The following titled "After the Shock" was previously published in
MONITORING TIMES, 140 Dog Branch Rd., Brasstown, NC 28902, in the July 1994 issue.
Sorry, no .gif files available for the photos from the article. You'll need to get a copy of the
magazine for those...
EARTHQUAKES - AFTER THE SHOCK
In the United States, whenever people think of earthquakes, they naturally think of California.
While we certainly have our share of earth shaking events out here, they are in no way limited
to the west coast, as evidenced by recent moderately strong temblors that shook areas of the
Dakotas, Wyoming, and even the northeast. Earthquakes can occur almost anywhere. The are
among the most costly of all natural disasters - in terms of both dollars and in human lives.
Whenever the ground shakes, so do people's nerves. Phones ring, emergency services are
activated, and the radio comes alive with traffic. Communication lines become jammed as
people report damage, call for help, or try to contact their family and friends. The effects are
stimulating to some, devastating to others.
Most people dread earthquakes. For the scientist of the Office of Earthquakes, Volcanoes, and
Engineering Division of the United States Geological Survey (USGS), an earthquake is a subject
for study. It's an opportunity to examine data, search for correlations, and test theories,
ultimately with the hope of learning how to predict when and where an earthquake will occur.
At the Western Region Headquarters for the USGS in Menlo Park, California, data is collected
from more than 500 different recording locations. The state is criss-crossed with the largest
seismic network (CALNET) in the United States. It is made up of sensors, telemetry links, and
analog and digital recording devices. Not all of the components of the network belong to the
USGS. The CALNET system is made up of sites maintained by USGS, Lawrence Livermore
National Labs., University of Nevada at Reno, University of California at Berkeley, and the
California Dept. of Water Resources.
Data is collected from several different types of sensors that measure different motions or, that
are especially responsive to specific frequencies of waves. Much of the data arrives at the
Menlo Park center by VHF, UHF and microwave radio links. Data processing systems monitor
the network and can produce real-time locations of earthquakes with magnitudes (M) between
M 1.5 and M 3.5, within minutes. Whenever seismic waves are detected by instruments at 4
different recording locations, an "event" is declared and the data is routed to additional
computers for further analysis.
When a seismic event occurs some devices, called vertical gain accelerometers, use a spring
loaded mass mounted in a sensor about the size of a small tomato juice can. The mass is free
to move relative to the earth. A magnetic field is maintained around the mass. As the mass
moves, changes in the magnetic field generate a signal. The signal, typically around 1 Hz., is
amplified up to 90 Db and converted to frequencies in the voice range. It can then be
transmitted by conventional means (radio or telephone) to a central processing center for
analysis.
Seismic data transmissions can be identified by the listener by their continuous tone. Ground
movement modulates the signal which changes the tone. As the spring loaded mass moves in
one direction the tone increases pitch, movement in the other direction decreases the pitch of the
tone. The analog data produced can be displayed on the rotating drum seismographs that we see
on the 11 o'clock news after an earthquake. During an "event", this analog data is digitized at
a rate of 100 samples per second and saved in the data processing center for analysis.
VHF and UHF frequencies used to transmit the data are usually found in the US Government
frequency allocations. Occasionally you may find telemetry on frequencies licensed to
universities. Transmitter output power is typically less than 1 watt. Many sites provide reliable
data with only 100 milliwatts of output power. The signal is transmitted by horizontally
polarized beam antennas and may be relayed several times before it reaches a processing site.
Emissions are narrow band FM. (See photo 2)
Routine monitoring of the voice channels of the USGS does not produce a "hot bed" of
excitement. Voice traffic is normally between technicians testing equipment or making
transmitter adjustments. Some traffic concerns the daily checks which are performed on all data
channels to insure signal integrity. However, following a significant earthquake, traffic can be
heard concerning epicenter location, evidence of surface ruptures, placement of sensors,
microwave path alignment, and communication between scientist and engineers in the field.
