Earthquake Risk Mitigation - NIDM

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National Workshop on
Earthquake Risk Mitigation Strategy in North East
four great Indian earthquakes (1905, 1934, 1897 and 1950 from west to east) as detachment thrust
earthquakes. The impact of this publication on the geodynamics, seismotectonics and SHA for the
Himalaya in general and NER in particular was enormous.
From an exposed sub-vertical fault system with variable dip and geological slip-rake [see among
others; Evans (1964), Murthy et al (1969), Nandy (2001)], the Dauki Fault became considered to
be a shallow north dipping thrust fault of a nature similar to the Himalayan frontal thrust. Seeber
and Armbruster (1981) proposed a rupture dimension for the 1897 earthquake to the order of
550 km (E-W) by 300 km (N-S) and also attributed the same mechanism to explain the 1943 and
1947 earthquakes occurring in the so called seismic gaps between the 1897 and 1950 earthquake
ruptures. Various publications followed, dealing with tectonics and seismic hazard of this domain
and all implicitly or explicitly attesting to the thrust tectonics for the Dauki Fault vis-à-vis the 1897
earthquake, an idea originally expressed by Oldham (1899) but with cautions; Though apparently
the most probable this is not the only possible hypothesis (Oldham, 1899, p. 167). The rupture
area of the great earthquake was brought down to the order of 170 km x 100 km (Gahalaut and
Chander, 1992) within depths from 15 km (at the Dauki front) to 23 km beneath the Brahmaputra
valley. These authors proposed a midcrustal detachment, a low angle thrust fault responsible for
the 1897 earthquake, which is different from the shallower detachment of Seeber and Armbruster
(1981). Chen and Molnar (1990) give a succinct summary of the problem in their paper on ‘Source
parameters of earthquakes and intraplate deformation beneath the Shillong Plateau and northern
Indoburman Ranges’; They initiate the discussion as; There is no unanimity about the origin
and tectonic history of the Shillong Plateau, but two ideas, which are not mutually, exclusive,
prevail. These points of view center on the Dauki fault, which bounds the southern edge of the
Shillong Plateau. But then at the end supports the prevailing view as; but there is a consensus that
this earthquake probably involved largely thrust faulting on a plane dipping gently northward.
In one of our contribution (Nandy and Dasgupta, 1991) we observed that the Shillong Plateau
was seismically less active than commonly presumed and the 1897 earthquake was not related
to the activity of the Dauki fault. The entire concept on the geodynamics of terrain which we are
discussing was due for a drastic change with the publication of another important contribution by
Bilham and England (2001). The origin of the Shillong Plateau vis-à-vis that of the great 1897
earthquake was put forward; they modeled a ESE-WNW trending 110 km long buried reverse
‘Oldham Fault’ at the northern end of the plateau, that dips 57° towards SSW. A slip of 16m on this
prodigious fault lifted the northern edge of the Shillong Plateau at least by 11m during the 1897
earthquake. The steep dip nature of the Dauki Fault was however re-instead to develop the pop-up
origin of the Shillong Plateau. Several articles since published supported the existence of a south
dipping fault bordering the northern margin of the plateau. The contribution by Rajendran et al
(2003) is worth mentioning who while attesting for a steep south dipping fault located north of the
plateau finds evidence to place it further north along the course of the mighty river and referred it