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Geological Background of Sichuan Earthquake - Mechanism of Thrust Fault Movement -

Hideo Takagi,
Professor of Faculty of Education and Integrated Arts and Sciences, Waseda University

Eearthquake of Mw 7.9 (Mw: moment magnitude) occurred with the focal depth of 19 km at about 90 km northwest from Chengdu, the capital of Sichuan, at 14:28 on May 12, 2008 (local time). The energy of the earthquake is about 30 times as that of the Kobe Earthquake in 1995 (Mw 6.9); due to the collapse of houses with poor earthquake-resistant design, great damage including more than 300 thousand casualties and missing persons (released by the Chinese Government) occurred. This earthquake following the major disaster by a cyclone that had occurred in Myanmar just one week before reminds us anew of the fearfulness of natural disasters. Based on the information offered by the US Geological Survey (USGS), which has collected and released quickly geological information about this earthquake (hereinafter referred to as the Sichuan Earthquake), the geological background that caused this earthquake will be described.

Large altitude difference and weak fault zone

Although the Sichuan Basin is a one with an altitude of about 200 - 750 m, the east part of the Tibetan Plateau, which lies in the west of the basin, has an altitude of higher than 4,000 m, and the boundary has a very large altitude difference (Figure 1). Therefore, this basin has the largest potential capacity for hydraulic power generation in China, but it also has high danger due to landslides.

Figure 1: Bird's-eye view of Sichuan Basin and its surroundings (Source: http://worldmap.at.webry.info/200610/article_1.html)

The active fault that caused this earthquake is located on the boundary between the Tibetan Plateau and the Sichuan Basin, and there is a fault zone (the Longmenshan Thrust Zone) running from northeast to southwest in Sichuan (Figure 2). The Longmenshan Thrust Zone consists of multiple reversed faults with northwest dip and slip sense of thrusts toward southeast (Figure 3).

Figure 2: Distribution of the epicenters of the main shock (yellow circle) and aftershocks (red stars), and relative displacement rates (yellow arrows) with Chengdu set as the fixed point by GPS (obtained from HP of Google Earth and MIT: http://quake.mit.edu/~changli/wenchuan.html). The Longmenshan Thrust Zone exists from northeast to southwest along the epicenters.

Figure 3: Movement image of Longmenshan Thrust Zone (correction to Kroner and Sengor, 1988, Episodes, v.11). Thrust movements (= reversed fault movements) to southwest accompanying folds.

The width of the thrust zone is estimated as about 60 km, and the convergence rate as about 4 - 6 mm/year (He and Tsukuda, 2003).
The aftershock distribution (Figure 2) and the shake map (Figure 4) show that the area of at least 250 km moved in the running direction along the earthquake source fault. In this district, an earthquake of M 7.5 occurred at a place about 100 km north of this epicenter in August 1933, and more than 9,000 people were killed including the victims due to the dam break that occurred after the earthquake.

Figure 4: Seismic intensity map by the USGS
The seismic intensity scale used is Mercalli scale, different from the one used in Japan; in the vicinity of the earthquake source fault, it reaches X (extreme).

Reversed fault observed at the site

The focal mechanism of this earthquake shows a reverse fault running from northeast to southwest (Figure 4). This figure shows by stereographic projection (lower hemisphere projection) the focal mechanism drawn by a USGS based on the first motion of the seismic waves recorded by seismometers around the world; black areas show pushing waves from the focus and white areas show pulling waves to the focus. In other words, this figure shows that the compression stress in the pulling direction moved the fault. This result, as though striped patterns of a "beach ball" are viewed sideways, shows reverse fault movement; an assumption that a reversed fault striking northeast and dipping northwest (red curve in Figure 5), one of two curves (called great circles) that make the boundary between pushing and pulling, moved agrees with the attitude of the Longmenshan Thrust Zone.

