Amplification of seismic response of a large deep-seated landslide in Tokushima, Japan

Engineering Geology 249 (2019) 218–234

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Amplification of seismic response of a large deep-seated landslide in Tokushima, Japan ⁎

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Ning Maa, , Gonghui Wangb, , Toshitaka Kamaib, Issei Doib, Masahiro Chigirac a

Graduate School of Science, Kyoto University, Gokasho, Uji 611-0011, Japan Research Center on Landslides, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611-0011, Japan c Division of Geo-Disaster Mountain Hazard Section, Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji 611-0011, Japan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Deep-seated landslide Long-term seismic monitoring Seismic site response Amplification Effect of topography Effect of material contrast

To better understand seismic site responses of deep-seated landslides, we carried out long-term seismic monitoring of a target landslide reactivated by a rainstorm in 2004 accompanying Typhoon Namtheun in Naka Town, Tokushima Prefecture, Japan. Five seismometers with high sensitivity were installed at different locations on landslide areas with different elevations. By examining numerous records of earthquake events, we distinguished the effects of material contrast and topography on these localized amplifications for different areas, and summarized their features, with help of detailed geological and geophysical surveys. To analyze differences in amplification, the HVSR method (calculating the horizontal to vertical spectral ratio of ground motion) was applied. The amplification on a landslide block with an HVSR peak amplitude smaller than 5 and resulting from topography is predominantly perpendicular to the direction of elongation of the mountain ridge. However, the amplification (revealed by HVSR peak amplitude of 3 to 5) resulting from material contrasts in multiple strata showed differing amplification directions. By examining the relationship between the HVSR peak amplitude and shear velocity profiles, we found that the amplifications at different frequencies are greatly affected by the extent of material contrasts. This may explain why the toe part of the slope, with ancient landslide deposits, generally shows the greatest ground motion (characterized by HVSR peak amplitude normally > 10). This type of strong amplification due to material contrasts may result in severe damage to residential areas in mountainous areas during an earthquake and should be considered for the construction of buildings on landslide areas.

1. Introduction Coseismic landslides can cause severe damage to local properties and often result in great loss of lives (Keefer, 1984, 2002; Wang et al., 2002; Petley, 2010; Gorum et al., 2011). For example, the 1920 Haiyuan Earthquake triggered numerous landslides, and investigation by Close and McCormick (1922) reported that about 100,000 people were killed directly by these landslides (Zhang and Wang, 2007). More recently, the 2008 Wenchuan Earthquake in China triggered > 200,000 landslides (Fan et al., 2018), directly resulting in about 20,000 casualties (Wang et al., 2014; Yin et al., 2011). Therefore, preventing or at least mitigating possible damage from coseismic landslides is an urgent and challenging issue for both engineers and geologists. Recently, for landslide risk assessment, great efforts have been made to better understand the seismic response of slopes using field seismic monitoring or numerical experiments. Field seismic monitoring results show that there is a correlation between landslide initiation and

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topographic amplification (e.g. Davis and West, 1973; Pedersen et al., 1994; Spudich et al., 1996; Sepúlveda et al., 2005; Buech et al., 2010; Hartzell et al., 2014; Massa et al., 2014; Stolte et al., 2017), and that remarkable amplification of ground motion occurs at the top of hills, ridges or crests, with a preferred orientation perpendicular to the relief main elongation. On the other hand, some studies based on numerical experiments show that not only the properties of seismic waves (such as incident wavelength, angle and wave phase), but also the scale and shape of the slope, can influence the topographical amplification (e.g. Bard, 1982; Géli et al., 1988; Ashford et al., 1997; Paolucci, 2002; Meunier et al., 2008; Hartzell et al., 2014). In addition, some studies point out that near-surface geological contrasts in materials can also play a significant role in the amplification of seismic motion on slopes. Due to anisotropy in the properties of slope materials, the preferential orientation of amplification normally coincides with the general direction of the displacement (if the slope is moving), or is parallel to the direction of maximum slope (e.g.

Corresponding authors. E-mail addresses: [email protected] (N. Ma), [email protected] (G. Wang).

https://doi.org/10.1016/j.enggeo.2019.01.002 Received 17 September 2018; Received in revised form 3 January 2019; Accepted 4 January 2019 Available online 08 January 2019 0013-7952/ © 2019 Elsevier B.V. All rights reserved.

