MAY 26, 2003 MIYAGI-OKI (JAPAN) EARTHQUAKE


1 General
2 Seismotectonics
3 Seismology
4 Ground motions
5 Damage


1 General

An earthquake with magnitude 7.0 occurred in northern Japan at 6:24 pm local time (0924 UTC) May 26, 2003. Its focal depth is 71 kilometers below the sea floor, off the coast of northern Miyagi Prefecture. The epicenter was located lat. 38.8 N., long 141.68 E. (about 100km northern Sendai city) as shown in Figure 1. The very high PGA values (1.1G at MYG011, 1.0G at IWT007) and high JMA intensity (at many sites 6-(about 9 in MMI scale)) were observed. The distribution of seismic intensity (in JMA scale) is shown in Figure 2. This earthquake caused total 171 injured persons (slightly injured person: 148, seriously injured person: 23). No severe structural damage, death or missing persons were reported. The economic losses due to the quake are 97.3 million dollars (FDMA). The damage due to the earthquake is extremely small comparing with recorded very high PGA values and seismic intensity.



Figure 1. Location of the May 26, 2003 Miyagi-Oki Earthquake (After JMA)



Figure 2. The distribution of seismic intensity in JMA scale (After JMA)


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2 Seismotectonics

The tectonics of this region is controlled by the convergence of the Pacific plate and the Eurasian plate that generates numerous earthquakes in this area (Figure 3). The Pacific plate subductes west beneath the northeastern Japan island arc at rate of about 80 mm/year (DeMets et al., 1990, 1994). The surface expression of the boundary between these plates is the north-south trending Japan Trench that is about 220 km east of the epicenter. The Wadati-Benioff zone is made up of two distinct planes that are vertically separated by about 30km. The depth of hypocenter and aftershock distribution (Figure 4) show that this earthquake occurred at the upper seismic plane and is a typical intraplate earthquake that resulted from the release of compressional stresses within the Pacific plate that continuously develop as it descends into the mantle (USGS).



Figure 3 Past Earthquakes occurred in this region (After NIED)



Figure 4 Location of Miyagi-Oki earthquake and aftershocks (After NIED)


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3 Seismology

The parameters of the May 26, 2003 Miyagi-Oki Earthquake as given by JMA and USGS are presented in Table 1

Table1 Parameters of the May 26, 2003 Miyagi-Oki Earthquake

The fault plane parameters of the May 26, 2003 Miyagi-Oki Earthquake as given by Earthquake Research Institute, University of Tokyo, Japan are presented in Table 2.

Table2 Fault plane parameters of the May 26, 2003 Miyagi-Oki Earthquake



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4 Ground motions

The ground motions of May 26, 2003 earthquake were recorded by K-Net and Kik-Net (Kyoshin Network and Digital Strong-Motion Seismograph Network deployed by National Research Institute for Earth Science and Disaster Prevention, Japan). These digital records are released on their websites (http://www.k-net.bosai.go.jp/k-net/ and http://www.kik.bosai.go.jp/kik/). The locations of these stations are shown in Figure 5 and Figure 6 (NIED). The distribution of peak surface and underground accelerations are illustrated in Figure 7 and Figure 8, respectively (NIED). As the general ground motion distribution, the attenuation along the Pacific Ocean coast is small comparing with that of direction west-east. On the other hand, the amplitudes of surface acceleration are larger than that of underground acceleration, and the surface accelerations have a complex shape comparing with underground acceleration because of the difference of amplification of site (NIED). Some recorded large peak ground accelerations are list in Table 3. More information can be found on the K-Net and Kik-Net websites.


Figure 5 Locations of K-NetFigure 6. Locations of Kik-Net


Figure 7 Distribution of Peak Surface ACC.Figure 8 Distribution of Peak Underground ACC.


