ИСТИНА

The deployment of seismometers and tsunami-meters (pressure gauges) at the ocean-bottom was firstly proposed and implemented in the 20th century. At present, deep-water measurements are actively applied for monitoring earthquakes and tsunamis. The main purpose of this talk is to share our experience of processing ocean-bottom observatory signals registered during the 2003 Tokachi-oki earthquake and the 2011 Tohoku-oki earthquake. The first data-set was recorded by the seafloor observatory Kushiro-Tokachi/JAMSTEC and the second one – by 10 stations of the DONET-1 that were in operation by 11.03.2011. Both data-sets are really unique. The 2003 data-set includes the first records ever obtained at the ocean-bottom in a tsunami source. The 2011 data-set includes the first records ever obtained by a dense network of 10 seafloor stations. First we discuss the nature of signals registered by ocean-bottom sensors and introduce the necessary physical concepts. Then, by example of the 2003 data-set we describe what characteristics of ocean-bottom motion can be calculated (estimated) from pressure variations data recorded by a single station. Making use of this data-set we also demonstrate how the coincidence of the spectra of pressure and acceleration within the frequency band “forced oscillations” was revealed for the first time. After that we point out some important advantages of the DONET observatories as compared with Kushiro-Tokachi station. Turning to the 2011 data-set, we show how a dense network of tsunami-meters (pressure gauges) can be used for the retrieval the horizontal motion of a water layer during the passage of a tsunami wave. Here we also discuss simple method of short-term tsunami forecasting based on the data of a dense network of PGs and suggest principles of optimal positioning of deep-water stations for tsunami forecasting. Ultimately we introduce phenomenon of “tsunami-forerunners” - free gravity waves generated in the ocean by low-frequency surface seismic waves. "Tsunami-foreunners" were firstly revealed by analysis of the 2011 data-set. Physical mechanism of “Tsunami-forerunners” formation is briefly discussed. This work was supported by the Russian Foundation for Basic Research, projects 10-05-92102, 13-05-92100, 16-55-50018, 16-05-00053.