In a short span of about a week, a region about 40 km from Herat, Afghanistan was struck by four shallow focus earthquakes of 6.3 magnitude. The first earthquake of 6.3 magnitude occurred at a depth of 14 km at around 11 am local time on October 7. This was followed by another earthquake of 6.3 magnitude (at 13.5 km depth) about 30 minutes after the first quake. Two more shallow focus earthquakes of 6.3 magnitude at 9 km and 6.3 km depths struck the region close to Herat on October 11 and October 15, respectively. All four earthquakes occurred on east-west striking fault planes that dip to either the north or south. The earthquakes occurred within the Eurasia plate in an intracontinental mountain belt.
Aftershocks, by default, have magnitudes less than the main event. However, all the four earthquakes near Herat have the same magnitude. “Because these two earthquakes [on October 7] and the two subsequent earthquakes [on October 11 and October 15] are all approximately the same magnitude, we would call them ‘multiplets’ rather than mainshocks, foreshocks, or aftershocks,” geophysicist Dr. William Bill Barnhart, Assistant Coordinator at the USGS Earthquake Hazards Program told The Hindu by email.
On whether the earthquakes had occurred in the same fault, Dr. Barnhart said: “Preliminary, though uncompleted analysis, indicates that all four M6.3 [magnitude] earthquakes occurred along the same fault or fault system. They did not occur in the exact same spot; rather, they ruptured different portions of the same fault along its length. It is rare for an earthquake to rupture the entire length of the fault that the earthquake occurred on, so it often requires multiple earthquakes, spread out over some unknown amount of time, to fully rupture a geologic fault.”
