Evaluation of the Seismic Alert System of Mexico (SASMEX) during the June 23, 2020, Oaxaca Earthquake (Mw 7.4)

The Mexican Seismic Alerting System (SASMEX) successfully alerted for the June 23, 2020, La Crucecita earthquake (Mw 7.4) in southern Mexico. The time between the alert broadcast and the initiation of strong shaking ranged from 30 s in the city of Oaxaca to 134 s in Mexico City. SASMEX messages are presented from the time the earthquake was detected to the generation of maps of peak ground acceleration in Mexico City. A first pre-alert message was issued based on the tS-tP algorithm by a station located 3 km away from the epicenter. Confirmation of the magnitude (Mw > 6) came from an adjacent station located 43 km from the epicenter, 16 s after the origin time. The decision to broadcast the alert took longer than in previous large earthquakes, due to the relatively large interstation spacing in this region. An algorithm called tP + 3, that estimates magnitude based on the first three seconds after the onset of the P wave, would have issued the alert 4 s earlier. However, the triggering procedures in place require that the same algorithm is used to issue the alert. SASMEX warned all cities where it operates with ample time. Peak ground acceleration in key sites in Mexico City was estimated 3.5 min after the origin time of the earthquake and one minute after strong shaking began, allowing authorities to launch civil protection procedures and building inspections. Also, the earthquake of June 23, 2020 provided important lessons to further improve the performance of SASMEX.