The four earthquakes in October near Herat, Afghanistan, should be called as ‘multiplets’ rather than mainshocks or aftershocks
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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.”
Explaining how the second earthquake on October 7 with 6.3 magnitude had occurred just 30 minutes after the first one, Dr. R.K. Chadha, former scientists at NGRI and currently a Raja Ramanna Fellow at NGRI had told The Hindu that “the release of stress in one fault [in Herat] can result in the loading of stress at another fault. The loading of stress can result in another earthquake which can be of similar magnitude or even higher magnitude.”
Dr. Barnhart elaborating on this further said: “Understanding the relationships between these earthquakes will take further research. However, given the close spatial and temporal proximity of all of these earthquakes, it’s reasonable to infer that the stress changes from one earthquake potentially encouraged the subsequent earthquakes. These stressing relationships — where stress changes from one earthquake encourage a subsequent earthquake — are quite common in nature.”
Since all the four earthquakes occurred due to thrust faulting, where one block moves up relative to the other, the area where the earthquakes had occurred would experience upliftment. “Observations of surface motion from satellites indicate that there has been at least 55 cm of uplift caused by these earthquakes,” Dr. Barnhart said. “Each earthquake causes both uplift and subsidence, with the primary deformation being uplift. The earthquake sequence has led to an accumulation of uplift along the fault that is rupturing. The USGS does not yet have observations of the most recent [October 15] M6.3 earthquake to assess the cumulative uplift from all earthquakes so far. The earthquakes are far enough away from Herat that uplift and/or subsidence within Herat is likely negligible.”
On what explains the clustering of four 6.3 magnitude shallow focus earthquakes in a span of about a week, he said that there “isn’t yet a consensus scientific answer to this question”.
