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however, have concluded that essentially all of
the geodetic convergence is ultimately passed
along the sole thrust to the leading edge of the
plate interface during megathrust earthquakes
(Avouac, 2003). Although the rupture patches
can apparently span thousands of square
kilometers during
M
≥
8 earthquakes and thereby
pose a major hazard (Bilham
et al.
, 2001),
no surface ruptures have been previously
described during three megathrust earthquakes
(
M
w
=
7.8-8.5) in the 20th century. Until recently,
neither the amount of slip during these
megathrust events nor the earthquake magnitude
of pre-20th-century earthquakes have been
known. New paleoseismological studies along
the Himalayan Main Frontal Thrust have helped
to resolve these questions. Trenches across this
fault (Fig. 6.31) have revealed a Medieval
earthquake (
∼
1100AD) with dip slip of
∼
17 m, a
lateral extent probably exceeding 240 km, and an
estimated moment magnitude of 8.8 (Lavé
et al.
,
2005). Although not as large as the 2004 Sumatra
earthquake, a repeat of this Medieval rupture
would wreak havoc with the lives of several
million people who currently live directly above
the sole thrust.
delineate deformation. Although some innovative
techniques for defining displacement on blind
thrusts have recently been described (Dolan
et al.
, 2003), relatively few buried faults have
been evaluated and, for those that have, the
interpretation is highly dependent on a geometric
model for growth strata
vis-à-vis
coseismic rock
uplift. Such models may need to be constructed
independently for each fault that is studied.
We have seen that trenching along strike-slip
faults can yield a history of multiple earthquakes,
even if each earthquake causes displacements of
several meters (Fig. 6.8). What practical means
can be used to develop similar histories for dip-
slip faults with large offsets during each earth-
quake? Deeper trenches could help, but such
trenches are often dangerous places in which to
work. Can aspects of the stratigraphic record be
tied unambiguously to recurrent fault movements,
as has been attempted at El Asnam in Algeria
(Fig. 6.23)? Can high-resolution lidar topography
reveal event-driven hanging-wall deformation
that could be tied to individual earthquakes?
Determining precise timing for deformational
events is a primary concern and an almost
never-ending problem for paleoseismologists
(and for many geomorphologists!). In this
chapter, applications of high-precision radio-
carbon dating,
230
Th dating, cosmogenic exposure
ages, lichen diameters, and annual growth rings
in corals and trees have been discussed. A
perennial search is ongoing for ways to improve
existing techniques and for new ways to date the
record of deformation. The limitations, assump-
tions, applicable age range, accuracy, and pre-
cision should be evaluated prior to applying any
technique to a particular dating problem. As
researchers confront the chronological problems
and possibilities of trenches and outcrops, new
ways to use existing techniques, as well as new
techniques themselves, should be forthcoming.
Similarly, more ways to assess vertical and
horizontal accelerations and the distribution of
seismic shaking in prehistoric earthquakes need
to be developed.
Despite the many challenges and problems
confronting paleoseismologists, tremendous
progress has been made in the past few
Remaining problems
Among the paleoseismic approaches described
in this chapter, many are generally suitable for
assessing past activity along faults that break the
surface or for defining deformation that occurred
where a good spatial reference frame against
which to measure displacement is available.
When successful, such studies reveal when
earthquakes occurred in the past, the magnitude
of offset, and the extent of rupture associated
with each earthquake. Numerous paleoseismo-
logical problems remain to be solved, however,
and some will require innovative approaches
that are presently untested.
One major unresolved problem is to define
the seismic record of faults in terrestrial settings
that do not rupture the surface. Unlike coastal
records, terrestrial regions commonly lack a
reliable reference frame against which to
