JULY 26, 2003 NORTHERN MIYAGI (JAPAN) EARTHQUAKE


1 General
2 Seismotectonics
3 Seismology
4 Ground motions
5 Damage


1 General

A powerful shallow inland earthquake occurred in Miyagi Prefecture, northern Japan at 07:13 AM. local time July 26, 2003 with magnitude M6.2. The epicenter is located lat. 38.40 N., long. 141.175 E. (about 25km northeastern Sendai city), with focal depth 12km, as shown in Figure 1 (From USGS). The same day, foreshock and aftershock of magnitudes -M5.5 and M5.3 - hit the same area at 00:13 AM. and 04:56 PM. with epicenters about 3km and 11km north of the mainshock’s, respectively. The maximum PGA value (2005.1gal at thk.NARUSETY Station) in seismogram history was recorded in the 00:13 event and very high JMA intensity (at many sites 6+(about 10 in MMI scale)) were observed in all the three earthquakes. The distributions of seismic intensity (in JMA scale) for three earthquakes are shown in Figure 2, 3 and 4.(From JMA). These earthquakes caused total 676 injured persons (slightly injured person: 626, seriously injured person: 50), and 11,341 buildings were damaged, 1,017 of which were collapsed. Fortunately, no death or missing persons were reported. The economic losses due to these earthquakes are 195.4 million dollars (FDMA).



Figure 1 Location of the July 26, 2003 Northern Miyagi Earthquake (After USGS)



Figure 2 The distribution of seismic intensity of 00:13 Event (JMA scale) (From JMA)



Figure 3 The distribution of seismic intensity of 07:13 Event (JMA scale) (From JMA)



Figure 4 The distribution of seismic intensity of 16:56 Event (JMA scale) (From JMA)


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2 Seismotectonics

The epicenters are near east coast of Honshu, Japan, where the Pacific plate subductes west beneath the northeastern Japan island arc at rate of about 80 mm/year (DeMets et al., 1990, 1994). The depth of hypocenters and focal mechanisms show that this earthquake sequence is typical inland crustal earthquake (Figure 5) (From NIED). This earthquake sequence took place around an active fault named "Asahiyama flexure ", as shown in figure 6, which has North-South strike. The Asahiyama flexure inclines to the west with 8km long extending from Naruse Town to Kanan Town, and is a reverse fault with the west rock above this fault moving up. There is no recorded earthquake occurred due to this fault according to the Sendai Meteorological Bureau. About one hundred active faults in Japan have been surveyed after Hanshin earthquake, 1995, however, at the time of writing this report, little detail information about this flexure was available. No clear surface trace of fault rupturing was reported in this region after shock.



Figure 5 Location of 07:13 Event (From NIED)



Figure 6 Asahiyama flexure (From Active faults in Japan (Univ. Tokyo Press))


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3 Seismology

The parameters of the July 26, 2003 Miyagi Earthquakes as given by JMA are presented in Table 1

Table 1 Parameters of the July 26, 2003 Miyagi Earthquakes

The fault plane parameters of the July 26, 2003 Miyagi Earthquakes as given by NIED are presented in Table 2.

Table 2 Fault plane parameters of the July 26, 2003 Miyagi Earthquakes


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4 Ground motions

The strong-motion network deployed by JMA and local officer in this area is very dense and ground motion recordings (PGA and JMA Intensity) close to the epicenter are available. The maximum PGA (2005gal) in seismogram history was recorded in the 00:13 event at thk.NARUSETY Station deployed by local office 3.5km from the epicenter. However, the highest PGA recorded by the 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) is just 350gal recorded by the MYG004 station 39km from the 07:13 event epicenter, and the damage is mainly distributed in the region near the epicenter, which is different from the May 26, 2003 Miyagi-Oki earthquake which focal depth is 71km. As shown in figures 10-15, the records of the July 26 earthquakes have much high-frequency content, which is similar to the records of the May 26 earthquake (see http://www.disaster.archi.tohoku.ac.jp). The difference of damaged area may be due to the difference of magnitude and focal depth. Although the May 26 earthquake occurred in the subducting slab, the July 26 earthquakes occurred in the shallow crust, where the attenuation rate is much larger than that in the slab (e.g., UTSU, 1977). Some large peak ground accelerations recorded by JMA and local officer station (Figure 7) are list in Table 3 (Sendai Meteorological Bureau). Unfortunately, most of these data are lost because of overwriting by many aftershock data.


