Analysis of strong ground motions and site effects at Kantipath, Kathmandu, from 2015 Mw 7.8 Gorkha, Nepal, earthquake and its aftershocks
© Dhakal et al. 2016
Received: 2 November 2015
Accepted: 29 March 2016
Published: 12 April 2016
Strong ground motions from the 2015 Mw 7.8 Gorkha, Nepal, earthquake and its eight aftershocks recorded by a strong-motion seismograph at Kantipath (KATNP), Kathmandu, were analyzed to assess the ground-motion characteristics and site effects at this location. Remarkably large elastic pseudo-velocity responses exceeding 300 cm/s at 5 % critical damping were calculated for the horizontal components of the mainshock recordings at peak periods of 4–5 s. Conversely, the short-period ground motions of the mainshock were relatively weak despite the proximity of the site to the source fault. The horizontal components of all large-magnitude (Mw ≥ 6.3) aftershock recordings showed peak pseudo-velocity responses at periods of 3–4 s. Ground-motion prediction equations (GMPEs) describing the Nepal Himalaya region have not yet been developed. A comparison of the observational data with GMPEs for Japan showed that with the exception of the peak ground acceleration (PGA) of the mainshock, the observed PGAs and peak ground velocities at the KATNP site are generally well described by the GMPEs for crustal and plate interface events. A comparison of the horizontal-to-vertical (H/V) spectral ratios for the S-waves of the mainshock and aftershock recordings suggested that the KATNP site experienced a considerable nonlinear site response, which resulted in the reduced amplitudes of short-period ground motions. The GMPEs were found to underestimate the response values at the peak periods (approximately 4–5 s) of the large-magnitude events. The deep subsurface velocity model of the Kathmandu basin has not been well investigated. Therefore, a one-dimensional velocity model was constructed for the deep sediments beneath the recording station based on an analysis of the H/V spectral ratios for S-wave coda from aftershock recordings, and it was revealed that the basin sediments strongly amplified the long-period components of the ground motions of the mainshock and large-magnitude aftershocks.
KeywordsGorkha earthquake Kathmandu basin Site effects Long-period ground motion
Event locations and magnitudes from USGS
Local origin time
April 25, 2015, 11:56
April 25, 2015, 12:30
April 25, 2015, 12:41
April 25, 2015, 14:40
April 26, 2015, 05:01
April 26, 2015, 12:54
April 26, 2015, 22:11
May 12, 2015, 12:50
May 12, 2015, 13:21
In this study, we used the recordings from the KATNP site for the nine events listed in Table 1. We also used ground-motion data (Galetzka et al. 2015) from two global positioning system (GPS) stations, KKN4 and NAST (shown in Fig. 1), for the mainshock. The KKN4 GPS station is located outside the Kathmandu basin at a hard rock site, whereas the NAST GPS and KATNP strong-motion stations are located in the basin. Several papers (e.g., Bhattarai et al. 2015; Dixit et al. 2015; Takai et al. 2016) already discussed the main features of the ground-motion recordings at the KATNP site for the mainshock. This paper differs from the previous papers in that we analyzed the strong-motion data at the KATNP site for a greater number of events and compared the observed ground-motion parameters of these events with those calculated using ground-motion prediction equations (GMPEs) for Japan. This paper discusses the degree of nonlinearity during the mainshock in some detail. Additionally, a one-dimensional (1D) velocity model for deep sediments was constructed, and this paper describes the long-period site amplification effect with reference to this newly constructed velocity model.