Where to search for telemetry signals
162.000 Mhz.- 174.000 Mhz.U.S. Govt.
216.000 Mhz.- 220.000 Mhz.U.S. Govt.
406.100 Mhz.- 420.000 Mhz.U.S. Govt.
________________________________________________________________________
California Telemetry Frequencies
San Francisco Bay Area - Northern California
163.0500 163.4400 163.6050 163.9100
164.8450 165.8100 166.4000 166.8250
167.8050 170.3100 171.0000 172.8600
217.6000 217.6900 218.2500 406.1900
407.3520 408.5120 409.6000 410.5500
412.2500 413.5100 414.6650 415.2000
415.2250
Southern California
162.5940 162.5970 162.8060 162.8090
163.3500 163.3970 163.6060 163.6090
163.7935 163.7970 163.9375 164.0060
164.0095 164.8440 164.8470 165.8065
165.8095 166.4190 166.4220 166.6565
166.6595 167.1940 167.1970 167.8065
167.9085 171.2190 171.2220 171.4065
173.1940 175.2550
Nationwide Federal Frequencies Shared With USGS
164.1000 164.5250 164.6750 164.8000
165.4875 166.2750 166.3500 166.3750
166.8000 166.8750 166.9500 166.9750
167.0750 167.1250 167.9500 168.2750
168.5000 168.5500 169.5750 169.6250
169.8250 172.4250 172.6750 172.7250
407.4250 407.5250 407.5750 408.0750
408.5500 410.5750 411.6250 411.6750
412.1750 412.3750 412.7000 412.8250
412.8750 412.9500 412.9750 414.8250
417.4000 417.5750 417.6250 419.8750
419.9000 419.9250 419.9500 419.9750
Also check U.S. Govt. Dept. of the Interior frequencies for USGS activity.
Aftershock Early Warning System
William Bakun is a seismologist for the Office of Earthquakes, Volcanoes, and Engineering in
Menlo Park. He was watching a television newscast that showed rescuers as they crawled
through the rubble of the collapsed Interstate 80, in Oakland, California. The structure collapsed
and trapped many motorist under tons of concrete and steel as a result of shaking caused by the
October 17th., 1989, Loma Prieta earthquake. Aftershocks of the 6.9 quake were frequent.
They were of great concern to the rescuers. Additional shaking of the damaged structure
threatened additional collapse and a potential for loss of life. As a branch chief, he discussed the
problem with other members of his staff and came up with an idea - deploy an Early Warning
System for aftershocks.
The prototype system consisted of four elements: 1) ground motion detectors and telemetry
transmitters placed around the epicenter of the earthquake, 2) a radio receiver and central
processing unit in Menlo Park, 3) a mountain top radio repeater, and 4) alerting monitors.
The key to the system is the difference in speed that radio waves travel as compared to seismic
waves.
There are several different types of ground waves that are generated when portions of the earth's
crust break during earthquakes. The speed of the waves depends upon the density and rigidity
of the surrounding rocks. P waves are compressional or push-pull type waves. They are the
first to arrive locally. In the San Francisco Bay Area, earthquakes that occur between 5 and 15
kilometers below the surface, typically produce P waves that travel about 6.2 miles per second.
S waves are the second to arrive. They usually cause most of the damage due to the severe
shaking that is produced by the high amplitude waveform. S waves generated by the Loma
Prieta earthquake traveled about 2.5 miles per second. Compared to the 186,000 miles per
second of radio waves, the ground waves generated by earthquakes are real slow movers!
How It Works
The central processor evaluates the data supplied from the epicentral ground motion sensors and
determines the magnitude of the aftershock. The system is designed to transmit an alert on all
aftershocks with a magnitude greater than 3.7 on the Richter Scale. The alert consists of two,
dual tone, multi-frequency signals that activate alarms at the remote receiving locations.
In the San Francisco Bay Area, the signal was transmitted by microwave to a repeater on top
of Monument Peak, which overlooks the entire San Francisco Bay, Oakland, and San Jose areas.
The alert was repeated on the VHF frequency of 169.825 Mhz. Every 60 seconds, a test
transmission was automatically sent to verify that the links were alive and well. If an aftershock
was detected that exceeded the 3.7 threshold, a different set of tones activated the alarms and
warned the workers.
In the first 6 months of operation following the 1989 quake, 12 aftershocks were detected with
magnitudes greater than 3.7. The system triggered alarms successfully each time. It did not
trigger any alerts on aftershocks with a magnitude of 3.6 or less. One false alarm was sent due
to a minor design flaw which has now been corrected.
The farther away from the epicenter that you are, the more time you have between the time of
the alert and the arrival of the first ground waves. The Loma Prieta earthquake epicenter was
about 62 miles (100 Km) from the severely damage areas in San Francisco and Oakland. The
Early Warning System for Aftershocks developed by the scientists at USGS in Menlo Park,
provided between 20 - 27 seconds of warning for workers demolishing the damaged structures.
20 seconds may not seem like a lot unless you're the one under tons of concrete. Any warning
that allows you to seek refuge is a blessing. I know. I was there.
The system works. It is not earthquake prediction. It is a method of rapid notification of
approaching seismic waves. When a seismic event occurs, the signals are analyzed immediately
by the micro processor. If certain criteria are met, the alarm triggers. This all happens almost
instantaneously - without human intervention. The current status of the system is that it is neatly
packaged in the basement of the USGS in Menlo Park waiting to be sent wherever it is needed.
Refinements have made it smaller and more compact than it was in 1989. It can be flown to a
site and be quickly deployed to transmit the alert on any of the preexisting, nationwide, USGS
frequencies. Remote receivers are provided by the USGS that respond to the alert tones.
