New insights into mega 1897 Assam earthquake

New Delhi, Dec 22: The Earth’s internal structure is made of different layers. The outermost layer is broken into several pieces called tectonic plates. These pieces slowly move with respect to each other and deform the surface. The Himalaya is the result of a large piece, the Indian plate moving northeastwards and pushing against the Eurasian plate. In general, the motion between the two plates is blocked due to friction, but at rare moments there is very rapid rupture leading to earthquakes. The Himalayan arc has experienced many-many earthquakes, big and small, since its birth.

One such earthquake took place in the area of the Shillong Plateau in Meghalaya nearly 125 years ago, in 1897. It was a mega-quake with a magnitude of more than 8 on the Richter scale and caused widespread destruction. This quake was out of the normal as it did not rupture the surface at a well-defined fault line. Its location (epicentre) is also a topic of great debate among experts across the world. The main challenge for determining the location is that globally only a few ground-vibrations measuring instruments (seismographs) were operating at that time.

In a new research paper published in the journal, The Seismic Record, two scientists from the Institute of Earth Sciences, University of Lausanne, Switzerland, have come up with some new insights. After an extensive study of records obtained from seismic stations in different parts of the world and also reports of station masters of railway stations covered by the East Indian Railway which operated trains between Calcutta and Delhi at the time of the disaster, they have determined that the earthquake started at about 11:06:46 Greenwich Mean Time, on Saturday, June 12^th, 1897and that its location was 26.0 degrees latitude north and90.7 degrees longitude east, about 20 km from Goalpara city in Assam as the crow flies. This is on the south of the Brahmaputra river and on the northern side of the Shillong Plateau.

Geologically, this location is at the junction of two faults: the short“Chedrangfault” that ruptured the surface in 1897 as documented by the geologist Richard Dixon Oldham, and the long “Oldham fault”, a structure that is not visible at the surface but was proposed based on land survey data.

In an online interview to India Science Wire, the scientists, ShibaSubedi and GyörgyHetényi said that the study not only found where and when the 1897 Assam earthquake happened, but it has also clearly brought out that great, magnitude 8 or larger earthquakes can happen on hidden faults too, without a visible trace at the surface.

“Our results clearly highlight the need to consider both types of faults: those visible at the surface, and those that are hidden but can be located by their seismic activity, while making seismic hazard assessments. We need to know more about what is happening or has happened underground and expand our understanding of subsurface faults. Ultimately, these will help better understand how earthquakes and faults interact in the Himalayan region and thus enable more reliable seismic risk assessment”.

Reacting to the paper, Dr. Vineet K. Gahalaut, Chief Scientist, CSIR-National Geophysical Research Institute, and former Director of National Seismology Centre, said, “This is one of the most enigmatic events. It could possibly be the largest intraplate earthquake in the Indian subcontinent. Knowing its occurrence process (seismogenesis) is of great importance for future earthquake hazard assessments. There are several models for its occurrence and this is the new one. The findings are interesting”. (India Science Wire)