It has previously been reported that regional seismograms of earthquakes that occur near the Middle America trench are relatively deficient at high frequencies. Based on this observation, an algorithm has been proposed for detecting potentially tsunamigenic earthquakes and issuing tsunami alerts. It is reasonable to expect relatively low peak accelerations during these earthquakes. In this note, we present evidence that this is indeed the case. This explains why the seismic alert system for Mexico City, with sensors located along the coast, does not trigger during some earthquakes. Low peak accelerations from near-trench earthquakes also have important implications in seismic hazard estimation.

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