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Nepal at Risk: Interdisciplinary Lessons Learned from the April 2015 Nepal
(Gorkha) Earthquake and Future Concerns
GSA TODAY | JUNE 2016 Elizabeth J. Catlos, Jackson School of Geosciences, Univ. of Texas The challenge of the rugged and steep terrain of the Himalayas,
at Austin, Austin, Texas 78712, USA, ejcatlos@gmail.com; Anke M. coupled with its large size, have resulted in an incomplete under-
Friedrich, Dept. of Earth & Environmental Sciences, Geology, Univ. standing of its paleoseismicity and tectonic history. Unknowns
Munich, Luisenstr. 37, Munich, 80333, Germany; Thorne Lay, Earth & include the northward extent of the Indian craton prior to colli-
Planetary Sciences, Univ. of California Santa Cruz, 1156 High Street, sion (Lippert et al., 2015) and the role of previously unrecognized
Santa Cruz, California 95064, USA; John Elliott, COMET, Earth or underappreciated fault systems that accommodated convergence
Sciences, Oxford, UK; Sara Carena, Dept. of Earth & Environmental in historical times (Taylor and Murphy, 2015). Segmentation of
Sciences, Geology, Univ. Munich, Luisenstr. 37, Munich, 80333, the MHT is also unclear. Structural variations along the Himalayas
Germany; Bishal N. Upreti, Dept. of Geology, School of Mines, Univ. control the extent of rupture of large earthquakes, and the conver-
of Zambia, Lusaka, Zambia; Peter DeCelles, Dept. of Geosciences, gence rate is not constant. The paleoseismic record is limited to
Univ. of Arizona, Tucson, Arizona 85721, USA; Brian Tucker, ground-rupturing events (Wesnousky et al., 2015); the Gorkha
GeoHazards International, 687 Bay Road, Menlo Park, California earthquake left little surface record that would be identified by
94025, USA; and Rebecca Bendick, Dept. of Geosciences, Univ. of trenching. Models of the Himalayan seismic cycle based on only
Montana, Missoula, Montana 59812, USA mapped surface ruptures lead to misfits between geodetic rates
and estimated recurrence intervals.
INTRODUCTION
Space-geodetic measurements of present-day strain accumula-
In response to the devastation caused by the 25 April 2015 MW 7.9 tion across active fault systems directly test structural geological
Nepal (Gorkha) earthquake and its aftershocks, the Geological models. Earthquakes help to illuminate detailed fault geometry,
Society of America convened an interdisciplinary session at its but event observables must be interpreted in context. In the past,
2015 Annual Meeting in Baltimore. The forum allowed researchers verification of geometric and kinematic relationships depended
from diverse disciplines to exchange information and develop on rare earthquake occurrences on a fault. Space-geodetic and 3D
meaningful paths toward reducing the societal impacts of future fault-geometric data will need to be integrated and made available
large earthquakes in the Himalayan region. Major seismic hazards to earth scientists prior to an earthquake. Novel integration tech-
exist near Kathmandu and along the Himalayan front due to niques may result in quicker and better hazard estimation.
incomplete rupture of the Main Himalayan Thrust (MHT)
(Avouac et al., 2015; Bendick et al., 2015; Elliott et al., 2015; Lay, EVENT INFORMATION FROM SEISMOLOGY AND GEODESY
2015) and thousands of co-seismic landslides (Andermann et al.,
2015; Gallen et al., 2015; Ohja and DeCelles, 2015; Poudel, 2015). The Gorkha earthquake occurred on Saturday, 25 April 2015, at
Surprisingly, the 2015 event ruptured a limited region. Given 11:56 NST, with an epicenter ~75 km WNW of Kathmandu (e.g.,
shortening rates and interseismic geodetic indications that the Avouac et al., 2015; Lay, 2015). The event started along the eastern
MHT is almost uniformly locked along strike, larger earthquakes side of a millennial-scale seismic gap and ruptured eastward to the
may occur along the collision zone. 1934 Bihar-Nepal earthquake zone. It did not break to the surface
as in 1934, which leads to concern about limited paleoseismic
GEOLOGICAL FRAMEWORK recognition of past events (Bendick et al., 2015; Wesnousky et al.,
2015; Upreti, 2015).
The 2015 Gorkha earthquake occurred within the India-
Eurasia convergent plate boundary, defined by the >2500-km- Interseismic strain could proceed to the sub-Himalaya via post-
long Himalayan orogenic system. Major Himalayan faults sole seismic creep along the unruptured portions of the MHT, or a
into the MHT, a pervasive décollement that separates the down- large earthquake could occur along the shallower portion of the
going Indian plate from the Himalayan orogenic wedge (Brown MHT, feeding slip to the surface (Wesnousky et al., 2015).
et al., 2015). Above the MHT, the Lesser Himalayan Duplex is Another major earthquake is expected near Kathmandu, because
the locus of an ~50-km-wide seismogenic zone of predominately the Gorkha event ruptured only a portion of the MHT and its
moderate earthquakes, up-dip of which the MHT has low back- up-dip region remains locked with minor afterslip occurring
ground activity but intermittent large slip events (Khattri and south of Kathmandu (Avouac et al., 2015; Bendick et al., 2015;
Tyagi, 1983). Elliott et al., 2015). Rupture of a shallower, highly strained portion
of the MHT may involve higher stress drop failure and possibly
GSA Today, v. 26, no. 6, doi: 10.1130/GSATG278GW.1.
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