Report on the surface ruptures of the Taiwan earthquake on Sept. 20, 1999

Professor, Geodynamics Division, Earthquake Research Institute, University of Tokyo   

Tetsuzo Seno

 I joined the research party of Ministry of Education of Japan (headed by Prof. Hirokazu Iemura, Kyoto Univ), and accompanied Prof. Kenshiro Otsuki (Tohoku Univ) and Prof. Chao-Nan Yang (Nat. Taiwan Univ), who survey the surface faults, with Prof. Wang (retired) and Prof. Wuu-Liang Huang (Nat. Taiwan Univ). Our survey started on Oct. 9, and I and the other two returned to Taipei on Oct. 11, leaving Drs. Otsuki and Yang in the fault area.
 By this survey, surprizing facts came out. I then make a short report on these.      From the right, Drs. Otsuki, Yang, Wang, Huang, and Seno
       Japanese version   Last update on March 28, 2000, 1st version on Oct. 14, '99
What's NEW Number of visitors since Oct. 21, '99
Explanation of liquid like behaviour 10/16/99 Explanation of a large uplift at the NW corner 10/17/99
Future research plans 10/19/99 GPS data support the inelastic deformation at the NW corner 10/21/99
Implications for tsunami earthquakes 10/21/99 Where do tsunami earthquakes occur? 10/22/99
Image of faulting 10/22/99 Pictures of the dam damage, the fault scarp in the peach farm looked from SW 10/23/99

The Chichi earthquake as analogues of off-Sanriku, Japan Trench events, and the Nankai Trough events 10/24/99 The revision of the explanation for the SW slips, and the surface rupture looks more powerful than landslides 10/24/99
Corrections to the coseismic deformation pattern revealed by GPS data 10/25/99 Revision of the deformation pattern of sediment in the trench wedge due to faulting 10/29/99
Revision of the tectonic interpretation 10/30/99 The detailed pattern of the surface displacement suggests ductile deformation 1/21/00
Withdrawal of the slow slip of the surface faulting 1/27/00 Abstract: The 1999 Chi-Chi, Taiwan Earthquake: A subduction zone earthquake on land 3/28/00
Abstract: The Sept. 21, 1999 Chi-Chi Earthquake in Taiwan: Implications for Tsunami Earthquakes 6/9/00 Abstract: Sediment effect on the tsunami generation of the 1896 Sanriku ttsunami earthquake 4/6/01

Surface faulting occurred very slowly (See correction below), and some part of the material of the hanging wall behaved in a ductile way.

1.The surface rupture did not generate seismic waves, or strong ground motions.

2.Although the slip vector of the event is N55 deg W(Harvard CMT), there are at least three fault segments which showed slips in the SW-NE direction.

3.At the northern bend of the fault, there are multiple frontal thrusts and buldges which are not conneted.

4.Some ruptures did not cut the neotectonic morphological features.

 Based on these, I conclude that the surface faulting occurred very slowly, and some material on the hanging wall behaved in a ductile way with a small rigidity. Therefore the surface faulting cannot be regarded as earthquake faulting. This surface faulting should be rather regarded as a slow tsunami generating event which occurred in the sedimentary wedge of a deep-sea trench, but in Taiwan, exposed on land (See tectonic interpretation).

 The slow faulting mentioned above was not correct as far as lookingstrong motion records close to the fault. I then withdraw this. However, to explain the peculiar damage features, the faulting near the surface would not have accompanied stress drops, i.e. with a low coefficient of friction and a low rigidity.

    

1.No strong ground motions Pictures See Index map below and Analogues for locations.
As well known already, the surface ruptures appeared in a N-S direction along the foothill over 80 km in length. However, the ruptures did not generate seismic waves or strong motions. There is no damage for buildings which do not cross the fault, even if they are very close to the fault trace. I have seen only the northern half, but Dr. Yasutaka Ikeda, Univ of Tokyo, who surveyed the area extensively with Dr. Hiroshi Sato, Univ of Tokyo, told me that in the south, the feature of the damage is the same as in the north.

Examples

Photo 1 Middle of the fault A factory in Chu-Tzu Keng northwest of Wufeng

Photo 2 Northern part of the fault Stores in Bu-Tzu Keng

Photo 3 Northeastern bend of the fault Hanging wall of the fall in the river

  

Grabels in the river floor or in the former river floor did not move, which indicate no acceleration.

Photo 4 Peach farm northeast of the fall

Photo 5 A river floor in Chu-Tzu Keng

2.Some faults show SW-NE slips
The slip directions almost coinside with the slip of the earthqauke mechanism solution in most locations (personal communication with Drs. Ikeda and Sato). However, there are at least three segments which showed SW-NE slips.

Photo 6 Middle of the fault An athletic field in Kuang-Fu New Village south of Wufeng

Photo 7 East of the northeastern bend A load in Shicheng

Index Map(Based on the map from Central Geological Survey)

2-3-4.Multiple thrusts and etc

The fault bends to the northeast at the northern end (See fault traces). This part is out of the trace from the Chelungpu fault which had been recognized geologically. In this part multiple thrust and buldges occurred, and they are not well connected with each other. Generally the slip directions of the faults here are consistent with the slip of the earthquake mechanism. Prof. Yang showed us a frontal thrust, which is located on a small hill north of the dam damage site (see location). Prof. Otsuki and I found a NW striking thrust at the northeast slope of the hill, which showed a SW slip, perpendicular to the earthquake slip.

There appeared a large uplift amounting to 10 m in this area. This anomalous feature may be related to the liquid like behavior of the hanging wall (See explanation and the revised version).

We tried to collect water level change data using wells of private houses by asking four or five persons. However, wells have not been used at present in this area. No sand eruption has been witnessed by our party.

5.Conluding remarks

Since just after the occurrence of this earthquake, I have been insisting that the event represents an interplate event (See tectonic interpretation). However I had not imagined that the faults on the surface are much different from the so-called active faults on land in Japan. However, the characteristics of the surface faults are quite different from those active faults, such as the Nojima fault of the 1995 Kobe earthquake. The surface faults of the Taiwan earthquake should rather be regarded as faulting under the sea, near the trench like the Japan Trench and the Nankai Trough, which ruptured the sedimentary wedge. However the trench wedge and accretionary prism are exposed on land in Taiwan. The sediment would have a small rigidity and explain the slip in the peculiar directions and the anomalous uplift at the NW corner (See also new interpretation). If the event occurred under the sea, the uplift amounting to 10 m at the northwestern corner would have generated a tsunami.

     Japanese edition

      Tectonic interpretation

  

Since the initial rupture occurred at the hypocenter, the earthquake faulting would have continued further to the shallower part of the accretionary prism and to the deeper part along the decollement (See image of faulting). Note that strong motions and seismic waves were generated in the deeper part of the earthquake fault and caused damages on the hanging wall to the east far from the surface ruptures (personal comm. with Prof. Y.-B. Tsai, National Central Univ). So the present event is regarded as a tsunami earthquake in the shallow part near the trench and an ordinary event in the deeper part (See analogues). More intimate surveys are necessary in the near future before the morphological features are collapsed down and damages are repaired (See future research plans).

The party shared the idea on the ductile behavior of the fault, and agreed that it is difficult to explain the deformation by the left lateral motion in the N-S striking shear zone. However, the idea on the slow slip cutting the sedimentary wedge came out on my mind in the last night on Oct. 11 in Taipei.

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