Tsunami source area of the 2011 off the Pacific coast of Tohoku Earthquake determined from tsunami arrival times at offshore observation stations
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2011
Received: 7 April 2011
Accepted: 22 June 2011
Published: 27 September 2011
The source area of the tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake estimated from tsunami arrival times recorded at offshore wave gauges, GPS buoys, cabled ocean-bottom pressure gauges, and tsunami buoys is about 500-km long with a maximum width of approximately 200 km. The 2011 tsunami source area includes several segments of previous recurring large earthquakes. However, the northern and southern parts of the large Japan Trench segment were not included in the source area. The southern off-Boso-Peninsula part of the Japan Trench segment has the potential to generate a large tsunami earthquake in near future.
The 2011 off the Pacific coast of Tohoku Earthquake occurred at 05:46:23 on 11 March, 2011 (UTC). The epicenter of the earthquake (38.104°N, 142.861°E) was southeast of Sendai City and its moment magnitude (Mw) was 9.0, according to the Japan Meteorological Agency (JMA), which means that it was probably the largest earthquake in the recorded history of Japan. The tsunami that accompanied the earthquake was detected at various offshore observation stations, including coastal wave gauges (Nagai, 1998; Nagai et al., 2005), real-time kinematic global positioning system (RTK-GPS) buoys (Kato et al., 2005), cabled deep ocean-bottom pressure gauges (OBPG) (e.g. Fujisawa et al., 1986; Hirata et al., 2002), and Deep-ocean Assessment and Reporting of Tsunami (DART) buoys (González et al., 2005). Coastal wave gauges and RTK-GPS buoys are part of Japan’s Nationwide Ocean Wave Information Network for Ports and Harbours (NOWPHAS).
Several series of large recurring earthquakes with source areas on the plate boundary of the subduction zone under the Japan Trench have been identified by the Earthquake Research Committee (ERC, 2005). Determining which earthquake segments have ruptured during individual earthquakes is of crucial importance in the evaluation of the potential for subduction-related seismic activity along the Japan Trench in the near future.
We determined the areal extent of the source area of the tsunami caused by the 2011 off the Pacific coast of Tohoku Earthquake by using tsunami arrival times recorded at offshore observation stations. We then determined which of the segments of recurring earthquakes in the area were within the tsunami source area.
Tsunami waveform data recorded during the tsunami of the 2011 off the Pacific coast of Tohoku Earthquake.
Instrument type, number, location and bathymetry (m)
Initial wave arrival time1
Arrival time for each phase2 Trough Local crest Primary crest
Coastal wave gauges a
Sendai New Port
RTK-GPS buoys a
Off Central Iwate
Off Southern Iwate
Off Northern Miyagi
Off Central Miyagi
Off Tokachi 1 b
Off Tokachi 2 b
Off Kamaishi 1 c
Off Kamaishi 2 c
Off Boso 2 d
Off Boso 3 d
Tsunami buoy e
SE off Etorofu Island
DART buoys f
NE off Tokyo
SE off Tokyo
To find the edge of the tsunami source area, Huygen’s principle was applied to back-propagate the tsunami from each observation station. for these calculations we used Geoware tsunami travel-time software (TTT v. 3.0) and bathymetric data at one-minute intervals (ETOPO1; Amante and Eakins, 2009). The phase velocity of tsunami propagation was assumed to be equal to the square root of gravity multiplied by bathymetry. A tentative tsunami source area was then estimated as an area surrounded by the back-propagation lines.
In general, the travel time of a tsunami from its origin to a particular observation station is approximately equal to the time difference between the occurrence of the main shock and the tsunami arrival. However, for very large earthquakes the difference between the time of the main shock and the generation of the tsunami is not negligible (Seno and Hirata, 2007). Therefore, in this study, we modified the tsunami travel times by 1 min corresponding to a distance of 120 km from the epicenter to the contact point of the back-propagation line and the tentatively-determined tsunami source area (Table 1). This correction is to account for typical differences between the time of the main shock and the generation of the tsunami; this is almost equivalent to assuming an averaged apparent (i.e. projected to the seafloor) fault rupture velocity of 2 km/s. The final tsunami source area was then determined with back-propagation lines using the modified tsunami travel times.
Back-propagation methods were also applied to the primary crests to discuss the location of major seafloor uplift in the tsunami source area. However, this was done only for the primary crests observed by OBPGs and GPS buoys, in order to limit data to near-field tsunami in deep sea, so that using data strongly affected by non-linear effects or dispersions was avoided as much as possible.
4.1 Tsunami source area
4.2 Back-propagation of the primary crest
5.1 The accuracy of the estimated tsunami source area
As shown in Fig. 1, the tsunami arrival times have some ambiguity. Especially, in the case where the direction of the initial deflection of the tsunami waveform is downwards, the ambiguity can be as much as several minutes. The northern limit of the tsunami source area shown in Fig. 2 is constrained only by the tsunami arrival time data (WS602, WS202, and WS203) whose initial deflections are down. Taking into consideration the water depth at this point (Fig. 2), we estimate that an error of 1 min in the arrival time can cause an error of up to 4 km in the location of the northern edge of the source area. On the other hand, because stations GPS804, GPS802, GPS803, and GPS801 are within the tsunami source area, the western limit of the tsunami source area must be west of these stations (as described in Section 4.1). However, the data available to us do not allow us to determine where the western limit of the source area lies between these GPS buoys stations and the Sanriku coastline. Accordingly, we believe that the maximum error in our estimation of the size of the tsunami source area is, at most, a few tens of kilometers for its northern and western limits.
5.2 Possible highly-uplifted area in the tsunami source area
If the seafloor uplift area identified was confined only within the small area through which most back-propagation curves of the primary crests go (Fig. 3 and Section 4.2), most of the arrival-time data of the primary crests observed at GPS buoys or OBPGs (Table 1) can be reasonably explained; a highly-uplifted area in the tsunami source area is possibly located several tens of kilometers east of the epicenter. The area of large slip obtained from the inversion of seismic strong-motion waves (Yoshida et al., 2011) is almost coincident with the small area marked in Fig. 3. However, we suppose that this is but one of the possible solutions which can explain the timing of the primary crests.
One reason is, of course, that uplifted area from a great earthquake such as the 2011 off the Pacific coast of Tohoku Earthquake may be excited at multiple locations (e.g. more than one seafloor uplifted area) in the source area. In this case, it may be difficult to find the location of the maximum uplift area from the back-propagation curves of the primary crests from each station.
The other reason is that the back-propagation analysis is based on the assumption that the tsunami phase-velocity is equal to the square root of gravity multiplied by bathymetry. Nonlinearity of the phase velocity results in wave crests moving faster than this assumption; on the other hand, dispersion results in crests moving slower. Because these effects may cause some estimation errors in the highly-uplifted area, the results of this study do not allow us to discuss this in more detail.
The latter reason might be why it is difficult to explain the back-propagation curves from GPS807 and GPS806 (Fig. 3), unless the seafloor-uplift area has some extension in the north-to-south direction, instead of assuming the only seafloor uplift exists east of the epicenter.
5.3 Segments of known recurring large earthquakes within the 2011 tsunami source area
We appreciate the valuable comments of Professor Y. Tanioka and an anonymous reviewer. We used the Generic Mapping Tools software of Wessel and Smith (1998) to prepare our figures. Waveform data were acquired from MLIT (each coastal wave gauge and RTK-GPS buoy), JAMSTEC (stations KPG-1 and 2), ERI at the University of Tokyo (stations TM-1 and 2), JMA (stations Boso-2 and 3), NOAA (DART 21413 and 21418), and RFERHR (DART 21401).
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