Effects of topographic and geological features on building damage caused by 2015.4.25 Mw7.8 Gorkha earthquake in Nepal: a preliminary investigation report
© Wang et al. 2016
Received: 23 December 2015
Accepted: 29 April 2016
Published: 10 May 2016
The 2015.4.25 Gorkha earthquake affected about eight million people in Nepal. Most injuries and loss of life were due to building collapse and damage. This work aims to investigate the topographical and geological effects on the severe damage caused by this earthquake.
In one-week field investigation in the earthquake-affected areas, several severely damaged areas with different topographic and geological features were surveyed, as well as the site of Kaligandaki River landslide dam failure. Some general tendency related to the building damage and landslide dam failure was obtained.
Through the field investigation, it was found that geological and geomorphological characteristics of a site, combined with the structure feature of the building, such as the short column effect, amplified the seismic vibration and caused severe building collapse and damages, i.e., 1) For buildings on flat area consisting of lacustrine deposit or diluvial deposit, resonance effect might be the main reason, while for the buildings on the top of hills or narrow ridges, topographic effect and sometimes, short column effect should take the main responsibility; 2) For buildings located on the gentle slopes or landslides, the settlement in the infill side caused by the strong seismic vibration can be the main reason; 3) Besides of the building failure on lacustrine deposit, failure patterns in three types of topographic and geological features, i.e., narrow ridges formed by landslides, diluvial deposits and alluvial fans, and landslides, were proposed as the possible mechanism of the building damage caused by the earthquake. For landslide dam failure, it was found that landslide dam could easily breach or collapse, when the landslide-dam-deposits were fine.
The Gorkha earthquake occurred at 11:56 NST on 25 April 2015. The focal depth is about 15 km. The epicenter is 77 km northwest of Kathmandu. Based on the information from the United Nations, about eight million people have been affected by the strong earthquake in Nepal, which is more than a quarter of the Nepal’s population (Goda et al. 2015; Dahal and Timilsina 2015).
To understand the damage caused by the earthquake, and find a solution to mitigate the geo-disasters in Nepal, a preliminary investigation on the 2015 Gorkha earthquake was conducted from 2 to 9 June 2015.
Cultural heritage and building damage in Katmandu city (located on lacustrine deposits)
Building damage in Gorkha Palace, Chautara and Changu Narayan (located on a narrow mountain ridge)
Building damage in Sankhu (located on an alluvial fan)
Building damage in Purano Naikap (located in a possible landslide area)
An earthquake-induced landslide dam in Kaligandaki River
Methodologies and findings
Investigation on cultural heritage and building damage in Katmandu city (lacustrine deposits)
According to a previous study (Sakai 2001), lacustrine deposits are widely distributed in the Katmandu valley. The maximum depth reaches 600 m, and the average thickness at the center of the Katmandu basin is about 200 m. Many ancient buildings and temples were severely damaged or destroyed in Katmandu city.
Damage at the Swyambhunath world heritage site
During the investigation, it is found that the damages are more severe where the buildings and temples are closer to the slope. It may mean that the shaking is much stronger at the slope shoulder than at the mountain top. After the investigation in the same site, Hashash et al. (2015) pointed out that the topography effects refer to the modification (frequently amplification) of incident ground motion due to energy focusing effects at convex topographic features (hills, ridges, canyons, cliffs, and slopes), complicated subsurface topography (sedimentary basins, alluvial valleys), and geological lateral discontinuities (e.g., ancient faults, debris zones). These features can significantly alter the intensity, frequency content and duration of ground shaking during earthquakes compared to the shaking that the same site would have experienced, had it been on flat ground. In the Swyambhunath site, the topographic effect of hills and slopes may have occurred.
Damage in Dharahara and Durbar Square
Dharahara and Durbar Square areas are the central areas of Kathmandu city. It is also an attractive resort with many famous temples and buildings. During the earthquake, those ancient temples and buildings suffered heavy damage.
Through a study of local amplification effect of soil layers on ground motion in the Kathmandu Valley using microtremor analysis, Paudyal et al. (2012) concluded that the multiple amplified frequencies in a particular area can create a resonance effect both for low rise and tall buildings. They pointed out that the behavior of the surface layer as well as the layer underneath should be taken into consideration for seismic risk studies in the Kathmandu Valley. Because of the resonance effect, the low-rise to medium-rise buildings, and improperly designed buildings were selectively damaged.
From a viewpoint of long-term safety of the buildings, reconstruction of these building is recommended. However, the number of damaged buildings is so large that the budget required to rebuild them may have to be evaluated by the government.
Building damage in Gorkha Palace, Chautara and Changu Narayan (narrow mountain ridge)
The residential areas located on narrow mountain ridges suffered severe damage. This damage may be caused by the topographic amplification effect of the narrow mountain ridge. During the investigation, we visited three typical sites: Gorkha Palace, Chautara town and Changu Narayan. The following gives the damaging situation in details.
Damage in Gorkha palace
Gorkha Palace is the source of the Shah dynasty, and it is a very important culture heritage in Nepal. Because it is located near the epicenter, it was exposed to different effects of the earthquake.
