GEOPHYSICAL AND GEOTECHNICAL FEATURES OF THE LEVEE SYSTEMS DAMAGED BY THE EAST JAPAN EARTHQUAKE Tomio INAZAKI, Public Works Research Institute, Tsukuba, JAPAN Abstract Levee systems in Kanto Region, central Japan, were severely damaged at many places caused by the long-lasting strong ground motion of the magnitude (Mw) 9.0 East Japan Earthquake, which occurred at 05:46 UTC on Friday, 2011 March 11, even located more than 400 km far from the epicenter. Since 2005, we have conducted integrated geophysical surveying for the safety assessment of levee systems at 39 actual levee sites in Japan. Among them, severe damage took place in two sites by the East Japan Earthquake just at the anomaly part delineated by the survey. The anomaly part in one site was characterized as low S-wave velocity and low resistivity both for levee body and substrata. After the earthquake, we conducted comparative surveying on the same levee but the damaged part of which had been soon repaired. As a result, the characteristic low S-wave velocity and low resistivity zone was again identified just at the damaged or repaired part where substantial top subsidence had occurred. This suggests a physical model that nonlinear loosening of underlying clay layers had caused the ground failure and resulted in the damage of levee systems. The other site, where large sliding had taken place on a levee slope during the earthquake attack, was featured by the existence of high resistivity anomaly in the levee body. The anomaly was also identified by the comparative surveying at the same part in the line where the slope sliding had occurred. A different type of levee failure mechanism was interpreted as resulting from high contrast of physical properties in levee body, based on our integrated geophysical surveys. Thus the corresponding survey results lead us to the usefulness of the integrated geophysical surveying for understanding levee failure mechanism and for the assessment of present conditions of levee systems attacked by the earthquake. Introduction A devastating earthquake of magnitude (Mw) 9.0, named formally the 2011 off the Pacific coast Tohoku Earthquake, but commonly called as the East Japan Earthquake, occurred at 14:46 JST on Friday, 2011 March 11 with the epicenter approximately 70 km off the Tohoku Region, northeastern Japan. It was the largest earthquake Japan ever had in its history and one of the 5 largest earthquakes in the world since 1900. The earthquake caused a devastating tsunami disaster in northeast Japan. Actually, tsunami waves traveled up to 10 km inland by overtopping maximum 15 m high tsunami walls or through river channels followed by rupturing 5 m high river levees. Accordingly, coastal areas more than 500 square km were inundated along the Pacific coast region and more than 19,000 casualties were lost or missed by tsunami. Even worse was the fatal nuclear accident at ill guarded Fukushima Nuclear Power Plant. The nuclear accident forced more than 200,000 local residents to evacuate to the outside of the 30 km exclusion circle zone. Furthermore, the accident exposed worldwide the hazardous nature of nuclear power generation technology. On the other hand, the accident anew reminded us the importance of scientific risk assessment based on detailed geological and geophysical investigations. This is the primary lesson should be learned from the East Japan Earthquake.
SAGEEP 2012 Tucson, Arizona USA http://www.eegs.org Second feature of the East Japan Earthquake was its long duration time of strong motion. For instance, strong ground motion was recorded during more than 2 minutes at Tokyo, 400 km far from the epicenter. This unusual length of strong motion caused serious ground failure as typified by liquefaction in many places in the Kanto Plain, the largest tectonic sedimentary basin in Japan. Consequently, more than 1,000 sections of earthen river levee systems were severely damaged (Fig. 1). Ministry of Land, Infrastructure and Transportation (MLIT) was undertaking rushed repair of failure levees before the rainy season beginning from June in 2011 and is now conducting full-spec repairment. Concurrently to the repair work, drilling survey was conducted to clarify the internal conditions of levee systems. As easily imagined, drilling is not an Figure 1. A map showing two sites where comparative appropriate method to interpret levee survey was conducted (pink stars), and heterogeneous structure. In contrast, major sites where significant ground failure geophysical survey can provide 2-D or 3-D took place (red circles) caused by the 2011 structure involving discontinuity. We have East Japan Earthquake. proven the usefulness of geophysical methods to the safety assessment of levee systems through our demonstrative surveying at 39 actual sites and confirmed the advantages of integrated method or combination of seismic and electrical method (Inazaki, 2007). Among them, two sites located in the Kanto Plain (Fig. 1) were heavily damaged by the East Japan Earthquake just at the anomaly parts identified beforehand, which eventually demonstrated the practical usefulness of the geophysical investigation for the assessment of levee systems not only for Figure 2. A digital elevation map showing the levee system on seepage risk but also for seismic which geophysical surveys were repeatedly conducted. risk. This indicated that we Failure occurred at 35.0K post where the levee overly on geophysicists should have driven an abandoned channel. the expansion of both research r P t n i r p e
and routine survey for the vulnerability assessment of levee systems using integrated geophysical method more powerfully. After our proposal, Kanto Regional Development Bureau, a branch of MLIT, has decided to adopt our integrated geophysical method and apply it widely to the safety assessment of the damaged levee systems. In order to conduct this routine surveying successfully, it is necessary to clarify the criteria to discriminate potential damaged parts in levee systems. We therefore re-analyzed the survey data obtained before the earthquake, and compared them with those along the same line but acquired after the earthquake. Geophysical Characterization of Failure Parts As described above, two sites were severely damaged by the East Japan Earthquake, where integrated geophysical surveys had been conducted, and levee failure occurred just at the part that we had delineated heterogeneous structure in the geophysical profiles as interpreted as an anomaly. Figure 3. Integrated geophysical survey results along the levee of Kokai River, about 300 km away from the epicenter of the 2011 East Japan Earthquake (Kokai_35L in Fig.1). Field survey was conducted in 2005, and had depicted anomaly zones in levee body. Levee failure took place at the shaded part about 80 m in width. Top of the levee settled down about 0.6 m with slope failure. Fissures and accompanying liquefaction were observed on the ground adjacent to the levee body in the failure zone. (a): S-wave velocity structure reconstructed from LS_SW data; (b): Resistivity profile along the levee inverted from CCR data; (c): Seismic resistance vulnerability section classified into 4 categories based on S-wave velocity and resistivity; (d): Landform along the levee interpreted from aerial photos. Note that the levee failure occurred just on an abandoned channel (revised from Inazaki, 2007).
