Earthquakes - Extreme Events: A Physical Reconstruction and Risk Assessment - page 162

Because of these problems it is likely that coseismic surface displacement
is a better parameter for estimating prehistoric earthquake magnitude than
surface rupture length. Establishing the extent of surface displacements is far
more accurate than attempting to establish the boundaries of surface rupture
lengths as the latter are often affected by geological and climatic processes over
time. The magnitude of the palaeoearthquake can be estimated by comparing
themean displacement and magnitude of modern earthquakes with the mean
displacement of the surface rupture. This can be achieved by:

(1) determining the slip associated with modern earthquakes of known

magnitude before any measurements of palaeoearthquake slip are

undertaken; and

(2) calculating the mean displacement for the whole surface rupture and

then determining the statistical uncertainties from the range of pre-
served displacements as any one site may not be representative of the
displacement of the entire rupture.

An assumption is made here that the relationship between the distribution
of measured displacements of palaeoearthquakes is similar to that occurring in
modern earthquakes. There is little doubt, however, that differences are likely to
occur between the two data sets due to erosion and burial, and also geological
sampling differences between historic and prehistoric displacement sites. Esti-
mates of palaeoearthquake magnitudes using surface displacements have so far
not distinguished between different earthquake styles, for a further assumption
is often made when using this method that earthquakes create similar geolog-
ical evidence over both time and space. It is likely that this assumption is to a
certain extent incorrect. Nevertheless, displacement curves for different earth-
quakes probably do not display very significant differences. Such differences are
also likely to be minimised through the random sampling of a number of dis-
placements of both prehistoric and historic displacements (Hemphill-Haley and
Weldon, 1999).

Landslide dammed lakes

Most earthquake-triggered landslides are rock avalanches or rock falls.
The rocks fall rapidly from ridge crests and form a pile at the base of the
slope and sometimes as shallow features against the opposite wall of the val-
ley. These landslides can result in the creation of lakes due to the damming of
streams. Such features have been observed in New Zealand, the United States
of America, central Asia and the Himalayas. Adams (1981)suggests that these
dams are sufficiently common to be used to estimate palaeoseismicity. Two