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Borcherdt, 1970; Havenith et al., 2002; Del Gaudio and Wasowski, 2007, 2011; Del Gaudio et al., 2008, 2013, 2014). This kind of amplification may occur because seismic energy can be trapped and redistributed within the boundaries of the materials within the slope (e.g. Bourdeau and Havenith, 2008; Bozzano et al., 2008; Danneels et al., 2008; Gischig et al., 2015). Some studies also attribute the localized amplification of some fractured rock slopes to the open tension fractures and near-surface weathering (e.g. LeBrun et al., 1999; Burjánek et al., 2010, 2012; Moore et al., 2011; Durante et al., 2017). Although some studies have been conducted to analyze earthquake amplification effects on different types of landslides (e.g. Graizer, 2009; Bourdeau et al., 2017; Hartzell et al., 2017; Zare et al., 2017), our understanding on the seismic response of landslide areas is still poor, because there is a lack of long-term and permanent field observations. Especially for deep-seated landslides, geological and topographical conditions can be very complicated, and the relative contributions made by topographic and geological effects to the amplification of ground motion at different locations could vary significantly. As well known, Japan is frequently affected by strong earthquakes. In the last two decades, several devastating earthquakes have occurred, resulting in numerous landslides. For example, the 2004 Mid Niigata Prefecture earthquake triggered > 1300 landslides (Sato et al., 2005), most resulting from reactivations of pre-existing landslides. The 2016 Kumamoto earthquake (Mj7.3) triggered > 3400 individual landslides (Xu et al., 2018). Furthermore, a very strong earthquake has a probability of 70–80% of occurring in the coming 30 years along the Nankai and/or Tonankai megathrust. Because there are many potential deepseated landslides in Japan's accretionary prism mountains, it will be of high potential that these landslides be reactivated by a mega earthquake. To mitigate possible damage from these landslides, a better understanding of the seismic responses of deep-seated landslides will be of great importance. Thus, in this study, we targeted a deep-seated landslide in Azue district (hereinafter termed the Azue landslide) in southwestern Japan for in-situ seismic monitoring. Through examining the geological properties by means of geophysical surveys and analyzing the numerous ground motions recordings at different locations in the landslide area at differing elevations, we analyzed (1) the amplification patterns on different landslide areas, (2) features of localized amplifications resulting from different factors, and (3) interpretation of extreme amplification phenomena in the landslide area. 2. Azue landslide Typhoon Namtheun, the 10th tropical storm in the western Pacific in 2004, brought rainstorms to the southwest part of Japan from July 30 to August 2, with > 2000 mm of cumulative precipitation. During the typhoon, many landslides were triggered in Kisawa Village, Naka District, Tokushima Prefecture (Wang et al., 2005) (Fig. 1a). Azue landslide, one of the area's giant landslides, is situated on the left side of Sakashu-Kito River (Fig. 1b). Azue is an ancient landslide. During the rainfall, the lower part failed and moved away, while the upper part was reactivated, with the opening of cracks and some downslope movement. The lower displaced landslide materials (about 1–2 × 106 m3 in volume) moved downslope rapidly (with a speed > 20 m/s) (Hiura et al., 2004), dammed the river, swept away an entire big road bridge on the opposite bank, and climbed up the opposite slope to a height of about 50 m above the river bed (Wang et al., 2005; Nakaya et al., 2006). Fig. 2a shows the geological setting of the landslide area. The bedrock outcropping in the landslide area is composed mainly of Paleozoic greenstone (Kurosegawa terrane) (Yokoyama et al., 2006), which consists of pillow lava, pillow breccia and pyroclastic rock. Jurassic sandstone (about 20 m thick), sandwiched in the greenstone, outcrops on the middle upper slope areas of the valley. In addition, limestone (about 50 to 80 m thick) outcrops along the river bank extending to the middle upper part of the valley. A fault (strike of N52°W

Fig. 1. (a): Location of the research area (Naka town, Tokushima Prefecture, southwestern Japan); (b): Oblique aerial view of theAzue landslide area and the unstable blocks (Block-A, B and C) after August 2004 (photo by Shikoku Regional Development Bureau, Ministry of Land, Infrastructure and Transport, Japan). The red frame indicates the zone of subsidence due to the collapse occurred in 2004, along the ancient landslide scarp, shown in the Fig. 3a photo. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

~ N60°W and dip of about 30° ~ 62° to S) passes through the landslide area from the river bank to the bottom of the landslide mass, but no crushed zone was found between the sandstone and greenstone. The geomorphology of the Azue landslide area (Fig. 2b) is characterized by deep river valleys with steep slopes, and in 2004 a landslide occurred on a steep chute near the ridge of the hill slope. In general, the elevation difference from the river bank to the ridge is about 500 m, and the 219

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Fig. 2. (a): Geological map of Azue landslide area (revised by Prof. Chigira based on Yokoyama et al., 2006); (b): Slope-terrain map of Azue landslide area – oblique view. 220

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Fig. 3. (a): Collapse occurred on the scarp, whose location is marked by the red frame shown in Fig. 1b; (b): Outcrop showing the well-consolidated ancient sliding surface. The photos were taken on August 21, 2004. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

component seismometer (KVS-300) with a natural frequency of 2.0 Hz and a data logger (EDR-7000) that records seismic movement continuously at a sampling rate of 250 Hz, archiving one file per minute. These seismic stations are placed on different geological units (see Fig. 6b). Station P1 (300 m in elevation) sits on bedrock with greenstone, on the opposite bank of the Kito-Sakashu River. This station serves as a reference site because it is located on bedrock outside of the landslide. Station P2 is on old landslide deposits located at the toe of the slope near the river, at an elevation of 311 m. Station P3, at an elevation of 682 m, is located on a relatively flat area on the middle part of landslide sub-block-A. The substrata below station P3 consist of gravelly soil with a depth of approximately 10 m, overlying strongly weathered greenstone (Fig. 5b). Station P4 (at an elevation of 723 m) is close to the boundary of landslide block-A. Due to the previous landslide in 2004, this region has some tension cracks (Fig. 2a) and is composed of shallow gravelly soil layers (roughly several meters thick). In addition, P4 is on a local ridge of the hill slope with sharp topography, and slope angles are > 30- and 40-degrees downslope to the west and east sides, respectively. Station P5 (at an elevation of 713 m) is on bedrock exposed by the construction of a new road on the top of the hill slope. The continuous recording system of the seismometers enabled us to obtain a substantial amount of earthquake data. To remove the influence of the azimuthal location of earthquake sources from the data, we analyzed earthquake events with a wide azimuthal distribution around the Azue landslide. The sources of these earthquakes are listed in Table 1 and their locations with reference to the Azue landslide are plotted in Fig. 7. The magnitudes of these earthquakes mainly range from 2.4 to 6.0, with five of them > 6.0.