Nepal and the Himalayan regions in general have not obtained the number of strong-motion recordings necessary for seismic hazard analysis because of the sparse and underdeveloped strong-motion monitoring network in the region (Parajuli et al. 2008; Nath and Thingbaijam 2011). The first country-wide seismic hazard analysis project in Nepal (HMG and UNDP/UNCHS 1994) adopted the GMPE developed for Japan by Kawashima et al. (1984), mainly because this GMPE employed data from plate interface thrust events and the sites were sufficiently similar to be applicable to Nepal. During a comprehensive earthquake disaster mitigation study in the Kathmandu Valley (JICA and MoHA 2002), the GMPE developed by Boore et al. (1997) for western North American earthquakes was used, mainly because this GMPE accurately described the derived ground-motion data for the Ms 6.6 Udayapur earthquake that occurred in the eastern part of Nepal. It should be noted that the Udayapur earthquake was a relatively deep event with a focal depth of 57 km (Dixit 1991) and that it was different from plate interface events (Ghimire and Kasahara 2007). Parajuli et al. (2008) selected the GMPE developed for subduction zone events by Atkinson and Boore (2003) for use in probabilistic seismic hazard analysis in Nepal without further explanation regarding this choice. Goda et al. (2015) adopted the GMPE by Kanno et al. (2006) to assess the ground motions of the mainshock in Nepal on the grounds that this GMPE was found to be superior to other applicable GMPEs regarding its ability to predict peak ground accelerations (PGAs) at rock sites in North India and Nepal in an extensive analysis of worldwide GMPEs by Nath and Thingbaijam (2011). Goda et al. (2015) also selected the GMPE by Kanno et al. (2006) because of its applicable magnitude ranges and suitable distance definition for large-magnitude events.
Based on the above discussion and because the Gorkha earthquake and its major aftershocks occurred along the Main Himalayan Thrust (MHT), which is a megathrust plate interface (Avouac et al. 2015), it is reasonable to employ GMPEs developed for events that occurred along other megathrust plate interfaces, such as in Japan, to assess the ground-motion parameters for the Nepal earthquakes. However, in the Himalayan region, the plates that are separated by the thrust interface are continental in nature and do not resemble typical thrust interfaces in subduction zones where an oceanic plate subducts beneath a continental plate. Morikawa and Fujiwara (2013) updated the database used by Kanno et al. (2006) with additional data and obtained a GMPE applicable to different tectonic environments as well as sites located on deep sediments. Therefore, this paper compares the ground-motion parameters, namely the PGAs, peak ground velocities (PGVs), and response spectra, observed at the KATNP site with those obtained from the GMPE developed by Morikawa and Fujiwara (2013) for Japan for both plate interface and crustal events. To elucidate the epistemic uncertainties associated with the GMPEs, the observed PGAs and PGVs were also compared with those obtained from the GMPEs developed by Si and Midorikawa (1999), which have been used by the Headquarters for Earthquake Research Promotion of Japan to create national seismic hazard maps for Japan.
Figure 4 also shows that the PSVRS for the Mw 6.6 event were larger than those for the Mw 6.7 event by a factor of approximately 1.8 at a peak period of approximately 3.5 s despite the fact that the source-to-site distances of the two events were similar (see Fig. 1 for the locations of the events). The focal depth of the Mw 6.6 event was 10 km, whereas that of the Mw 6.7 event was approximately 23 km (Table 1). The difference between the focal depths of the two events may be one of the reasons for the difference in their response amplitudes because shallow events can excite stronger long-period ground motions. The difference between the peak response amplitudes of the two events may also indicate the dependence of the basin response on the azimuth of the incident wave field (e.g., Kagawa et al. 1992), as the waves impinged on the basin from opposite directions.
During the mainshock, mostly low-strength masonry buildings, such as those made of bricks and mud mortar and those constructed without reinforcement elements, collapsed or were severely damaged at several sites in the Kathmandu basin, whereas the reinforced concrete buildings in the area remained standing (Dhakal et al. 2015a, b; Galetzka et al. 2015; Goda et al. 2015; Hashash et al. 2015). The level of acceleration generally considered sufficient to produce ordinary damage to low-strength structures is approximately 0.1 g (Richter 1958). Hence, the selective damage of buildings during the mainshock may be attributable to the smaller PGAs and short-period ground motions in the Kathmandu basin. Here, it should be noted that in the northwestern portion of the Kathmandu basin several reinforced concrete buildings were damaged or collapsed (Goda et al. 2015; Hashash et al. 2015). Because of the lack of strong-motion recordings at the sites of damaged buildings, it is not clear whether the damage was due to large ground motions. An analysis of the design and construction of damaged buildings may reveal the intensity of the ground shakings in the area. Hashash et al. (2015) reported that some of the damage to the reinforced concrete buildings in the area may have been due to topographic and basin edge effects.