Figure 5: Focal mechanism (movement tensor solution) of Sichuan large earthquake and schematic view of reversed fault (Created by author based on the Global CMT by the USGS)

Professor Lin Aiming of the Shizuoka University conducted a field survey immediately, observed an earthquake fault having a throw (vertical displacement) of 3 m and a net slip of 6 m, and reported to the Japanese Society for Active Fault Studies as the first report in the early morning on May 17 that a reversed fault striking northeast and dipping 30属to northwest in the Longmenshan Thrust Zone moved as shown in Figure 5. Incidentally, the conversion of the displacement (D = 6 m) of this fault to magnitude by the empirical equation by Matsuda et al. (1975), Log D = 0.6 M - 4.0, shows M = 8.

Although reversed fault movement having compressing direction from west-northwest to east-southeast like this is similar to that of the Niigata-ken Chuetsu Earthquake in 2004 and the Niigata-ken Chuetsu-oki Earthquake in 2007 occurred successively in Japan, their geological backgrounds of occurrence are different from each other. Now then, why did the reversed fault occur in this district?

The Himalaya Range and the Tibetan Plateau, which lies in the north of the Himalaya Range, are called the roof of the world; they formed a very thick earth crust as thick as 70 km when the Indian Subcontinent on the India-Australia Plate collided with the Eurasian Plate about 50 million years ago and the continental crusts overlapped each other. The boundary of the Sichuan Basin at the east end of the Tibetan Plateau, which is pushed at a speed of 5 cm/year from south to north bring about a compressed zone nearly from northwest to south east (Figure 6) at eastern margin of the plateau; it is considered that this earthquake occurred due to the movement of the reverse fault (yellow thick line in the figure) to eliminate the strain. As surveys will be conducted from now, the actual state of the earthquake fault that appeared on the ground will become clearer.

Figure 6: Compressing directions acting on the Tibetan Plateau due to the collision of the Indian Subcontinent. Correction based on He and Tsukuda (2003) (from HP of the Earthquake Research Institute of the University of Tokyo: http://www.eri.u-tokyo.ac.jp/topics/china2008/). The yellow line is the Longmenshan Thrust Zone, and the green line lying in the south of the Longmenshan Thrust Zone is the Kangding Fault Zone where many large earthquakes have occurred./p>

Reduction of earthquake damage by historical study on active faults

In Japan where about 10% of earthquakes in the world occur, since there is no collision zone of continentals like this, no earthquake due to the geological background similar to that in the Sichuan Earthquake occurs. In the Japanese Islands, however, many active faults exist; it has particularly been known recently as described above that the active faults having caused large damage due to reversed fault movement exist along the Japan Sea. In addition to this, for example, the Kannawa/Kozu-Matsuda Fault Zone, Fujigawa (River Mouth) Fault Zone, and Inadani Fault Zone are of reversed fault type and contain active faults ranked as Class A (Figure 7). The reverse faults are considered to have been formed in these active faults because compressing forces (red arrows in Figure 7) act in the directions spreading out in a fan-like form against to collision (blue arrow) of the Izu-Ogasawara Arc with the Honshu Arc in the direction of northwest, which has been continuing from about 15 million years ago till today. It is necessary to study sufficiently the history of movements of these active faults in the past and make use of it for the prediction of the occurrence of future inland earthquakes and the reduction of earthquake damage.

Figure 7: Collision front of the Izu-Ogasawara Arc and main active faults having reversed fault movement (Correction to Active fault distribution map, Geological Survey of Japan, 1982)

Hideo Takagi
Professor of Faculty of Education and Integrated Arts and Sciences, Waseda University

Profile

brief sketch of portrait, academic background, literary works, etc.
March 1978: graduated from Department of Earth Sciences, Faculty of Science, Chiba University.
March 1980: finished the master course of Department of Earth Sciences, Graduate School of Science, Nagoya University.
Doctor of Science (Nagoya University, 1986)
Specialized assistant of Earth Sciences, School of Education, Waseda University from April 1982.

Literary works

Basic Earth Science, partner author, Asakura Publishing Co., Ltd., October 2002
Global Environment System (Chapter 2: Earthquake and Active Fault), partner author, Gakubunsha Co., Ltd., March 2004
Field Geology 7 "Metamorphism and Deformation", editor and partner author, Kyoritsu Shuppan Co., Ltd., May 2004
Earth, Environment, and Resources, editor and partner author, Kyoritsu Shuppan Co., Ltd., September 2008 (scheduled)
Etc.