Developments like these from the scientists and engineers of the USGS, can help save lives
following other major earthquakes which will certainly occur. Current technology cannot predict
when or where earthquakes will happen. Perhaps someday predictions can be made, but that
day is somewhere in our future. This system is available now. It can be used to notify
emergency workers to evacuate hazardous locations and seek shelter. Receivers, placed at radio
broadcast stations, can provide a tone alert and warn the general public instantly, without
disrupting commercial broadcast. Use of such a system for public notification will require a
massive public education campaign to be effective and not create panic. Science and technology
has provided us with a tool. It is up to the policy makers in our legislatures and emergency
services to include systems like this in their disaster preplans. Japan, long known for its strong
earthquakes, uses a similar system to detect aftershocks and automatically slow their high speed
Bullet trains before the approach of strong ground waves.
H.A.R.P.S.
Highway Advisory Radio Portable System
When traveling into or out of a disaster damaged area look for signs advising of temporary
Travelers Information Stations. Photo 6 shows a H.A.R.P.S. station licensed to CALTRANS
(California Dept. of Transportation). It was rapidly set up in a rest area 150 miles north of Los
Angeles on Interstate 5. It broadcast on 530 Khz. and informed motorist of delays and offered
alternate routes into the earthquake damaged areas following the Northridge Earthquake of Jan.
17, 1994.
According to the manufacturer, Information Station Specialist in Zeeland, Mi., 13 HARPS units
have been built for the State of California at a cost of a bit over $30,000 each. Each unit can
be remotely controlled by a cellular telephone link. While unattended, the messages can be
checked or changed as needed. Different messages can be programmed to start and stop as
required. Power can be provided either by 110 volt shore power or by a 2.5 kw generator
which can also be started or stopped by the cell phone interface. Each trailer mounted unit has
two transmitters, one on 530 Khz. and the other above 1600 Khz. Transmit output power is
limited to less than 10 watts. The typical range is about 5 miles.
Public Seismic Network BBS
The Public Seismic Network is a computer billboard dedicated to sharing information regarding
seismology and earthquakes. Weekly earthquake reports from the USGS and California Institute
of Technology are made available for downloading to your computer. Other information is
provided by members of the network that allow you to plot distance from seismograph locations
to epicenters of earthquakes. The network has four nodes that you may access 24 hours a day.
(901) 360-0302 PSN Memphis, Tn.
(818) 797-0536 PSN Pasadena, Ca.
(408) 226-0675 PSN San Jose, Ca.
Photo Legend
Photo 1 3/16/94
Bank of 24 seismograph recorders display activity from CALNET Network in central and
northern California. (Horizontal cropping)
Photo 2 3/21/94
Bank of 24 seismograph recorders display activity from CALNET Network in central and
northern California. (Vertical cropping)
Photo 3 2/11/94
Seismograph recording aftershocks of Northridge Mag. 6.6 earthquake.
Photo 4 3/21/94
Seismograph recording aftershocks of Northridge Mag. 6.6 earthquake taken 3/21/94.
Photo 5 2/24/94
Menlo Park Headquarters. Rooftop antennas.
Photo 6 3/21/94
USGS Menlo Park Headquarters and telemetry tower.
Photo 7 2/11/94
Closeup of rooftop antennas from rear. Microwave telemetry tower in background.
Photo 8 3/21/94
Rooftop antennas for telemetry, satellite uplink / downlink, and WWVB time standards.
Photo 9 2/24/94
Telemetry Tower in Menlo Park.
Photo 10 2/21/94
Solar powered seismic sensor installation.
Photo 11 2/24/94
Highway Advisory Radio Portable System. Portable Travelers Information Station.
Photo 12 2/24/94
Highway Advisory Radio Portable System. Portable Travelers Information Station.
Photo 13 2/21/94
Logo of USGS.
Photo 14 3/21/94
Seismic Alert Receiver used in Oakland in 1989.
Photo 15 3/21/94
Telemetry calibration equipment used daily in Menlo Park.
References
Early Warning System for Aftershocks, W.H. Bakun, F.G. Fischer, E.G. Jensen, and J.
VanSchaack, 1994, advance abstract from Bulletin of American Seismologist.
Real-Time Earthquake Monitoring, National Research Council, 1991, National Academy Press
Elementary Seismology, Charles Richter, 1958
Interviews with:Wesley Hall, USGS Telemetry Section
William Bakun, USGS Seismology
Stan Silverman, USGS GEOS Program
John VanSchaack, USGS Telecommunication Div.
Selected Southern California frequencies from PSN BBS, Pasadena, Ca.
Field research by Ken Navarre Jr., 1993 - 1994.
Northern California frequency field research, Ken Navarre, 1994.