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The authors of the article are J.-L. Mugnier, A. Gajurel, P. Huyghe, R. Jayangondaperumal, F. Jouanne, B. Upreti. They are affiliated to Université de Savoie - France,
Tribhuvan University - Nepal and Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand - India
Tribhuvan University - Nepal and Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand - India
A major question about the Himalaya remains open: does a
great earthquake (like the Mw ~ 8.1 1934 earthquake) release all the
strain stored by the Tibet–India convergence during the preceding
interseismic period and only that strain, or can it also release a
background store of energy that remained unreleased through one or more
earlier earthquakes and so potentially engender giant events or a
relatively random sequence of events?
To consider this
question, the history of the great earthquakes of the last millennium
is investigated here by combining data provided by the historical
archives of Kathmandu, trenches through surface ruptures, isoseismal
damage mapping, seismites, and the instrumental record. In the Kathmandu
basin, the location of the epicenter of the 1934 earthquake was
determined from the arrival of high-energy P-waves that caused
sedimentary dikes and ground fractures perpendicular to the epicenter
azimuth. The epicenter of the Mw ~ 7.6 1833 earthquake can therefore be
determined analogously from dike orientation, and its location to the NE
of Kathmandu indicates an overlap with the Mw ~ 8.1 1934 rupture. The
1934 earthquake released strain not released by the 1833 earthquake.
Comparison of the historical records of earthquakes in Kathmandu with 14C
ages from paleo-seismic trenches along the Himalayan front suggests
that: (1) the 1344 Kathmandu event ruptured the surface as far away as
Kumaon and was therefore a giant Mw ≥ 8.6 earthquake; and (2) the 1255
event that destroyed Kathmandu is attested by surface ruptures in
central and western Nepal and by seismites in soft sediment as far away
as Kumaon.
Geometric and rheologic controls for the
different types of ruptures during the medium (Mw ~ 7), great (Mw ≥ 8),
and giant (Mw > 8.4) earthquakes are illustrated in structural
cross-sections. It is found that the epicenters of great Himalayan
earthquakes are located on the basal thrust farther north or close to
the locked zone, which is defined from geodetic measurements of regional
deformation during the interseismic period; this suggests that great
earthquakes initiate in a wide transition zone between exclusively
brittle and exclusively creeping regimes, the extent of which depends on
the dip of the Main Himalayan Thrust.
The succession
of the great earthquakes during the last millennium has released all the
20-m millennial Himalayan convergence; even in the central seismic gap
which has been locked since 1505, the millennial seismic release rate is
close to the convergence rate. Nonetheless, no evidence of a succession
of characteristic earthquakes has been found: the ~ 1100, 1833, and
1934 earthquakes in the eastern Himalaya are characterized neither by
constant displacement nor by constant recurrence. Furthermore, some
great earthquakes do not release all the strain elastically stored by
the Himalayan and Tibetan upper crust: after the 1255 event, there was
still the potential for a slip of several meters for the Mw ~ 8.1 1505
event. This suggests a rather random release of seismic energy; great
earthquakes could occur anytime and in any part of the central Himalaya.
Furthermore, a future giant earthquake of Mw ≥ 8.6 cannot be excluded.
Affiliations:
ISTerre, Université de Savoie, CNRS, Université Joseph Fourier, Batiment les Belledonnes, Université de Savoie, F-73376, Le Bourget du Lac Cedex, France
J.-L. Mugnier
A. Gajurel
P. Huyghe
F. Jouanne
Department of Geology, Tribhuvan University, Ghantaghar, Kathmandu, Nepal
A. Gajurel
B. Upreti
Wadia Institute of Himalayan Geology, Dehradun, Uttarakhand, India
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