Damage in Chautara town
The damage to the Chautara town was very severe though it is far from the epicenter and earthquake fault. The reason may be the amplification of the narrow mountain ridge, combined with the special structure of the buildings built on the slope along the highway. However, looking from another side of the town, it seems like that the whole town of the Chautara might be located on a large-scale landslide. If it is true, the Chautara town should consider moving to a safer place; otherwise, it will most likely suffer similar damage in the next large earthquake event. To solve this problem, more detailed investigation and landslide mapping are useful and effective.
Damage to Changu Narayan temple at Bhaktapur
Building damage in Sankhu (Alluvium fan)
Building damage in Purano Naikap (landslide area)
Landslide dam in Kaligandaki river
Fortunately, before the major slope failure happened, the local authority understood the situation and resettled the local people to safe place. 124 people living in the village that was located at the opposite side to the cliff were evacuated.
So roughly, it can be estimated that it took about 38 hours from the time of the small-scale rockfall to change into a major failure. It took 14.5 hours for the landslide dam lake to become fully filled, and 30 minutes for the landslide dam to complete collapse by overflow.
For the purpose of time prediction for landslide dam failure, we learned from that the collapse of the landslide dam by overflow only took 30 minutes. One of the possible reasons might be the crushing of the thin rock mass when the rockfall fell to the ground. The bedrock of the cliff is thin bedding phyllite which is relative soft and composed of fine scale-like minerals. Because of the strong collision from about a 700 m high position, the structure of the rock mass was pulverized. We concluded that the landslide dam consisting of loose deposits of the rockfall might be vulnerable for overflow, and the longevity of the dam will be short.
Figure 29(a) shows the case on the narrow mountain ridges. The Chautara town belongs to this type. Because the ridge is narrow, buildings used non-homogeneous foundations. One side of the building is directly located on the ground, and the other side is supported by piles, bricks, or concrete structures. Generally, the mountain ridge experienced stronger shaking, and this may cause the failure at the outside foundations. If the outside foundation is located in a landslide area, the foundation will be prone to collapse even if the landslide does not show obvious displacement; if all of the mountain ridge is located in a landslide, the amplified shaking can cause any types of failure because the ground shaking may cause not only horizontal displacement or vertical displacement, but also rotational displacement. The building damage occurred in Gorkha Palace and Changu Narayan were mainly caused by the strong shaking due to the topographic amplification as they are almost at the center of the mountain ridges. The situation at the Swyambhunath World Heritage site is not fully understood, however, at least the failure distribution on the main temple in the center and minor temples closing to the slopes coincided with type (a) in Fig. 29.
Figure 29(b) represents the geological structure in Sankhu town. In the upstream side, gravels and surface water is abundant, while as it goes to downstream side, the grain size of the soil becomes finer and finer, surface water becomes to groundwater, and the groundwater gradually becomes deeper. The reason for the concentrated building damage in Sankhu town may be resonance effect of the buildings to the deposit layers.
Figure 29(c) shows the failure pattern at Purano Naikap area. The similar pattern may be widely distributed around the Kathmandu basin. In this kind of area, the structure of the lacustrine deposit overlaying the bedrock can form an enhanced structure for the occurrence of landslides. If there is no earthquake, the gentle slopes are good for agriculture and housing. However, during a large earthquake, existing landslides can be reactivated. Because the lacustrine deposit landslide mainly consists of clay, generally they will not move for long distance during earthquake, but they can amplify the vibration and cause general destruction to the buildings, especially if the buildings in this area were located on shallow foundations on cut-slopes at the upper side, and infilled ground at the lower side. The amplified shaking may cause intensive settlement at the infill and cause non-homogeneous deformation at the foundation. When a building is located at the boundary of a landslide, it will suffer much more severe damage.
For buildings on flat area consisting of lacustrine deposit or diluvial deposit, resonance effect may be the main reason, while for the buildings on the top of hills or narrow ridges, topographic effect and sometimes, short column effect should take the main responsibility;
For buildings located on the gentle slopes or landslides, the settlement in the infill side caused by the strong seismic vibration can be the main reason;
Besides of the building failure on lacustrine deposit, failure patterns in three types of topographic and geological features, i.e., narrow ridges formed by landslides, diluvial deposits and alluvial fans, and landslides, are proposed as the possible mechanisms of the building damage caused by the earthquake.
For landslide dam failure, it is found that landslide dam will easily breach or collapse, when the landslide-dam-deposits are fine. The Kaligandaki landslide dam failure event gave us a good lesson that geo-environmental disasters can be reduced by catching the precursory phenomena and necessary managements.
This work was financially supported by JSPS KAKENHI Grant Number A-2424106 for landslide dam failure prediction (Principal Investigator: Fawu Wang). International Consortium on Geo-disaster Reduction (ICGdR) also financially supported the field investigation. Mr. Tek Bahadur KC, the Chief District Officer of Beni and Mr. Uddhab Prasad Timalsena, the Chief District Officer of Gorkha, kindly gave us permission and supported our investigation. The authors thank Lynn Highland of USGS for her helpful and constructive comments on the contents when the first author presented the results in GSA2015 in Baltimore, USA. She also reviewed and edited the first manuscript of this paper.
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