Loma Prieta Earthquake – California – October 18, 1989

Damage was extensive and large as twenty-five miles of the San Andreas Fault slipped.

On October 18, 1989, an earthquake of magnitude 6.9 struck a branch fault of the San Andreas near the city of Santa Cruz in the southern Santa Cruz Mountains. It came to be known as the Loma Prieta earthquake after the name of a 3,000-foot mountain in the Santa Cruz area that was close to the epicenter. A series of thousands of destructive landslides was triggered all along a stretch of coast and in the central valley from north of San Francisco to points forty-five miles south of Santa Cruz. This earthquake took sixty-three lives, cost $10 billion, and damaged 27,000 structures. The source of the quake was a slip along a twenty-five-mile segment of the San Andreas Fault where it crosses the Santa Cruz Mountains sixty miles south of San Francisco. It was the most powerful earthquake to strike this part of California since the 1906 San Francisco quake.

Extensive studies were made in the wake of this quake to ensure adequate preparation for any future similar event. For the most part, places that were damaged by landslides were checked out and where necessary, as in some coastal locations, homes were removed or other remedial action taken. One particularly weak area was the Marina District of San Francisco. Despite the experience of 1906 when this part suffered severe damage, city officials went on to fill the area with sand and rubble from the quake, in order to use it as a site for the 1912 Panama–Pacific International Exhibition. In later years it became a very popular section of the city. The lurking danger, which was ignored, was liquefaction in the event of an earthquake. When Loma Prieta struck, the Marina area immediately sunk five inches. This was followed by widespread liquefaction as water saturated sand turned into a liquid. Buildings shifted off their foundations and many collapsed.

Thousands of landslides generated by the quake were found all over an area half the size of the one hit in 1906. Loma Prieta thus provided the first opportunity to study the effects of a major earthquake on landslides. Previous landslide-producing earthquakes, apart from the 1906 quake, were either too small or too poorly documented for this purpose. Techniques for identifying slopes susceptible to failure that had been developed over the previous ten years were proved correct in the studies that followed the Loma Prieta earthquake. At the same time there was recognition of new types of landslide hazards not fully appreciated in the past. The most severe property damage occurred in San Francisco and Oakland. The earthquake was felt over most of central California and in part of western Nevada.

It was fortunate that the epicenter was in a sparsely populated area because the amount of shaking was very strong. In homes furniture was moved several feet and in one case a built-in oven was ejected from its cabinet. The city of Watsonville was badly damaged as were older buildings in downtown Santa Cruz. Around the margins of San Francisco Bay the shaking that was experienced in 1906 was much stronger than in 1989. Most publicity was focused on the collapse of a section of the freeway that connects downtown Oakland to the Bay Bridge. The freeway’s bridge was built in 1936 and was intended to withstand moderate earthquakes. Such designs were common at that time before people were acquainted with the damaging influence of earthquake motions.