SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU stanislav lenart, junichi koseki and yukika miyashita about the authors Stanislav Lenart Slovenian National Building and Civil Engineering Institute (ZAG) Dimiceva 12, 1000 Ljubljana, Slovenia (formerly IIS, The University of Tokyo) E-mail: stanislav.lenart@zag.si Junichi Koseki Institute of Industrial Science (IIS), The University of Tokyo 4-6-1 Komaba, Meguro-ku, Tokyo, Japan E-mail: koseki@iis.u-tokyo.ac.jp Yukika Miyashita Institute of Industrial Science (IIS), The University of Tokyo 4-6-1 Komaba, Meguro-ku, Tokyo, Japan E-mail: ymiya@iis.u-tokyo.ac.jp Abstract A brief report about the liquefaction damage in the Tone river basin, caused by the 2011 earthquake off the Pacific coast of Tohoku, is presented. It includes sand boiling, damage to river dikes, the settlement and tilt of superstructures, the uplift of light underground structures and lateral spreading. A history of land reclamation along the Tone river is briefly presented to understand why extensive liquefaction took place predominantly in reclaimed land. The recorded ground motions near the river were analyzed and compared to near-source ground motions. The effects of the site location and the ground conditions during the peak ground acceleration are discussed. Keywords 2011 earthquake off the Pacific coast of Tohoku, liquefaction, case history 1 INTRODUCTION HISTORY OF TONE RIVER BASIN The Tone river (in Japanese, Tone-gawa) originates in the volcanic area of the northwestern Kanto region and flows approximately 320 km southeast, crosses the Kanto Plain with the Tokyo Metropolitan area, and finally enters the Pacific Ocean at Choshi City. No other river in Japan has been so modified by human activity [1]. Its course has been altered and its entire length is confined by dikes. The ancient Tone river flowed to Tokyo Bay; however, its course was diverted to the Pacific Ocean during the 17th century (Fig. 1). Although the main purpose of the project was to protect the capital of Japan, Tokyo, from floods, there have been infinitely many floods in the rest of river basin since then. Figure 1. Eastward diversion of Tone river (modified from [2]): original course (left) and diverted course (right). ACTA GeOTeCHNICA SLOVENICA, 2012/2 25. S. LENART ET AL.: SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU Figure 2. "Kirisho-numa" swamp (left) and its reclamation using dredged soils (right) [3]. One of the floods caused the breaching of the Tone river dike near Fusa, Abiko city in Chiba prefecture in 1870 [2]. The embankment for controlling the waters of the river collapsed over a length of more than 80 m and submerged the whole Fusa area. A large swamp called "Kirisho-numa" with an area of more than 5 ha was created. This area suffered from frequent flood disasters. Therefore, a project to improve the Tone river channel and the dike was initiated in 1952. The dredged soil was transported through pipes to reclaim the swamp. Finally, the reclaimed area was converted into a residential area. The "Kirisho-numa" swamp and the process of its reclamation using dredged soils are shown in Fig. 2. with a magnitude of 5.0 or above up until the 15th of March at 5 p.m. [5], indicating a tendency of tectonic plates and faults to stabilize ground conditions, which have been greatly altered by the earthquake. The existence of two faults might be proved by the near-source acceleration waveforms. Two remarkable distinct phases of ground motion can be seen on the earthquake ground-motion records from seismographs in the north part of Honshu island [6], which are the closest to the epicenter, while on the acceleration waveforms recorded in the south, in the Tone river basin, the first phase is not visible at all (Fig. 6). 