Rama Mohan Pokhrel

The 2015 Gorkha Nepal Earthquake: Insights from Earthquake Damage Survey

The 2015 Gorkha Nepal earthquake caused tremendous damage and loss. To gain valuable lessons from this tragic event, an earthquake damage investigation team was dispatched to Nepal from 1 May 2015 to 7 May 2015. A unique aspect of the earthquake damage investigation is that first-hand earthquake damage data were obtained 6 to 11 days after the mainshock. To gain deeper understanding of the observed earthquake damage in Nepal, the paper reviews the seismotectonic setting and regional seismicity in Nepal and analyzes available aftershock data and ground motion data. The earthquake damage observations indicate that the majority of the damaged buildings were stone/brick masonry structures with no seismic detailing, whereas the most of RC buildings were undamaged. This indicates that adequate structural design is the key to reduce the earthquake risk in Nepal. To share the gathered damage data widely, the collected damage data (geo-tagged photos and observation comments) are organized using Google Earth and the kmz file is made publicly available.

GIS-Based Study on Liquefaction-Induced Soil Subsidence in the Urayasu Area Due to the 2011 off the Pacific Coast of Tohoku Earthquake

Soil liquefaction is one of the geotechnical-related effects of earthquakes especially in cities built on young alluvial deposits and reclaimed land. Geotechnical problems associated with liquefaction in these areas include ground subsidence. Therefore, a detailed study on the occurrence and extent of ground subsidence after severe liquefaction following an earthquake is essential in such an area. Urayasu City in the Tokyo Bay area, Japan was selected as the study area. This area is a young reclaimed land where severe liquefaction occurred following the 2011 off the Pacific Coast of Tohoku Earthquake (M = 9.0). Twenty-three borehole locations were selected for the study of liquefaction potential and a geostatistical method of interpolation was applied to attain a spatial variation of liquefaction potential within the area. The ground subsidence was estimated by using airborne Light Detection and Ranging (LiDAR) images before (2006) and after (2011) the earthquake, giving the spatial distribution of soil subsidence. The liquefaction potential and ground subsidence were determined using a P L distribution map and ground subsidence map, respectively. By studying these maps, the relationship between liquefaction potential and ground subsidence was developed. The relationship shows that high ground subsidence is observed in the area with high liquefaction potential.

Investigation into the multiple recent sinkholes in Pokhara, Nepal

Since November 2013, numerous sinkholes have been forming in the Armala area of Pokhara Valley, Central Nepal, posing serious threat to local residents. In order to provide countermeasures for reducing sinkhole risk, detailed investigations into the cause and the formation mechanism of the sinkholes are crucial. Preliminary surveys were conducted in June 2014 and November 2014. Comparison of photos, taken in the two surveys, clearly indicates not only the formation of new sinkholes, but also the re-activation of filled sinkholes. By means of dynamic cone penetration tests and surface wave investigations, qualitative characterization of the soil profile was attained, and shallow weak soil layers which are believed to be the location for future sinkholes could be identified. On the basis of the preliminary field investigation, possible sinkhole formation mechanisms are considered. A risk of sinkhole does not seem to disappear as white turbid water continuously springs. It indicates that the internal erosion of white clayey silt layer is still in progress. In August 2015, a boring was carried out beside one of the largest sinkholes. The overall structure of ground layers was first revealed and a 2.5m high cavity at 7.5-10m deep from the ground surface was found within a thick white clayey silt layer. Further ground investigations including surface wave exploration were conducted in December 2015 and the results are reported.

Preliminary Field Assessment of Sinkhole Damage in, Pokhara, Nepal

Since November 2013, numerous sinkholes have been forming in the Armala area of Pokhara Valley, Central Nepal, posing serious threat to local residents. In order to provide measures aimed at reducing sinkhole risk, investigations into the cause and features of the sinkholes are crucial. This paper presents early research results based on two damage surveys conducted in June 2014 and November 2014 in the Armala area. Comparison of photos, taken in the two surveys, clearly indicates not only the formation of new sinkholes, but also the re-activation of filled sinkholes. By means of dynamic cone penetration tests and surface wave method investigations, qualitative characterization of the soil profile was attained, and shallow weak soil layers which are believed to be the location for future sinkholes could be identified. On the basis of the field investigation results, possible sinkhole formation mechanisms are identified for the Armala area. Furthermore, results of a reconnaissance survey conducted in the Armala area in early May 2015 (following the 2015 Gorkha Nepal Earthquake, which occurred on April 25th) are also reported. Although the epicentral distance to Pokhara was closer than Kathmandu, which suffered from severe damage, no major apparent effects of the earthquake were observed in the sinkhole damaged area.