Tipsheet for June issue of BSSA

June 04, 2008

June 5, 2008--Numerical earthquake models are helping seismologists understand the physical processes taking place in earthquakes, and thereby predict what will happen during future large events. These simulations are providing a means to mitigate earthquake damage and loss of life through better science-based decision-making in the public policy and structural design arenas. Several papers in the June issue of the Bulletin of the Seismological Society of America highlight these developments.

"Surface Rupturing and Buried Dynamic-Rupture Models Calibrated with Statistical Observations of Past Earthquakes" by L. A. Dalguer, H. Miyake, S. M. Day and K. Irikura.

The intensity of ground shaking during an earthquake has been found to change systematically, depending upon whether the earthquake fault slip extends to the Earth's surface or remains buried. The computer simulations in this paper, by L. A. Dalguer, H. Miyake, S. M. Day, and K. Irikura, aim for a better understanding of the physical processes responsible for the comparatively reduced shaking intensity in earthquakes that slip at the surface. They show that surface-rupturing earthquakes may have more areas of their faults that absorb seismic energy than buried earthquakes do, leaving less energy available for ground shaking.

"Scale-Model and Numerical Simulations of Near-Fault Seismic Directivity" by Steven M. Day, Sara H. Gonzalez, Rasool Anooshehpoor, and James N. Brune.

"TeraShake2: Spontaneous Rupture Simulation of M 7.7 Earthquakes on the Southern San Andreas Fault" by K. B. Olsen, S. M. Day, J.B. Minster, Y. Cui, A. Chourasia, D. Okaya, P. Maechling and T. Jordan.

During an earthquake, short, intense pulses of ground movement are focused preferentially in the direction that the earthquake fracture itself is traveling (a bit like how water waves pile up at the bow of a ship). Simulations can help us understand these potentially damaging pulses, and how they vary with earthquake magnitude, position with respect to the fault, and geologic conditions. Research described in two papers combine such computer simulations with scale-model experiments and data from past earthquakes to form a more complete picture of this directional focusing effect.

"Simulated Ground Motion in Santa Clara Valley, California, and Vicinity from M6.7 Scenario Earthquakes" by S. Harmsen, S. Hartzell, and P. Liu.

The Santa Clara Valley, south of San Francisco, has experienced a significant transformation from a sparsely populated agrarian economy to a high technology economy with a population equal to that of the San Francisco Peninsula. Several major fault systems are present: San Andreas, Hayward, Calaveras and Monte Vista/Shannon, among others. S. Harmsen, S. Hartzell, and P. Liu used models to consider 20 scenario earthquakes along the various area fault systems, finding that the details of the ground motion depend strongly upon assumptions about various attributes of the earthquakes themselves, such as the hypocenter location and rupture velocity. However, a systematic effect is that the three-dimensional structure of the local rock can greatly amplify ground motion, especially over sedimentary basins, some of which are relatively far from the fault. This finding contrasts with standard ground-motion prediction equations which claim diminishing ground motion with distance.
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BSSA is published by the Seismological Society of America. The peer-reviewed papers referenced above are available upon request.

Seismological Society of America

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