size? How short will the rise time be for much larger earthquakes in which the slip may exceed 10 meters? Is it limited by the geometry of the fault plane or the dynamics of friction at high slip velocities?

  • What physical mechanisms explain the deep-focus earthquakes that occur in the descending lithosphere down to depths of nearly 700 kilometers? How do these mechanisms differ from shallow seismicity?

5.5 WAVE PROPAGATION

Earthquake damage is caused primarily by seismic waves. Seismic shaking is influenced heavily by the details of how seismic waves propagate through complex geological structures. In particular, strong ground motions can be amplified by trapping mechanisms in sedimentary basins and by wave multipathing along sharp geologic boundaries at basin edges, as well as by amplifications due to near-site properties. Although near-site effects such as liquefaction can be strongly nonlinear, most aspects of seismic-wave propagation are linear phenomena described by well-understood physics. Therefore, if the seismic source can be specified precisely and the wave velocities, density, and intrinsic attenuation are sufficiently well known, it is possible to predict strong motions by a forward calculation.

A conspicuous success of earthquake physics has been the development of computational techniques for describing the propagation of seismic waves. These techniques yield approximate solutions to the forward problem of seismic-wave propagation, which is to predict the wavefield as a function of position and time knowing the source and a model describing the Earth’s elastic and anelastic constitutive properties (180). Such calculations can be used to predict the strong ground motions in the vicinity of an anticipated earthquake. Moreover, they provide the theoretical framework for solving the structural inverse problem (to estimate a set of constitutive parameters from recordings of the wavefield and knowledge of the source), as well as the source inverse problem (to estimate a set of source parameters from recordings of the wavefield and knowledge of the structure). The effects of source excitation and wave propagation are coupled in seismograms, which complicates their separation. Recent progress on solving these coupled inverse problems, outlined in the previous chapter, has enhanced the predictive capabilities of wavefield modeling. At present, numerical simulations using good propagation models can reproduce the recorded waveforms of low-frequency motions (less than 0.5 hertz) from events such as the 1994 Northridge earthquake and match the spectral amplitudes at higher frequencies with moderate success (181). However, matching the waveforms at higher fre-



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