Written by Michael Fend and John Galetzka
6 August 2015
Just before noon local time on April 25, 2015 at 11:56 am, a moment magnitude (Mw) 7.8 earthquake struck central Nepal and was felt in the capital, Kathmandu. Please see the U.S. Geological Survey (USGS) summary and Nepal’s National Seismological Centre summary of the Gorkha earthquake for more information about the event and aftershocks. Hours after the event, UNAVCO rolled into action at the request of Professor Jean-Philippe Avouac at the California Institute of Technology (Caltech) to collect high sample rate (HSR) GPS data from the GPS network installed by Caltech and the Nepal Department of Mines and Geology a decade earlier.
John Galetzka — UNAVCO’s project manager for TLALOCNet in Mexico — landed in Kathmandu on April 29 to lead the effort to collect data and get stations online. Before joining UNAVCO in March 2014, John worked for Caltech from 2002 to 2013, and installed 28 GPS stations in Nepal during that time frame.
Professor Roger Bilham from the University of Colorado, Boulder teamed up with John several days later, and on May 10 UNAVCO field engineer Michael Fend arrived in Kathmandu with spare parts, additional tools, and relief supplies from donors in the Boulder area. Other colleagues from the USA, Nepal, France, the United Kingdom and Italy joined Mike, Roger and John to help recover GPS data, repair stations, add telemetry where possible, survey pre-existing survey markers, and install new GPS stations. All of the institutions involved in this project pledged to have their data archived at UNAVCO and allow the data to be openly accessible to the public. Please see the UNAVCO Community Event Response for more information and more data.
The partnering institutions include:
The rapidly produced GPS and InSAR data products from the Mw 7.8 mainshock, and the occurrence of a Mw 7.3 aftershock on May 12 to the east of the mainshock altered the focus of the group. Five stations in difficult to reach western Nepal were ignored and priority was shifted to recovering HSR data and densifying the GPS network south of the mainshock and around the large aftershock.
At the time of the April 25th earthquake, nine Caltech GPS stations were still telemetering data, but the fate of the 19 other stations was unknown. Two months later when UNAVCO staff departed Nepal and the monsoon rains arrived, 21 Caltech stations were up and running, two were found vandalized and have not been re-established, and five could not be visited and have no telemetry.
The high rate GPS data products from the mainshock were analyzed by David Mencin, UNAVCO Project Manager in Boulder with support from Professor Roger Bilham in Nepal. In particular, the motions of GPS station NAST in central Kathmandu at the Nepal Academy of Science and Technology, showed how the earthquake shook the city. Kathmandu is located in a sedimentary basin, against higher ridges of denser igneous rocks. Once the seismic waves reached the basin, they continued to shake the city by bouncing (resonating) back and forth from one side of the basin to the other. The seismic resonance of the basin is somewhat analogous to a bathtub filled with water and then the water is disturbed creating a wave that sloshes back and forth in the tub. A video of the horizontal motion of GPS station NAST (within the basin) compared to GPS station KKN4 (on a ridge outside of the basin) illustrates the resonance of NAST within the city compared to little to no resonance at KKN4 outside the basin. The NAST station continues to move back and forth long after the KKN4 station stops moving. In another video, the horizontal motion of NAST is compared to a stationary camera video in a square in Kathmandu that shows how the people and other objects moved back and forth with the ground motion. Another video recorded by Michael Fend on a street in Kathmandu shows a similar basin resonance associated with the May 12th aftershock.
Video: Horizontal Motion at GPS stations NAST and KKN4
Video: Horizontal Motion at GPS stations NAST compared to ground shaking in Kathmandu square
Video: Michael Fend's filming of May 12th aftershock from street in Kathmandu
UNAVCO and our partners are grateful to Trimble Navigation Limited for donating seven NetR9 receivers and Morningstar solar controllers to Caltech, for providing funds for helicopter transport, and for sending engineer Michael O’Grady to Nepal to support the effort in many ways. UNAVCO would also like to thank Filters for Families, based in Colorado for providing relief supplies that were taken to Nepal by Michael Fend for distribution by relief agencies.
The high rate data were recovered thanks to a rapid intervention funded by NASA (US) and the Department of Foreign International Development (UK), and engineering services provided by UNAVCO through the GAGE Facility with support from the National Science Foundation (NSF) and NASA under NSF Cooperative Agreement No. EAR-1261833. NSF support through rapid response awards (RAPID) to Rebecca Bendick, University of Montana and Roger Bilham, University of Colorado, Boulder are also gratefully acknowledged (No. EAR-1546633).
Accomplishments of the international field geodesy collaboration in Nepal include:
Last modified: 2020-01-28 22:54:33 America/Denver