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Scientists map April 2015 Nepal Earthquake

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Washington –  The devastating 7.8-magnitude earthquake that jolted Nepal in April and killed nearly 9,000 people was a rupture consisting of three different stages, say scientists who have accurately mapped the movement of the quake.

The study could help a rapidly growing region understand its future seismic risks, researchers said. The Himalayan region is particularly prone to earthquakes and this study will serve as an important benchmark for understanding where future earthquakes may occur, especially since the area has experienced high population growth over the past few decades, they said.

The study assessed the presence of low frequency and high frequency waves over the three stages of the earthquake. High frequency waves cause more shaking, thereby posing the greatest risks for structural damages.

Low frequency waves are less violent and less damaging to buildings and infrastructure. “The Nepal earthquake is a warning sign that the region is of high seismic risk, and each earthquake behaves differently,” said Peter Shearer, from the Scripps Institution of Oceanography at University of California – San Diego.

“Some earthquakes jump from one fault line to another, whereas the Nepal quake apparently occurred on the same fault line in three different stages, moving eastward,” said Shearer.

“Using this research, we can better understand and identify areas of high seismic hazard in the region,” said Shearer.

Using the Global Seismic Network (GSN), Shearer and Scripps graduate student Wenyuan Fan were able to unravel the complex evolution of fault slips during this earthquake.

GSN provides high-quality broadband digital seismic data for monitoring earthquakes and learning about Earth structure. The study concludes that the rupture travelled mostly eastward and occurred in three distinct stages; Stage 1 was weak and slow; Stage 2 was near Kathmandu and had the greatest slip but was relatively deficient in high-frequency radiation; and Stage 3 was relatively slow as well.

Overall, this earthquake was more complicated, with multi-stage movements on multiple faults, than smooth models of continuous rupture on a single fault plane, researchers said.

“Using the GSN instead of regional array data really enhanced the spatial resolution of the back-projection images and helped us see that frequency-dependent rupture was one of the main features of this earthquake,” said Fan.

“Stage 2 was high-frequency-deficient and occurred closest to Kathmandu, which was probably why ground shaking was less severer than expected for such a high-magnitude earthquake,” said Fan.

“In general, understanding large earthquakes will inform our ability to forecast the nature of future earthquakes,” Shearer added. The study was published in the journal Geophysical Research Letters.

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