a) Introduction

At 5.04 p.m. Pacific Daylight time, while most families were watching the third game of the World Baseball Series, the San Francisco region was struck by a major EARTHQUAKE.

The EPICENTRE was sixty miles south-east of San Francisco in the remote area of Loma Prieta, in the Santa Cruz Mountains.

The earthquake was felt over an area of 103.6 million hectares from Los Angeles in the south to the Oregon border in the north and it registered on SEISMOGRAPHS throughout the world.

It was the first major earthquake to hit the San Francisco region since the great earthquake of 1906, but it was not unexpected because Loma Prieta lies on the San Andreas Fault.

b) The San Andreas Fault

This is a major fault - 800 miles long - which runs from north to south through Oregon and California. It is part of the system of faults which separate two of the great tectonic plates that make the outer shell of the Earth's crust. To the west lies the Pacific Plate; to the east the North American Plate. The fault is at least 11 miles deep and the plates are moving past one another at the rate of several centimetres a year. Since the formation of the fault 30 million years ago the Pacific Plate has moved 300 km north-west, relative to the American Plate. This movement is continuing today but it is not uniform. Movement can be very slow over a period of years but there will then be a sudden movement to compensate. When this happens a major earthquake occurs.

The LOMA PRIETA EARTHQUAKE is the first major earthquake in the area since 1906.

c) Damage assessment

The effects of the earthquake are summarised in the table below.

OTHER DAMAGE:

The extent of earthquake damage at any location depends primarily on:

• The size of the earthquake
• The distance of the location from the epicentre of the earthquake. Damage usually decreases rapidly with distance
• Geology - and in the case of Loma Prieta this proved to be of particular importance.The earthquake itself occurred in a remote, sparsely populated mountainous area and there was comparatively little damage. In fact, most damage occurred 90 km to the north in San Francisco and Oakland and here it was largely confined to the margins of the San Francisco Bay where roads, houses and business premises had been built on recently deposited sediments or on recent infill (both of which were unconsolidated). When the shockwave hit, these sediments behaved like liquids and this caused considerable structural damage to elevated roads, large buildings and the airport runways

Damage in the main earthquake area was also limited because there was little displacement on the surface.

d) Cause of the Loma Prieta earthquake

The immediate cause was a sudden movement (or rupture) of the San Andreas Fault in the Santa Cruz mountains:

• The rupture took place 16.8 km below the surface
• It affected 40 km of the fault system
• The rupture lasted 7 - 10 seconds and stopped 6 km below the surface
• The land to the west of the fault moved northwards relative to the land to the east
• Horizontal movement was 1.8 metres
• Vertical movement was 1.2 metres

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