Tsunami records due to the 2010 Chile Earthquake observed by GPS buoys established along the Pacific coast of Japan

For over 12 years we have developed the GPS buoy for detecting tsunami given that the early detection of tsunamis contributes to mitigating tsunami disasters (e.g., Kato et al., 2000, 2008). The system employs a real-time kinematic (RTK) GPS in which a GPS antenna is situated at the top of a buoy floating on the offshore ocean surface, while another antenna is situated at a ground base station near the coast (Fig. 1). A sampling frequency of 1 Hz is used to monitor the sea surface changes. The data obtained at the buoy is transmitted to the ground base station using radio and a baseline analysis is made in real-time by a PC using both the buoy data and the data taken at the base station. Assuming that the coordinate of the base station is known, the precise position of the buoy can be determined to better than a few centimeters of accuracy.

The determined positions of the buoy applies not only to the case of a tsunami but also for all kinds of sea surface changes including wind waves, tides, etc. In order to extract the relevant components of a tsunami, the record is applied with a low pass filter using a simple moving average of 120 seconds. Tides are also removed by subtracting a tidal component calculated using harmonic analysis of 90 days of data at the position of the buoy. Since the accuracy of such a harmonic analysis would be a few centimeters if no anomalous sea surface deviation occurs such as that due to the passage of low pressure or variations of current, tsunami data can be extracted with a certain accuracy if the tsunami perturbation exceeds a few centimeters (e.g., Shimizu et al., 2006). Both long period and shorter period waves are communicated via the dedicated webpage (http://www.tsunamigps.com/gpsreal.php).

The operational system was first deployed off Ofunato, northeastern Japan, in 2001. This system successfully recorded two tsunamis due to the June 23 (UTC) 2001 Peru earthquake (M_w 8.4) and the September 25 (UTC) 2003 Tokachi-Oki earthquake (Mw 8.3). Both records showed clear tsunami wave traces with about 10 cm of maximum height. After three years of operation, the system was abolished and a new GPS buoy was established about 10 km south off Cape Muroto, southewestern Shikoku, Japan, on April 11 2004. The buoy recorded the tsunami due to the September 5 (UTC) 2004 Off Kii Peninsula earthquake (M_w 7.4) (Kato et al., 2005). This system sank as a result of an accident in March 2006 and a new GPS buoy was established in April 2008. The current GPS buoy is now operational at about 10 km west of Cape Muroto, southwestern Japan. The location of the buoy is shown in the inset of Fig. 2 and the design of the buoy is shown in Fig. 3. The distance between the buoy and the land base station is about 13 km.

The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) has established a GPS buoy system for monitoring sea waves, with eleven GPS buoys along the Pacific coast of Japan, since the year 2008 as a part of the Nationwide Ocean Wave information network for Ports and HArbourS (NOWPHAS) system (http://www.mlit.go.jp/kowan/nowphas/). All of these GPS buoys are located within 20 km from the coast. More than 14 GPS buoys have been established in this project, of which twelve buoys are in place and in operation along the pacific coast of the Japanese Islands as of February 2011 (Fig. 2).

The tsunami record was compared with a numerically-simulated tsunami after removing shorter and longer periods of waves. The numerical modeling of mid-ocean tsunami propagation was carried out using the finite difference method of the linear shallow-water wave theory with a Coriolis force in a spherical co-ordinate system (Nagano et al., 1991). For tsunami modeling, we used the digital bathymetry data (GEBCO; Monahan, 2008) to resample and created a 5 arc-min grid. The modeled tsunami waveform at the GPS buoy is shown in Fig. 5 with the plot of the observed record after tide and wind-wave components have been removed.

Note that the modeled tsunami waveform has been manually shifted by 26 minutes to fit to the observed one, because a direct comparison suggested that the observed tsunami arrived about 26 minutes later than the arrival time predicted by the numerical modeling. We find that the longer period components of the tsunami of about 1 hour fit very well whereas the components of a shorter period of 10–30 minutes show significant phase shifts. The difference in arrival times of about 30 minutes is under investigation by considering various factors such as the sea bottom topography features on the path of the tsunami propagation, the spatial resolution of gridding, modeling errors, etc. The reason that the longer wave fits well with 30 minutes of phase shift while a shorter wave does not may be due to the effect of dispersion of the tsunami wave resulting in the longer wave propagating more rapidly than a shorter wave. Slip heterogeneity of the source might also be responsible for such dispersion, though the examination of these effects are left for future studies.

The GPS buoys of the NOWPHAS system at eleven sites also recorded tsunamis with 20 to 30 centimeters of maximum height (see also: http://www.mlit.go.jp/kowan/nowphas/). The tsunami record is processed similarly as the record taken at the Muroto GPS buoy (Kawai et al., 2010). Comparisons of the NOWPHAS records with the numerical model are conducted at eight NOWPHAS buoy stations: East Off Aomori, Off Central Iwate, Off South Iwate, Off North Miyagi, Off Central Miyagi, Off Owase, SW Off Wakayama, and West Off Kochi. The results are shown in Fig. 6. These results indicate a similar tendency as the record at the Muroto GPS buoy shown in Fig. 5.

The GPS buoy that has been operated nearby Cape Muroto, southwest Japan, successfully detected a tsunami due to the February 2010 Chile Earthquake. The maximum amplitude of the tsunami was about 20 cm from crest to zero in height. The tsunami had also been recorded at all the other GPS buoys established as part of the NOWPHAS system along the Japanese coast.

In order to use a GPS buoy for tsunami disaster mitigation, however, there are still some problems to be solved. One such problem is that the numerically simulated record predicted tsunami arrived about 30 minutes earlier than the observed tsunami. The causes of such a difference require clarifying and numerical simulation modeling should be improved. Also, the deployment of buoys further from the coast is indispensable for earlier detection and the evacuation of people. When such problems are solved, an array of GPS buoys for monitoring tsunami will provide us a powerful tool for mitigating disasters due to tsunami.

Finally, a very large earthquake of M_w 9.0 occurred offshore of the northeastern part—known as Tohoku—of Japan and a huge tsunami of more than 30 meters of runup heights devastated the whole area along the Pacific coast of Tohoku. The earthquake and tsunami caused more than 20,000 people to be accounted as dead or missing. The GPS buoys that were deployed a few years before the earthquake by the Minstry of Land, Infrastructure and Tourism (MLIT) recorded significant tsunami higher than 6 m at the offshore South Iwate (near Kamaishi City) before its arrival at the coast (Takahashi et al., 2011). The records at the GPS buoys were monitored at the Japan Meteorological Agency and led to an updated tsunami early warning of more than 10 meters at the coast. Detailed investigations of the tsunami and the effectiveness of the GPS buoys are still underway and will be reported elsewhere.