topography of the upper parts of hill slopes are steeper than the lower parts. After the landslide in 2004, numerous cracks with significant displacements were found on the slope above the main scarp and three sub-landslide blocks (Block-A, B and C) were identified (Figs. 1b and 2a). These blocks were thought to belong to an old deep-seated landslide, with the landslide mass estimated to be 4 × 106 m3. A road on the left side of the ancient landslide scarp was severed by a crack, with subsidence of about 2.0 m at the time of our first survey on August 21, 2004, which increased to 2.5 m over about two weeks (to September 4, 2004) (Fig. 3a). A well-consolidated sliding surface of the ancient landslide outcrops: it originated in bedrock with a strike of N44°W and a dip of 48°W (Fig. 3b). Due to the occurrence of these cracks and continuous movements of these unstable sub-blocks, the Japanese government immediately began efforts to investigate and mitigate the landslide risk. > 20 boreholes were drilled, and 12 drainage wells (with diameters of 6 m and depths of ~80 m) were constructed inside the landslide blocks. The toe part of sub-block A was stabilized by the construction of a large number of anchors and concrete frames (Fig. 4a) and nine large check dams were constructed below its toe (Fig. 4b). Geological drilling revealed that these sub-landslide blocks have thicknesses ranging from 20 to 50 m, and the sliding surfaces are mainly developed in a strongly weathered stratum. Fig. 5a and b show crosssections of sub-block A and B, respectively (locations marked in Fig. 2a) and their geotechnical context. They mainly consist of three strata – superficial gravelly soil, strongly weathered greenstone (fractured greenstone) and weakly weathered greenstone (bedrock). The sliding surfaces are developed in the strongly weathered greenstone and the maximum depth reaches approximately 50 m. Countermeasures for stabilizing the landslide were started in 2004 and are still in progress, with new additional drainage wells or other mitigation techniques being required and constructed almost every year, due to the large volume and complex movement mechanisms of the landslide.

4. Geophysical surveys of the landslide area To investigate the subsurface geological conditions of the landslide materials in different areas, we carried out geophysical surveys using multichannel analysis of surface waves (MASW) (Park et al., 1999; Hayashi and Suzuki, 2004) and electrical resistivity tomography (ERT) in the landslide deposit area (P2 area in Fig. 6a) and landslide sub-block A (P3 area in Fig. 6b). These methods allowed identification of the distributions of shear-wave velocities (Vs) and electrical resistivity within the ground. The detailed layout of the survey lines is shown in

3. Seismic array and earthquake events Five seismic stations (P1 to P5, shown in Fig. 6a) were installed at different locations in the landslide area. Each station consists of a three221

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Fig. 4. (a): Close-up view of the anchored scarp; (b): A view of the constructed check dams along the valley and stabilization facilities on the scarp. The photos were taken on March 13, 2018.

earthquake event. Fig. 9 shows an example of the recorded original data for the EW component for different stations. Before further analysis, the quality of the recorded seismic signal was tested at first using the signalto-noise ratio (SNR) criterion, following the method used by Massa et al. (2010). Seismic S-phases (shear waves in a seismogram) are normally characterized by the strongest amplitudes (up to maxima corresponding to PGA or PGV), while the coda waves (tail waves in seismograms) are independent of the location or the orientation of the earthquake source (Seekins et al., 1996). Therefore, the records of Sphase and coda waves (Fig. 9) were used for the analysis and comparison of seismic amplifications in this study. After being pre-processed by baseline correction and removal of mean and linear trends, the selected raw seismic signals were filtered in the range between 0.5 Hz to 40 Hz by a fourth-order Butterworth band-pass filter. A method based on the spectral ratio horizontal-to-vertical component of the records (referred to here as HVSR) was used to evaluate the amplification effects of local ground motion. This method was initially used to estimate local site effects using ambient noise records (Nakamura, 1989) and then was also widely used to investigate site response under strong motions in different research fields (e.g., Lermo and Chávez-García, 1993; Panzera et al., 2017; Lin et al., 2018, among others). Recently, this method was also applied to evaluate amplification on slopes or landslides (e.g., Havenith et al., 2002; Del Gaudio and Wasowski, 2007). The HVSR method calculates the quantity: Fig. 5. Cross section of the landslide sub-blocks A and B along the lines I-I′ and II-II', respectively, shown in Fig. 2 (a). The data is based on an in-situ borehole (from Japan Conservation Engineering Co., Ltd).

H(ω) = V(ω)

H2EW (ω) + H2NS (ω) V 2 (ω)

(1)

where V is the Fourier spectrum of the vertical component, HEW and HNS are the Fourier spectra of the east-west and north-south components, respectively, and ω is the angular frequency. To calculate the HVSR curves, we followed the criteria suggested by the SESAME (2004) project, by using the Geopsy software (www. geopsy.org-SESAME Project). For each event, the pre-processed threecomponent waveforms taken from the S-phase and coda waves were firstly transformed to Fourier spectrums by means of the fast Fourier transform (FFT) method applied to 10% cosine-tapered windows, then a window function named Konno-Ohmachi (Konno and Ohmachi, 1998) was used for smoothing the spectrums. Finally, the values of HVSR were calculated using Eq. (1). Through searching the significant peaks of the HVSR curves, we can obtain the corresponding resonance frequencies,

Fig. 8. In the MASW survey, seismic surface waves were generated at the ground surface by the impact of a sledgehammer (about 8 kg), and the signals of the travelling waves were recorded by 24 geophones (natural frequency of 4.5 Hz) set on the ground at intervals of 1 or 2 m. In addition, the ERT survey was conducted using pole-pole arrays. 5. Data processing and methods 5.1. Analysis of seismic recordings All stations record the seismic motions synchronously for any 222

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Table 1 Parameters of earthquakes whose recordings were employed in this study. Number

Date (Y-M-D)

Longitude(°)

Latitude (°)

Magnitude

EQ1 EQ2 EQ3 EQ4 EQ5 EQ6 EQ7 EQ8 EQ9 EQ10 EQ11 EQ12 EQ13 EQ14 EQ15 EQ16 EQ17 EQ18 EQ19 EQ20 EQ21 EQ22 EQ23 EQ24 EQ25 EQ26 EQ27 EQ28 EQ29 EQ30 EQ31 EQ32

2014-06-29 2014-07-08 2014-07-11 2014-07-11 2014-08-26 2014-10-04 2014-10-05 2015-02-06 2015-02-14 2015-03-25 2015-05-30 2015-06-05 2015-06-14 2015-07-13 2016-04-14 2016-04-15 2016-04-16 2016-04-16 2016-04-16 2016-08-14 2016-08-15 2016-08-21 2016-09-03 2016-09-18 2016-10-01 2016-10-21 2016-10-21 2016-10-21 2016-10-21 2016-11-03 2016-11-17 2016-11-19

