Page 28 - Jabalpur_EQ

Chapter 2 Selsmologlcal Aspects

by down faulting of rock formation on both sides (Roy 1971) and a horst like
nature for the Satpura mountains running just South of the Narmada valley is

interpreted based on the broad high Airy-Heiskanen anomaly zone.
To the East and West of Burwaha, the Narmada valley shows different geological
setting (Fig. 2-1). To the East of Burwaha, the valley all along exposes older hard
rocks, comprising the upper Vindhyan sediments, archaean phyllites, quartzites
and granitoid rocks. To the West of Burwaha, the river bed and valley are passing
through Deccan trap flows which are late Cretaceous to early Paleocene in age.
Systematic and detailed geological mapping carried out on the various segments
across the Narmada valley did not show evidences in support of any rifting or
faulting. In view of this, it is conjectured that beginning with the immediate pre-
Trappean period up to the post-Trappean period, the area covering the Narmada
valley and the surrounding regions had been warping up with the formation of
broad crestal region elongated in the direction of the present lineament. It is
thought that the tension fractures at the crestal region are the vents through
which lava erupted. The predominantly longitudinal tension fracture and proba-
ble depression in the crestal region controlled the initiation of the valley (Ghosh
1976). Therefore, according to this model NSL represents an erosional post-Dec
can Trap Narmada valley, formed at the crest of a domal upwarp with tension
fractures and probably shallow depressions. This was later superimposed in part
on earlier topography.

West (1962) first drew the attention to the significant distribution of the Vindhyan
and Gondwana rocks with reference to the NSL and pointed out the absence of
Gondwana rocks to the North of the NSL and Vindhyan rocks to the South of the
NSL. On the basis of this observation it was concluded that the NSL perhaps rep-
resents an ancient line of weakness and the land to North and South of the NSL
had been experiencing up and down movements in the geological time. The NSL
is interpreted as a horst crustal block bounded by deep faults which are active
and extending up to the Moho (Kaila et al. 1989). In the NSL, the crust is divided
into four major blocks by deep faults at Narsinghgarh, Katangi and Jabalpur. The
crustal block between Narsinghgarh and Katangi forms a graben, and between
Katangi and Jabalpur is a horst block which represents the NSL (Kaila et al. 1987).

SEISMICITY AND SEISMOTECTONICS

Indian Peninsula although an ancient land mass is characterized by moderate
size intraplate earthquake activity. Several attempt have been made to study the
seismicity and seismotectonics of the Peninsula and to investigate the cause of the
intraplate earthquakes, for which plate tectonics theory offers no explanation.
Some of the significant earthquakes which have rocked the Peninsular India caus-
ing considerable damage and loss of life are listed in Table 2-6. Notable among
these earthquakes are: Kutch, June 16, 1879(MMI=XI); Son valley, June 2, 1927
(M-6.5); Satpura, March 14, 1938 (M=6.3); Balaghat, August 25, 1957 (M=5.5);
Koyna, December 10, 1967 (M=6.5); Broach, March 23, 1970 (M=5.4) and Latur,
of
September 30, 1993 (M=6.4). The Kutch earthquake June 16, 1879 caused crustal

18
Jabalpur Earthquake of May 22, 1997