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU One of the world's largest recorded earthquakes in history, with a moment magnitude Mw = 9.0, occurred and affected the east part of Japan on the 11th of March 2011 at 14:46 local time. It was caused by tectonic movements of the North American plate and the Pacific plate. The Geospatial Information Authority of Japan [4] constructed a fault model using co-seismic, surface-displacement data observed by the GPS Earth Observation Network System. A fault model that consists of two rectangular faults with a uniform slip in an elastic halfspace shows that the total major rupture length reached approximately 380 km with a fault width of 90-130 km. The slip amounts of the northern and southern segments were estimated to be ~25 m and ~6 m, respectively [4]. Fig. 3 shows the surface displacements caused by the main shock. A star indicates the epicenter of an earthquake. Two rectangular faults were assumed. The circles indicate the epicenters of the aftershocks determined until the 15th of March. There have been 408 aftershocks Figure 3. Calculated and observed co-seismic horizontal surface displacements [4]. ACTA G£OT£CHNICfl SLOVENICA, 2012/2 13. S. LENART ET AL.: SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU 2 GROUND-MOTION CHARACTERISTICS DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU THE EFFECT OF SITE LOCATION Although the damage due to the great Tohoku earthquake occurred at many places on the east part of Honshu island, only five typical sites with liquefaction occurrence in the Tone river basin are presented in this paper. Their locations are shown in Fig. 4b. Fig. 4a presents the overall location of the earthquake's hypocenter, the Tone river basin and the seismographs, the records of which are discussed in this paper. To understand the transfer of ground shaking from near-source to more distant sites, we compare the earthquake motion records from six sites of the Japanese seismograph network. The seismographs CHB004, IBR016, MYG004 and MYG010 are part of K-NET, a Japanese, nation-wide, strong-motion seismograph network. They are all installed on the ground surface. Seismograph CHBH13 is part of KiK-net, a Japanese, strong-motion, seismograph network, which consists of pairs of seismographs installed in a borehole as well as on the ground surface. Basic data about these seismographs are listed in Table 1. Their location is shown in Fig. 4a and Fig. 4b. It is evident from Table 1 that the levels of ground shaking recorded during the 2011 earthquake off the Pacific coast of Tohoku were very high and the values of the peak ground accelerations (PGAs) at some seismograph locations, e.g., PGA=3.0g at MYG 004, exceeded all reasonable expectations. It is difficult to explain such values, particularly as there was no extra damage observed on bridges or buildings in Tsukidate, where this record was made [8]. It is supposed that the design values of the PGAs less than 1.0g were used in the design. A detailed insight into the acceleration record shows a sharp spike. It might not be caused directly by seismic-induced ground motions, but by some other impacts that happened nearby (some banging) during the earthquake's MYG 004 { MYG 01 Of j Hypocenter ^ -, MYG 011 ★ ! r„, .¿S" Figure 4. Map of the sites discussed in the paper (a) and their detailed locations (b) along the Tone river (modified from http://maps.google.com). a) Table 1. Details about the K-NET and KiK-net seismographs [7]. Seismograph Epicentral distance [km] Altitude [m] Peak acceleration* [cm/s2] Depth [m] Base CHB 004 323 13.9 300.7 surface Ash clay, sandy soil CHB H13 340 12 233.9 surface sandstone, siltstone, mudstone and conglomerate (surface) 56.9 1300 Chert (bedrock) IBR 016 350 20.4 516.7 surface Ash clay, sand MYG 004 175 40 2699.9 (spike) surface Clay, rock MYG 010 143 2 458.2 surface Sand MYG 011 121 13.2 921 surface Rock * maximum value of NS/EW components ACTA G£OT£CHNICfl SLOVENICA, 2012/2 13. S. LENART ET AL.: SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU duration. If this spike is eliminated the maximum PGA of this record is reduced by more than half. The subsoil condition with a firm base in the cases of two extreme PGA values (MYG 004 and MYG 011) differs slightly from the subsoil conditions at the other discussed locations, where sandy soil is observed in the base. There is no evidence that this could be a reason for the sharp spike