Pune: Distant earthquakes and their aftershocks could trigger tremors in the vulnerable Koyna-Warna region, a recent study conducted by scientists at the CSIR-National Geophysical Research Institute (CSIR-NGRI) in Hyderabad and institutes in the US has found.The study, published in the international journal ‘Journal of Geophysical Research(Solid Earth)’, said increase in seismic activity after the temblor in the Indian Ocean on April 11, 2012 lasted for up to four days, including a magnitude 4.8 earthquake on April 14, 2012 in the Koyna-Warna region.Human activities, including the construction of the reservoirs, has made the region highly vulnerable—so much so that earthquakes far away could trigger tremors or earthquakes here, the study indicated.Such earthquakes are known as ‘remotely triggered’ earthquakes, Abhey Ram Bansal from CSIR-NGRI, Hyderabad, who spearheaded the research, said.“These earthquakes or tremors, which could be as intense as 5.5 magnitude, are caused by the passing of surface waves of large distant earthquakes,” he added.Recent research studies have shown that such tremors or earthquakes are most prominent in the regions with earthquakes induced by human activities.“Our study found that the magnitude 8.6 Indian Ocean earthquake of April 11, 2012 and its aftershock of magnitude 8.2 were the two events that triggered seismic activity in the Koyna-Warna region. The increase in this activity in the region led to a magnitude 4.8 earthquake here on April 14, 2012,” he added.Researchers found that a small stress of the order of 20 kilopascal (kPa)—a unit of pressure measurement— carried by the surface waves of distant earthquakes may be sufficient to trigger earthquakes in the Koyna regions. So, while other, more stable surrounding regions in the way of these waves may feel nothing, a vulnerable region like Koyna may experience seismic activity, they added.The team conducted a systematic search for dynamic triggering after 20 large distant earthquakes with dynamic stresses of at least 1 kPa in the region and found triggering during the 2012 Indian Ocean earthquake and its largest aftershock.“Not all earthquakes spark others because triggering depends on many factors. Triggering during a single quake is also important enough to understand the vulnerability of the region and how little stresses from outside can trigger the techtonic events in the region,” Bansal said.An earthquake in one distant part of the Indian Ocean could trigger tremors or an earthquake in Koyna because the region is critically stressed, adding that the trigger also depends on the rupture direction of the main earthquake, he added.“Koyna region is already stressed due to the reservoir. It is disturbed by other stresses generated by far away earthquakes. Had the stresses caused by the reservoir not been present in Koyna-Warna region, then the far away earthquakes may not trigger seismicity here. This study is significant to suggest that human activities could disturb stress conditions below the earth’s surface, making the regions more sensitive to earthquakes occurring in distant places,” he said.

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