Saturday, January 20, 2007

Earthquake Case Study: 1989 San Francisco Earthquake

On 17th October 1989, at 5.04 pm, an earthquake measuring 6.9 on the Richter scale, and lasting only 15 short, but devastating seconds, hit San Francisco (see shake intensity map) The following YouTube video gives an overview of some of the images of the aftermath of the earthquake. You will need to learn the case study of the San Francisco Earthquake. You need to have an understanding of both the causes of the quake and its effect on people, the environment and the economy.

Cause of the 1989 San Francisco Earthquake

San Francisco is located on the San Andreas Fault Line which marks the boundary between the North American and Pacific Plate (a conservative Plate boundary). As the two plates move past each other, pressure builds up due to friction. The earthquake occurred due to the sudden release of the built up pressure. The epicentre of the earthquake was under a mountain known as Loma Prieta, 10 miles NE of the city of Santa Cruz. The 1989 earthquake (measuring 6.9 on the richter scale) was the largest earthquake to have hit California since the 8.2 earthquake of 1906. This movie, shows the shaking intensity and movement of seismic waves during the 1989 quake.

Effects of the 1989 San Francisco Earthquake

The effects of the 1989 earthquake included:
- 67 Deaths
- 6,000 Homes damaged / destroyed
- 2000 people made homeless
- Upper deck of the Nemitz highway collapsing onto the lower deck crushing people in their cars
- A section of the San Francisco-Oakland Bay Bridge collapsing
- Fire resulting from gas explosions
- Massive economic costs ($4.4 billion)
- Damage to infrastructure - electricity / gas and water mains cut

3 Areas in San Francisco were p articularly badly affected:

1. The Marina / Bay District
Here the buildings were wooden and not securely attached to their foundations. The area was also located on an old landfill site, which was created following the devastation caused by the 1906 earthquake. As a result of the weak sub-surface sediments, when the ground shook in the 1989 earthquake, the process of liquefaction occured. This is where as the ground lost its sheer stength and acted more like a liquid as water moved up through the sediments resulting in reduced strength and causing buildings to collapse due to lack of support.

2. Collapse of the Double-Decker Nimitz Highway and San Francisco-Oakland Bay Bridge

A number of roadways were damaged during the earthquake, including the collapse of the upper tier of the double-deck er Nimitz Highway (Interstate 80) onto the lower deck, killing and trapping motorists in their cars. The collapse was caused partly by soil faiture and also the unsuitable design of the supporting piers. A section of the Bay Bridge also collapsed killing motorists.

3. Older Buildings of Downtown San Francisco

Many of the older (50-100 year old) buildings which were not designed to withstand earthquakes were most severely damaged. Low rise building were also worse affected than taller buildings which swayed with the quake / ground motions.

Follow Up Links:
Remember to get full marks in a case study answer you need to learn specific facts and figures and include locational detail (i.e. name specific places, areas, buildings etc. affected). As well as class notes, see the following links for further detailed information on the 1989 quake.

1989: Earthquake hits San Francisco (BBC - On this day)
San Francisco Virtual Museum - 1989 Earthquake Reports and Photographs
1989 San Francisco Earthquake Photographs
Wikipedia - Loma Prieta Earthquake
The October 17, 1989 - Loma Prieta, California Earthquake (USGS)
USGS Summary - 1989 Loma Prieta Quake

Key Term Check:

Liquefaction - where the ground begins to act like a liquid during an earthquake
Focus - the start of the earthquake underground
Epicentre - the point on the grounds surface directly above the epicentre

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