134.44 134.04 134.49 134.97 134.89 134.88 135.15 134.37 133.88 134.04 140.68 135.07 134.25 131.85 130.81 130.78 130.76 130.90 131.19 133.26 131.92 133.36 135.15 134.69 134.13 133.85 133.83 133.87 133.83 134.06 134.47 135.46

34.07 34.06 34.06 33.84 33.36 33.38 33.68 33.73 33.96 33.65 27.86 33.74 33.96 32.99 32.74 32.70 32.75 32.86 33.02 34.07 33.49 33.83 33.49 33.98 33.82 35.38 35.41 35.36 35.43 33.68 33.90 33.84

3.0 2.6 2.4 3.5 4.2 3.8 3.9 5.1 4.0 3.2 8.1 4.2 3.0 5.7 6.5 6.4 7.3 5.9 5.9 3.8 4.3 3.2 4.0 3.2 3.4 6.6 4.4 5.0 4.3 3.4 2.4 5.4

6. Results 6.1. Features of HVSR curves The HVSR curves for stations P1 to P5, calculated from earthquake recordings by using the S-phase are presented in Fig. 10a–e, while those based on coda waves are presented in Fig. 10f–j. In general, for each station, the HVSR curves obtained from S-phases show similar tendencies to those obtained from coda waves (Fig. 10). However, in some cases, the features of these curves differ for different seismic stations. For station P1, the HVSR curves have no predominant peak in the frequency bands, and the average HVSR peak amplitude is < 2 (Fig. 10a and f). Therefore, based on the HVSR criteria used in SESAME (2004), we can conclude that there is no, or very weak (if any), amplification effect at the reference location P1. For P5, the HVSR curves show a predominant peak at a frequency of around 2.1–2.6 Hz, with HVSR peak amplitude of < 4 (Fig. 10e and j). P2 shows the greatest HVSR peak amplitude approximately at the frequency band of 5.1–5.3 Hz, and the average amplitudes are approximately 10, with some individual values even > 20 (Fig. 10b and g). On the other hand, HVSR curves for P3 and P4 have two predominant peaks (2.5–3 Hz and 9–10 Hz for P3 and 2.6–2.8 Hz and 14–18 Hz for P4, respectively), and the average amplifications range from HVSR peak amplitude of 2–5 for both (Fig. 10c and d, h and i, respectively). In order to examine the directional features of amplification, we rotated the HVSR curves clockwise by steps of 10°, starting from zero (North). This approach has been applied to analyze directional amplifications by Del Gaudio and Wasowski (2007) and has been applied in some other research (e.g. Pischiutta et al., 2014; Stolte et al., 2017; Bottelin et al., 2017; Napolitano et al., 2018, among others). Examples of results derived for stations P1, P2 and P3 from the analysis of the earthquake recorded on February 16, 2015 (M5.1 in Tokushima Prefecture) are shown in Fig. 11 as polar diagrams. In Fig. 11, the H/V ratio

Fig. 6. (a): Aerial view of topography and locations of the 5 seismometers (station P1 to P5) installed over the landslide area; (b): Cross section along the line A–B of (a), showing the location of the 5 seismometers. Ellipses indicated in (a) as P2- and P3-areas mark the zones where geophysical surveys were carried out.

which are interpreted as indicative of site resonance properties. Although the HVSR peak amplitude may not be representative of the actual amplification factors, due to the geologically and geomorphologically complex site conditions of the landslide areas (Del Gaudio et al., 2013), it may be considered as an index representative of the relative differences of the amplification, since the HVSR peak amplitude is generally higher at those places or along those directions where the amplification factor is also higher.

5.2. Multichannel analysis of surface waves and electrical resistivity tomography surveys The shear-wave velocity (Vs) of the ground was obtained by analyzing the data derived from the MASW technique. We used the software called SeisImager developed by OYO Cooperation in the analysis, which enabled us to produce a 2-D Vs profile along the survey line. The 2-D profile of the electrical resistivities was analyzed and obtained using the software Eleclmager/2D, which is also developed by OYO Corporation. 223

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Fig. 7. (a): Distribution of epicenters of earthquakes recorded from June 2014 to November 2016. The colours of the stars indicate the magnitudes of the earthquake events; (b): Distances and azimuths between the epicenter and the Azue landslide area. In (b), logarithms of epicenter-landslide distances are plotted along diagram radii.

Fig. 8. Detailed layouts of the geophysical surveys in P2-area (a) (landslide deposits area) and P3-area (b) (middle landslide block-A), marked in Fig. 6a). M and E represents the MASW and ERT survey lines, respectively. 224

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Fig. 9. Time-history (EW component) curves recorded at seismic stations P1 to P5 on the Azue landslide area during a M4.0 earthquake event on September 3, 2016.

surficial soil layers are relatively homogenous. Fig. 12d and e show the distribution of electric resistivity (ER) in areas P2 and P3, respectively. In landslide sub-block-A (P3-area), the gravel soil layers shallower than 20 m have resistivities ranging from 300 to 1000 Ωm on the centre part of the survey line, and > 1000 Ωm at the two ends of the line. However, in the strongly weathered greenstone stratum, ER is > 1500 Ωm. Compared to this stratum, the gravel soil stratum shows more sharp changes of ER. In the landslide deposit area (P2-area), ER is 2500 Ωm in a thin (< 5 m) surface layer, then is higher up to a depth of 15–20 m (with ER > 3500 Ωm). However, it decreases at depths > 20 m.

is presented using a colour scale as function of frequency, reported radially, and by azimuth. It can be seen that:

• For both S-phase and coda waves (Fig. 11a and d), the HVSR peak •

•

amplitude at P1 is < 3 with a weak directional maximum at N110° ( ± 55°). P2 shows H/V directional maxima at N80° ( ± 35°) and N60° ( ± 20°) in S-phases (Fig. 11b) with the HVSR peak amplitudes being > 50 in the frequency bands of 4–6 Hz and 20–25 Hz, respectively. However, the amplitudes of the H/V ratio are 20–30 in coda waves in these frequency bands, with weaker directivities (Fig. 11e). P3 shows clear directional maxima in the different frequency bands. In the S-phase (Fig. 11c), HVSR peak amplitude is approximately 3–5, oriented along N167° ( ± 42°) and N155° ( ± 55°) in the frequency bands of 2–4 Hz and 8–10 Hz, respectively. However, in coda waves (Fig. 11f), the HVSR peak amplitude is about 4–6, and amplification directions are oriented along 175° ( ± 55°), N157° ( ± 42°), and N130° ( ± 40°), in the frequency bands of 2–4 Hz, 8–11 Hz, and 20–25 Hz, respectively.