Table 3 Recorded peak ground accelerations (Event: 00:13)

Table 3 Recorded peak ground accelerations (Event: 07:13)

Table 3 Recorded peak ground accelerations (Event: 16:56)



Figure 7 The locations of some stations

Figure 8 Distribution of Peak Surface ACC. (Form NIED) Figure 9 Distribution of Peak Underground ACC. (From NIED)


Based on the recorded seismogram by the K-Net and Kik-Net, the distribution of peak surface and underground accelerations for Event 00:13 are illustrated in Figure 8 and Figure 9, respectively (NIED). These figures show that the strong ground motion is centered the region near the epicenter because of the shallow focus.
Some acceleration time history and 2%, 5% and 10% damped pseudo velocity response spectra of ground accelerations are provided in following figures.


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5 Damage

The strong shallow earthquakes caused severe damage in region near the epicenter such as Kanan, Yamoto, Naruse and Nangou Town (location is shown in Fig. 7), northern Miyagi Prefecture, where JMA intensity 6+ or 6 is observed. The damage by this earthquake is more serious than that of May 26, 2003 Miyagi-Oki earthquake (M7.0). The main damage is just centered on the area about 5-10km from the epicenter because of the middle sized earthquake, shallow focus and high frequency predominance. July is the rainy season in Japan, so the continuous rain may contribute to the heavily damage. The earthquake did not generate tsunami.


Figure 10 Acceleration time history and pSv at station MYG010 (Event 00:13)

Figure 11 Acceleration time history and pSv at station MYG012 (Event 00:13)

Figure 12 Acceleration time history and pSv at station MYG010 (Event 07:13)

Figure 13 Acceleration time history and pSv at station MYG012 (Event 07:13)

Figure 14 Acceleration time history and pSv at station MYG007 (Event 16:56)

Figure 15 Acceleration time history and pSv at station MYG010 (Event 16:56)

Figure 16 Velocity time history, displacement orbit and velocity orbit of station E08 (Event: 07:13)

Figure 17 Velocity time history, displacement orbit and velocity orbit of station MYG012 (Event: 07:13)

Figure 16 and 17 are the velocity time history, displacement orbit and velocity orbit of station E08 and MYG012 for mainshock, respectively. The displacement and velocity orbits in figure 16 show that the direction of ground motion near station E08 (Ishinomaki) is W-E. The direction of ground motion near the epicenter has been studied through investigating the turnover tombstones in these areas by Disaster Control Research Center, Tohoku University, Japan.


 5.1 Ground Motion

Ground crack, liquefaction, rock falls and landslides were reported due to strong shaking. The continuous rain during the rainy season before earthquake may contribute the ground failure. Total 288 locations of roads suffered cracking or slumping damage and 3 damaged bridges were reported (Figure 18 and 19). Some liquefaction damages were observed in Naruse Town (Figure 20). Along the line with the same direction of Asashiyama Flexure, from Naruse Town to Yamoto Town, end at Kanan town, several landslides were reported (Figure 21). Landslides contributed to crop damage.
The characteristic of the ground motion has been discussed by investigating the damaged tombstone and other turnover thing near the epicenters of July 26, 2003 earthquakes and by observing the strong ground motion feature of these inland earthquakes, such as the recorded larger acceleration, velocity and ground motion directivity.
The WG1 group has investigated the tombstone and other turnover thing in order to study the amplitude and directivity of ground motion near the epicenter. Here is the report about the damage to the tombstone, stone monument et.al.


 5.1.1 Introduction

The WG1 group began their investigation on July 26, the day when the earthquake occurred. Then they investigated the turnover rate of tombstone on July 27, July 29, July 30, August 4 and August 5. The moving direction of the tombstone was checked on August 1, August 7 and August 9.
Total 40 cemeteries were investigated (Table 4). Specially, the evaluation of ground motion is not only judged from the tombstone itself but also the movement direction of the bottom stone, the middle stone and the turnover direction of stone lantern which directions were comparatively stable. The movement and turnover conditions of the monument, the big stone, arbor and chimney were also investigated. The relationship between the damage to the roof tile and the ground motion was studied.

 5.1.2 Investigation Place and Results

The investigation place, ground condition, turnover rate and the predominant direction of ground motion judged from the turnover direction are shown in table 4. The result is also shown in figure 22 with the Asahiyama Flexure. Figure 23 is the typical damage to tombstone, for example, turnover top stone, movement of the upper stone and the middle stone.