Table 3 Recorded peak ground accelerations



Figure 9 Acceleration time history and damped pseudo velocity response spectra (MYG11)


Some acceleration time history and 2%, 5% and 10% damped pseudo velocity response spectra of ground accelerations are provided in following figures. These figures clearly show that this intraplate earthquake mainly generated high-frequency wave. No severe structural damage caused by this earthquake probably since it had no enough power at frequency range important to structural response.




Figure 10 Acceleration time history and damped pseudo velocity response spectra (MYG02)





Figure 11 Acceleration time history and damped pseudo velocity response spectra (MYG03)





Figure 12 Acceleration time history and damped pseudo velocity response spectra (MYG06)





Figure 13 Acceleration time history and damped pseudo velocity response spectra (MYG10)





Figure 14 Acceleration time history and damped pseudo velocity response spectra (IWT014)


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5 Damage

This earthquake hit the northeastern Japan and rocked buildings as far away as Tokyo about 450 km to the south. Total 171 people were injured according to government, but no deaths and severe structural damage were reported. Tohoku Earthquake Disaster Investigation Committee was held the day after the Miyagi-Oki earthquake. This committee consists of six groups to investigate the ground motion, performance of building, facility effect, lifelines, damages to human and historical building. The organization relationship among different groups is presented in figure 15. The committee will provide detail reports about the damage later.


Figure 15 The organization relationship among groups


 5.1 Ground Motion

The SWG1 group has investigated some areas near stations recorded large peak accelerations (Figure 16). The K-Net (MYG011) recorded the maximum peak PGA (1.1G) is located at the top of cliff (Figure 17). The effect of this topographical irregularity may be included in the ground motion at this station. This earthquake caused damage in many cemeteries. A typical failure is tombstone rotation (Figure 18). A few tombstones were toppled down during earthquake (Figure 19). Detail information about the damage to tombstone can be found on the homepage of Disaster Control Research Center, Tohoku University, Japan (http://www.disaster.archi.tohoku.ac.jp/).


Figure 16 Map of Surveyed Area



Figure 17 K-Net (MYG011) is located on the top of cliff



Figure 18 Rotation of tombstone



Figure 19 Toppled down tombstone


This earthquake caused some large cracks in the road (Figure 20). Several landslides, rock and debris fall are reported (Figure 21). Foundation settlement caused damage to concrete plate at seaside (Figure 22). The liquefaction damage was observed in Ohfunato, Iwate (Figure 23)



Figure 20 Cracks of Route 397 in Ohfunato (After Mizusawa Regional Development Bureau)



Figure 21 Debris flow and Landslide in Karakuwa (After EDM, NIED)



Figure 22 Damage caused by foundation settlement in Ofunato (After EDM, NIED)



Figure 23 Liquefaction in Ofunato (After PARI, Japan)


 5.2 Performance of Structures

The communities of Iwate and Miyagi Prefecture felt the shaking strongly. The houses in these areas are mainly wooden building. Damage to wooden house was minor, only 2 severe and 8 moderate damaged houses building were reported in Iwate and Miyagi Prefecture according to government. The typical damage to residents is overturned and spilled contents (Figure 23). A few fall of roofing tile was reported.


Figure 24 Books fell from the bookshelf at Miyagi Library (After Asahi Shinbun)



Figure 25 Damage of reinforced concrete building in Tohno (After IEMS, Univ. Tsukuba)


Overall damage to concrete building was also minor. A few reinforced concrete buildings were damaged at column (Figure 25). The common damage was minor shear cracks in wall and columns, especially in old building (Figure 26). The column of elevated bridge of Tohoku Shinkansen near K-Net station (IWT014) damaged by this earthquake (Figure 27)



Figure 26 Cracked north wall of primary school



Figure 27 Damages to Elevated Bridge of Shinkansen (After Asahi Shinbun)


 5.3 Lifeline Performance

No severe damage has been reported to lifelines. The third machine of Onagawa nuclear power station stopped automatically just after begin of the earthquake. The local telephone network was somewhat disrupted after the main shock. The earthquake caused 3 fires in Sendai City and one in Iwate Prefecture. Some water pipe burst which caused suspension of water supply in some areas.


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