FURTHER EVIDENCE OF SEISMICITY IN THE SUBDUCTING COCOS PLATE AT A DEPTH OF ~ 100 KM BELOW NEVADO DE TOLUCA VOLCANO IN CENTRAL MEXICO
Seismicity in the subducted Cocos plate below central Mexico, defined by moderate and large earthquakes, ceases well before reaching the active volcanic front. Recently, an intraslab earthquake, which occurred on 19 May 2015 (Mw 3.3) at a depth of 98 km, was reliably located 38 km to the west of Nevado de Toluca Volcano. It provided evidence, for the first time from an earthquake, of the subducted slab at a depth of ~100 km below the volcano. Recordings of the event also revealed high attenuation of shear waves as they propagate through the mantle- wedge asthenosphere. Here we report on another intraslab event which occurred on 18 June 2025 (M w 3.2). The earthquake was located at a depth of 89 km, 34 km southwest of Nevado de Toluca and had normal-faulting focal mechanism. Recordings of the event exhibit large variability at high frequencies (f ≥ 3 Hz) because of different site effects and Q. Under various reasonable assumptions we find very high attenuation of seismic waves (at f ≥ 3 Hz) as they traverse through the Nevado de Toluca Volcano structure to a station in Toluca (TOVM). Q along this path is roughly half of what it is along paths to stations in the Mexican Volcanic Belt and to forearc stations. Detection of these events raises the issue of whether moderate and large intraslab earthquakes can occur below Toluca. Such an event may be highly damaging in the epicentral and the sprawling metropolitan area of the Valley of Mexico, a scenario similar to the earthquake of 19 September 2017 (M w 7.1).