Development of a new real-time GNSS data analysis system in GEONET for rapid Mw estimates in Japan

The 2011 off the Pacific Coast of Tohoku Earthquake (Mw 9.0) occurred on March 11, 2011. The earthquake and following tsunami caused serious damages to the broad coastal area of east Japan. Japan Meteorological Agency (JMA) operates the Tsunami Warning system, which is designed to forecast the tsunami height and its arrival time around 3 minutes after a large event. However, the first estimated magnitude of Mj, which was used for Tsunami Warning issuance, was far below the real one at the Tohoku event because of a saturation problem. In principle, as well as most other magnitude scales, Mj is saturated at certain values around 8.0. On the other hand, Mw represents the earthquake energy itself and it can be directly calculated by permanent displacements derived from geodetic measurements without the saturation problem. GNSS Earth Observation Network System (GEONET) is one of the densest real-time GNSS networks in the world operated by Geospatial Information Authority of Japan (GSI). The GEONET data and recent rapid advancement of GNSS analysis techniques motivate us to develop a new system for tackling the tsunami disasters. In order to provide the more reliable magnitude for Tsunami Warning, GSI and Tohoku University have jointly developed a new real-time analysis system in GEONET for quasi real-time Mw estimation. Its targets are large earthquakes, especially ones of Mw > 8.0, which would be saturated by the Tsunami Warning system. The real-time analysis system in GEONET mainly consists of three parts: (1) real-time GNSS positioning, (2) automated extraction of displacement fields due to the large earthquake, and (3) automated estimation of Mw by an approximated single rectangular fault. The positions of each station are calculated by using RTKLIB 2.4.1 (Takasu, 2011) with the baseline mode and the predicted part of the IGS Ultra Rapid precise orbit. For the event detection, we adopt the 'RAPiD' algorithm (Ohta et al., 2012) or Earthquake Early Warning issued by JMA. This whole process is done within 10 seconds at most and the estimated results are immediately announced to GSI staffs by e-mail. We examined the system by using the recorded 1Hz GEONET data of past several large earthquakes in Japan. The results showed that it could estimate reliable Mw within a few minutes like Mw of 8.9 for the 2011 Tohoku earthquake (Mw 9.0) after 172 seconds, Mw of 7.6 for the 2011 off Ibaraki earthquake (Mw 7.7) after 107 seconds and Mw of 8.0 for the 2003 Tokachi-oki earthquake (Mw 8.0) after 93 seconds respectively. GSI launched its prototype in April of 2012 with 146 GEONET stations for covering mainly Tohoku district and now is planning to extend it to the whole area of Japan. We assure that this system would become one of the powerful tools for supporting Tsunami Warinng in order to prevent or mitigate the severe damages of future disastrous tsunamis.