Estimation of 1-D velocity models beneath strong-motion observation sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes
© The Author(s) 2017
Received: 27 February 2017
Accepted: 14 July 2017
Published: 24 July 2017
The collision of the Eurasian and Indian tectonic plates forms an active plate boundary with many seismic events. The Nepal Himalaya regularly experiences small seismic activities, and large earthquakes occur over certain time intervals (Sapkota et al. 2013). The occurrence of strong ground motions in the Nepal Himalaya is comparatively less than other seismically active regions around the world. As a result of the collision, the Himalayan orogeny has formed a number of tectonic valleys in the region, including the Kathmandu Valley in the Nepal Himalaya. The formation and eventual drying of a lake has left the valley with thick (>600 m) unconsolidated sediments (Moribayashi and Maruo 1980; Sakai 2001).
The manifestation of an earthquake effect is a combination of source, path, and site characteristics. In addition to the earthquake magnitude, site conditions play a vital role in the effect of earthquakes on infrastructure. An earthquake with seemingly no effect above hard ground can be felt as a strong tremor and cause severe damage in areas above soft or unconsolidated sediments due to the amplification of seismic waves. There are accounts of more than 20 devastating earthquakes occurring in or near the Nepal Himalaya after the thirteenth century (Dixit et al. 2013); the 2015 Gorkha Earthquake is the most recent. The Kathmandu Valley, along with a large part of central and eastern Nepal, suffered heavy loss of life and property due to the mainshock and ensuing aftershocks of the 2015 Gorkha Earthquake. According to Government of Nepal (2015), 8856 people lost their lives and more than 60,000 buildings were completely damaged. The Kathmandu Valley had 1739 casualties, and about 13% of buildings were estimated to have been completely damaged. A location in a seismically active region, and the presence of thick sediments that amplify seismic waves, have made the Kathmandu Valley a seismically vulnerable region. Moreover, the increasing tendency for haphazard building construction without proper engineering considerations has added to the potential for catastrophe.
A thorough study of ground motion amplification is required to prepare safe and up-to-date building codes that reduce loss of life and property during an earthquake. Characterizing subsurface velocity structures is a necessary precondition for achieving that goal. Previously, there was a study of 1-D velocity models in a few places in Kathmandu based on microtremors (Pandey 2000). A subsurface velocity model was prepared based on geological maps (Shrestha et al. 1998) and borehole logs collected during an earthquake disaster mitigation study (JICA 2002). Piya (2004) used available borehole data and geological information to prepare soil profiles of the valley sediment for liquefaction hazard analysis. The borehole logs available are predominantly from the groundwater wells in the Kathmandu Valley. The velocity logging below depths of 30 m is not publicly available. A study of basement structure using microtremor recordings from different locations in the valley, some of which lie in vicinity of the sites described in the present study, was carried out in 2012 (Paudyal et al. 2012).
Geological setting of the Kathmandu Valley
The Kathmandu Valley is a tectonic basin filled with fluvio-lacustrine deposits and surrounded by hills on all sides. The rapid uplift of the mountain range south of present-day Kathmandu dammed the proto-Bagmati River (Sakai et al. 2002) and formed the paleo-Kathmandu Lake (Fujii and Sakai 2002). The lake breached the hills and eventually dried out (at about 10 ka) leaving behind a sediment-filled valley (Sakai 2001). Since then, fluvial deposits from the Bagmati River system have overlain the older lake deposits. Previous studies (Moribayashi and Maruo 1980; Sakai 2001) have shown the sediments to be thicker than 600 m at the center of the valley. The hills surrounding the valley are formed of meta-sedimentary rocks with some granitic intrusions of Cambrian to Devonian origin (Stocklin and Bhattarai 1977). It is clear that the sediments in the valley are sourced from these basement rocks, as the lake in the past and the Bagmati River system at present are fed with water flowing from these very hills (Fig. 1). Figure 1 is based on the Engineering Geological Map of Kathmandu (1:50,000) published by Department of Mines and Geology (Shrestha et al. 1998).
The bottom and fringes of the valley have coarser sediments of the proto-Bagmati river system (Sakai 2001), which give way to finer sediments in the upper and central parts of the valley. The sedimentary layers in the valley can be generalized as coarser sand and gravel layers at the bottom superimposed with layers of sand and clay deposit of lacustrine facies; fluvial deposits from the Bagmati river system make up the topmost layer (Yoshida and Igarashi 1984; Dangol 1985; Shrestha et al. 1998; Sakai 2001). The sediment types in the valley generally vary from south to north, and the lithology has been differentiated into three main groups: southern, central, and northern (Sakai 2001; Piya 2004). The predominant sequence of thick black clay formed from a lacustrine environment in the southern and central region gives way to the sand-dominant sediments in the northern region. The clay layer diminishes in the north and pinches out to sandy/silty sediment (Sakai 2001) which has its origin in the granitic intrusion (Stocklin and Bhattarai 1977) in the northern surrounding hills.
