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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Oral Presentation 1747
Possibility of tsunami source detection by using ocean radar
Dr. Tomoyuki Takahashi
Department of Civil and Environmental Engineering Akita University IASPEI
Shinpei Takahashi, Shoichiro Kojima
In the 2004 Indian Ocean tsunami, many people who don't know tsunamis were killed by the tsunami.
It shows that disaster information is important to mitigate damage caused by tsunamis. To provide
appropriate disaster information for the administrations and residents, a tsunami warning system is
necessary. A reliability of the tsunami warning system depends largely on its capacity for detecting
tsunamis. The present tsunami warning systems don't detect tsunami generation directly, but they try
to do indirectly by using observed seismic waves. Unfortunately, the detection method may
underestimate some kinds of tsunamis. Then, it is expected to carry out a new observation of the ocean
surface directly, widely and two-dimensionally. To realize such a tsunami source monitoring, remote
sensing is promising. The aim of our research is to develop the tsunami detection system by using
remote sensing and to add the new system to the present tsunami warning system. In this study, we
focus on an ocean radar as one of the remote sensing. The ocean radar has some advantages, such as
a moderate price and easy maintenance, by comparison with other remote sensing systems, e.g.
satellites. The present ocean radar, however, requires a certain time for analysis and needs large
velocity on the ocean surface, because it uses doppler spectrum to analyze the ocean surface condition.
It is difficult to detect tsunami source and its initial propagation in the deep region by the present
analyzing method. Then, a new detection index is proposed in this study. It is sea surface disturbance in
the tsunami source. We assume that an abrupt ocean bottom crustal deformation increases small waves
in the sea surface and they cause heavy back scattering of electromagnetic wave transmitted by the
radar. Hence the back scattering strength is used directly as the detection parameter to investigate the
tsunami source. The ocean radars are established in Ishigaki Island and Yonaguni Island in Okinawa
Prefecture, Japan. The analyzed radar data were recorded from April 1, 2004 to March 31, 2006. In this
period, 46 earthquakes were occurred in the observed region and the significant three earthquakes
(M6.5, M4.4 and M4.3) are selected for analysis. The back scattering strength is examined between 1
minute before and 2 minutes after their main shocks. Therefore, no particular transition of the back
scattering strength is found for M4.4 and M4.3 earthquakes. On the other hand, for M6.5 earthquake,
the back scattering strength became larger rapidly around its epicenter when the main shock occurred.
The result, however, is not enough to confirm that the back scattering strength can reveal the sea
surface disturbance, because the M6.5 earthquake is not large for tsunamigenic earthquakes. Then,
larger sea surface disturbance due to Typhoon Matsa in 2005 are also analyzed. The typhoon had the
minimum central atmospheric pressure of 950 hectopascals and the maximum wind velocity of over 40
m/s. Accordingly, it is confirmed that the back scattering strength became larger when it passed over
the ocean radar observation region.
Keywords: sea surface disturbance, back scattering strength, tsunami warning system