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8-4. Division of Disaster Mitigation Science

Earthquake Engineering

　The objective of earthquake engineering is to prevent or mitigate disasters, especially damage to structures, caused by earthquakes.
Academic or practical methods of simulating or evaluating 1) design earthquake motion, 2) response, 3) seismic performance, 4)
damage, or 5) risk, are developed and improved through observation, experiment ( Fig. 1 and Fig. 2 ), analysis, and theory, for application
to seismic design, repair and strengthening of structures and ground.

Fig.1. Shaking table test of 6-story one-third scale reinforced concrete wall-frame building structure with soft
   first story, July 2000. (a) Test specimen before test. (b) Response of overall and first story drift angles to the
                             maximum input motion of equivalent Takatori 135 kine.
 

Fig.2. Performance verification test on a simple and economical method of strengthening reinforced concrete
    columns against axial load collapse during major earthquake, August 2000. (a) Reinforced concrete specimen
   strengthened with sheet after cyclic lateral load test. (b) Hysteresis of reinforced concrete column specimen
 without strengthening failed in shear and axial load collapse. (c) Hysteresis of the column specimen strengthened
                       with sheet maintaining axial load capacity until amazingly high drift.

Damageability of strong ground

　In order to mitigate earthquake disaster, we have to investigate damageability of strong ground motions in addition to investigation of
strong ground motions and structures. Damageability of strong ground motions, i.e., structural damage, is determined by the
relationship between intensity of strong ground motions and strength of structures, therefore, we have to grasp characteristics of
both strong ground motions and structures. The result of the investigation leads to index of representing damageability of strong
ground motions, such as J.M.A. (Japan Meteorological Agency) seismic intensity. Such index is essential to grasp actual structural
damage rapidly and accurately for quick post-disaster response. Slow post-disaster response enlarged human damage in the 1995
Hyogoken-Nanbu Earthquake. However, many strong ground motions records are need to be obtained under the same structural
condition for such investigation. In the 1999 Chi-Chi earthquake, Taiwan, many strong ground motions records were obtained under
the same structural condition. This made it possible to investigate index of representing damageability of strong ground motions from
actual structural damage for the first time. The relationship between often used index and proposed index (elastic response with a 5%
damping factor around 1.0 sec) and actual damage (area damage level) is shown in Figs. 3 and 4, respectively. Almost no correlation was
found for PGA and a weak correlation for PGV, SI and JMA intensity whereas, there is good correlation for proposed index with area
damage level.

Fig.3. Relation between index of representing damageability and area damage level

  Fig.4. Relation between elastic response (5% damping for period 1s (left) and 0.8s (right)) and area damage level.


 



Applied Seismology

　Seismologists are responsible for mitigating earthquake disasters, so precisely estimating seismic ground motions is one of our most
important tasks. We are working on any problems related to this task, and our recent research subjects are as follows:

1) Modeling earthquake faults and their rupture processes as sources of seismic ground motions (e.g. Rupture process of the 1995
Kobe earthquake, Fig. 5).

2) Exploring detailed images of underground structures, which should influence seismic ground motions (e.g. three dimensional ray
tracing in a subduction zone and tomographic analysis of explosion data).

3) Numerical simulation of seismic ground motion caused by fault rupture in a three dimensional heterogeneous structure (e.g.
simulation of strong ground motion caused by the 1995 Kobe earthquake, Fig. 6).

Fig.5. Fault model (lower) and its slip distribution (upper) for the 1995 Kobe earthquake (after Yoshida, Koketsu et
                                                  al. 1996).


 

  Fig.6. Numerical simulation of seismic ground motion caused by the 1995 Kobe earthquake. The upper and lower
   diagrams represent the distribution of peak ground velocities and their temporal snapshots, respectively (after
                                         Furumura and Koketsu, 1998).