In spite of the proximity of the KATNP site to the source fault, the PGA of the mainshock was relatively small; this may be attributable to the earthquake rupture characteristics (e.g., Galetzka et al. 2015) and soil nonlinearity (e.g., Dixit et al. 2015). Several researchers’ source inversion analyses (e.g., Kobayashi et al. 2015; Yagi and Okuwaki 2015) have shown that the Kathmandu basin is oriented in the direction of forward rupture directivity and is close to large-slip areas. Previous studies (e.g., Ide et al. 2011; Lay et al. 2012) of megathrust subduction zone events demonstrated that high-frequency seismic waves emanate from deeper areas of the rupture plane, in contrast to the large total slips that occurred at shallower parts of the rupture plane. The model of high-frequency radiation sources of the mainshock proposed by Yagi and Okuwaki (2015) shows that stronger high-frequency radiations occurred in deeper areas in the source fault rather than at the shortest fault distance from the KATNP site. The indirect analysis of soil nonlinearity conducted in the present study demonstrated that the KATNP site indeed experienced a considerable nonlinear site response, as described in the next section. Thus, in summary, it may be inferred that the rupture characteristics and soil nonlinearity greatly contributed to the reduced PGAs and short-period ground motions, resulting in less damage and fewer casualties in the Kathmandu basin than expected (e.g., JICA and MoHA 2002).
Long-period ground-motion intensities at the KATNP site
Hypocentral distance (km)
Observed long-period intensity
Predicted long-period intensity
Observed peak absolute velocity response (cm/s)
Median predicted absolute velocity response (cm/s)
Local site condition and site characteristics
To understand the site characteristics, such as the predominant period of the ground motion and the site amplification, the horizontal-to-vertical (H/V) spectral ratios for the S-waves and S-wave coda were analyzed. The peak H/V spectral ratio corresponds to the predominant period of the ground motion at which the input seismic motions are most strongly amplified (e.g., Lermo and Chavez-Garcia 1993). The H/V spectral ratios for the S-waves and S-wave coda are plotted in Fig. 5a, b, respectively. The S-wave plot (Fig. 5a) shows that the spectral ratios at periods shorter than 0.5 s (i.e., frequencies lower than 2 Hz) for the mainshock were systematically smaller than the mean spectral ratios for the aftershocks; furthermore, the predominant period of approximately 0.4 s for the aftershocks shifted to a period of approximately 0.7 s for the mainshock. The lower H/V ratios for S-waves at short periods and the greater predominant period are characteristics of a nonlinear site response during strong shaking (e.g., Wen et al. 2006).
Noguchi and Sasatani (2008, 2011) introduced a quantitative index called the degree of nonlinearity (DNL), which is a measure of the area between the S-wave H/V ratio curve for the mainshock and the curve of the mean S-wave H/V ratio for the small events. The area is zero when the site response is linear. However, considering the fluctuations in the calculated spectral ratios, Noguchi and Sasatani (2011) suggested that DNL values of at least 4 indicate nonlinearity. The DNL value for the data plotted in Fig. 5a is 9.7. This large DNL value and the reduction in the short-period H/V ratios for the mainshock suggest that the KATNP site suffered a substantial nonlinear site response during the mainshock. Conversely, the H/V ratios for the S-waves for the mainshock at periods longer than 0.8 s do not show any systematic trend compared to the scattering of the spectral ratios for aftershocks from the mean spectral ratios. This suggests that the ground motions at longer periods were not affected by the nonlinearity. Previous studies (e.g., Aguirre and Irikura 1997) have reported that vertical-component ground motions are negligibly affected by site response nonlinearity in comparison with horizontal-component motions. These findings are supported by the richer short-period ground motions and larger PGA of the vertical component in comparison with those of the horizontal components during the mainshock (see the acceleration recordings in Fig. 2 for the mainshock).
