Geology Reference
In-Depth Information
140
1901
1881
1866
historical
earthquake
dates
0.5-mm class interval
1929
n = 5,650
120
probable
pre-historic
earthquakes
100
80
1948
60
1968
40
20
Lichenometrically Dated Rockfalls
0
40
50
0
10
20
30
Largest lichen maximum diameter (mm)
Fig. 6.25
Histogram of maximum lichen diameters, New Zealand.
Data were collected from more than 5000 rockfall blocks distributed across 20 000 km
2
on the South Island,
New Zealand. Note the clustering of maximum diameters that rise above the background level. This clustering
suggests that discrete events caused episodic input of rockfall blocks. Despite the spatially broad collection area, the
presence of peaks that are separated by less than 3-4 mm suggests that lichen growth rates cannot vary significantly
between or within the study sites. The fact that several historical South Island earthquakes can be correlated to
discrete peaks in the histogram suggests that the lichenometric dating is robust. Modified after Bull
et al.
(1994).
Box 6.2
Ground acceleration during earthquakes.
Although the vertical acceleration in past
earthquakes is of great interest to construc-
tion engineers and urban planners, measure-
ments of vertical accelerations in prehistoric
earthquakes have been rare. Neither the
stratigraphic nor the geomorphic record
commonly yield data that reveal peak vertical
accelerations, because these data sets record
net ground displacement, rather than the rate
of slip, or its derivative, the acceleration. Two
geomorphic approaches using boulders can
sometimes provide useful limits on vertical
acceleration. If seismic accelerations exceed
gravitational acceleration, unattached objects
will be thrown in the air. In the 1989 oblique-
slip Loma Prieta earthquake in California,
there were numerous reports of massive
objects (cars, stoves, fireplaces) that were
thrown into the air, indicating that at least
locally the ground accelerations exceeded 1
g
.
Boulders that are lying on the surface can
actually be moved some distance laterally if
the accelerations are oblique and approach
or exceed 1
g
in the vertical direction. During
the 1992 (
M
s
7.4) Suusamyr earthquake in
Kyrgyzstan, boulders weighing up to 1.5 tons
were displaced 2 m horizontally and were
overturned on a nearly horizontal surface
(Ghose
et al.
, 1997). The overturning of a
boulder suggests that it partially rolled into
its new position, but first it had to move
upward out of the depression (
=
∼
30 cm deep)
in which it had been situated.
A second approach to estimating vertical
and horizontal acceleration in past earth-
quakes uses “perched” boulders to place
limits on the amount of shaking in the past
(see figure). In arid regions, jointed bedrock
