THE 2011 TOHOKU EARTHQUAKE IN JAPAN. VSU Lyuben Karavelov, Sofia, Bulgaria. Key words: Tohoku earthquake, strong ground motion, damage

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THE 2011 TOHOKU EARTHQUAKE IN JAPAN Radan Ivanov 1 VSU Lyuben Karavelov, Sofia, Bulgaria Abstract: This earthquake which occurred on March 11, 2011, had a magnitude of 9.

1 THE 2011 TOHOKU EARTHQUAKE IN JAPAN Radan Ivanov 1 VSU Lyuben Karavelov, Sofia, Bulgaria Abstract: This earthquake which occurred on March 11, 2011, had a magnitude of 9.0, which places it as the fourth largest in the world since 1900, and the largest in Japan since modern instrumental recordings began 130 years ago. It will surely become a landmark earthquake, and one which will shape seismology and earthquake engineering research for many years to come, just as the 1995 Kobe earthquake did. This paper attempts to give an early account of the source mechanisms that caused the earthquake, the distribution of seismic intensity and damage, as well as the characteristics of the ground motion at selected strong motion recording stations. A summary and analysis of the damage attributable to strong ground motion and tsunami is also included. Key words: Tohoku earthquake, strong ground motion, damage 1. Tectonic background The main shock occurred on 14:46 of March 11, The hypocenter was located off the Sanriku coast at 130km ESE of Oshika Peninsula with focal depth of 24km. The magnitude of the earthquake was initially announced as M7.9 and was finally determined as M9.0. The earthquake was confirmed to be of thrust type with a pressure axis in the WNW-ESE, i.e. the earthquake was generated as a typical inter-plate earthquake which is caused by the rebound of a continental plate (North American plate) against a subducting oceanic plate (Pacific plate) at the Japan trench, Fig. 1. JMA named this earthquake 2011 off the Pacific coast of Tohoku Earthquake. Fig. 1. Tectonic setting of the earthquake; a) plate configuration; b) source mechanism [1] 1 Associate Professor, Ph.D., 175 Suhodolska St., 1373 Sofia, Bulgaria, r_ivanov@vsu.bg I - 169

Fig. 2 shows the fault model and the slip distribution along the fault. The largest slip is estimated to be about 23m.

The fault area is approximately a rectangle 20

Fault model and distribution of slip along the fault surface [2] The huge slip along the fault surface resulted in very large permanent displacements of the crust and the surface of Japan islands.

2 Fig. 2 shows the fault model and the slip distribution along the fault. The largest slip is estimated to be about 23m. It is obvious from the figure that the earthquake did not occur as a rupture originating from a single source, but rather had multiple sources triggering at about the same time. The fault area is approximately a rectangle 200km by 500km in plan. Fig. 2. Fault model and distribution of slip along the fault surface [2] The huge slip along the fault surface resulted in very large permanent displacements of the crust and the surface of Japan islands. Fig. 3 shows the displacement field caused by this earthquake as detected by the GPS network of the Geospatial Information Authority of Japan. Taking the fixed point at Misumi, Hamada City in Shimane Pref., Pacific side of eastern Japan moved several meters to ESE direction. Displacement of 4.4m was observed at Shizugawa, a Minami-Sanriku Town in Miyagi Prefecture, and the largest displacement of 5.3m was detected at Ojika, Ishinomaki City, while displacement at Japan Sea side was around 1m, causing a large extensional field in the eastern Japan. Vertical subsidence of several tens of centimeters was detected in a wide area along the Pacific coast region. Subsidence of 75cm was observed at Shizugawa, and the largest subsidence of 120cm was detected at Ojika. Such a subsidence makes sea water brought by tsunami difficult to drain back. Also, the descent of breakwater and land itself results in increased vulnerability to the next tsunami. Fig. 3. Permanent displacements at surface level; a) horizontal; b) vertical [1] I - 170

2. Strong ground motion Seismic intensity of 7 in JMA (Japan Meteorological Agency) scale was recorded at Kurihara City, Miyagi Prefecture, and intensities of 6+ or 6- were observed in wide area

Peak ground acceleration of 2,933gal (vector sum of three components) was observed at Tsukidate, Kurihara City, at a NIED K-NET station.

3 2. Strong ground motion Seismic intensity of 7 in JMA (Japan Meteorological Agency) scale was recorded at Kurihara City, Miyagi Prefecture, and intensities of 6+ or 6- were observed in wide area along the Pacific region ranging from Iwate Pref. to Ibaraki Pref. Peak ground acceleration of 2,933gal (vector sum of three components) was observed at Tsukidate, Kurihara City, at a NIED K-NET station. It was the third time that intensity of 7 was recorded in Japan following the 1995 Kobe Earthquake (M7.3) and 2004 mid-niigata Earthquake (M6.8). The distribution of instrumental intensity as well as its evolution in time for several stations is shown in Fig. 4. Fig. 4. Instrumental intensity distribution and its evolution in time [1], [2] There are two characteristic intensity evolution patterns, related to the time histories they are associated with. The curves with faster rising time are those for which the waves propagating from the two principle ruptures overlap. Most of the curves however are of the second type where the arrival of the two waves does not overlap, and have a substantial rising time to intensity 6, which means that people in areas with such records must have had enough time to escape from their homes. The time history of the record with the larges PGA is shown in Fig. 5 Fig. 5. Time history at station MYG004 I - 171

3. Tsunami It is usual that an inter-plate earthquake at a trench region is accompanied by a tsunami. Since the magnitude of this earthquake was as large as M9.