7. Discussion 7.1. Comparison of amplification between S-phase and coda waves To analyze the properties of amplification in the landslide area in different earthquake events, we compared the HVSR curves obtained from the analysis of S-phase (Fig. 10a to 10e) and coda waves (Fig. 10f–j). Some previous research has indicated that for the same site, the shapes of the HVSR curves from S-phase and coda waves are similar in the same earthquake event, but the amplitudes of the HVSR curves are different (e.g. Seekins et al., 1996; Massa et al., 2014). From this point of view, our results presented in Fig. 10 show good consistency in general. We then compared the HVRf and its corresponding peak amplitude for all the HVSR curves shown in Fig. 10. The HVSR peak amplitude and the corresponding HVRf for different stations obtained from the S-phase and coda waves are presented in Fig. 13a and b, respectively, reporting for each station the HVRf appearing most frequently in the HVSR curves of Fig. 10. It is seen that an HVRf of < 10 Hz occurs similarly in both Sphase and coda waves. However, in the range of 10–30 Hz, the coda waves enabled us to distinguish more peaks than the S-phase. However, greater amplification effects occur in the S-phase than in the coda waves, especially for the stations located on the landslide deposit area (P2) and close to the landslide boundary (P4). Therefore, it is inferred that the coda waves in an earthquake record can provide more information on the occurrence of resonance peaks, and thus provide more evidence for understanding the ground motion amplification.

As shown in Fig. 11, both S-phase and coda waves present similar distribution patterns in general. Nevertheless, the coda wave enabled us to identify some extra HVRf in high frequency bands of 20–25 Hz (e.g. station P3 shown in Fig. 11f). This aspect will be discussed further later. 6.2. Vs-profile and electrical resistivity The Vs profile along line M1 (see Fig. 8b for location) is presented in Fig. 12a. It is seen that the values of Vs vary gradually from 320 to 360 m/s at a depth of 6 to 8 m. The borehole data revealed that the slope in this area consists of subsurface gravelly soil, which may be the reason for the small value of Vs in the upper soil layer. The layer consisting of strongly weathered greenstone has a Vs of around 400 m/s at a depth of 6–8 m, and up to about 500 m/s in the deeper part (near a depth of 15 m). The 2-D Vs profiles of M2-1 and M2-2 for the landslide deposit area around station P2 (see location in Fig. 8a) are shown in Fig. 12b and c, respectively. Fig. 12b and c, respectively. The soil layer with a Vs of around 240–300 m/s is up to 4 m thick and gradually becomes thicker to the northern end. This layer consists of landslide deposits (mainly gravelly soil). The Vs becomes greater (up to 400 m/s) when the depth reaches 10 m. Compared to the P3 area, the values of Vs in these

7.2. Reasons for the resonance frequency (HVRf) and amplification The resonance frequency (HVRf) in various frequency bands can be 225

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(caption on next page)

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Fig. 10. HVSR curves of spectral ratios between the horizontal (H) and vertical (V) components of seismic motion for different stations. (a) to (e): HVSR curves derived from the S-phase; (f) to (j): HVSR curves from coda waves. Black straight lines in graphs indicate the mean value and black dash lines delimit the 95% confidence interval. The colour curves are HVSR values derived from a single earthquake among those listed in Table 1.

Fig. 11. Polar diagrams of the H/V ratio values calculated at azimuth intervals of 10° from the recording of the February 6, 2015 earthquake, acquired at seismic stations P1, P2 and P3. (a) to (c): Results derived from the S-phase; (d) to (f): Results from coda waves. In the polar diagrams, colours represent the H/V ratio value as a function of frequency, reported radially, for ground motion components along different azimuths. Note that values at P2 are much higher than for other stations and hence the scales of the polar diagrams are different. 227

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Fig. 12. (a) to (c): Distribution of shear wave velocity (Vs); (d) and (e): Distribution of electrical resistivity; (a) shows the profile M1 in the P3 area. (b) and (c) show the profiles M2–1and M2–2, respectively in the P2 area. (d) and (e) show the profiles of E2 and E1, respectively in the P2 and P3 areas. The lithological boundariesstratum boundaries in (a) and (e) are based on borehole data.

lithological conditions, but may result from the normal mode of vibration of the mountain ridge, showing phenomena similar to those presented in Roten et al. (2006). In order to examine above-mentioned inference, we compared these data with the theoretical resonance frequency for the topographic effects (ft) for this slope. We employed an empirical relationship used in other studies (e.g. Panzera et al., 2011; Hartzell et al., 2014) to estimate ft, i.e.

