The Mw7.6 Kashmir 8 October 2005 earthquake occurred in a region where
a great plate-boundary earthquake has long been considered overdue.
Although the earthquake resulted in widespread devastation, it is
doubful that it has released more than one tenth of the cumulative
elastic energy that has developed since the previous great earthquake
in the region in 1555 or earlier. This overview places the earthquake
in a historical and structural context.
The figures below, published in May 2001 and in May 2005 respectively,
illustrate the slip potential developed along the arc (left: red=certain,
pink=possible) and the possible size and locations of earthquakes
in these regions based on recent findings (right). The area of each
yellow region is proportional to estimates of potential elastic
energy to be released (Bilham and Wallace, 2005). The Kashmir earthquake
overlapped the extreme left-hand rectangle. Click for larger image
with the Kashmir earthquake superimposed.
Historical Earthquakes in the Himalaya Also see
Stacey Martin's ASC page. previous catastrophic earthquake in Kashmir
occurred in 1555 but we have insufficient data to assign it a magnitude.
Five decades earlier a moderate Mw≈7.3 earthquake damaged
Kabul, and a great 600-km-long 8.2<Mw<8.6 rupture in the central
Himalaya destroyed monasteries in Tibet and buildings in Agra. Moderate
earthquakes similar to the 2005 Kashmir event occurred in 1885 (Mw=7.5),
1905 (Mw7.8 Kangra), in 1842 (Mw7.5 Kunnar), and in 1974 (Mw=7.4
Using internet responses and reports in newspapers,
a map of the decay in shaking intensities as a function of distance
from the epicenter has been developed by Stacey Martin at the ASC.
The preliminary version of this map shown here illustrates the long
reach of the earthquake ranging from Intensity 12 near the epicenter
(red=total destruction), to barely felt (violet), beyond distances
of hundreds of km. Click for original. Compare with Susan Hough's
intensity distribution for the 1905 Mw=7.8 earthquake.
How bad was it? If reports from
the Kashmir epicentral region are confirmed, the number of fatalities
exceeds 40,000 making it the most fatal earthquake ever to occur
in the Indian subcontinent. The number of fatalities in an
earthquake is linked to the vulnerability of local buildings, population
density, and shaking intensity. In 1935 a Richter magnitude M7.5
strike-slip earthquake near the city of Quetta, the only large settlement
in an otherwise sparsely populated region of Afghanistan, Pakistan
and Baluchistan, resulted in an estimated 35,000 dead. The M7.8
Kangra earthquake in 1905 caused 20,000 fatalities, and the Mw=7.6
Bhuj 2001 earthquake 18,500.
Deaths vs earthquake magnitude for earthquakes throughout
the world 1900-2004 compared to the Kashmir 2005 earthquake. Although
a simple relation between earthquake magnitude and the number of
resulting deaths can be discerned (gray shading), the fatal consequences
of large earthquakes depends more on their proximity to urban populations,
the vulnerability of dwellings, and the time of day, than on the
energy released. (fig. from Hough & Bilham,2005). The
Sumatra/Andaman/Nicobar earthquake resulted in epicentral building
collapse and damage, and huge loss of life along a thin but concentrated
population on a coastal strip throughout the Indian ocean and Andaman
Is this a foreshock to
a larger quake? Probably not, but we have so few well-documented
examples of Himalayan earthquakes that it is conceivable that another
earthquake could occur. The damaging Kashmir 1555 earthquake was
preceded in 1501 by a damaging earthquake of unknown magnitude also
in Kashmir. Also it is known that Himalayan earthquakes can trigger
others. For example in 1833, two major earthquakes preceded the
Mw=7.8 mainshock in Nepal by 5 hours and 15 minutes respectively,
alerting local residents and resulting in a very minor loss of life.
The 1909 Afghan earthquake consisted of two events 1 minute apart,
and the 1905 Kangra earthquake had a M7 earthquake embedded in its
coda some 7 minutes after the mainshock. We know of no great earthquake
triggered days to months after a major earthquake in the same region.
However, speculation initiated by the recent Sumatra earthquake
that the rupture zones of even quite large earthquakes can re-rupture,
suggests that recent M≤7.9 ruptures cannot be considered immune
to premature failure in a larger earthquake (Bilham and Wallace,
2005, Feldl and Bilham, 2005).
Landslides in the region may result in floods in the next
few months. This Aster image processed by Eric Fielding shows the
3 km path of a landslide near the epicenter that has over-ridden
a facing spur and dammed two tributaries of the Jhelum river top
right. Lakes will grow upstream (to the left and below) until the
dam is breached. A catastrophic downstream flood may occur should
the dam not breach early in reservoir development.
Tectonic setting Earthquakes occur
throughout the Indian subcontinent but most are located along the
plate boundaries to west, north and east. The 1935 Mw=7.5 Quetta
earthquake occurred on part of the left-lateral Chaman fault system
on the western boundary. The 2004 Mw=9.3 Sumatra/Andaman earthquake
ruptured the right lateral eastern plate boundary. In contrast,
the Kashmir 2005 earthquake occurred near the western end of the
Himalaya. Thrust earthquakes here signify the descent of India beneath
Tibet. The structure on which the mainshock occurred - the Hazara
syntaxis - is known to be among the most active in the Himalaya,
with local uplift rates in the past several thousand years of ≈1
Earthquake details Teleseismic
data indicate that a 100x50 km wide plane oriented N27W slipped
unevenly 1-6 m with most slip 2/3 from the NW end. Chen Ji at Caltech
using the USGS mainshock location of 34.432N & 73.537E indicates
a preferred nodal plane dipping 37deg NE with dimensions 90-100
km along strike by 35-50 km down-dip. The moment release is 2.1x10^27
dyne.cm ( Harvard 2.7x10^27 dyne.cm). The mean slip derived from
the geometric moment for Mw7.6 assuming a 100 km long, 50 km wide
rupture is roughly 2 m. The Harvard CMT indicates 1.4 m. Using these
preliminary values we have calculated approximate surface strain-fields
and displacement fields (below). The surface dilation calculated
from Coulomb 2 (Toda et al.) is shown in map view and in profile.
Red areas have been stretched and violet regions squeezed by 1 part
in 100,000 by the earthquake.
GPS Points in the area The right
hand map (click for larger version) shows aftershocks to 12 Oct
with Chen Ji's slip solution for the rupture and principal faults
(top right Karakorum fault). It is probable that some relocation
of the aftershocks and rupture area will occur as improved models
are calculated. In particular the mainshock/aftershock geometry
is parallel to the projected strike of the Himalaya, whereas the
synthetic rupture and mainshock strike is skewed 20 deg clockwise
parallel to the Hazara syntaxis. Points of a sparse GPS network
measured once collaboratively with the University of Peshawar are
located South, West and NE of the rupture (red triangles). New points
are being re-occupied to the east of the rupture zone by Indian
geodesists in a search for afterslip. Synthetic models for convergence
suggest that afterslip will be pronounced in the 100 km NE of the
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