4 3. Tsunami It is usual that an inter-plate earthquake at a trench region is accompanied by a tsunami. Since the magnitude of this earthquake was as large as M9.0, the scale of generated tsunami was also huge. In Japan, large tsunami attacked the Pacific coast ranging from Hokkaido to Okinawa 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. At Kamaishi, Ishinomaki, and Ofunato, the first arrival of tsunami was at 14:46, which means that the tsunami reached to these coastal cities at the same time of the earthquake occurrence. The tsunami of maximum height attacked these cities around 15:20, i.e. 30 minutes after the earthquake. Fig. 6 shows the distribution of maximum height of tsunami along the coast around Japan. Heights of more than 8.5m were recorded at Miyako, Iwate Pref., more than 8.0m at Ofunato, Iwate Pref., more than 7.3m at Soma, Fukushima Pref., 4.2m at Oarai, Ibaraki Pref., etc. The Japan Meteorological Agency issued Tsunami Warning (Major tsunami) at 14:49, i.e. 3 minutes after the earthquake, to Iwate, Miyagi, and Fukushima Prefectures. It was extended to Aomori, Ibaraki, and Chiba at 15:14, and was followed by Japan Sea side, Bonin Islands, Sagami Bay, Shizuoka and Wakayama Prefectures. They were in series downgraded to Tsunami Warning (tsunami) and Tsunami Advisory for each region, and were completely cleared on 17:58, March 13. Fig. 6. Tsunami height distribution; a) along the Pacific coast; b) along the Sea of Japan, [1] (heights larger than 5 m are omitted) 4. Damage Given the magnitude of the earthquake and the intensity of ground shaking, the damage brought by the earthquake was also considerable people are either dead or missing, hundreds of thousands are left homeless. The largest damage was brought about by the tsunami which devastated the lowlands of the Pacific coast. The usual damage modes of buildings such as soft first floor collapse were also present, but to a much lesser extent. It is fare to say that damage to buildings was small given the scale of the earthquake. Figs. 7 to 10 show the devastation brought about by the tsunami. On careful inspection of Fig. 9 one can see that the buildings on higher ground are standing, confirming that the damage to buildings was caused by tsunami rather than by strong ground shaking. It still remains to be seen, but probably the most significant damage will prove to be the secondary damage to the reactor of Fukushima I NPP, Fig. 10, where due to a structurally minor damage of the cooling system, the reactor buildings of the NPP were almost brought to a state of meltdown. This led to unplanned power shortages and most importantly, to a release of radioactive substances in the air and in the water. On the optimistic side, the road shown in Fig. 12 is a confirmation of the efficacy of Japan in dealing with disasters. The road was back in operation just 6 days after the earthquake. I - 172

5 Fig. 7. Damage - Sendai neighborhood ( Google) Fig. 8. Damage - Oshika ( Google) Fig. 9. Damage - Minami Sanriku ( Google) Fig. 10. Damage - Ofunato Fig. 11. Damage to Fukushima NPP I - 173

Fig. 12. Road repaired six days after the earthquake Conclusions The scale of this earthquake was enormous in terms of magnitude, intensity, and severity of ground shaking.

On the optimistic side, the damage attributable to ground shaking was less than expected from such a powerful earthquake.

6 Fig. 12. Road repaired six days after the earthquake Conclusions The scale of this earthquake was enormous in terms of magnitude, intensity, and severity of ground shaking. The ensuing damage was also considerable. The devastating tsunami was the main reason for the damage to buildings and facilities. On the optimistic side, the damage attributable to ground shaking was less than expected from such a powerful earthquake. The total number of dead and missing people is high, but given the large area hit by the earthquake, it could have been much worse. In fact, given the circumstances, the human loss may be considered as moderate. The most tragic event was the accident at the Fukushima NPP, where the tsunami disabled the cooling system, resulting in an explosion, uncontrolled release of radioactive material, and ultimately nationwide and indeed worldwide distress. The accident is a clear example of disproportionate damage, and will surely lead to rethinking of the way NPPs are designed, maintained and managed, including the degree of involvement of the private sector in the nuclear power business. This earthquake was not unexpected, given the millennia old record of destructive earthquakes along the Japanese Pacific coast. Regardless of the damage it caused, it will likely not have long lasting adverse effect on the Japanese economy, provided the nuclear accident is contained. REFERENCES [1] NIED, Preliminary report of the 2011 off the Pacific coast of Tohoku Earthquake, [2] NIED, 2011 Off the Pacific Coast of Tohoku earthquake, Strong Ground Motion, I - 174

Coseismic slip model

Coseismic slip model Figure 3 - Preliminary highly smoothed model of coseismic slip for the 11 March UCL Institute for Risk & Disaster Reduction Magnitude 9.0 (JMA scale) earthquake Occurred at 02:46:23 pm local time near