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7．Researches of Earthquakes and Volcanic Eruptions in the  Recent Few Years　

7-1． The 1999 Chi-Chi, Taiwan, Earthquake

　In the early morning of September 21, 1999, the destructive earthquake of Mw7.7 struck central
Taiwan near the small town of Chi-Chi. There were 2297 deaths and about 6800 buildings destroyed. It
was accompanied by extensive faulting, which trends about north-south and was traced for about 80
km along the Chelungpu fault. The principal surface expression of faulting appeared on the riverbed and
it created a new waterfall (Fig. 1).

　Taiwan is located in a collision zone between the Asian continent and the Luzon Arc on the
Philippine Sea plate. The Tertiary sediments on the continental margin have been off-scraped and
sliced by thrusts due to the collision, forming the uplifted mountain range. One of the thrusts is the
Chelungpu fault on which the Chi-Chi earthquake occurred (Fig.2). Because the collision is young, the
geological structure is close to that of the subduction zone.

Fig.1. The Tachia River in Fengyuan was cut by an 8 m displacement that created a new

                          water fall and destroyed a bridge.

Fig.2. Geological cross-section of central Taiwan. The Longitudinal Valley is the suture
 zone of the collision, but the relative plate motion is also consumed at the thrusts and
  the decollement beneath the Western Foothills. The ChiChi earthquake occurred on
                                one of such thrusts.

Fig.3. Maximum shear strains of the Taiwanese islands estimated by the Least Square Prediction method for the period 1990-1995. Shallow earthquakes (d≦30km) of  magnitude greater than or equal to 4.0 for the period from 1990 to 1995 are plotted  (ISC database). Black asterisk shows the epicenter of the Chi-chi earthquake and the red line denotes the surface faults.


In order to help understand tectonic background of the 1999 Chichi, Taiwan earthquake, ERI analyzed
data from the repeated GPS surveys conducted during a period of 1990-1995 in Taiwan. Figure 3
shows obtained pattern of maximum shear strain in the period. Shallow earthquakes in the same period
are also shown. The figure indicates that the 1999 earthquake occurred in the area of relatively low
strain rates between the eastern Coastal range and the southwestern area of high strain rates.

　Soon after the earthquake, ERI set up the special page on the web site, and tried to provide
information about relevant researches and surveys on the Chi-Chi earthquake. A preliminary result of
the source process was obtained on the following days from teleseismic data, notifying that the major
moment release occurred about 40 km north of the epicenter. Figure 4 shows a spatial distribution of
the fault slip obtained from the joint analysis of teleseismic data and strong motion data. The maximum
slip amounts to about 9 m near the end of seismic fault.

　In the strong motion data, we find very large long-period ground velocities around the northern part
of the Chelungpu fault, but they did not cause very severe damage. Damaging ground motions with large
velocities and accelerations happened in the epicentral area and the east of it, which are 10 to 20 km
or more away from the fault. ERI investigated the damage to the buildings close to observation stations,
and derived the relation between the actual damage and strong ground motion by carrying out
earthquake response analyses with strong motion records as inputs.

　The surface ruptures with a large displacement, associated with the Chi-Chi earthquake, appeared
along the Chelungpu fault, which is an out-of-sequence thrust in the fold-and-thrust belt in the
western Taiwan. To determine the exact amount of net-slip and direction, ERI carried out field survey
in October 1999. The maximum amount of net-slip obtained was 10.1 m in the northern part of the
fault. The slip directions are concordant with those from GPS measurements. The surface ruptures
occurred along the pre-existing active faults determined by tectonic geomorphology, except for the
short segment in the northern part of the fault. The survey of tectonic geomorphology across the
Chelungpu fault in 2000, revealed the vertical crustal movement last 100 ka, suggesting that the
amount of slip along the frontal thrust (Changhua fault) is as large as one of the out-of-sequence
thrust (Chelungpu fault).

　ERI have conducted an aftershock observation of the 1999 Chi-Chi, Taiwan, Earthquake. We deployed
20 seismographs in and around the focal area. For two months observation we recorded a large number
of aftershocks. Taking the lateral heterogeneity in the crustal structure into account, we have a clear
distribution of aftershocks (Fig.5). There are three particular trends in an east-west cross section: an
east dipping distribution, a very low angle distribution, and a deeper distribution. They correspond to the
fault plane of the main shock, the supposed decollement between the accretionary wedge and the
upper boundary of the Eurasian Plate, and activities in the Eurasian plate.

　A plausible range of the number of major aftershocks of the 1999 Chi-Chi, Taiwan earthquake was
tried to predict. Based on the modified Omori formula, the number expected in a certain time period
was estimated. Among 13 trials of prediction, 11 cases were successful during 2 months after the main
shock. Such an attempt will contribute to the temporary planning of disaster prevention and elimination
of over-anxiety among the people in the epicentral area.

　In order to observe postseismic deformation, ERI deployed eight single frequency and two dual
frequency GPS receivers about three weeks after the Chi-Chi Earthquake. The single frequency
receiver is developed in ERI. Ten GPS receivers formed the linear array perpendicular to the strike of
the earthquake fault. This GPS line array crosses the fault trace on the surface from Taichung City to
central mountain area because the surface projection of the maximum slip area on the fault is about
40 km east of Taichung City. We detected the movement to the west at the sites on the hanging wall
in the eastern area, while no movement on the footwall in the western area. The displacement rates
are 1 to 1.5 cm/month, suggesting heterogeneous postseismic deformation.

Fig.4. Spatial distribution of the fault slip obtained from teleseismic data and strong motion data. Star indicates　the epicenter and solid curve indicates the Chelungpu fault

 

 

Fig.5. Aftershock distribution with a station correction. Epicenters (upper) and a depth distribution (lower) are shown. A star and inverse triangles indicate the main shock andobservation stations, respectively.

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