Furthermore, borehole logs (Sakai 2001; JICA 2002; Piya 2004) have demonstrated a number of different layers and lenses formed due to varying depositional environments. The basin topography is highly undulated, and there are several rocky hillocks that breach through the thick sediments to the surface as bedrock exposures. One of these exposures can be seen in Kirtipur, where one of the accelerometers is installed (Fig. 1). The variation in basement topography, geology, and the depositional environment make it difficult to generalize the subsurface geologic structures and ground response of the valley as a whole. The lack of proper data on subsurface geology, velocity logs, and soil profiles makes the task challenging.
A collaboration between Hokkaido University, Japan, and Central Department of Geology (CDG), Tribhuvan University, Nepal, began in 2011 to study the strong-motion characteristics in Kathmandu Valley (Takai et al. 2016). Four strong-motion accelerometers (Mitsutoyo JEP-6A3-2) were installed at KTP, TVU, PTN, and THM, in a west to east array (Fig. 1). These accelerometers are bolted to the ground floor of reinforced concrete (RC) buildings, except at PTN where the building is a single story masonry structure. These accelerometers operate continuously at a sampling rate of 100 Hz, and the time calibration is carried out using GPS. Although the data loggers are powered with regular 200 V AC supply, backup batteries are employed due to inconsistent electricity supply in the valley. The seismometers recorded a number of earthquakes including the Gorkha Earthquake on April 25, 2015, and its subsequent aftershocks. The seismic activity has largely increased after the Gorkha Earthquake. Four more temporary stations (BKT, RBN, PPR, and KPN) were deployed for 3 months (May 8 to August 6, 2015), after the Gorkha Earthquake, to observe aftershock activity.
The station KTP lies in Kirtipur, west Kathmandu, on a rock site. The stations TVU (CDG, Tribhuvan University), PTN (Pulchwok Campus, Patan), and THM (University Grants Commission, Sano-Thimi) are the sediment sites (Fig. 1). The shear wave velocity (V s) of the shallow subsurface layer measured during their installation shows V s ~700 m/s at KTP and ~200 m/s at other sites (Takai et al. 2015). The absence of prominent peaks in the average H/V ratio of microtremors at KTP and early arrival of S-waves at KTP compared to other sites during earthquakes (Bijukchhen et al. 2015) indicate that KTP is a rock site; there is likely a weathered and fractured layer of exposed bedrocks overlying the basement rocks at depth. The temporary stations, BKT (Bhaktapur), RNB (Ranibu), PPR (Panipokhari), and KPN (Kapan), were installed more or less normal to the existing W–E profile (Fig. 1). All four were installed over the sediment sites.
The forms of the Fourier spectra of the rock site (KTP) depend on f 2 for low frequencies (f < 1 Hz) except for the Mw5.5 event. This spectral form resulted from incidence of the low-frequency S-wave beneath the KTP site. The higher frequencies (f > 1 Hz) do not depend on f 0 effect of a weathered rock layer at the top. The spectra of the sedimentary sites show larger amplitudes in 0.2 < f < 2 Hz than those at KTP. There is sharp increase in amplitude at low frequencies between 0.2 and 0.4 Hz at the sedimentary sites for all earthquakes. It should be noted that as the window length for the spectral analysis in Figs. 2, 3, 4, 5 and 6 is 80 s, and the high amplitude observed is pertinent to S-wave amplification as well as excitation of basin-induced surface waves.
The long-period (0.1–0.5 Hz) transverse component of the acceleration waveform from the rock site (KTP) during the mb4.9 earthquake was considered as the incident wave, and simulated waveforms from the three sediment sites were calculated using Eq. (7). Although the bedrock at KTP is overlain with a shallow weathered rock layer, this material has little or no effect on the long-period waves we are using in this study. As this earthquake originated at >50 km depth, we assumed that the seismic waves impinged on the bedrock beneath the basin perpendicularly. The information regarding damping of soil layers in Kathmandu Valley is not available, so we fixed Q = 0.1 V s (V s in m/s) which is commonly used for long-period strong-motion simulation (Olsen et al. 2000; Satoh 2004).
Available borehole data (Sakai et al. 2001; JICA 2002; Piya 2004), a geological map (Shrestha et al. 1998), and geological cross-sections were synthesized to create the initial subsurface models. The shear wave velocity was based on the earthquake disaster mitigation report (JICA 2002), and the number of layers was based on the geological cross-sections. The shear wave velocity of bedrock was fixed as 3.2 km/s from a regional velocity model of the Himalaya (Monsalve et al. 2006; Ichiyanagi et al. 2016). We used a pulse of the band-pass-filtered (0.1–0.5 Hz) acceleration waveform from KTP as input motion passing through the initial models and obtained simulated ground motions at the sediment sites. The bandwidths were chosen considering the spectral ratio in the low-frequency range in all the earthquake records. The thicknesses of the layers were then adjusted using trial and error to match the simulated waveforms with observed ones. These adjusted 1-D velocity models were then used to simulate the long-period S-waves of the three aftershocks, two Mw5.1 and a single Mw5.5, after the Gorkha Earthquake. These simulated S-waves were compared with the observed ones to verify the adjusted models.