Figure 5b shows that the mean H/V ratios of the coda waves achieve a larger peak at longer period, and the difference between the H/V ratios for the mainshock and aftershocks at short periods is not so strong as it was for the S-waves, suggesting that the coda waves were mainly composed of the long-period surface waves. Considering these facts, the 1D S-wave velocity model depicted in Fig. 5c was constructed by trial and error to reproduce the peak period on the long-period side (1–10 s) of the H/V spectral ratios for coda waves by utilizing the available geological and geophysical information discussed above. The material densities were estimated using the empirical relationship between the density and the S-wave velocity obtained by Ludwig et al. (1970). The S-wave velocities of the basin layers estimated in the present study are 200, 350, and 500 m/s from surface to underlying hard rock, respectively; the thicknesses of the corresponding layers are 30, 200, and 240 m, respectively (see Fig. 5c). The theoretical H/V ratios for fundamental-mode Rayleigh waves and amplification factors for vertical incident plane SH-waves for the new velocity model are shown in Fig. 5d, e, respectively; the corresponding values from Pandey (2000) are plotted in the same graphs for comparison. The structure proposed by Pandey (2000) achieves peak amplification at a period of approximately 2 s, which is not supported by the observed ground-motion data, whereas the structure proposed in this study achieves a peak amplification period of approximately 4.0 s, which corresponds to the peak response periods of the large-magnitude events, as shown in Fig. 4. A plot of the ratios of the 5 % critically damped PSVRS at KATNP to those at KKN4 and those at NAST to those at KKN4 (see Additional file 2) shows that the peak response ratios at periods of approximately 4 and 1.5 s correspond well to the peak amplification periods depicted in Fig. 5e. The results also indicate that the velocity structure at the NAST site may be similar to that at the KATNP site.
As a preliminary validation of the proposed velocity model for long-period ground-motion simulations, we simulated velocity waveforms assuming a plane SH wave incidence at the base of the sediments. The transverse component of velocity records obtained by the differentiation of 5 Hz GPS displacement data obtained at the KKN4 site, which is a hard rock site, was used as input motion after halving the amplitudes to cancel the free surface effect. Because information on the damping factor Qs of the sediments in the Kathmandu basin is not available, we assumed a frequency-independent Qs equal to one-tenth of the S-wave velocity (unit: m/s) for each layer. It was found that the results discussed below were not significantly altered if the variation in Qs remained within a factor of two.
Strong ground motions from the Mw 7.8 Gorkha earthquake and its eight aftershocks recorded by a strong-motion seismograph at the KATNP site were analyzed to understand the characteristics of strong ground motions and site effects. The GMPEs developed for crustal and interplate events in Japan were found to generally well describe the observed PGAs and PGVs at the Kantipath site, except for the PGA of the mainshock. A comparison of the observed response spectra with those from the GMPEs indicated that the ground motions at the KATNP site were strongly influenced by the local site condition at long periods; hence, appropriate deep soil correction factors for the Kathmandu basin must be developed. An indirect analysis of the recordings for soil nonlinearity suggested that the KATNP site experienced a substantial reduction in short-period ground motions during the mainshock because of the nonlinear site response. To fully explain this nonlinearity, a broadband ground-motion simulation considering details regarding the surface soil layering, propagation path, and rupture characteristics of the earthquake is necessary. A 1D velocity structure model was developed for the deep sediments beneath the recording station based on the H/V spectral ratios for the S-wave coda. A simple validation of the model by waveform simulations demonstrated that the proposed velocity model is able to explain the observed large-amplitude velocity waveforms at the peak periods of approximately 4–5 s for the mainshock. Thus, we conclude that the deep sediments beneath the recording station at the KATNP site strongly amplified the long-period components of the ground motions during the mainshock and its large aftershocks.
YPD analyzed the strong ground-motion data, modeled the subsurface velocity model, interpreted the data, and drafted the manuscript. HK, WS, and TK interpreted the strong ground-motion data and edited the manuscript. SA and HF designed the study and edited the manuscript. All authors read and approved the final manuscript.
We thank the United States Geological Survey for providing us with strong-motion data and earthquake source locations. We also acknowledge the Department of Mines and Geology, Tribhuvan University, and the California Institute of Technology for providing us with the GPS data. We are grateful for the helpful comments given by two anonymous reviewers. We also would like to thank Wessel and Smith (1998) for providing us with Generic Mapping Tools, which were used to make Fig. 1 of this paper.
The authors declare that they have no competing interests.
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