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Occurred in the offshore of SW Taiwan on 26 December 2006 with a magnitude of 7, the Pingtung earthquake had triggered numbers of submarine landslides. This event provides an excellent opportunity to incorporate the back analysis approach to evaluate the in situ shear strength parameters. According to the chirp sonar images of the seabed near the SW Xiaoliuqiu obtained before and after the earthquake were adopted to establish the slope profile and identified the location of a circular sliding surface. Consequently, the in situ, effective strength parameters under the critical condition can be calculated by back slope stability analysis. Submarine sediment sampler was obtained via gravity sampling method and the laboratory tests were performed to determine the index properties and strength parameters. Test results indicate the cored sediment has the characteristics of normally consolidated (NC) clay. The effective friction angle (φ’) is 15.3° with cohesion (c’) of 19.4 kPa. The effective and total stress methods were used to perform the back analysis. The strength parameters derived from back analysis of effective and total stress methods all indicate values approach the CIU triaxial tests results. Consequently, the representativeness of the marine sediment characteristics obtained from laboratory tests is identified. The total stress approach yields an undrained strength ratio cu/σ'vo of 0.26 which well fit the ratio used in geotechnical practice for estimating NC clay. According to the analytical approach, the landslide was applied seismic forces (seismic coefficient kh = 0.14) and generated excess pore pressure of 31 kPa at the sliding surface.

Submarine landslides frequently occur in passive continental margins or active margins (Hampton et al. 1996; Wynn et al. 2000; Mienert et al. 2002; Korup et al. 2007; Twichell et al. 2009; Cukur et al. 2016). Submarine landslides have been studied extensively not only for scientific research but also for submarine geohazards. Submarine landslides could jeopardize marine infrastructures, such as offshore drilling platforms or submarine telecommunication cables, and could even trigger disastrous tsunamis (Bondevik et al. 2005; Harbitz et al. 2006; Hornbach et al. 2007, 2008; Hsu et al. 2008; Su et al. 2012; Tappin et al. 2014; Li et al. 2015). For instance, one disastrous tsunami hitting the coastal area of southwestern Taiwan in 1781 or 1782 was reported (Chen 1830; Hsu 1983); the tsunami event was probably generated by submarine landslides in the offshore area of southwestern Taiwan (Li et al. 2015). Moreover, several submarine landslides triggered by the 2006 Pingtung earthquake have induced turbidity currents off southwest Taiwan and destroyed about 14 submarine telecommunication cables off SW Taiwan (Hsu et al. 2008). The area of southwest Taiwan currently has a dense population (more than 3 million people in total), one deep-water Kaohsiung Port, several tanks of liquefied natural gas and a nuclear power plant on the coast (Fig. 1). Numerous submarine telecommunication cables exist off SW Taiwan. If a considerable tsunami event would hit again the costal area of SW Taiwan, the damage could very serious. Likewise, there are two nuclear power plants on the coast of northern Taiwan (Fig. 2), and the population in northern Taiwan has more than 10 million people. Submarine telecommunication cables also exist off northern Taiwan. In any case, it is important to understand the status of seafloor stability in the offshore areas of SW and NE Taiwan. For that, this special issue of submarine geohazard records and potential seafloor instability is aimed to provide some research results, hoping to have a general reconnaissance of submarine landslide potential off Taiwan.

