Comments on 1989 Loma Prieta Ground Motion Movie

Color Scheme in Movie

The movie shows the propagation of seismic waves away from the epicenter, which lies in the Santa Cruz Mountains, about ten miles northeast of the of the city of Santa Cruz. The residual colors indicate the peak shaking intensity at locations up to the time in seconds indicated near the top center of the movie. The current intensity, at the time indicated, is indicated by shading of the colors.

Comparison with 1906 Earthquake

One striking observation for those who experienced the 1989 Loma Prieta earthquake's shaking is the comparison of the extent and intensity of shaking with the 1906 earthquake. The Loma Prieta rupture was about 30 times smaller in energy than the great 1906 earthquake.

The rupture in the Loma Prieta earthquake began at a depth of about 12 miles and appears to have ruptured a 25 mile long portion of the San Andreas fault. Unlike the 1906 earthquake, the rupture in the Loma Prieta earthquake did not reach the surface. As in the 1906 earthquake, the strongest shaking was concentrated along the fault. In 1989 the two areas of most intense shaking were north and south of the epicenter in the Santa Cruz mountains.

Shaking in the Marina District and Oakland

The simulations do not single out either the Marina District or Oakland as having experienced especially strong shaking. These computer simulations resolve features as small as 650 feet in scale. However, this means that they do not include the amplification of shaking or liquefaction that occurs as a result of thin sedimentary deposits, such as bay mud. Additionally, the computer simulations do not include the shaking at frequencies higher than 0.5 Hz. Including these additional small-scale effects explains the extensive damage in these areas. Future versions of the computer ground motion simulation models will also include these effects.

Role 1989 Loma Prieta Earthquake in 1906 Simulations

The 1989 magnitude 6.9 Loma Prieta earthquake was used to validate the computer simulations, because a large number of instrumental observations are available from that event. If the models had not been able to reproduce the shaking in the Loma Prieta event, we would not have confidence in the reconstructions of he 1906 earthquake. In fact, the simulations produce motions that agree quite well with the observations from the Loma Prieta earthquake.

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