Strong ground motion

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Strong ground motion is usually defined as the strong shaking that occurs close to the causative fault (less than about 50 km). The strength of the shaking usually overwhelms a seismometer, and must be recorded with an accelerograph (or strong ground motion accelerometer).

Now that seismic instruments (and accelerometers in particular) are becoming more and more common, it becomes necessary to correlate expected damage with instrument readings. The old Modified Mercalli Intensity Scale is a relic of the pre-instrument days, but it is still useful in the sense that each intensity level is an observable difference in seismic damage.

After many years of trying every possible manipulation of accelerometer time histories, it turns out that the best correlation with damage is the extremely simple Peak Ground Velocity (PGV). [1] [2] PGV is merely the peak of the first integration of the acceleration record. There are now accepted formulae linking PGV with MM Intensity. Note that the effect of soft soils is built into the process, since it is expected that these foundation conditions will amplify the PGV significantly.

The increasingly-popular ShakeMap [3] [4] systems tie all of this together into a useful product. These are systems that use seismometers and accelerometers to give a near-instantaneous map of expected MM Intensities after a significant earthquake. As well, people can send in their observations of earthquake effects to help fill in the maps, which are useful to disaster-relief teams and other agencies.

As well, strong ground motion deals with the variations of fault rupture, both in total displacement, energy released, rupture velocity, etc. An interesting paper describes how the Alaska Pipeline dealt with the toughest engineering challenge of all: straddling an active fault. [5]