We used band-pass-filtered transverse components of the aftershock records at KTP as the input motions. Because these earthquakes have shallow hypocenters, the incident angle was considered to be 30°, based on trial and error.
Moderate earthquakes considered for the HVSR method for fixed stations. The magnitudes of the earthquakes range from M5 to M5.5
The HVSR method has been used to generate a 1-D velocity structure in Tohoku, Japan (Nagashima et al. 2014), confirming that the method could be employed to estimate the 1-D structure at our study sites. The temporary stations added after the main shock do not have records of the m4.9 earthquake, so we employed the HVSR method for the 1-D velocity model estimation.
Earthquakes considered for the HVSR method for four temporary stations
Despite the availability of a number of aftershock records, many were small and low energy in the long-period range, so they were not considered in this study. Moreover, a number of moderate-sized aftershocks in our database were affected by continuous smaller aftershocks occurring for a few seconds. The quality of the records was a constraint in choosing the earthquake for the modeling of the observed long-period S-wave. However, for larger earthquakes, because they are affected by the nonlinear site response of valley sediment (Dhakal et al. 2016; Rajaure et al. 2016), the nonlinear response needs to be taken into account when used for similar calculations.
The 1-D velocity models (Figs. 8, 13) of the sediment show varying sediment thicknesses (bedrock depths) for the sites in the Kathmandu Valley. The depth of bedrock varies from 155 m at KPN to 440 m at THM. Because THM is not in the central part of the basin, the depth might increase at the center. As indicated previously, the dominant clay layer in the central valley transitions to a sandy layer in the north; this observation has been taken into account while adjusting the velocity models for the northern sites, PPR and KPN (Fig. 13). As we consulted and based our initial models on the geological cross-sections and borehole logs near the PPR site (JICA 2002), which showed the dominance of sandy layer with lenses of clay layers, the adjusted models (Fig. 13) show a velocity inversion at PPR. Another feature of the velocity models (Figs. 8, 13) is a large velocity contrast at the bedrock depth. This is due to a geological unconformity in the lithological sequence, where soft sediments of the proto-Bagmati lake deposited over the layer of weathered basement rocks during the valley formation.
We estimated 1-D velocity models for seven sites in the Kathmandu Valley using strong-motion records from moderate-sized earthquakes. First, the initial 1-D velocity models were constructed based on available geological data, borehole logs, and cross-sections. Second, we adjusted velocity models from the initial models, by using forward modeling of the observed long-period S-wave from mb4.9 deep earthquake. In this modeling, the observed long-period S-wave at the rock site was assumed as incident wave at the bedrock beneath the sedimentary sites. Third, the adjusted velocity models were verified by comparing the long-period simulated S-waves with the observed ones for three aftershocks (Mw5.1, Mw5.1, and Mw5.5) of the 2015 Gorkha Earthquake (Mw7.8). We have obtained fairly good agreement between the observed and simulated waveforms.
Finally, we examined the horizontal-to-vertical spectral ratios (H/V ratios) of earthquake ground motion to verify the adjusted velocity models estimated by forward modeling of the observed long-period S-wave. The observed H/V ratios agreed with the theoretical H/V ratios (Kawase et al. 2011) calculated from the adjusted velocity models in the low-frequency range; however, there were discrepancies between the observed and theoretical H/V ratios in the high-frequency range. We hypothesize that the HVSR method can also be used to estimate the subsurface geology in the Kathmandu Valley where the velocity contrast is high. The HVSR method is also a tool at our disposal to validate 1-D velocity models. The adjusted 1-D velocity models estimated by this study show the variation of sediment thickness beneath the sites. The bedrock depth varies from 155 to 440 m indicating an undulating basin topography of the Kathmandu Valley. The models show a high velocity contrast at the bedrock depth which results in significant S-wave amplification at the sediment sites. Our longer-term goal is to prepare a full 3-D model of the subsurface geology of the valley using seismic waves, and this study is a first step in that task.
NT, TS, and MS designed the study. NT, MS, TS, MI, and SB aided in the installation and managing observations. YS collected the geological data and prepared the cross-sections. SB analyzed the data and drafted the manuscript. All authors read and approved the final manuscript.
We acknowledge Prof. M.R. Dhital of Tribhuvan University, and Mr. S. Rajaure of Department of Mines and Geology, for help in installing and maintaining the instruments. Figure 1 is based on the Engineering Geological Map of Kathmandu printed by the Department of Mines and Geology (Shrestha et al. 1998). The Himalayan Main Frontal Thrust outline in Fig. 1 inset is based on Lave and Avouac (2000). We used GMT (Wessel et al. 2013) to prepare some of figures in the paper. The data for the location of the epicenters, depths, origin times, and magnitude were obtained from the United States Geological Survey (USGS 2015) portal. Part of this study was supported by the SATREPS program of JST/JICA and J-RAPID program of JST and JSPS KAKENHI (Grant Numbers 16K06586, 16K16370, and 17H06215). We are grateful to two anonymous reviewers who gave their time to thoroughly review the manuscript and helped improve it.
The authors declare that they have no competing interests.
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