Nepal earthquake anniversary: What's cooking up underground? Gorkha-Dolakha fault segment first slipped then got locked after April 25 earthquake, residual energy yet to be released There were no foreshocks or any other significant seismic activities to set off the alarm bells before the big earthquake hit Nepal on April 25 last year. Given its duration and visible devastation, people suspected it could be the same much-feared mega earthquake after the Great Earthquake of 1934. Nepal's National Seismological Centre put the magnitude of the earthquake -- epicentre of which was close to Barpak of Gorkha district -- at Local Magnitude (ML) 7.6, while the United States Geological Survey (USGS) said it was of Moment Magnitude (M) 7.8. All these calculations showed the April 25 event, however, was not one of those great earthquakes that scientists said was due for over eight decades in Nepal and the Himalayan region. Further analysis of seismological and satellite data shows a section – extending from Gorkha in the west to Dolakha in the east Nepal – of the Main Himalayan Thrust (MHT) fault slipped southward but it did not rupture enough to cause a bigger earthquake, according to Prof Bishal Nath Upreti, a senior geologist and academician at Nepal Academy of Science and Technology (NAST). What is more, observation for six months following the April-May earthquakes suggests that there is no further southward movement along this fault. This means the energy built up for decades in the MHT fault segment, which runs beneath this part of Nepal affected by the April 25 earthquake, due to the gradual pushing of the Indian tectonic plate under the Tibetan plate has not been fully released. Scientists believe that the April 25 earthquake caused less damage and weaker shaking than expected in the Kathmandu Valley despite its magnitude. USGS seismologistSusan Houghexplains: "The biggest effect within the Valley was what we call non-linearity. While sediments in an old lake bed zone generally amplify shaking in small earthquakes, in very big earthquakes it is like trying to shake a sand box very hard, according to her. Sediment grains start to shift slightly, such that some of the potentially damaging shaking is actually absorbed. Detailed investigations show that damaging shaking was actually lower in the deepest part of the Valley than along the edges of the Valley. Another factor was that, while the earthquake rupture extended to almost directly beneath the Valley, the radiation of damaging shaking was not uniform along the fault segment that moved. The most damaging shaking was radiated from the northern edge of the rupture, relatively far from the Valley."