Conversely, the actual levee failures gave us valuable information on the vulnerability assessment process, or how to interpret geophysical survey results. Figure 2 shows the levee system on which we have repeatedly conducted many kind of geophysical surveys including high-resolution surface wave method (Hayashi & Suzuki, 2004) using Land Streamer (Inazaki, 1999) for the S-wave measurement (LS_SW), capacitively-coupled resistivity (CCR) survey using OhmMapper (Geometrics, 2001), Slingram electromagnetic (EM) survey using GEM2, GPR survey, and high-resolution S-wave reflection survey since 1999. The survey line was set on earthen levee along the left side of Kokai River, a branch of Tone River, about 34 km upstream from the confluence. The levee system lies basically parallel to the present channel, straightened through repeated improvements over the years, and crosses over abandoned channels at 35.0 and 34.2 K posts. There remains a small oxbow lake in the landside. The failure just occurred along the levee at 35.0 K about 80 m in width. Top of the levee settled down about 0.6 m with slope failure. Fissures and accompanying liquefaction were observed on the ground adjacent to the levee body in the failure zone. Figure 3 summarizes integrated geophysical survey results obtained in AUG 2005, before the earthquake. S-wave profile along the levee is characterized as low in levee body about 140 m/s and moderate but partly low in the substrata (Fig.3 (a)). Compared with S-wave profile, resistivity section shows significant heterogeneous structure in the levee body (Fig.3 (b)). This indicates that coarse or potentially permeable materials were used partly to embank the levee body. As clearly shown in Fig. 3 (d), the levee failure just occurred on an abandoned channel at 35.0 K. However, no clear sign on the failure was observed at other two zones where abandoned channel underlay. Figure 3 (c) is an interpreted section on the vulnerability of seismic resistance along the levee. The section is classified into 4 categories based on crossplot analysis of S-wave velocity and resistivity. Here threshold values of Fig. 4 Comparison of integrated geophysical survey results along the same line reconstructed from the data of field survey conducted in July, 2011 after the Earthquake. (a), (b), and (c): ditto to the above. Note that the profiles are basically concordant with those shown in Fig. 3 (revised from Inazaki, 2011).
140 m/s in S-wave velocity and 100 Ω-m in resistivity were adopted for the classification empirically. As shown, the failure occurred at one of the zones characterized as low S-wave velocity and low resistivity zones both in levee body and substrata. Figure 4 shows an integrated geophysical section for the survey conducted in JUL 2011, just after the earthquake. Both S-wave profile (Fig.4 (a)) and resistivity profile (Fig.4 (b)) along the levee are basically similar to those shown in Fig. 3. Note that warm colored parts or low velocity zones expand their area over the S-wave profile. This indicates loosening of ground and levee materials due to the earthquake. To make clear the change in geophysical values before and after the earthquake, we calculated average values from the profiles. Figure 5 compares the averaged geophysical values along the line for levee body down to 5 m from the surface and substrata from 6 to 12 m. As shown in Fig. 5, averaged resistivities in levee body slightly increased across the line, whereas those in substrata were basically same owing to the saturated condition below ground water level near the river. In contrast, averaged S-wave velocities showed complicated change, but it is able to distinguish general lowering both in levee body and substrata. Moreover, it is notable that the failure part is characterized by low S- wave velocities and low resistivities both in levee body and substrata for the averaged values. This indicates that these geophysical properties are useful for classifying seismic vulnerability along with seepage risk we proposed before (Inazaki, 2007). Overall increase in resistivity in the levee body after the earthquake is interpreted as the change in moisture condition in vadose zone, but may be due to the effect of some sort of coseismic or post seismic change. Fortunately, we have conducted resistivity survey repeatedly along the levee, and are able to infer time lapse change in resistivity. Figure 6 compares resistivity profiles along the line obtained from 2004 to 2011, after the earthquake. The profiles were reconstructed from OhmMapper survey data and slingram EM data using GEM2. As shown, the Fig. 5 Comparison of average profiles along the survey line calculated from the sections shown in Fig. 3 and Fig. 4. Resistivities in the substrata show basically same values. Slight decrease in S-wave velocity was recognizable both in levee body and substrata after the Earthquake. Failure part (shaded by pink) is discriminative as relatively low Vs and low resistivity (revised from Inazaki, 2011).