used to understand why different types of amplifications occur. We analyzed the value of HVRf and its corresponding peak amplitude for all the HVSR curves shown in Fig. 10, trying to distinguish whether the amplification results from topography or material contrasts, although it is difficult to separate these two factors clearly in most cases (Graizer, 2009; Massa et al., 2010). At station P1, in both S-phase and coda waves, the HVSR peak amplitude is almost always smaller than 2 (Fig. 13), indicating that there is no amplification there, based on the criterion mentioned above. However, peaks > 2 appear for P3, P4 and P5 in both S-phase and coda waves in the frequency band of 1.1–3.8 Hz. Amplification caused by topography always occurs when the wavelengths of seismic waves are of the same order as the horizontal width of the slope (e.g. Faccioli et al., 1997; Paolucci, 2002). Stations P3 and P4 are located on landslide blocks (Fig. 6) with P4 being close to the mountain ridges where the topographic effect could be strong, while P5 is installed directly on the bedrock of the mountain ridge. Amplifications appearing in the narrow frequency band of 1–3.2 Hz for these three stations indicate that this kind of amplification may be less or not related at all to the local

ft = Vs / L

(2)

in which ft is the topographic resonance frequency, Vs is the shear wave velocity and L is the base width of the hill slope. For the hill slope where Azue landslide is located, L is estimated to be about 1800 m (as one can see from Fig. 6b, where half of the hill slope width is shown from station P2 to P4). Concerning Vs, Hartzell et al. (2014) indicate that its value should be adopted from the bedrock to avoid being contaminated by the material contrasts within the shallow strata. Because we did not directly measure the Vs for the bedrock (greenstone stratum) of Azue landslide, we used a Vs value of 228

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fm = V0 (1 − x)/[4(1 + h)1 − x − 4]

where V0 is the shear-wave velocity at surface, z’ = z/z0 (with z0 = 1 m), x represents the depth dependence of velocity and h is the thickness of the soil layer. The value of x can be calculated through the regression of Vs vertical profile data derived from MASW surveys. Using the data relative to profile M1, the value of 0.329 was estimated. According to Fig. 12b and c, the thickness of the shallow material ranges from about 2.5 to 5 m, thus with a mean V0 of 240 m/s (averaged from the results in Fig. 13a and b), the fm was estimated to be about 17.2 to 32.1 Hz. These frequency bands contain the HVRf of 28 to 31.9 Hz in Fig. 13b. In addition, Fig. 12d indicates that the stratum of weathered greenstone, which reaches depths from 12 to 21 m, shows a contrast with the deeper stratum, and we then inferred that this may result in another HVRf in the frequency band of < 10 Hz. Due to limitations of the investigated depth (about 10 m) from the MASW survey in the P2 area (Fig. 12b and c), we applied a passive seismic method named HVTFA (Poggi and Fäh, 2010) to retrieve the Vs velocity in the deeper stratum (> 10 m) through analyzing the ambient noises recorded at P2. By using code Dinver in the Geopsy software (a platform for solving inversion problem by using Neighborhood Algorithm as detailed by Wathelet, 2005), we analyzed these HVSR peaks (up to 10 Hz) shown in the ellipticity of fundamental mode Rayleigh waves (Fig. S1a), and obtained the 1-D Vs profile (Fig. S1b). It is seen that Vs changes obviously at a depth of about 16 m. Based on Eqs. (3) and (4), an fm of 7 Hz was obtained. This value showed similar frequencies corresponding to the peaks of HVRf between 4.1 and 8.8 Hz (as shown in Fig. 13). Therefore, we infer that the amplifications at P2 may mainly result from the material contrasts within multiple strata. As seen in Fig. 12a, the Vs profile along M1 shows lateral heterogeneities in the shallow soil stratum (< 8 m in depth) within landslide sub-block A and a velocity contrast occurs at a depth of about 7 to 11 m along the survey direction. We then also used Eqs. (3) and (4) to estimate the possible resonance frequency due to material contrasts. Assuming a mean V0 of 320 m/s (averaged from the results shown in Fig. 12a) and thicknesses of about 7 to 9 m and 5 to 7 m for the gravel soil strata where station P3 and P4 are located, respectively (according to borehole data provided by Japan Conservation Engineering Co., Ltd), fm values from 14.6 to 17.7 Hz and from 17.6 to 23.1 Hz are obtained for P3 and P4, respectively. These results are close to the HVRf of 14.9 to 19.8 Hz for P3 and 12.3 to 18.3 Hz as shown in Fig. 13. In addition, at P3 there is another HVRf peak in the frequency band from 4.3 to 10.1 Hz (Fig. 13a) and 6.9 to 11.6 Hz (Fig. 13b), which may result from the material contrast in the deeper stratum of strongly weathered greenstone. Assuming a thickness of 13.5 to 45 m, based on borehole data, fm was estimated to be from 4.5 to 10.8 Hz by means of the Eqs. (3) and (4), which are close to the frequencies of the P3 HVRf. Therefore, the amplification in the frequency band > 4 Hz for stations P3 and P4 may result from the material contrasts in multiple strata. Another common method, standard spectral ratios (SSR) between two stations (Borcherdt, 1970), was also used to evaluate the amplification effects on landslides (e.g. Havenith et al., 2002; Hartzell et al., 2014). In this study, we applied this method to examine the amplification effects for different areas of landslides and compared the results between the SSR and HVSR methods. Station P1 (shown in Fig. 8) on a bedrock area off the landslide area was treated as the reference site. The results of SSR curves (averaged over the events in Table 1) at stations P2, P3 and P4 from S-phase and coda waves are shown in Fig. S2. The results are similar to those from the HVSR method (as shown in Fig. 10) and the detailed comparisons between the two methods are shown in Fig. S3. In general, the resonance frequencies of the spectral ratio curves from two methods present good consistent in frequency bands < 10 Hz (Fig. S3a and S3b) for both S-phase and coda waves for station P2, P3 and P4, but show clear difference at the frequencies > 10 Hz in coda waves. Furthermore, the resonance frequencies for high frequency bands 10–20 Hz on curves for P4, revealed by the HVSR

Fig. 13. HVSR peak amplitude in the HVSR curves of Fig. 10 as function of resonance frequencies (HVRf). HVRf identified from S-phase and coda waves are reported in (a) and (b), respectively. The HVRf for each station shown here is that appearing most frequently in all HVSR curves of Fig. 10. The error bar indicates the range of HVRf and the corresponding range of the HVSR peak amplitude in all HVSR curves of Fig. 10.

