The 19 September 2017 (Mw7.1) Intermediate‐Depth Mexican Earthquake: A Slow and Energetically Inefficient Deadly Shock.

We investigate dynamic source parameters of the Mw7.1 Puebla‐Morelos intermediate‐depth earthquake (h = 57 km) of 19 September 2017, which devastated Mexico City. Our simple, elliptical source model, coupled with a new Particle Swarm Optimization algorithm, revealed rupture propagation within the subducted Cocos plate, featuring a high stress drop (Δτ = 14.9±5.6 MPa) and a remarkably low radiation efficiency (ηr = 0.16 ± 0.09). Fracture energy was large (G = (1.04 ± 0.3) × 1016 J), producing a slow dissipative rupture (Vr/Vs = 0.34 ± 0.04) with scaling‐consistent radiated energy (Er = (1.8 ± 0.9)·1015 J) and energy‐moment ratio (Er/M0 = 3.2 × 10−5). About 84% of the available potential energy for the dynamic rupture was dissipated in the focal region, likely producing friction‐induced melts in the fault core of 0.2–1.2 cm width due to heat production (700–1200 °C temperature rise). Such source features seem to be a universal signature of intermediate‐depth earthquakes. Plain Language Summary: Devastation in central Mexico produced by the intermediate‐depth (magnitude 7.1) Puebla‐Morelos earthquake of 19 September 2017 has raised important questions about the seismic hazard in Mexico City, where more than 20 million people live. Probabilistic hazard in the city from intermediate‐depth and shallower subduction events (i.e., as the 1985 quake that killed 15,000 people in the capital) is about the same despite that the first type of earthquakes is much less frequent. Understanding the source process of intermediate‐depth earthquakes has been a major research topic since decades ago because they occur at depths where mechanical considerations for shallow earthquakes are no longer valid. Recent research has led to unexpected conclusions for this kind of events that were thought to be the consequence of fast and energetically efficient rupture processes. Through a novel technique, we investigate the Puebla‐Morelos earthquake to better understand the generating physics of the strong ground motions. We show that the event was extraordinarily slow and inefficient and that this does not contradict the large ground accelerations. More than 84% of the earthquake energy was dissipated in the source region likely producing rock melting in the geological fault hosting the rupture process. Key Points: We performed the dynamic source inversion of the 19 September (Mw7.1) 2017 intermediate-depth Mexican earthquake with a Particle Swarm Optimization algorithmThe event was highly dissipative with a slow rupture propagation and scaling-consistent radiated energyThe temperature rise associated with the specific fracture energy likely produced friction‐induced melts in the fault core [ABSTRACT FROM AUTHOR]