We explore the versatility of three parameters computed from recordings at station CU, located in Mexico City, ∼300–600  km from the trench, for quick detection (≤7  min after the origin time) of near‐trench earthquakes for potential use in tsunami warning. The parameters are (1) ER, the ratio of total to high‐frequency energy, (2) Sa*(6), the pseudoacceleration response spectrum with 5% damping at 6 s normalized by peak ground acceleration (PGA), and (3) RESN, the PGA residual with respect to a newly derived ground‐motion prediction equation at CU. Because near‐trench earthquakes are relatively deficient in high‐frequency radiation, we expect ER and Sa*(6) to be relatively large and RESN to be negative for such events. Tests on CU recordings of 65 interface earthquakes occurring along the Mexican subduction zone (4.8≤Mw≤8.0; 270≤R≤615  km) show that if ER≥100, Sa*(6)≥0.70, and RESN≤0, then the earthquake is near the trench. Such an event has greater tsunami potential and produces low acceleration. Few misidentifications and missed events are probably a consequence of poor location, although unusual depth and source characteristics may also be responsible in some cases.

Online Material: Data used for the construction of ground‐motion prediction equation (GMPE) at station CU, Mexico City.

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