Earthquakes can generate seismic disturbances that propagate vast distances and trigger landslides that can achieve high-speeds. It remains difficult to identify the co-seismic ground motion of these landslides and their triggering earthquakes. In this paper, we report on the analysis of co-seismic ground motions generated by the initiating fracture and deposition impact of the Tsaoling landslide. The landslide, with a source volume of 125 × 10 6 m 3 , was triggered by the 1999 Chi-Chi earthquake in Taiwan. The ground motion was recorded by a strong ground motion station, CHY080, near the scar area. The polarization of the seismic waves indicates that the peak acceleration was parallel to the dip direction. Modified ensemble empirical mode decomposition (EEMD), with additional clustering analysis, was applied to decompose the seismic signals. Two instances were found in the seismic records of a series of peculiar wave packets, with the first being associated with the landslide initiation and the second the landslide impact on the deposit valley. To confirm the first landslide breakage, the decomposed signals were compared with the predictions of the analytic elastic wave model and Newmark analysis. The landslide impact was verified with a computational fluid dynamic simulation. Comparison between the EEMD decomposed signals, elastic wave theory, Newmark rigid body analysis, and numerical simulation demonstrates the claimed landslide motion.

Tsaoling landslide is the largest and best documented landslide among several large landslides induced by the 1999 Taiwan Chi-Chi earthquake. Pliocene sedimentary rocks of about 125 Mm 3 in volume slid along very flat bedding planes dipping by 14° with an average speed of 35–40 m/s for about 1650 m, before hitting the bank of the Chinshui River during the landslide. Detailed analysis of DTMs before and after the earthquake using a GIS software leads to an accurate determination of the locations of the centroids of landslide mass, revealing the horizontal and vertical displacements of the 2524 m and 524 m, respectively. Those displacements and landslide mass give an apparent friction coefficient of 0.21 and the release of the potential energy of 1.6 × 10 15 J. We conducted rotary-shear high-velocity friction experiments on fault gouge from bedding-parallel faults under semi-wet conditions and at 3 MPa normal stress corresponding to the overburden pressure of the landslide mass. We also compiled reported data on the frictional properties on shale powders and fault gouge from the landslide site under both dry and wet conditions, and proposed a velocity–displacement dependent friction law that can describe most experimental data. Newmark analysis of landslide motion with six scenarios for different landslide materials and conditions, assuming a simple rigid block sliding and using measured frictional parameters, revealed that the landslide did not occur with dry frictional properties, and that the landslide occurred at 38–39 s with accumulated displacements of 0.62 m–1.09 m and reached at the river bank at 82–87 s after the generation of Chi-Chi earthquake at its epicenter. Those timings are consistent with high-frequency signals at 32–40 s and at 76 s recorded at a nearby seismic station and with a survivor's witness that the landslide initiated 10 s after he felt strong ground motion, possible S wave arrival at 25.2 s. Slip-weakening is essential in initiating the landslide and low friction coefficient (0.08–0.1) allowed high-speed of the landslide possible. The landslide was caused by a few peaks of northeast-oriented strong accelerations of the ground motion. Frictional work during the sliding of the landslide mass was estimated to be of about 23% of potential energy, and the rest of the released energy is likely to have been consumed during the stopping phase of the landslide after hitting the river bank in complex processes such as fragmentation, heat dissipation, and spreading of the landslide deposits.

Pliocene sedimentary rocks of about 130 Mm3 in volume slid along bedding planes dipping by 14°, with an average speed of about 35 m/s, during the Tsaoling landslide. We conducted friction experiments to reproduce the initiation processes of this landslide, by idealizing landslide movements during the earthquake as accelerating/decelerating motion. Experiments were done on shale from the field, at 3 MPa normal stress corresponding to the overburden pressure. Results indicate that the accelerating/decelerating motion causes weakening and strengthening at each oscillation cycle and results in overall slip weakening which can be approximated as an exponential slip weakening. Behaviors during oscillatory slip are fairly similar to those during sliding at constant slip rates. Newmark analysis with measured frictional properties reveals that the landslide can be triggered with wet gouge properties, but the landslide motion stops with parameters for dry shale gouge. Delayed initiation of the landslide is consistent with a survivor's witness.

