The Complex and Long-Duration 2002 April 18 <i>M</i>w 6.7 Near-Trench Earthquake in the Guerrero Seismic Gap, Mexico

SUMMARY The seismic behaviour of the near-trench plate interface of the Guerrero seismic gap and other segments of the Mexican subduction zone is likely to play a critical role in the seismic and tsunami hazard of the region. In this context, a detailed study of the near-trench 2002 April 18 Mw 6.7 earthquake that occurred about 55 km off the coast of Guerrero and generated a small tsunami attains particular importance. From an analysis of the teleseismic P waves and S waves, local recordings and aftershock distribution, we find that the rupture most likely began at a subducted seamount, propagated unilaterally towards NW, parallel to the trench for ∼ 54–58 km and a duration of ∼ 68–70 s. The moment rate function is highly rugged, with two dominant pulses separated by about 50 s. Although relatively small in magnitude, the earthquake has all the characteristics of a tsunami earthquake: the slip occurs very close to the trench, the rupture speed is slow (∼ 1 km s−1), the high-frequency radiation is deficient, and, in common with tsunami earthquakes, the moment-scaled radiated energy is low (ER/M0 = 1.45 × 10−6). We confirm that the duration of the event (∼ 70 s) is extraordinarily long compared to that expected from scaling relations (∼ 12.8 s), consistent with it being the most anomalous of all the events studied in the last 40 yr. Our results support a conditionally stable upper 15 km of the plate interface in the region reported from recent offshore seismic observations.