2015 Nepal earthquake offers clues about hazards

The Himalaya Mountains are a geological wonder, drawing adventurers and religious devotees from near and far to the world’s highest peaks. The mountains were created along a fault where the Earth’s plates crash into each other and press up toward the sky. But the same fault that formed the piercing summits of the Himalayas produces large earthquakes that can cause immense loss of life in the densely populated plains of northern India and southern Nepal. On April 25, 2015, the magnitude 7.8 Gorkha earthquake struck near Kathmandu in central Nepal, killing about 9,000 people and injuring thousands. It damaged or destroyed more than 600,000 buildings in the area and its initial shock and magnitude 7.3 aftershock were felt throughout the region.

Scientists’ understanding of the Main Himalayan Thrust geological fault, where the Indian Plate has pushed under the Eurasian Plate along the Himalayas, has been largely based on historical records of earthquakes that occurred before the advent of modern seismometers. The 2015 Gorkha earthquake offered researchers an opportunity for a much-needed update. Seismic data from the Gorkha quake showed that it started to rupture, and also stopped rupturing at two places where the rock types change, adjacent to the fault. The research, led by an international team of scientists and co-authored by Simon Klemperer, a geophysics professor in the School of Earth, Energy & Environmental Sciences (Stanford Earth), appears in Science Advances June 26.

The new information offers clues about where, why and how earthquakes occur, and increases our understanding of earthquake hazards in India and Nepal. As a result of these observations, researchers now want to image the entire 1500-mile-long Himalayan front, where the plates overlap, to determine the shape of the Main Himalayan Thrust and the factors that control the maximum rupture that can occur in different parts of this convergent zone. Klemperer offers his perspective in this Q&A.

What was unique about the 2015 Gorkha earthquake from a research perspective?

KLEMPERER: Although we thought we had a reasonable understanding of Himalayan seismicity, that understanding was based on rather limited data. Gorkha filled a large knowledge gap, though as always leaves more questions unanswered that will probably remain until we see a great, over magnitude 8, earthquake.

What have you learned that could improve our understanding of earthquake hazards in that region?

KLEMPERER: In the past, for lack of data, earthquake seismologists have typically assumed that the Himalaya is uniform – cylindrical or co-axial – along-strike, along the curving arc of the Himalaya from west to east. The rupture pattern of the Gorkha event and our new results strongly imply that the earthquake both started and stopped at significant along-strike (west-east) changes in fault geometry. If in the future we can identify the locations of these significant changes before earthquakes happen, we would have a better idea of where future earthquakes would be triggered, and how large they might be.

The Main Himalayan Thrust transitions from gently sloping at its southern tip to more steeply sloping further north, forming what geologists have called the “Lesser Himalayan Ramp” that concentrates and builds up the stress ultimately released in an earthquake. But the location and tilt angle of this ramp structure cannot be well constrained by surface geology. Our seismic study showed that the ramp of the Main Himalayan Thrust changes from west to east, and is steeper beneath the mainshock area, and flatter and deeper beneath the eastern end of the aftershock zone.

Was there anything surprising about the way this earthquake behaved?

KLEMPERER: Although the Gorkha earthquake started out in an anticipated location, it surprised seismologists by rupturing only to the east-southeast, as opposed to in all directions; and by being too small to be the expected “great” earthquake over magnitude 8, yet still large enough to rupture for over 90 miles, reaching beneath Kathmandu Valley.

What about the geology of this region makes it susceptible to major earthquakes?

KLEMPERER: India is forcing its way beneath the Himalaya at 0.7 inches per year. Along the earthquake fault between the two plates, stress builds up for tens to hundreds of years, then is released in a matter of seconds – in this case moving the ground surface 23 feet southward and 3 feet vertically up.

What are the current methods for monitoring and minimizing the threat of earthquakes in India and Nepal?

KLEMPERER: The Gorkha earthquake was captured by distant and local seismometers, by a dozen global positioning system (GPS) receivers operating near and above the rupture zone and by satellite synthetic aperture radar imagery. All these methods will continue to be available in the future. It was a matter of good fortune that lead author Prof. Ling Bai had deployed a temporary array of seismometers along the Himalaya just north of the Nepali border, otherwise, the number of local seismometers would have been much smaller, as the Nepal national seismographic network is currently sparse and still developing. The best way to minimize the threat is better – and better enforced – building codes.

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The lead author of the study is Ling Bai at the Institute of Tibetan Plateau Research (ITP), Chinese Academy of Sciences. Bai was also a Blaustein Visiting Professor in the Department of Geophysics at Stanford University while conducting this research. Work for the study, “Lateral variation of the Main Himalayan Thrust controls the rupture length of the 2015 Gorkha earthquake in Nepal,” was funded in part by the National Science Foundation.