scale"

Rumors persist the Mount Borah quake sent seismographs in Boise and at
Ricks College "off scale."

The Richter scale has no upper limit. In that sense "off scale" is
impossible. Near an epicenter the various waves are together, and
reinforced echo waves bounce off nearby rocks. A seismograph near the
epicenter would have yielded no meaningful information. The seismogram
from Mackay would have been a crazed jumble of squiggles.

"Large earthquakes are rare," states Spencer H. Wood, geologist at
Boise State University," so some seismographs are set for increased
sensitivity. This makes it possible for seismologists to study the
small earthquakes that are always occurring. When a large earthquake,
such as at Borah occurs, these instruments go "off scale." Other
seismographs set at lower sensitivities are not affected."

A common example of the same thing is when a C.B. operator turns down
the squelch dial and turns up the volume, he is increasing the
sensitivity of his radio. He is trying to receive weaker signals. He
also receives more static.
Should an overpowered transmitter begin to broadcast nearby, he would
be blasted out of his rig. His receiver would have gone "off scale."

Hydrology

Perhaps one of the most significant aftereffects of the Borah quake
will be the effect on springs and groundwater. The warm springs on the
Ingram Ranch near Challis has completely dried up. If the flow does
not return, hundreds of acres of hayground will be lost. Wells near
Mackay have been reported to have up to 30% more water. Flooding on
farmground near Chilly Butte has occurred.

The Mount Borah earthquake has changed the flow of groundwater in many
areas. Some will benefit, others will lose. Complex legal question are
apt to arise concerning water rights. How does a rancher prove "his"
water is producing the new spring on his neighbor's land? How would he
get it back? Where did the Ingram Ranch water go? Perhaps it will
return. Perhaps studies will find it. Perhaps it will remain a
mystery.

The Quake: Some Benefits

Soil scientists from the Salmon office of the Bureau of Land
Management and from the Challis National Forest have a 10 to 15 mile
long scarp near Mount Borah on which to study the soils.
The fault area is easy to reach and is not in a heavily wooded region.
Access to study will be fairly simple. From these studies geologists
will gain more insight into the nature of major earthquakes.
Prevention will remain elusive for many years, but prediction of major
earthquakes will improve as a result of these studies.

Carolyn Wurts, one of 30 students from Boise State University camped
on Doublesprings Road last weekend summed up the scientific importance
of the Mount Borah quake, "It's a perfect example to see first hand
what happens. It's great! I learned a lot from being here."
[Challis Messenger; November, 1983]

Photo Journalhttp://www.seis.utah.edu/NEHRP_HTM/1983bora/1983bo1.htm#Photos

Petra Challus Quake Predictions
Where Safety and Prediction Come Togetherhttp://www.petrachallusquakepredictions.com

What are sand boils? They don't look like acne.

.