about 2000 m/s based on data from local boreholes drilled by NIED, Japan (http://www.hinet.bosai.go.jp) for the construction of the KiKNET system. Using these data, ft is estimated to be 1.2 Hz, which is close to the HVRf obtained by both S-phase and coda waves (Fig. 13). Therefore, we infer that the amplification on landslide sub-block A in frequency band 1–3.8 Hz may mainly result from the topographic effect. For P2, the amplification shows that the HVRf in the frequency band 4.1–8.9 Hz appears in the S-phase (Fig. 13a), while in the coda waves, two peaks are identified with the values of HVRf located in the frequency bands of 4 to 8.2 Hz, and around 30 Hz, respectively (Fig. 13b). Considering that there is no irregular topography in the P2 area, and the elevations for P1 and P2 are approximately the same (about 300 and 310 m for P1 and P2, respectively), we inferred that the amplification at P2 might result from the material contrasts. To examine this inference, we used an empirical relationship proposed by Ibs-von Seht and Wohlenberg (1999) to estimate the possible resonant frequency of ground with different soil layers. According to Ibs-von Seht and Wohlenberg (1999), if the shear wave velocity Vs of a soft layer overlying a hard rock basement is depth dependent according to the equation

Vs (z) = V0 (1 + z′) x

(4)

(3)

the resonant frequency fm of the soil layer can be calculated from equation.

Vs (z) = V0 (1 + z′) x 229

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method (as shown in Fig. 13), are not present in the SSR curves. For the peak amplitude, the values from the SSR method are 1.5 to 3 times greater than those from the HVSR method (Figs. S3c and S3d). Havenith et al. (2002) indicated that the amplitudes between two methods (HVSR and SSR) are not always consistent and the HVSR peak amplitude can be considered as the lower bound of the amplification effects in some cases. However, Fig. S3c and S3d indicate that the peak HVSR amplitude on station P2 are even larger than those of the SSR ratio from coda waves. Concerning this further examination will be needed and is planned in the future. 7.3. Directivity of the amplification To understand this feature of amplification, we analyzed the direction of the HVRf for all the earthquake events given as examples in Fig. 11. To evaluate the specific directivities of amplification, we followed the criteria proposed by Del Gaudio et al. (2013) by using the specific directivity coefficient (SDC), which is defined as:

SDC =

HVSR max (f) HVSR min (f)

(5)

where HVSRmax and HVSRmin are the maximum and minimum values of HVSR at the same resonance frequency f, obtained by rotating the horizontal component used to calculate the HVSR curve (and typically are found along approximately orthogonal directions). According to the criteria given by Del Gaudio et al. (2013), the amplification can be regarded as having specific directivity when SDC is > 1.5. Fig. 14 shows the results obtained from S-phase and coda waves, respectively. It is seen that the SDC mean values for all the HVRf are smaller than 1.5 for both P1 and P2, indicating that they have no predominant amplification directions on stations. Although the mean values of SDC on station P3, P4 and P5 on landslide are > 1.5 on different HVRf, the directivity of amplification on P3 is always weak with the values of SDC being slightly > 1.5. However, strong directivities occur on P4 at any resonance and P5, and their mean values of SDC are close to or even > 3 for HVRf in different frequency bands. To have a general overview of the directional amplifications, Fig. 15 summarizes all results presented in Figs. 13 and 14 for all the stations. In Fig. 15, it is seen that weak amplification effects (HVSR peak amplitude generally smaller than 2) with largely unspecific directivities appear in bedrock areas where P1 is located. Directional amplifications caused by topographic effect (marked as T) are found at P3, P4 and P5. On the ridge of the hill slope where P5 is located, the site response shows specific directions along N107° ( ± 42°), with an HVSR peak amplitude of 2 to 6, which is approximately perpendicular to the ridge direction (N20° ± 10°). Area P4 shows amplitudes of H/V ratio 2 to 6 at N115° ( ± 40°), which is also perpendicular to the ridge direction. The landslide sub-block area where P3 is located shows an HVSR peak amplitude of 2 to 4 along the direction of 120° ( ± 35°). This features of directional amplification by topography are consistent with previous studies (e.g. Massa et al., 2014; Hartzell et al., 2017). For amplification resulting from material contrasts (marked as M in Fig. 14) in multiple strata, the landslide deposit area where P2 is located shows a strong amplification effect (HVSR peak amplitude > 8) but does not show any specific directivity. However, the amplification (HVSR peak amplitude of 2 to 6) in the upper area where P4 is located shows a predominant orientation along N47° ( ± 37°). On the landslide block where P3 is located, the amplification (HVSR peak amplitude of 4 to 6) due to the strongly weathered greenstone stratum is predominantly oriented along N52° ( ± 37°) in S-phase and N40° ( ± 35°) in coda waves. Notably, the amplification effect (HVSR peak amplitude of 4 to 6) on the landslide block with a gravel soil stratum is predominantly oriented along N115° ( ± 45°), which is significantly different from the amplification directions by the strongly weathered greenstone strata.

Fig. 14. (a) and (b): Specific directivity coefficients (SDC) of HVSR peak amplitude resulting from different resonance causes, respectively in the S-phase and coda waves. Amplification here is embodied by HVSR peak amplitude at the resonance frequencies HVRf (shown in Fig. 13). The letters in brackets represent the resonance cause. A value of 1.5 is always treated as a threshold for significant site response directivity (Del Gaudio et al., 2013). A value > 1.5 indicate a predominant direction, otherwise an almost isotropic amplification is assumed. The error bar indicates the range of SDC estimated statistically from HVRf.