Strong Motion Seismology

　The major objectives of our research group are to understand the nature of near-field strong ground motions and to quantitatively
estimate the effects of complex surface geology on strong motions by mainly observational approaches. Strong ground motions at the
near-field contain high frequencies in the accelerogram (Fig. 7, the first trace) as well as quasi-static movements due to near field
terms before the S-wave arrival in the displacement, as shown in Fig.7 (the third trace). On the other hand, events that are supposed
to be simple rupture have also been observed. Figure 8　shows the example of simple and similar accelerograms, however, their high
frequency spectra and the waveform at later parts of pulses are significantly different. This difference can be interpreted by introducing
a rupture deceleration model (Nakamura and Kudo, 1997).

　Surface geology affects very much on strong ground motions. Figures 9 and 10 shows the relative intensity of 1-2 seconds ground
motions in Ashigara valley referring to a rock site. Areas showing high intensity are found in the central and in the south-west of the
valley and they are correspond to a deepness of sedimentary basin.
 



Fig.7. Observed acceleration, integrated velocity and displacement (EW) at IAR form the 1997 Ito-oki earthquake
                                                    (M5.7).

 Fig.8. Two simple events differ at only high frequency. The difference can be interpreted as that the rupture stops
                      suddenly (event A) and its velocity decelerates during 30m (event B).
 

Fig.9. Distribution map of relative intensity of 1-2 sec. motions in Ashigara valley estimated by using the records
                            of remote and large earthquakes (Uetake and Kudo, 1998).

Fig.10. Underground structure models estimated by refraction surveys and array microtremor measurements
                                              (Kanno et al., 1998)

Strong Motion Estimates

　Strong ground motion depends upon the causal earthquake fault, the propagation path, and a variety of local sites near the ground
surface. We need to understand strong motion characteristics for earthquake-resistant structure design and urban
earthquake-disaster prevention. Structure-soil interaction effects must be taken into account in a soft soil area with a high seismic
risk. Fig. 11 shows the acceleration amplification of seismic wave (-86m, -30m, 0m) due to very soft surficial layers.

Fig.11. Acceleration amplification due to very soft surficial layers.

Numerical simulation of strong ground motion

　The seismic wavefield is significantly affected by 3D variations in crustal structure both in the source zone itself and in propagation
to some distance. Such effects can be modeled in large-scale numerical 3D simulation of seismic wave propagation by using parallel
computers. We developed a hybrid simulation code with pseudospectral representation for horizontal coordinates and finite-difference
in depth. This arrangement improves parallel efficiency with high speed-up rate using large number of processors (Fig. 12). Numerical
modeling of strong ground motion with a realistic 3D model for structure in western Japan (Fig. 13) provide a good understanding of
seismic wave propagation from a subduction zone event such as the 1946 Nankai earthquake.

Fig.12. Schematic illustration of the PSM/FDM hybrid calculation. The hybrid method offers fairly good speed up
               rate even using large number of processors (Furumura, Koketsu and Takenaka, 2000).

Fig.13. Snapshots of horizontal ground motion from 3D simulation of wave propagation from a Nankai subduction
                                   earthquake (Kennett and Furumura, 2000).

Tsunamis

　A tsunami is a sea wave generated by a submarine earthquake. It has been clarified that the magnitude of tsunamis in the sea regions
of Indonesia and Philippines exceed by one to two grades larger than those generated by earthquakes of the same magnitude of the sea
region of the Japanese Islands (Fig. 14).

　At midnight on June 3, 1994 an earthquake with a magnitude of 7.6 occurs in the sea region of East Java, and the inhabitants of
Pancer village felt a small tremor, but a huge wave with a height of 9 meters washed away almost all of the houses in the village.
 


Fig.14. Almost all of the houses were washed away in Pancer village by the tsunami generated by the 1994 East
                                               Java earthquake.

Historical Seismology

　We started the historical study on earthquakes in 1975. The documents collected, which contain descriptions of earthquakes and
tsunamis, were published as a series of books, "Shinto-Nihon Jishin Shiryo (New collection of materials for the study of historical
earthquakes in Japan)", which consists of 21 volumes, amounting to 16, 812 pages altogether. The materials collected are used in the
long-range prediction of earthquakes and the mitigation of disasters caused by large earthquakes and tsunamis (Fig. 15).

       Fig.15. The tsunami records in "Tanabe-cho Daicho" (The official diary of the mayor of Tanabe City).


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