Figure 18 Cracks in road at Naruse Town (From Asahi Shinbun)Figure 19 Cracks in road at Naruse Town (From Asahi Shinbun)
Figure 20 Liquefaction at Naruse Town (From GEL, Tohoku Univ.)Figure 21 Landslide at Kanan Town (From Asashi Shinbun.)


Table 4 and Figure 22 show that the Toukou Temple and Koukoku Temple on the soft ground along the Naruse River at Nangou Town, Kojima Cemetery at a slope and Tosyuin at Akai of Yamoto Town have suffered larger turnover rate, some over 70% turnover rates (70% turnover rates approximately corresponds to 70 - 90 cm/s, Kaneko and Hayash, 2000).
The motion direction of tombstone shows that the direction of ground motion is East-West which is strike-normal, except the Ono and Ushiami at Naruse Town located in south of the Asahiyama Flexure and the northern Kanan Town located in north of the Asahiyama Flexure. The tendency that the ground motion dominates in East-West direction is not only observed around faults but also spread to the Kashimadai and Ishinomaki. That matches the seismogram recorded at the JMA Ishinomaki Station (E08) which ground motion is also mainly in the east-west direction (Figure 16). The ground motion direction at the north of fault is mainly the South-North, which matches the recorded wave at the Wakuya Station. The ground motion direction at Ono and Ushiami of Naruse Town located at south of the Asahiyama Flexure is from NE-SW to NNE-SSW.

 5.1.3 Turnover and Movement of Monument

An example of turnover monument at Hirobuchi elementary school and Ohshio elementary school is shown as an investigation of the movement and turnover conditions. Figure 24 is a monument (height 380cm, width 150cm) that fell down to the east, and Figure 25 is the school monument fell down in the same direction. However another monument that is near the turnover ones and faces to the normal direction did not fell down, which suggests that the ground motion direction is E-W.
The base seat (64x60x 93cm) of a bust (Ohsaki) at Yamato Town Ohshio elementary school was also suffered a movement (10cm) in the east (the east-northeast) (Figure 26). The base seat of a monument in the same school moved in the same direction. Another bust (Yamato) at Ohshio elementary schools was moved to the east-southeast (the east, 15.5cm, the south, 11.5cm).
The monument (about 3ton) located at the entrance of Naruse Town Kougaku Temple jumped to northeast (about 50cm), which is consistent with the damaged tombstone moved in the same direction (Figure 27).
Figure 28 is a big stone (about 2 ton) at the park near the Nangou JA building northern Nangou elementary school that is moved in E-W by the earthquake force. A stone (about 50cm) at the Sousen Temple was moved 6cm to the east.


An arbor at the courtyard of Hukaya Hospital at Kanan Town fell down in the west, and the banking was moved to the west (Figure. 29). Moreover, from the damage condition of RC chimney of Hukaya Hospital, we can also conclude that the ground motion direction is mainly in E-W, which is consistent with the damaged tombstone in Kouen Temple.

 5.1.4 The Relationship Between Ground Motion Direction and Damage to Roof Tile

The damage to roof tile of Koufuku Temple (collapsed) is an example showing the relationship between ground motion direction and damage to roof tile (Figure 30). A big Stone lantern in front of the Koufuku Temple fell down to east and an incense burner was moved to east, which shows the ground motion direction was mainly in the E-W. The roof tile of south side suffered damage, while that of north side did not, because the north wall is stronger than the south wall, which results that the shake in south side was stronger than the north side (Figure 30). The damage of residential building confirms that.
The entrance gatepost of Houshou Temple at Naruse Town fell down in the south-southwest. Corresponding to that, the beam of the inner temple jumped out.
Figure 31 shows that the entrance roof of Rikkyoudou of Kitamura elementary school at Kanan Town where RC school building was severe damaged fell down to the east, which suggested that the strong ground motion is in E-W. The damage conditions of the school and the gymnasium also confirm that.

 5.1.5 Conclusion

The following conclusion can be obtained from the investigating the damage to tombstone and turnover thing.

1.




Many graveyards where the turnover rate of the tombstone exceeded 70% were on soft ground around the epicenters. The graveyards at the hill area from the northwestern to the southwestern of Kanan Town Hirobuchi, south Kitamura, Nangou Town Kojima where the ground conditions are good suffered turnover rate 60% or more. That may be relative with the position of fault asperity.
2.