SAGEEP 2012 Tucson, Arizona USA http://www.eegs.org resistivity profile inverted from GEM2 data is quite different with other profiles, perhaps due to inappropriateness of inversion process. Resistivity profiles obtained by OhmMapper are basically similar with each other, but slight changes are identified especially in levee body, or vadose zone. High resistivity anomalies in the levee body at 34.4K and 35.4K vary unevenly, but the strongest anomaly at 35.2K shows relatively small change in resistivity. Low resistivity anomaly in levee body is clearly delineated at 35.0K, just at the failure part, not only in the post seismic profiles but also in those obtained before the earthquake. Because the failure part had been soon repaired before conducting geophysical surveying, this low resistivity zone in the post seismic profiles hinted that fine fill materials were used for the repair. However low resistivity in the pre-seismic profiles indicates the failure part had been filled originally with soft and loose materials. Owing to unsaturated condition, resistivity in levee body would vary reflecting the moisture content. This indicates attention should be paid when t n i r p e r P Fig. 6 Time lapse resistivity profiles along the survey line reconstructed from the data obtained using OhmMapper (CCR) or a slingram EM tool.
Figure 7. Schematic diagram assessing seepage and liquefaction vulnerability (left) and plastic deformation during strong earthquake (right), on crossplot chart of S-wave velocity and resistivity based on general relationship between geophysical and soil properties (revised from Inazaki, et al., 2011). classifying target levee systems for the vulnerability assessment by means of resistivity data, namely classification criteria are flexible and should be determined making use of not only geophysical data but also of geological and geotechnical information. Fortunately, we were able to utilize several drill data conducted along the levee as shown in Fig. 3 and Fig. 4 for seismic vulnerability assessment, and adopted 140 m/s for S-wave velocity and 100 Ω-m for resistivity as the threshold values. Figure 7 shows schematic classification charts for assessing seepage property, liquefiability, and plastic deformation characteristics of unconsolidated fill materials and sediments, with reference to grain size and stiffness. That is, the coarser in grain size and the looser in stiffness, the more unsafe in seepage vulnerability and liquefiability. Similarly, seismic resistance of soft soil is also characterized by grain size and stiffness but in different way. Namely, the softer in stiffness and the finer in grain size, the more unsafe on plastic deformation. Then when the relationship between these physical properties and geophysical properties of soils and sediments is clarified, we can directly estimate vulnerability condition of a target levee system from the geophysical properties. Conclusions Geophysical investigation method is expected to play an important role for the safety assessment, but it was needful to demonstrate its usefulness. We have successfully shown its usefulness to seepage assessment. In addition, reexamination of survey results for levee failure sites caused by the East Japan Earthquake revealed practical usefulness of the geophysical investigation for the seismic risk assessment of levee systems. Further applications of the integrated geophysical method to damaged levees are essential to find appropriate survey parameters and to establish the optimal criteria for both seepage and seismic safety assessment.
Acknowledgements The author wishes to thank Dr. K. Hayashi of Geometrics for his supports on field surveys and data analysis. A part of this study was conducted in cooperation with the SEGJ Levee Consortium. The authors acknowledge them who took charge of field measurements and tool operations. The author also gratefully acknowledges to Shimodate River Office for many conveniences afforded to conduct field work and provided useful data for this research. References Geometrics, 2001, OhmMapper TR1 operation manual, Geometrics Inc., 147p. Hayashi, K. and Suzuki, H., 2004, CMP cross-correlation analysis of multi-channel surface-wave data, Exploration Geophysics, 35, p7-13. Inazaki, T., 1999, Land Streamer; a new system for high-resolution S-wave shallow reflection surveys, Proceedings of the 12th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP1999), 207-216. Inazaki, T., 2007, Integrated geophysical investigation for the vulnerability assessment of earthen levee, Proceedings of the 20th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP2007), 8p. Inazaki, T., Hayashi, K., and SEGJ Levee Consortium, 2011, Utilization of integrated geophysical investigation for the safety assessment of levee systems, Proceedings of the 24th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems (SAGEEP2011), CD-ROM, 9p. Inazaki, T., 2011, Geophysical features of the levee systems damaged by the East Japan Earthquake, Proceedings of the 125th SEGJ Conference, 17-20. (in Japanese with English abstract).