On September 20, 1999, the M s 7.6 Chi-Chi earthquake ruptured the Chelungpu fault in central Taiwan. After the earthquake, two deep boreholes cutting the fault were drilled. The seismic velocities (P- and S-wave velocities denoted by v p and v s ) were well-logged at a 2000-m deep hole. The values of density, porosity and permeability of ten rock samples obtained at different depths were measured in the laboratory. Well-logged and measured results are used to study the following problems: (1) the depth variations in seismic velocities, porosity, and permeability; (2) the relationship between P- and S-wave velocities; (3) porosity-dependence of P- and S-wave velocities and their ratio; and (4) porosity-dependence of density. Results show that the polynomial can describe the depth variations in seismic velocities. The porosity slightly decreases with increasing depth. The permeability is depth-dependent and can be described by a polynomial, but the functions are different for different rock types. The porosity and permeability in the fault zone cannot be evaluated from the related depth-dependent functions inferred from wall rocks. A linear relationship, which is different from v s = 0.58 v p for the perfectly elastic materials, exists between v s and v p . Seismic velocities linearly decrease with increasing porosity. The ratio of v p to v s slightly depends on the porosity. The porosity-dependent functions of bulk and shear modulus are constructed and their values for dry rocks are also evaluated.

The Chi-Chi earthquake (Mw = 7.6) of September 21, 1999 triggered many landslides in central Taiwan. Two of these landslides, Hungtsaiping (HTP) and Jiufengershan (JFES) were situated as close as 2 km from each other but had significant differences in their kinematics. JFES landslide was a catastrophic rockslide-avalanche and the HTP landslide was relatively slow-moving. The authors conducted a study to explore the reasons for such differences. Factors such as the characteristics of strong ground motion, sliding direction of landslide, and friction angle of the sliding surface were considered in the study. An analysis of 12 strong-motion records collected in the study area showed that the distribution of horizontal pseudostatic coefficients, earthquake energy ratio and permanent sliding-block displacements (Newmark displacement) were anisotropic with their predominant direction mostly in the E/W–ESE/WNW trending. This direction is perpendicular to the axis of the main geological structures of the studied area. The computed Newmark displacement in the sliding direction of the JFES landslide is larger (44%) than that of the HTP landslide with sliding surface inclination of 21° and friction angle of 28° We can conclude that the seismic anisotropy and the corresponding sliding direction are important contributing factors to the kinematics of studied landslides. The back-calculated friction angle of the sliding surface that corresponds to a critical Newmark displacement for the JFES landslide is about 3.5° higher than that of HTP landslide. The material (colluvium) on the sliding surface in HTP should be less velocity-dependent than that of the JFES landslide (rock) according to the back calculations. The importance of seismic anisotropy, sliding direction, and mechanical properties of sliding surface on the kinematics of deep-seated landslides is demonstrated.

SUMMARY Taiwan Chelungpu-fault Drilling Project (TCDP) was initiated to understand the physical mechanisms involved in the large displacements of the 1999 Taiwan Chi-Chi earthquake. Continuous measurements of cores (including laboratory work) and a suite of geophysical downhole logs, including P- and S-wave sonic velocity, gamma ray, electrical resistivity, density, temperature, electrical borehole images and dipole-shear sonic imager, were acquired in Hole-A over the depth of 500–2003 m. Integrated studies of cores and logs facilitate qualitative and quantitative comparison of subsurface structures and physical properties of rocks. A total of 10 subunits were divided on the basis of geophysical characteristics. Generally, formation velocity and temperature increase with depth as a result of the overburden and thermal gradient, respectively. Gamma ray, resistivity, formation density, shear velocity anisotropy and density-derived porosity are primarily dependent on the lithology. Zones with changes of percentage of shear wave anisotropy and the fast shear polarization azimuth deduced from Dipole Shear-Imager (DSI) are associated with the appearance of fractures, steep bedding and shear zones. The fast shear wave azimuth is in good agreement with overall dip of the bedding (approximately 30° towards SE) and maximum horizontal compressional direction, particularly in the Kueichulin Formation showing strong shear wave velocity anisotropy. Bedding-parallel fractures are prevalent within cores, whereas minor sets of high-angle, NNW–SSE trending with N- and S-dipping fractures are sporadically distributed. The fault zone at depth 1111 m (FZA1111) is the Chi-Chi earthquake slip zone and could be a fluid conduit after the earthquake. The drastic change in fast shear wave polarization direction across the underlying, non-active Sanyi thrust at depth 1710 m reflects changes in stratigraphy, physical properties and structural geometry.