7.4. Multiple amplification effects due to material contrast The comprehensive results (Fig. 15) shows that the amplification effects due to material contrasts are greater than those caused by topography. In particular, multiple amplification effects from material contrasts occur in the centre area of the landslide block (see results for P3 in Fig. 15) and the landslide deposit area (see results for P2 in Fig. 15). Using the Eqs. (3) and (4), we evaluated the thickness of material where amplification occurs from the values of HVRf. The loose thin gravelly soil stratum (< 10 m on the landslide block and < 4 m on the landslide deposit) may trap the incoming waves and result in amplification (Fig. 16a and b). The presence of a deeply weathered greenstone stratum (at depth > 10 m for the landslide block and approximately 4–16 m for the landslide deposit) proved to be the most significant factor influencing the amplification effect (Fig. 16a and b). In addition, Fig. 15 also shows that the predominant amplification directions related to the superficial gravelly soil and the strongly weathered greenstone stratum are obviously different in the landslide block (see results for P3 and P4 shown in Fig. 15) and we inferred these

230

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Fig. 15. Features of the amplification effects (amplification factor and directivity) for different locations on the Azue landslide respectively, in the S-phase and coda waves. This map summarizes the results shown in Figs. 13 and 14. The colour scale indicates the mean values of the HVSR peak amplitude and the scale of azimuth bin indicates the occurrence rate of the mean amplification for different intervals of direction angle. The letters in brackets represent the probable causes for amplification (the same as those shown in Fig. 15). M and M' represent the amplification (caused by material contrasts) from the strongly weathered greenstone stratum and gravelly soil stratum, respectively. The dash line indicates the estimated boundary of the ancient landslide.

(N52° ± 37° and N40° ± 35°) in S-phase and coda waves, respectively) related to the strongly weathered greenstone stratum in P3 are approximately normal to strikes of joints (N58°W–58°SW, N8°E–66°W and N70°E–50°N as shown in Fig. 17b and c) that outcrop on the landslide scarp near P3. Station P2 is located on an ancient landslide deposit. It is possible that debris from the ancient large-scale landslide moved down to the

phenomena may be correlated with local site features. The predominant direction of amplification (N47° ± 37°) related to the gravelly soil stratum on P4 is approximately along the local slope aspect where P4 is located (southwest to west) (Slope-aspect map is shown in Fig. 17a). Meanwhile, the predominant direction of amplification (N115° ( ± 45°)) for P3 is also along the local slope aspect (west to northwest) (Fig. 17a). However, the predominant directions of amplification

Fig. 16. Relationship between the thicknesses of the material responsible for resonance (measured through the HVRf) and the corresponding HVSR peak amplitude for different stations, in the landslide block (a) and landslide deposit areas (b). The dashed lines indicate the material boundaries determined by the geophysical survey. The error bar indicates the scope of the measured thicknesses of the material and the corresponding range of the HVSR peak amplitude from all HVSR curves in Fig. 10. 231

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Fig. 17. (a): Relationship between slope aspect and amplification directions (caused by material contrasts related to the gravelly soil stratum), represented by rose diagrams, on P3 and P4; (b) and (c): relationship between the joint sets near P3 area and the amplification directions (caused by material contrasts related to the strongly weathered greenstone stratum) derived from S-phase and coda waves, respectively. The slope aspect map is shown in (a). The stereonets in (b) and (c) (equal area, lower hemisphere) shows joint planes measured on greenstone outcrops inside the landslide scarp in 2004 (data from Japan Conservation Engineering Co., Ltd). There are three predominant joint planes: N58°W58°SW, N8°E66°W and N70°E50°N. The dashed line indicates the mean amplification directions and the range of amplification directions are plotted as rose graph. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

topography and/or material contrasts for various areas of the landslide. Some conclusions are as follows.

river bank, producing a thick deposit which at present has been made thinner because of erosion and scour by the river. However, a strong contrast in materials is present between the weathered greenstone strata (approximately at a depth of 16 m, as shown in Fig. S1b) and the deeper solid bedrock. Thus, the stronger amplification effects, especially in the frequency band below 10 Hz, affects the toe of the hill slope covered by the ancient landslide deposit. It is also worth noting that consideration of this kind of amplification may play a key role in the construction of earthquake-resistant infrastructures in mountainous areas. For example, in Japan, many residential districts are located on old large-scale landslide areas, where house or building foundations may have material contrasts similar to those shown for location P2, and which may result in strong amplification during an earthquake, causing severe damage. Therefore, detailed investigations of the foundations for this type of residential area in mountainous areas will be of great importance for the construction of structures resistant to earthquakes.

(1) The HVSR curves (also the SSR curves in this study) show that different landslide areas have different amplification patterns resulting from the analysis of S-phase and coda of seismic waves. In general, the S-phase gives indications of greater amplification of the landslide area than coda waves. However, the coda waves can reveal multiple amplifications due to the existence of more resonance frequencies. Weak or no amplifications (multiple low maxima of HVSR in broad frequency bands) normally appear in bedrock areas, strong amplifications (predominant single HVSR peaks of < 10 Hz in S-phase, however two HVSR peaks in frequency bands < 10 Hz and 20–30 Hz, respectively in coda waves) appear in the landslide deposit areas. Moderate amplifications (generally multiple HVSR peaks range in frequency bands 2–20 Hz for S-phase and coda waves) normally appear in the landslide block. (2) Significant amplifications (revealed by HVSR peak amplitude of 2–6) due to topography occurs predominantly on the landslide blocks (P3 and P4) and on the crest of the hill slope where bedrock outcrops (P5), and their specific amplification directivities are approximately perpendicular to the direction of the local ridge axis. However, additional resonance phenomena, with HVSR peak amplitude of 2–6, occur on the landslide block because of material contrasts at different depths and with different directivities, e.g.

8. Conclusion In this study, seismic site responses on the Azue deep-seated landslide were analyzed based on in-situ earthquake monitoring. Based on numerous seismic records from 5 seismometers on different areas (station P1 to P5) on and near the landslide and the results of geophysical and geological surveys of the landslide, we analyzed and summarized the features of amplification resulting from both 232

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along the slope direction, related to the superficial gravelly soil strata, and normal to joint strike in the deep, strongly weathered greenstone. On the other hand, stronger amplifications shown by the HVSR peak amplitude of > 8, are induced by material contrasts between different strata on the landslide deposit areas, but without pronounced directivities. (3) Comparing the amplifications resulting from material contrasts in the landslide block, extreme amplifications occur in the toe part of the slope on ancient landslide deposits, due to the greater extent of material contrasts than those in the landslide block. This kind of strong amplification phenomenon may result in damage to residential structures in localized mountainous areas, even though the whole hill slope does not suffer from instability.

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