Improved 1D velocity model and deep long-period earthquakes in Kanlaon Volcano, Philippines: Implications for its magmatic system

Abstract We determined an improved 1D P-wave velocity model for Kanlaon Volcano, one of the most active volcanoes in the Philippines, for use in precise earthquake relocation. The inversion for velocity model was conducted using the program VELEST, which solved the coupled hypocenter–velocity problem from dataset of 251 local earthquakes recorded by the Kanlaon Volcano Seismic Network from 2014 to 2017. Using the improved velocity model, we relocated the volcano-tectonic (VT) earthquakes in Kanlaon with hypoDD, resulting in better-constrained hypocenter locations based on the decrease in RMS and corresponding error assessment tests. Subsequently, we relocated swarms of long-period volcanic earthquakes that occurred from September 1, 2017 to August 31, 2018 and prompted the Philippine Institute of Volcanology and Seismology to raise the volcano's alert level. These long-period earthquakes had emergent arrivals, hence we applied a cross-correlation procedure using REDPy to obtain an improved arrival time picks. These improved hypocenters revealed the existence of deep long-period (DLP) earthquakes that occurred at depths of 23 to 36 km beneath the volcano. These are the first DLP events documented in Kanlaon since the establishment of the Kanlaon Volcano Observatory in 1961. They were found to belong into families based on similarity of waveforms suggesting they were produced by repetitive, non-destructive sources. The occurrences of DLPs in Kanlaon reflected the pressurization of its deep-seated magmatic system. Their identification is critical in assessing the current state of Kanlaon Volcano especially since its last magmatic eruption was in 1902.