Shaking is Almost Always a Surprise: The Earthquakes That Produce Significant Ground Motion
Abstract
Ground-motion prediction equations (GMPEs) give us intuition about how much shaking should be expected from an earthquake of a given magnitude at a certain distance. However, they do not give us intuition about which earthquakes produce significant shaking at a given location. To answer that question, we want to know not the probability of ground motion given some earthquake source (which is what the GMPE calculates), but instead the relative likelihood that a particular earthquake source could have produced that level of shaking. That relationship, in turn, is dominated by the Gutenberg-Richter relationship: small earthquakes are much more common than large earthquakes. Thus, any level of shaking is much more likely to be caused by an outlier small earthquake producing stronger shaking than expected rather than by a rare large magnitude earthquake producing typical levels of shaking. We document this behavior both in synthetic ground motion catalogs and in databases of observed seismograms. While counter-intuitive, this behavior makes sense because log earthquake frequency declines more strongly with magnitude (slope of approximately -1) than log PGA (or similar ground motion amplitude measures) grows with magnitude (slope of 0.3). This suggests that shaking hazard will generally come from earthquakes smaller than might be expected simply by reading off the GMPE. However, to the extent that earthquake damage is not similarly skewed, i.e. to the extent that small earthquakes don't cause damage despite generating large ground motions, it suggests that damage must be correlated with ground motion features other than amplitude, such as shaking duration.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMNH32B..06M
- Keywords:
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- 1240 Satellite geodesy: results;
- GEODESY AND GRAVITYDE: 4331 Disaster relief;
- NATURAL HAZARDSDE: 4335 Disaster management;
- NATURAL HAZARDSDE: 4346 Emergency response and evacuations;
- NATURAL HAZARDS