MAGNITUDE AND EPICENTER
The
Loma Prieta earthquake occurred on October 17, 1989 at 5:04 p.m.
Pacific Daylight Time. It had a moment magnitude of 6.9 and a surface
wave magnitude of 7.1. The duration was 15 to 20 seconds.
The earthquake occurred due to slip in either the San Andreas Fault or a subsidiary fault.
Much
of the displacment was vertical motion typical of reverse or thrust
faults, according to Greg Beroza and Paul Segall of Stanford University.
The
epicenter was near Loma Prieta, which is a peak in the Santa Cruz
mountains. This location is 10 miles northeast of the city of Santa
Cruz.
A strong-motion seismograph gave an acceleration reading of 0.64 G near the source.
The focal depth was 11 miles (18 km), which is unusually deep. Typical California earthquake focal depths are 4 to 6 miles.
The fault rupture did not break the ground surface. Superficial ground cracks occurred in a number of locations, however.
This earthquake was the largest earthquake to occur in the San Francisco Bay area since 1906.
Furthermore, it was the most severe earthquake in California since the Kern County 1952 quake.
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DEATH TOLL
The earthquake caused damage throughout the San Francisco Bay area. The death toll was 62. About 3757 people were injured.
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PROPERTY DAMAGE
The
cost was 6 to 8 billion dollars. San Francisco had 22 structural fires
during the seven hours from the time the earthquake struck until
midnight.The
earthquake caused widespread damage to the War Memorial Opera House in
San Francisco. The House was closed for 18 months, and repaired
with a $49.5 million seismic retrofit, along with $28 million
improvement in backstage additions.
Many
buildings in downtown Santa Cruz were extensively damaged. Many of the
damaged structures were 50 to 100 years old, built with unreinforced
masonry that was brittle and weak. Furthermore, the buildings stood
upon the unconsolidated flood plain sediments of the San Lorenzo River
which were subject to intense shaking and liquefaction.
The earthquake damaged 242 buildings at Stanford University, more than 20 of them seriously.
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LIQUEFACTION
Some homes and buildings San Francisco's Marina
district suffered severe damage. These structures were built on loose,
sandy soil, permeated with water. As a result, liquefaction occurred. Liquefaction
is a process whereby the shaking motion and the weight of the buildings
causes water to be squeezed out from the soil. The soil thus
temporarily develops a liquid consistency, similar to quicksand.
Buildings may topple over or collapse when liquefaction occurs. |
LANDSLIDES
More
than 1,000 landslides and rockfalls occurred in the epicentral zone in
the Santa Cruz Mountains. One slide, on State Highway 17, disrupted
traffic for about 1 month. Areas outside of Santa Cruz, including the
towns of Watsonville, Hollister, and Los Gatos, also suffered heavy
damage.
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CYPRESS VIADUCT
The
earthquake caused the Cypress Viaduct to collapse, resulting in 42
deaths. The Viaduct was a raised freeway which was part of the Nimitz
freeway in Oakland, which is Interstate 880. The Viaduct had two
traffic decks.
Resonant vibration caused 50 of the 124
spans of the Viaduct to collapse. The reinforced concrete frames of
those spans were mounted on weak soil. As a result, the natural
frequency of those spans coincided with the forcing frequency of the
earthquake ground motion. The Viaduct structure thus amplified the
ground motion. The spans suffered increasing vertical motion. Cracks
formed in the support frames. Finally, the upper roadway collapsed,
slamming down on the lower road.
The remaining spans which were mounted on firm soil withstood the earthquake.
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BAY BRIDGE
A span of the top deck of the San Francisco Bay Bridge collapsed. The span fell at an angle blocking the lower deck.
One car drove off the edge of the gap, crashing onto the lower deck and killing the driver.
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WORLD SERIES
The Loma Prieta earthquake interrupted the baseball world series between the San Francisco Giants and the Oakland Athletics.
Eventually, the Oakland A's swept the San Francisco Giants in the 1989 Series, 4 games to 0.
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PERSONAL ACCOUNTS
Loma Prieta Earthquake, Oct 17, 1989
The
California government had mandated that all businesses with 200+
employees had to submit an Earthquake Preparedness Plan by Oct 15th,
1989. I was the Telecom Manager where I worked and had to be involved
in the process, so had been to several vendor presentations relating to
planning for a 'big one' .
Our
main corporate facility was in Torrance, CA and we had an office in
South San Jose (was Los Gatos when we purchased the property the year
earlier, before they rezoned) and had been named the Los Gatos Golf
Course. It was in beautiful surroundings, lush green hills, a bubbling
creek, deer would come to the windows to munch on our shrubbery.
When
I had to go there for the construction meetings I questioned the
location, knowing that the Guadalupe Creek was part of a fault line in
the area. I had lived in San Jose during the 1983 earthquake, near that
creek when we had a 6.3 earthquake and they had determined it to be
part of the fault. The building was mostly a concrete shell, so I got
to see the entire build-out process.
The
General Manager of our San Jose facility used to complain about the
poor service from our telecom equipment vendor and make me fly up there
to fix problems so often that we had a terminal installed so I could
monitor their phone system from Torrance. This allowed me to do some
maintenance and adds and changes to their system without having to fly
up there. AT 5pm on the evening of Oct 17th, I was in a meeting with my
MCI sales rep when the dedicated T-1 circuit to the San Jose facility
went off-line and I was unable to reconnect.
