Optimization of PRESTo Early Warning System for South-Eastern Korea

In September 2016 and again in November 2017, Korea witnessed some of the largest magnitude earthquakes occurred in-land in its recent history. They were located in the South-Eastern part of the peninsula, near the cities of Gyeongju and Pohang, with magnitude ML 5.8 and 5.4 respectively. The regional PRESTo Early Warning System has been in the testing phase at KIGAM (Korea Institute of Geoscience and Mineral Resources) since late 2013, using the nation-wide KIGAM and KMA (Korea Meteorological Agency) real-time accelerometric streams, after a calibration based on moderate-to-low seismicity. In this work the PRESTo algorithm parameters and regression laws were optimized and calibrated for the region of South Korea around the Gyeongju earthquake, specifically for a sub-network with a side of about 200 km. A waveform database spanning ten years was analyzed (99 earthquakes during 2007-2017, with M 2 ÷ 5.8, 15681 waveforms), to derive an optimized configuration for this network geometry and seismicity, including GMPEs and empirical regression laws for magnitude estimation from peak displacement, measured in short P-waves and early S-waves time-windows. This configuration enables a fast characterization of the local earthquake sources and effects, i.e. during the playback of onshore earthquakes with magnitude 3.0 and above the EW system produces, in 90% of the cases: a first alert within 6 seconds from the first pick; stable magnitude estimations (variations within 0.5 magnitude points); a final estimate performance against the reference bulletin of ∆M ≤ 0.5, ∆Repi ≤ 10 km.