So, just as people in Nepal began to think it safe to start again after the magnitude 7.8 (M7.8) earthquake of 25 April, the collision between India and Eurasia cursed them with another major tremor — this time registering M7.3.
Located around 76km to the east of Kathmandhu (the largest shock was almost exactly the same distance to the west of the Nepalese capital), the most recent large tremor is, according to the USGS tectonic summary, located just beyond the edge of the fault rupture generated by the earlier earthquake.
With the death toll from the first earthquake currently at above 8,000, and rescue work still ongoing, this new tremor can only be expected to increase the devastation and accentuate the misery experienced in Nepal.
Reports on the BBC suggested that “At least 16 people have been killed and 846 injured, according to Nepal’s information minister” and the number can be expected to rise.
The Indian-Eurasian Collision
Like the previous earthquake, this tremor of May 12 was the result of the collision between the Indian subcontinent, which has raced northwards (in geological terms) at a rate of around 45mm per year over a period of tens of millions of years. As recently as May 4, newly-published research in the journal Nature Geoscience attributed this unusually high speed to the previous existence of two subduction zones.
Such rapid collision has led to the uplift of the Himalayas through a method known as crustal shortening, in which layers of crust are effectively stacked one on top of another.
As convergence continues, movement occurs along these thrust faults to accommodate the strain. According to the United States Geological Survey’s earthquake summary, the tremor “occurred as the result of thrust faulting on or near the decollément associated with the Main Himalayan Thrust, which defines the interface between the underthrusting India plate and the overriding Eurasia plate to the north”.
Earthquakes and their Aftershocks
One major earth movement almost inevitably triggers another, and earthquakes generally occur in series. Sometimes they are preceded by smaller shocks; sometimes not. The mainshock in any sequence is self-defining; it is the largest to occur. Given this, we can never know which earthquake is the mainshock until that series is over.
The USGS is unequivocal that the May 12 tremor is an aftershock, but assuming that it is the case (it might conceivably be argued that it a separate quake triggered by the main one) then there are certain points to note. There are complicated physical laws relating to earthquakes and their distribution. Omori’s Law suggests that aftershocks decrease over time and, in so far as we can yet judge, the most recent tremor is consistent with that.
Was This a ‘Typical’ Aftershock?
Its magnitude, however, is of interest. In its advisory notice of aftershocks for the region, dated 8 May, the USGS noted that “A magnitude 7 to 7.8 aftershock is even less likely with about a 1-in-200 chance. This means that there is over a 99% chance that a magnitude 7 aftershock will not occur during the coming week.” Clearly, then, the M7.3 event must be considered unusual.
A further physical law relating to aftershocks, Bath’s Law, suggests that “the largest aftershock is, on average, 1.2 magnitudes smaller than its main shock, independent of main shock size…” and the May 12 aftershock was just 0.5 magnitudes smaller.
Neither of these statements proves in any way that the May 12 event was NOT an aftershock; it might merely be extremely unusual. But it does leave open the possibility that it might be the result of an earthquake triggered by stress. The USGS again: “There is evidence to suggest that earthquakes in one area can trigger seismic activity within a few hundred miles,’’ but they also go on to note that this activity includes aftershocks.
New Nepal Quake
Regardless of whether it’s a particularly unusual aftershock or not, the M7.3 tremor of May 12 is another blow to the people of Nepal. And the bad news is that there will almost certainly be large aftershocks still to come.