Insight into landslide kinematics from a broadband seismic network
© Lin; licensee Springer. 2015
Received: 26 July 2014
Accepted: 24 December 2014
Published: 17 January 2015
The kinematic features of the 2009 Hsiaolin landslide were analyzed using a broadband seismic network in Taiwan. Both the final impact velocity and acceleration of the landslide were calculated based on the traveling distance and time of the landslide. A distance of approximately 2,500 m was observed on the surface, and the time from the initial collapse to the final impact was of 60.38 s according to broadband seismic data recorded nearby. The initial collapse time was determined using very-long-period seismic signals (20 to 50 s) created by the elastic rebound of the shallow crust as the overlying landslide initially moved downhill. The final impact time was determined by detecting the largest amplitudes of high-frequency seismic signals (1 to 10 Hz). The final impact velocity of approximately 298 km/h exhibited by this landslide had never before been recorded and thus might mark a world record; these speeds can be attributed mainly to a low friction coefficient (approximately 0.12) and a long run-out (approximately 2,500 m) along a gentle dip-slope surface (approximately 15°).
In 1970, a forceful high-speed debris avalanche in Peru triggered by an earthquake killed approximately 20,000 people (Schuster and Highland 2001). In general, the damage caused by catastrophic landslides depends on their size and velocity. The damage area typically depends on the landslide volume, whereas human survival is strongly influenced by the velocity; people can easily escape from the area affected by slow landslides, whereas they cannot escape from fast landslides. To classify the velocity of landslides, a 7-level scale was created by Cruden and Varnes (1996). Level 1 represents extremely slow landslides with speeds of less than 16 mm/year, whereas Level 7 represents extremely fast landslides with speeds higher than 5 m/s (18 km/h). Some landslides even exceed this speed, but to determine the exact velocity of a landslide is difficult without reliable measures.
One of the most crucial parameters for determining the velocity of a landslide is the friction coefficient calculated according to the contact area that lies between the sliding material and the underlying layer. Less friction between the sliding material and the underlying layer engenders higher speeds. The friction coefficient typically depends on the materials, the textures of the surfaces, and given lubricants. Like the friction coefficient in the fault planes, the friction coefficient of rocks beneath landslides is a vital subject in the study of geodynamic processes (Sibson 1982; Scholz 1990; 1988). Determining whether an earthquake occurring along a fault plane exhibits high or low friction, it is interesting to know the seismogenic mechanism (Zoback et al. 1987). To determine friction coefficients along fault planes, the friction coefficients of various rock types have been measured in laboratories under simplified physical conditions (Dieterich 1978; Dieterich and Kilgore 1994; Blanpied et al. 1991). However, direct measurement of friction coefficients in the field remains difficult.
In this study, broadband seismic data were collected to analyze the Hsiaolin landslide, which occurred in Taiwan on August 8, 2009 and was one of the fastest landslides recorded worldwide. The friction coefficient of the contact area between the sliding material and underlying layer was retrospectively estimated by considering both the run-out distance and travel time.
Broadband seismic data
The seismic amplitudes of the high-frequency signals (1 to 10 Hz) recorded at Station TPUB increased in the first minute after the initial collapse and then decreased subsequently (Figure 3b). This pattern might represent the entire process of landslide movement. It is assumed that high-frequency seismic energy was largely released from the impact of the sliding momentum on a barrier or a particularly rough surface. A stronger impact generates seismic energy with a larger amplitude. At the early stage, small seismic amplitudes were generated by the slowly moving landslide, exhibiting low momentum; as the landslide accelerated, it generated larger seismic amplitudes. The high-frequency seismic signals exhibited the largest amplitude at 17 min 12.3 s at station TPUB, likely marking the time when the major part of the landslide hit and then stopped at Chishan Creek (Figure 4). However, the exact impact time of the landslide was reduced to 22 h 17 min 9.8 s if one considered the traveled time of 2.5 s from the landslide to the seismic station at a distance of 12.5 km by given a propagated seismic velocity of 5.0 km/s. Thus, the final impact of the landslide occurred approximately 60.38 s after the very-long-period seismic signals were first detected, indicating the initiation of the Hsiaolin landslide. Although some high-frequency signals were detected after the landslide produced its major impact, they might have been generated by subsequent landslides.
A velocity as high as that of the Hsiaolin landslide (approximately 298 km/h) had not been reported anywhere in the world prior to 2009. A high-velocity landslide of that scale is difficult to observe unless it can be predicted. The factors contributing to such a landslide includes a low friction coefficient, long run-out distance, and gentle dip-slope ground surface. The friction coefficient of the Hsiaolin landslide (approximately 0.12) is lower than that of most comparable landslides and debris flows (Scheidegger 1973; Brodsky et al. 2003). This low friction coefficient was can be attributed mainly to extremely heavy rainfall, which infiltrated the space between the sliding material and the overlying ground. In addition, the long run-out distance of approximately 2,500 m resulted from a gentle dip-slope surface extending from Hsindoshan to Chishan Creek. The dip slope has an average incline of approximately 15°, and the inclination varies from 10° to 20°. Consequently, the major portion of the collapsed landslide moved downhill smoothly until it reached the Chishan creek.
It is difficult to fathom the devastation caused by the Hsiaolin landslide when it reached Chishan Creek and Hsiaolin Village. Approximately, 25 × 106 m3 of debris (Tsou et al. 2011) slid down the slope at a maximal speed of approximately 298 km/h and destroyed all structures and buildings.
This detailed study of the 2009 Hsiaolin landslide in Taiwan demonstrated that the kinematic parameters of a landslide, such as the velocity (298 km/h) and acceleration (1.37 m/s2), can be derived directly from broadband seismic data. The starting time of the landslide was obtained from the very-long-period seismic signals (20 to 50 s) generated by the elastic rebound of the shallow crust beneath the landslide, whereas the end of the landslide was estimated using the largest amplitude of the short-period seismic signals produced by the impact on Chishan Creek. The results revealed that the velocity at the time of the final impact reached approximately 298 km/h, equaling the speed of the fastest high-speed trains and constituting a landslide velocity that had previously not been reported. In addition, the friction coefficient of the landslide was 0.12. Such low friction can be attributed mainly to extremely heavy rainfall, which infiltrated the space between the sliding materials and the overlying ground.
The author would like to thank both the Ministry of Science and Technology (MOST) as well as the Center of Sustainability Science, Academia Sinica, Taipei, Taiwan for the financial support. The data provided by the Institute of Earth Sciences, Academia Sinica, and the Central Weather Bureau, Taipei, Taiwan as well as the Google earth map are appreciated.
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