It’s unusual to hear about such a strong earthquake following a previous one, usually just smaller aftershocks (earthquakes that follow the first). There were nearly 100 M3+ aftershocks of the 25 April quake (M7.8), and yesterday’s M7.3 quake was by far larger than all the previous. This succession of quakes is essentially a series of slips over a large fault area, this second large quake was a slip just on the edge of the first, and continuation of the crust’s settling.
We often see locations of epicenters in depictions of earthquake locations, but earthquakes have a true activity center below the Earth’s surface, called the hypocenter. The epicenter is the location on the surface above the hypocenter. The distance below the surface is important, because the deeper it is, the more the crust will dampen the vibrations. Both of these large quakes had hypocenters relatively near the surface, which is one reason why they felt so strong.
In addition the magnitude and location of a quake, scientists measure the shaking felt on the surface from the quake. The higher the magnitude and the closer the hypocenter to the surface, the more likely the shaking on the surface is intense, though there are other factors like distance from the epicenter and soil conditions. The USGS has an excellent site for major earthquake events, and they produce ShakeMaps, which represent the intensity of the shaking felt on the surface, which correlates well with the intensity of the damage. The images of the ShakeMaps for the two major events in Nepal are included below. By comparing the two, one can see the varying intensity felt on the surface and how the second quake was much more concentrated in the east than the first.
Intensity doesn’t equate to damage, as some areas have more unstable structures or surfaces (landslides and avalanches). These areas east of Nepal, like Sindhupalchok, had been some of the hardest hit, and now have received additional intensity from the second. Time will tell how severe the damage has been, but we are so grateful to know that resources and rescuers, like Ang Tshering, were able to get out there after the first and before the second. Out thoughts and prayers are with all the locals and rescuers who are helping and have been struck by this second quake.