For the interpretation of the storm event-induced landslide distribution for an area, deterministic methods are frequently used, while a region's landslide susceptibility is commonly predicted via a statistical approach based upon multi-temporal landslide inventories and environmental factors. In this study we try to use an event-based landslide inventory, a set of environmental variables and a triggering factor to build a susceptibility model for a region which is solved using a multivariate statistical method. Data for shallow landslides triggered by the 2002 typhoon, Toraji, in central western Taiwan, are selected for training the susceptibility model. The maximum rainfall intensity of the storm event is found to be an effective triggering factor affecting the landslide distribution and this is used in the model. The model is built for the Kuohsing region and validated using data from the neighboring Tungshih area and a subsequent storm event – the 2004 typhoon, Mindulle, which affected both the Kuohsing and the Tungshih areas. The results show that we can accurately interpret the landslide distribution in the study area and predict the occurrence of landslides in the neighboring region in a subsequent typhoon event. The advantage of this statistical method is that neither hydrological data, strength data, failure depth, nor a long-period landslide inventory is needed as input.

Susceptibility analysis for predicting earthquake-induced landslides has most frequently been done using deterministic methods; multivariate statistical methods have not previously been applied. In this study, however, we introduce a statistical methodology that uses the intensity of earthquake shaking as a landslide triggering factor. This methodology is applied in a study of shallow earthquake-induced landslides in central western Taiwan. The results show that we can accurately interpret landslide distribution in the study area and predict the occurrence of landslides in neighboring regions. This susceptibility model is capable of predicting shallow landslides induced during an earthquake scenario with similar range of ground shaking, without requiring the use of geotechnical, groundwater or failure depth data.

In addition to the main surface rupture along the Chelungpu fault associated with the 1999 Chi-Chi, Taiwan earthquake, numerous secondary or branch ruptures on hangingwall were also observed. These secondary surface ruptures are parallel or sub-parallel to the main rupture within a distance of a few meters to 1–2 km. The rupture length of these secondary ruptures varies from a few tens of meters up to 5 km. The surface deformation resulted in serious damages of buildings. The present work studied the features of surface deformation on the hangingwall around the Chung-Cheng Park, Fengyuan, Taichung. Three distinct surface ruptures, minor ruptures and tension cracks were observed in this area. The observed distribution and types of building damage on the hangingwall are demonstrated. Due to the difference in geological condition and complex pattern of surface deformation, the resulted building damages on the hangingwall vary. A series of site investigation including field survey, drilling, seismic prospecting, P–S logging tests and laboratory tests were carried out in the interested area. A geological structure model was proposed on the basis of the results of site investigation. Numerical simulation was carried out to model the surface deformation as well as subsurface potential damage zone of an active fault. It reasonably explains the observed pattern of surface deformation and indicates that the surface deformation zone during a catastrophic earthquake is predictable.

Chien-chih Chen1, Jia-Jyun Dong2, Chih-Yu Kuo3, Ruey-Der Hwang4, Ming-Hsu Li5 and Chyi-Tyi Lee2 1Grad. Inst. Geophys. & Dept. Earth Sciences, Nat’l Central Univ., Jhongli, Taoyuan 2Grad. Inst. Applied Geology, Nat’l Central Univ., Jhongli, Taoyuan 3Research Center for Applied Sciences, Academia Sinica, Nankang, Taipei 4Dept. Geology, Chinese Culture Univ., Taipei 5Grad. Inst. Hydrological & Oceanic Sciences, Nat’l Central Univ., Jhongli, Taoyuan Taiwan