A Dedicated Seismic Early Warning Network: The Mexican Seismic Alert System (SASMEX)

The Mexican Seismic Alert System (SASMEX) initiated operations in 1993. It was the first to broadcast seismic early warnings to the general population. Today, SASMEX is composed of 97 monitoring stations that straddle practically the whole subduction zone of Mexico. The system also includes strong motion instruments that monitor the seismicity within the subducted Cocos plate. These earthquakes are located inland, close to the major population centers of Mexico. The monitoring stations are linked to control and distribution centers that receive, decode, and broadcast the alerts by a redundant telecommunications network. The dissemination of the early warning alerts is done through low-cost radio receivers, subscribing television and radio stations, and, in Mexico City, by the system of municipal loudspeakers installed in the streets throughout the city. From 1993 to 2017, the network has recorded 6896 earthquakes and has issued 158 seismic early warnings. The more recent examples of successful seismic alerts are the two earthquakes that occurred in Mexico in September 2017. In the case of the great Tehuantepec earthquake of 7 September 2017 (Mw 8.2), SASMEX gave almost 2 min of warning prior to the arrival of the strong-motion seismic waves in Mexico City. The second case was the Morelos earthquake of 19 September 2017 (Mw 7.1). The short epicentral distance to Mexico City of ∼120 km allowed only few seconds of warning prior to the arrival of the S waves.