[J04-2] Geohazard early warning systems II

[J04-2-02] A Fast Algorithm for Earthquake Early Warning Systems Based on the Energy release of P Waves in the Interval tS-tP

Armando Cuellar¹, ^○Gerardo Suarez², Juan Manuel Espinosa-Aranda¹ (1.Centro de Investigacion y Registro Sismico (CIRES), 2.Instituto de Geofisica, UNAM)

The Mexican earthquake alert system (SASMEX), operating since 1991, is in a unique situation because seismic waves that potentially damage Mexico City take approximately one minute to reach the soft soils of the city. Today, the system covers the entire Mexican subduction zone. As a result, cities close to the seismic zones were added to the system and alerted also of impending earthquakes. In those cases, the processing time of the original algorithm is relatively long, as it is based on the energy released in twice the S-P interval. We present another algorithm, which measures the maximum seismic energy released and the cumulative energy growth in the S-P period. The algorithm detects the P wave and measures the energy growth, which is in turn calibrated to magnitude and decides whether a magnitude threshold is exceeded. The algorithm stops when the S wave arrival is detected. Seismic field sensors need to be near the epicenter and at least two nearby instruments should confirm the potential alert. A regression model based on records from 144 subduction earthquakes 4.7>Mw<8.1 calibrated the magnitude estimator M(tS-tP). The magnitude is determined with a family of linear equations, which relate it to the sum of squared accelerations and the maximum acceleration. The algorithm was tested on a subset of 97 earthquakes. The threshold to activate the early warning system is magnitude M(tS-tP) >5.8. Out of 97 events, 93 were correctly classified with a magnitude larger than 5.8, demonstrating the robustness of the method. The south Napa earthquake (Mw6) is an excellent example where strong motion data was recorded nearby. The application of the tS-tP algorithm to the 24 August 2014, Napa earthquake shows that in this hypothetical scenario, the tS-tP algorithm would give a warning time of 10 seconds to the city of Berkeley. In comparison, the ShakeAlert algorithm provided a warning time of only 5 s.