Hydro-acoustic monitoring on the Kamchatka shelf: A possibility of early location of oceanic earthquake and local tsunami warning

Local tsunami warning remains a difficult task. A possible way to solve this problem is to process acoustic signals, which appear in the water a, he critical stage of a submarine earthquake preparation. An earthquake (EQ) preparation zone or "dilatant" zone is characterized by increasing micro-crack density. This process generates acoustic signals (AS) with frequencies in the range 10-100 Hz. The AS are decaying fast in the solid rocks but in the ocean these signals propagate far due to weak attenuation of the signal in incompressible water. We analyzed the observation data obtained by high-quality hydro-acoustic receiving array of hydrophones at the Kamchatka Pacific shelf and the local Kamchatka earthquake catalog data. Two types of acoustic signals were detected: the micro-earthquakes (MEQ) and the seismic rumble, which appears just before the main shock. The duration of MEQs was about 34 seconds, and their frequency was in the range 40-75 Hz. It was shown that the MEQ sources had shallow focuses, while the earthquake focus may locate significantly deeper. It was revealed that acoustic signals of MEQ occurred before oceanic EQ with M > 4 with advancing about 10 - 100 minutes. The results of the study show that the monitoring of hydro-acoustic signals in the ocean may be used for the early location of oceanic EQ preparation zone and for improvement of the local tsunami warning system.