Study Trip Report on
ONAGAWA TOWN OBSERVATION
By: Cigdem Tetik, Nolan Evangelista and Mehmet Baykal (S-course)

A great earthquake occurred on March 11th, 2011 Friday afternoon at 2:46:23 PM local time off the east
coast of Honshu about 130 kilometers east of Sendai. This massive earthquake’s magnitude was 9.0. The
Japan Meteorological Agency (JMA) named this earthquake “the 2011 off the Pacific coast of Tohoku
Earthquake”. This earthquake is the 4th largest earthquake in the world since 1900 to today. Also it is thebiggest earthquake for Japan. Nine major events of magnitude 7 or greater have repeatedly occurred
within the Japan Trench subduction zone since 1973. It was the third time that intensity of 7 was recordedin Japan following the 1995 Kobe Earthquake (M7.3) and the 2004 Mid-Niigata Prefecture Earthquake(M6.8).
The 2011 off the Pacific coast of Tohoku
Earthquake hypocenter locates off Sanriku at
130km ESE of Oshika Peninsula with the focal
depth of 24km. This earthquake occurred as a
typical inter-plate earthquake which was caused by
the rebound of a continental plate (North Americanplate) against a subducting oceanic plate (Pacificplate) along the Japan Trench. The focal
mechanism showed a reverse faulting movementwith a compression axis in a WNW-ESE direction(Figure 1).

According to NIED (K-net) records between
11.03.2011 to 15.07.2011 there were 358
earthquakes of M5.0 or over, 57 aftershocks of
M6.0 or over, and 5 aftershocks of M7.0 or overin Japan. Figure 2 shows the seismic activity ofthe main shock and aftershocks of M7.0 orlarger earthquakes (Figure 2). A red star markoff Miyagi Prefecture with an index 9.0 denotes
the hypocenter of the main shock; the otheryellow stars are those for aftershocks of M7.0 or
greater. At Tohoku large interplate earthquakes
had been predicted to occur in this region with
99% probability within thirty years and themagnitudes ranging from M7.5 to M8.0. The
March 11 earthquake was much bigger than
predicted, recording M9.0 and the rupture zone
stretched 510 km length and 210 km width.

Figure 1. Tohoku earthquake focal mechanism
solution (http://www.jma.go.jp)

Figure 2. Seismic activity of the main shock andaftershocks of M7.0 or larger which occurred between
11.03.2011 to 15.07.2011 (Earthquake location andmagnitude value from NIED web page)

At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to theEurasian plate at a velocity of 83 mm/yr. The Pacific plate thrusts underneath Japan at the Japan Trench,and dips to the west beneath Eurasia. The location, depth, and focal mechanism of the March 11
earthquake are consistent with the event having occurred as thrust faulting associated with subduction
along this plate boundary. Huge earthquake of M9.0 brought enormous crustal deformation on easternJapan. Depend on the GPS network of the Geospatial Information Authority of Japan; taking the fixed
point at Misumi, Hamada City in Shimane Prefecture, Pacific side of eastern Japan moved several meters1

to ESE direction. Displacement of 4.4 m was observed at Shizugawa, Minami-Sanriku Town in Miyagi
Pref., and the largest displacement of 5.3m was detected at Ojika, Ishinomaki City, while displacement atJapan Sea side was around 1m causing a large extensional field in the eastern Japan (Figure 3). On theother hand, as to the vertical displacement, subsidence of several tens centimeters was detected in wide
area along Pacific coast region. Subsidence of 75cm was observed at Shizugawa, and the largest
subsidence of 120cm was detected at Ojika (NIED). The areas hit by the Great Eastern Japan Earthquake
are known to be vulnerable to tsunami, as they were recurrently hit by tsunami. Large offshore
earthquakes occurred in the same subduction zone in 1611, 1896, and 1933. Each of earthquake causeddevastating tsunami on the coast. It is usual that an inter-plate earthquake occurred at trench region
accompanies tsunami. Since the magnitude of this earthquake was as large as M9.0, the scale of generated
tsunami was also huge one. In Japan, large tsunami attacked the Pacific coast ranging from Hokkaido toOkinawa and the tsunami was also observed at the coast of the Japan Sea, the Okhotsk Sea, and the East
China Sea. The tsunami also propagated to the coast of Hawaii, northern and southern America continents,and the Pacific countries.

Figure 3. Crustal deformation associated to the 2011 off the Pacific coast of Tohoku Earthquake (GSI)
Onagawa town was one of the most seriously tsunami-engulfed towns/cities along the Sanriku indented
coast line. Onagawa town is situated in an area where it is most vulnerable to tsunami because of its
geographical location. The Onagawa bay and its shallow bathymetry characteristics was a big factor thatmade the towering height of the tsunami as high as 15 meters. The inundation was as far as 3 km awayfrom the town. The town had a flat surface with no sufficient embankments. Most residential areas werelocated few meters away from the shoreline. For these reasons, the town was swept away by thetremendous energy of the tsunami (Figure 4).
There were at least four pile-supported buildings toppled and /or carried over some distances in Onagawa.
Buildings were uprooted, floated several meters away from their original positions. A three storey-hotel
fronting the pacific was totally inundated. Cars were like papers crumpled and thrown into small pieces
with the strong impact of the tsunami. Roads were subsided as well as the port facilities. Besides themassive destruction it brought to the town, 900 people were killed or missing in Onagawa Town. As
shown in Figure 5, the hospital was located approximately 14 meters high from the sea level. Based on
our observation, the trees were dried up which reveals that the tsunami reached and inundated the hospital2

which surpassed the height of the wall surrounding the area of the infrastructure.

Figure 4. Expected simulation of the tsunami andtopographic effectFigure 5. The high wall just near the hospital
about 12 m high. Tsunami exceeded from thewall and washed away first floor of the hospital.

Most of the buildings located near the seashore were completely destroyed and drifted by the strong
tsunami waves. Figure 6 shows one of the examples of building which was drifted by the tsunami about
70 meters. Most of the roads in Onagawa town especially those near the seashore were no longer visible.
The roads were subsided by approximately 80 cm as shown in Figure 7 which can no longer be used fortransportation. Some of the roads were still covered with tsunami debris and covered with sea water.
However, many roads were already built by the government to pave for continuous repair andreconstruction of the devastated town. Display of column steel bars was overturned by the backflow of
the tsunami wave (Figure 8).

Figure 6. Drifted building withtsunami. It was drifted about 70
meters by the waves.

Figure 7. The old road near theport subsided about 80 cm.

Figure 8. Backflow Effect3