The
meeting was almost over, so I was waiting until the rep left to do
anymore trouble-shooting. Within a couple minutes of loosing
connection, the head of our Security Dept came into my office, "All the
lines to San Jose are down and the guards aren't answering the
emergency phone lines, either. Do you know what is going on?"
I
was just about to walk out my sales rep and told the Security officer
that I was just getting ready to find out. He said, "well you know they
just had an earthquake in San Francisco?" To which I told him, "that is
what is wrong with the phone lines, they are all jammed up." and didn't
think much else about it. I went home and about 10pm that night got a
call from the Director of Facilities telling me I was 1 of 5 people who
had to go to the San Jose office on the first flight in the morning to
check out the damages and see how quickly we could get it back
operational.
It was one of
our offices that housed the software development staff and since they
were some of the highest paid employees, they wanted them back to work
quickly.
We met at
the Torrance Facility and took 1 car to the airport. We were on the
I405 when all the traffic came to a dead stop about a quarter mile from
the Century Blvd exit to go to LAX. It took 45 minutes from there to
get to the airport; normally it was about a 10 minute drive. We had to
pickup our tickets at the counter, which had lines 5 times as long as
normal, and were told that the power was going off at the San Jose
airport and they might have to cancel flights.
We
were still standing in line to get our tickets when our 9am flight left
for San Jose. A couple of the people with me were from the East
Hartford office and wanted to rent a car. I explained that driving
normally took 6 - 8 hours and we didn't even know if Hecker's Pass was
open. So we stood in line on stand-by for the next flight at 10am.
There
were several delays as they shut down the San Jose airport a couple
more times, causing us to have to wait on the runway in LA, circle in a
holding pattern while they got a runway available in San Jose and wait
on the runway in San Jose as they found an open gate for us. We finally
got to the front of the airport after 2pm only to find out that all the
taxis were commandeered for emergency services.
We
tried calling people's home phones, but phone service wasn't very
reliable yet. Finally contacted someone on a cell phone to come pick us
up from the airport and get us to a car rental place that still had a
car available. We didn't get to the facility until 5pm and it was
starting to get dark. A couple of us had pocket flashlights and went
inside. At this point we realized one of the people that was supposed
to be with us was the Structural Engineer to let us know if it was safe
to enter the building. He had missed the flight and all other flights
were cancelled that day.
The
building was a 3 level, split-level built into the side of the hill The
first thing we saw as we entered was the wall buckled out about 18
inches and the ceiling tile hanging on that side of the entryway. One
team member went up the stairs and I went into the bottom level cubicle
area. It looked like a bomb hit the place! I thought surely people died
the way things were thrown around and file cabinets overturned.
Most
of the employees were from Ann Arbor, MI and East Hartford, CT they all
dove under their desks like you are supposed to do. The only person who
was injured was the lady who grew up in CA and just sat in her chair to
wait out the shaking. A metal file cabinet fell on her and dislocated
her shoulder.
All the rings
around the sprinkler heads had shot across the room and the sprinkler
heads had eaten into the ceiling tiles 2 - 3 inches on each side as the
building rocked. All the overhead cabinets facing East or West had
their contents thrown out about 3 feet, looking like a very violent
initial shake had dislodged them from their normal locations. A
bookshelf near a hallway had books strewn at least 5 feet out into the
hall.
As I walked through
the facility I noticed that none of the T-bar that holds the ceiling
tiles were able to reach the far East wall causing the tiles to be
hanging in each area. I stated that it seemed that the building had
been stretched about a foot total. Later I was told that I was very
close to being accurate and they had to install more support beams to
secure the building before people would be allowed to come back to
work. In the telephone equipment room most of the equipment had been
bolted to the floor, except the voicemail equipment.
It
was a cabinet about 6' tall with a 20" X 20" footprint. It had walked
out from the wall and unplugged itself! Both hard drives in it had to
be replaced. Just as we were all gathering in the entry way to go to
the hotel a 5.0 aftershock happened. We were concerned about the
building undergoing more damage and quickly got out to the rental car.
The
next morning as we met over breakfast, the other team members were
remarking about all the earthquakes during the night keeping them
awake. I had slept through all of them. Apparently we had over 200
aftershocks with couple of them as big a 5.0. There were supposedly
over 1000 aftershocks during that week, which didn't help the cleanup
or rebuilding. We had constructions crews working 24 hrs a day and got
the facility back operational within 5 days and I was able to go back
home, to LA.
They said that
the retaining wall on the creek side of the building had acted like a
battering ram and kept hitting the building as the ground shook which
caused the building to stretch. I wish I would have taken a camera so I
could show what happens inside the office space when you don't lock
your overhead cabinets at work, not to mention all the other stuff that
went flying. We tend to be so complacent in CA when it comes to
Earthquake preparedness.
Now
I keep earthquake food in my home and in my office, even a little food
and water in my car and my overhead cabinets locked since that
experience. On the news we saw horrible pictures of the devastation in
San Francisco, as the fires from the broken gas lines, and aftershocks
were causing even more damage. I am grateful we didn't have a facility
there!
Thanks,
Sherry Ridge
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USGS - Loma Prieta
Earthquake: USGS-LP.pdf
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