Tsunami signals from the 2006 and 2007 Kuril earthquakes detected at a seafloor geomagnetic observatory

Abstract

A seafloor geomagnetic observatory in the northwest Pacific detected clear electromagnetic (EM) variations associated with tsunami passage from two earthquakes that occurred along the Kuril Trench. Previous seismological analyses indicated that the M8.3 earthquake on 15 November 2006 was an underthrust type on the landward slope of the trench, while the M8.1 earthquake on 13 January 2007 was a normal fault type on the seaward side. The EM measurements enabled precise monitoring of the tsunami propagation direction as well as particle motion of the seawater. The estimated horizontal water velocity differs significantly for the 2006 and 2007 tsunamis, in terms of initial motion and dispersive characters, being consistent with the hydrodynamic simulation results of the tsunamis. Namely, the tsunami‐induced horizontal geomagnetic components showed opposite signs for the rise and retreat waves as expected from the “electric current wall hypothesis.” The dispersion effect is more remarkable in the 2007 event with a smaller source region of its tsunamigenic earthquake. The 2007 tsunamis, therefore, tend to violate the long‐wave approximation. The Boussinesq approximation was required to reproduce the dispersive character of the 2007 event in our numerical simulation. In terms of tsunami forecast, an important advantage of EM sensors over conventional tsunami sensors, such as seafloor pressure gauges, is their capability of vector measurements: in addition to their ability to monitor particle motions, the first peak of the downward magnetic component always precedes the tsunami peak, suggesting a significant improvement in global tsunami warning systems if vector EM sensors are integrated into the existing systems.

Simulation of a Dispersive Tsunami due to the 2016 El Salvador–Nicaragua Outer-Rise Earthquake (Mw 6.9)

“…Because the outer-rise event occurred at shallow depth with a high angle fault, the tsunami contains much higher frequency component than those tsunamis caused by typical underthrust earthquakes. Toh et al (2011) shows that the far-field observed tsunami generated by the 2007 Kurile outerrise earthquake had a dispersive character, but the that generated by the 2006 Kurile underthrust earthquake did not.…”

TIARES Project—Tomographic investigation by seafloor array experiment for the Society hotspot

“…It is theoretically well understood that the movement of electricallyconductive ocean water in an ambient geomagnetic field induces secondary EM fields in the oceans (Sanford, 1971). However, until recent advances in high-precision measurement of the EM fields enabled the seafloor and island measurement of the tsunami signals (Manoj et al, 2011;Toh et al, 2011), this type of research was restricted by the lowsignal tsunami EM levels. The EM measurement enables the evaluation of tsunami propagation direction and particle motion of the seawater, which could not be obtained from the pressure measurement.…”

NEMO-SN1 Abyssal Cabled Observatory in the Western Ionian Sea

“…The tsunami detector prototype installed on SN1, called Tsunameter, has been previously tested in the Gulf of Cadiz for about two years in the framework of the European Commission (EC) project "NEAR shore sourcES of Tsunamis: toward an early warning system" (NEAREST, http://nearest.bo.ismar.cnr.it). Moreover, the observatory has been equipped to study acoustic and magnetic tsunami precursors [9], [10]. After a long series of tests in laboratory to check all the functionalities, the NEMO-SN1 observatory (i.e., SN1 abyssal station at TSN and O DE abyssal acoustic station at TSS) was successfully deployed on June 9, 2012, using the cable ship Certamen owned by Elettra Tlc (Catania, Italy).…”