Geoscience Reference
In-Depth Information
In the northern part of the strait banks were observed to crumble and cave in, under-
water cable lines were damaged.
Geophysical studies, performed by the Geological Survey of Canada, revealed
the existence of two separate deposition zones of bottom sediments, located be-
tween isobaths 30 and 120 m. Identification of these areas was carried out with
the aid of echo sounding gear for lateral observation and equipment for seismic
profiling of high resolution. Special underwater video shooting, performed in 1996,
revealed that lower down the slope there are a number of blocks of well-consolidated
sediments several meters thick, which obviously resulted from their breaking away
from the main mass and falling down the slope under the force of gravity. The main
block of sediments has corresponding areas with a very steep edges left after par-
tial collapse of the sedimentary mass. The lower boundary of this sedimentary layer
exhibits a very steep inclination everywhere—practically like a precipice.
The northern zone of the sedimentary cover is up to 38 m thick, and the inclina-
tion of the inner boundary toward the sea is, on the average, 7.5
◦
. The layer above
it is inclined at approximately 16
◦
relative to the boundary of the base layer and
extends about 400 m along the slope, exhibiting a thickness of about 300 m.
The hypothetical scenario of tsunami generation assumes that the earthquake
causes all the mass of sediments, accumulated in the northern zone of the sedi-
mentary cover, to break off from and to slide down the steep slope of the basalt
boundary of the bottom. Owing to a lack of geotechnical data on the properties of
sediments, simulation of the movement of the landslide is based on a broad set of
parameters of the material. The possibility of the southern and northern sedimentary
layers collapsing at the same time is not considered. Such a joint scenario would,
naturally, lead to the generation of waves of higher amplitudes.
Calculations were performed on a difference mesh with 365
×
197 nodes and
steps
y
= 25 m. In the initial state, the landslide body was considered to have
a rectangular shape with a parabolic profile over the thickness in both directions.
Calculations were performed for the following landslide parameters:
∆
x
=
∆
1,250,000 m
3
Volume:
Width:
200 m
Average width
30 m
49
◦
37.94
N, 124
◦
16.80
W
Coordinates of centre:
Average depth:
80 m
cm
−
3
Density (
ρ
2
):
2.0 g
·
s
−
1
Figure 4.3 shows fragments of numerical calculations of the movements of
the landslide and of tsunami waves in the strait. Unlike a solid-state body, re-
taining its size and shape, the viscous landslide moves along the slope, spreading
out and assuming the form of a sickle. Displacement of the landslide takes place
mostly along the normal to the west bank. Movement of the landslide gives rise
to radially diverging surface waves. The leading wave (positive) moves towards
the continent, while the negative wave (depression) moves in the opposite direc-
tion towards Texada island. The leading wave crosses the Malaspina strait and
0.01 m
2
Kinematic viscosity (
ν
):
·