The directivity of ground motion which is predominant in E-W direction (the direction of strike-normal) is not only near the fault but spreads west to Kajimadai and east to Ishinomaki city. However, Naruse Town located at the south of fault shook in the direction from NE-SW to NNE-SSW, while the north of Kanan Town located at the north of fault shook almost in the direction of N-S.
3.

The result from investigating the damaged monument is consistent with that of tombstone.
4.



The characteristic of ground motion, shaking mainly in E-W, is also consistent with the result made by the Society of Civil Engineers that the river flowing in N-S suffered heavily damage, while the river flowing in E-W suffered slight damage.
5.



The damage of building with weak resistant in E-W was severely damaged since the ground motion in that direction is strong. The characteristic of ground motion may make school building suffer heavily damage, since these building is general weak in the ridge direction.

Table 4 Damage distribution of tombstone

Figure 22 Damaged tombstone(turnover rate and direction)


Figure 23 An example of damaged tombstone and the terms of tombstoneFigure 24 Toppled down monument(Hirobuchi elementary school)
Figure 25 Toppled down monument(Hirobuchi elementary school)Figure 26 Moved Bust (Ohshio elementary school)
Figure 27 Moved monument(Entrance of Kougaku Temple)Figure 28 Moved stone (JA Park)
Figure 29 collapsed arbor(Hukaya Hospital)Figure 30 Damaged roof tile (Koufuku Temple)
Figure 31 Damaged entrance (Kitamura elementary school)


 5.2 Structural Damage

The earthquake caused severe damage to private building in Naruse, Yamoto, Nangou, Kanan and Kashimadai town in Miyagi Prefecture, where high JMA seismic intensity 6+ or 6- was observed. The residential buildings in these areas are mainly two-story wooden building. According to the government source, 1,017 housing units were collapsed, 2,245 housing units were heavily damaged and 8,079 housing units were lightly damaged. A great many of roofing tiles fell, which injured some person (Figure 32). Some collapsed housings by this earthquake were already damaged by the May 26, 2003 Miyagi-Oki earthquake and impaired their stability (Figure 33). The team of the Disaster Control Research Center, Tohoku University, Japan has investigated the damage to wooden building in these areas and more detail information will be released on the internet at http://www.disaster.archi.tohoku.ac.jp/.

Figure 32 Fell Roofing tiles at Kanan ( From Asashi Shinbun)Figure 33 Collapsed wooden house at Nankou (From Asashi Shinbun)


The structural type of government building (school, hospital, officer buildings) is generally reinforced concrete building up to three or four stories. Overall damage to reinforced concrete buildings designed by the 1981 code was minor throughout the region. Few concrete buildings designed by old code in the surrounding regions were observed to have sustained structural damage. Three reinforced concrete buildings of particular interest were Hukaya Hospital and Kitamura school located in Kanan Town and Kashimadai Hospital in Kashimadai Town.


Hukaya Hospital, constructed in 1969, was a four-story concrete frame (Figure 34, 35 and 36). Many columns in the 2nd storey suffered shear failure during the earthquake.


 
Figure 34 Hukaya Hospital

 
Figure 35 Shear failure at column of north side

 
Figure 36 Shear failure at column of south side




Kitamura school, constructed at 1971, suffered shear failure to column and shear wall (Figure 37-40).

Figure 37 Kitamura school, three-story concrete frame (From EDM )Figure 38 Shear failure at column (From EDM)
Figure 39 Shear failure at short columnFigure 40 Damage to shear wall


Kashimadai Hospital constructed in 1968 at Kashimadai Town is a three-story reinforced concrete frame that suffered severe structured damaged to column during the earthquake (Figure 41-44).

Figure 41 Kashimadai Hospital (From IEMS, Univ. Tsukuba)Figure 42 Shear failure at column (From IEMS, Univ. Tsukuba)
Figure 43 Damage to corner column (From IEMS, Univ. Tsukuba)
Figure 44 Shear failure at column


 5.3 Lifeline Performance

Some damage has been reported to lifeline. Some water pipes were damaged by the earthquake, which caused suspension of water supply for 13,925 families. Some local power lines were slightly damaged, which affected about 10,0000 families. The earthquake caused 3 fires. No damage to local telephone network was reported.


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