phenomenon in the case of MYG 004. Sharp spikes could also be caused by another kind of source, e.g., some banging near the instrument. In general, these kinds of spiky accelerations can be safely ignored as they do not allow sufficient time to cause large deformations. On the other hand, seismograph MYG 011 is located on a slope, suggesting possible topographical amplifications [9], while seismograph MYG 004 is located on flat ground. Nevertheless, we can also observe high PGA values at all the other listed locations, on average almost 0.4g. It should be noted that the PGA at the near-source location (Miyagi prefecture) is comparable to MYG 004 III Direction: N-S - O 50 100 150 200 250 3 Time [second] Figure 5. Ground-motion record with the largest observed peak ground acceleration PGA=2699.9 cm/s2 during to the 2011 earthquake off the Pacific coast of Tohoku, K-NET seismograph MYG 004. the PGA recorded at the locations in the Tone river basin, although the epicentral distance in the case of the latter is almost twice as large as in the case of the Figure 6. Ground-acceleration records at (left) the near-source K-NET seismograph (MYG 010) and (right) the K-NET seismograph near the Tone river (IBR 016). 8. ACTA G£OT£CHNICfl SLOVENICñ, 2012/2 S. LENART ET AL.: SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU Miyagi prefecture. One should refer to Fig. 3, where the assumed faults are shown. A similar distance to rupture is evident for both locations. Spectral analyses of the ground acceleration records are shown in Fig. 7. It is clear that the dominant period ranges up to 2 s at the near-source sites in Miyagi prefecture, while the dominant frequency increases up to a period of nearly 0.2 s at sites near the Tone river. THE EFFECT OF THE AMPLIFICATION OF MOTION IN THE SURFACE ALLUVIUM Ground motion records at two selected K-NET stations and one selected KiK-net station near the Tone river do not differ a lot. The durations of the shakings were rather long, 100-200 s, suggesting that the soil was subjected to a large number of load reversals. The selected stations are located at soft alluvium soils with shear-wave velocities of less than 500 m/s in the upper 20 m. The soil profiles are similar, consisting of sand or sandy soil covered by volcanic ash clay (Fig. 8). Considering the high level of ground water, as the Tone river is near, these ground conditions combined with a large number of load cycles are sufficient for triggering the liquefaction. The seismograph CHBH13 is part of the KiK-net network, and the motion records at the surface and at the bedrock (1300 m deep) are available. Soil profile from deep borehole of seismograph CHBH13 is presented in Fig. 9, while Fig. 10 shows the effect of surface alluvium layers on the wave propagation. A clear amplification of the acceleration in the top soft soil layers can be seen. Results of spectral analysis of ground motion records at bedrock and surface are compared on Fig. 11. Besides the amplification, one can observe that surface record results dominant period close to 0.2 s which is very similar to those obtained in other locations near Tone river (Fig. 7), while slightly smaller dominant period ranges of nearly 0.1 s is achieved at the bedrock. 100 -MYG010N-S MYG010E-W MYG010U-D S. 100 Period [second] Period (second] Figure 7. Spectral analysis of ground-motion data (5% damping) at (left) the near-source K-NET seismograph (MYG 010) and (right) the K-NET seismograph near the Tone river (IBR 016). Figure 8. Typical shallow (20 m) soil profile along the Tone river, K-NET station IBR 016 (data from [7]). ACTA G£OT£CHNICfl SLOVENICA, 2012/2 13. S. LENART ET AL.: SOIL LIQUEFACTION IN THE TONE RIVER BASIN DURING THE 2011 EARTHQUAKE OFF THE PACIFIC COAST OF TOHOKU Soil column S velocity [m/s] P velocity [m/s] 500 1000 B 0 1000 2000 3000 0 1000 2000 3000 4000 5000 6000 Sandstone, siltstone. mudstnne, conglomerate Sanations, siltstone, sandy siltstone Tuff, sandstone Metaniorphic roclc, chert Chert Figure 9. Soil profile from the deep borehole (1300 m) at the KiK-net station CHB H13 (data from [7]). £ 25