2011 - Cooperative Institute for Research in Environmental Sciences ...

Roger Bilham
Block Bounding Bookshelf-Faulting
in Baluchistan
FUNDING: NATIONAL SCIENCE FOUNDATION
In 2008, a pair of
earthquakes occurred
northeast of Quetta,
Pakistan, causing the collapse
of several thousand
buildings and the burial
of others in landslides.
As it happened, the
earthquakes occurred
within a line of collaborative
GPS survey points
installed by our colleague
Din Mohammad of the
University of Baluchistan
expressly to monitor
earthquake processes in
the region. They revealed
the existence of an interesting
new phenomenon
for earthquakes in the
Indian plate: bookshelf-faulting.
The earthquakes were unusual in that two identical
earthquakes occurred 11 hours apart—two mainshocks
followed by a sprinkling of magnitude 5 and magnitude 4
aftershocks in the next few months. The initial mechanisms
of the earthquakes indicated a sliding motion, and as is
always the case with strike-slip earthquakes, additional
information was needed to decide whether the far side of
a northwest-striking fault slipped to the right, or the far
side of a northeast-striking fault slipped to the left. In this
case, the decision seemed easy enough, because the two
mainshocks were aligned northwest. The earthquakes appeared
to have occurred on two adjoining segments of a 50
km–long northwest fault. End of story? Not quite.
The GPS points installed before the earthquakes were
shifted tens of centimeters when they were remeasured in
the following weeks, but far from confirming the contiguous
rupture theory, their motion equally satisfied the rupture
of two parallel faults separated by 15 km, we found.
We decided to investigate further.
We first gathered a few dozen seismograms from
throughout the world that had recorded the earthquakes
in detail, and to our surprise, we found that to emulate
the seismic waves wiggle for wiggle, most of the energy
had been released on northeast-trending planes parallel to
each other. For further verification, we examined satellite
images (from interferometric synthetic aperture radar, In-
SAR) of the earthquakes. These were severely distorted by
widespread avalanches that had occurred throughout the
region during the events, but enough of the deformation
pattern anticipated from a pair of parallel quakes had been
preserved to confirm the seismic solution.
The implication that two parallel faults had slipped in
2008 meant that perhaps other earthquakes in the region
had slipped similarly in the past. A search revealed three
26 CIRES Annual Report 2011
3 cm/yr
Chaman Fault
Quetta
0.6 cm/yr
1892
1995
2008
2008
1931
1931
100 km
b
Moderate earthquakes north of Quetta (dates of occurrence are shown)
appeared to fall along a line that suggested a large fault could slip (a).
We now know that they occurred on shalt faults bounding blocks that are
incrementing clockwise during earthquakes. Bookshelf-faulting means
that the magnitudes of the earthquakes are limited to about Mw=6.0.
Din Mohammad, left, and Bilham.
more. The first was the largest aftershock of the 2008 sequence,
indicating that this sequence had actually permitted
three parallel faults to slip. The second was a momentmagnitude
5.5 earthquake in 1995 to the northwest; and
the third was a 1931 moment-magnitude 7.1 earthquake
to the southeast. We now believe that these parallel faults
bound the edges of a series of contiguous blocks 15 km
wide and 20 km long, all rotating slowly clockwise allowing
northern Baluchistan to slide southeast relative to
the ranges east of Quetta. The process is called bookshelffaulting
and has hitherto been found in Iceland, Afar and
parts of California.
Most of the edges of the rotating blocks have slipped at
least once in the past century, and are likely to do so again
in the next. That is, the zone of bookshelf-faulting can be
defined as a source of persistent seismic hazard, an important
contribution to the characterisation of future seismic
risk in Baluchistan.
Din Mohammad was invited by the Seismological
Society of America to present these findings at its annual
meeting in Portland, Ore., this year, the first such honor
ever to be extended to an international guest.