Last April, a 7.8 magnitude earthquake rocked Nepal, making headlines across the world for all the wrong reasons. Nearly 9,000 people were killed, and more than 23,000 people were injured. A major study released this week in the journal Nature Geoscience details the pre- and post-earthquake architecture of the fault lines involved, highlighting in particular a still-quiet zone that becomes more and more likely to violently rupture with each passing year.
Roughly 40 million years ago, India collided with Asia in one of the world’s most monumental tectonic impacts. The Main Himalayan Thrust (MHT) fault is the primary fault line along which northern India slides under the Himalaya region at a rate of around two centimeters (0.79 inches) per year.
This complex fault network accumulates stress as massive continental chunks grind up or along each other. If a blockage occurs, this stress builds up, and the longer this goes on for, the more powerful the resulting earthquake will be. During the April 2015 event, a roughly 30-kilometer (18.6-mile) stretch of the antagonizing fault line moved eastwards at speeds of up to 3 kilometers (1.9 miles) per second, releasing an enormous amount of energy.
One recent study aimed to assess the new layout of the fault network around the MHT. The authors combined an analysis of the P-waves – a type of wave that includes sound waves – emitted during the earthquake with satellite data, which tracked the surface deformation.
Using this data, the authors calculated that the earthquake epicenter was 80 kilometers (50 miles) north of the Nepalese capital Kathmandu. The main fault slipped for 45 seconds, whereupon it impacted a “locked edge” – a region where faults cannot move – that stopped it. Several previous fault slips have likely encountered this locked edge, which have also failed to “unzip” it. When it finally does unzip, a major earthquake will occur.
This new study used similar techniques, predominantly satellite imagery, to determine the way the faults behaved during the event. The authors conclude that the MHT was ruptured, causing uplift in the area around Kathmandu, while causing the mountains nearby to slump. Despite this, they note that mountains are still able to slowly build themselves up during more quiet periods of tectonic activity.
Significantly, the authors note that the earthquake occurred in something called a “seismic gap,” a fault zone that has not experienced any major earthquake activity in a long time. These seismic gaps, while “quiet,” are regions that are in fact experiencing a large accumulation of stress.
When they eventually do give way, the consequences at the surface, as last April showed, are catastrophic. Worryingly, the seismic gap within the Nepalese region has now been divided: the eastern zone experienced a vast stress release, whereas the west remained inactive. This means that any western rupture would cause another earthquake similar to the one last April.
Unfortunately, as this part of the fault is much shallower, it is likely to produce far more damaging effects. “Unfortunately it is impossible to predict exactly when the next earthquake will happen in Nepal,” Dr. Elliott Smith, a postdoctoral researcher at Oxford and lead author on the paper, told IFLScience. This western region could rupture sooner due to the stress transferred from the east section to the west during the April event. “Therefore, rather than the typical century timescale between earthquakes in the same region of a fault, the timescale may be much shorter – for example, decade spacing, but we cannot be more definitive.”
