Geology Reference
In-Depth Information
Table 14.2
Example of soil categorisation based on shear-wave velocity.
Five soil types are defined on the basis of their shear-wave velocities (
V
s
)
and susceptibility to earthquake shaking amplification. The definitions are
those applicable in the Bay Area around San Francisco
Category
Description
A:
V
s
>
1500 m s
-1
Soil type A includes unweathered intrusive
igneous rock. Soil types A and B do not
contribute greatly to shaking
amplification
B:
1500 m s
-1
Soil type B includes volcanics, most
Mesozoic bedrock, and some Franciscan
bedrock. Does not contribute greatly to
shaking amplification
>
V
s
>
750ms
-1
C:
750 m s
-1
Soil type C includes some Quaternary
sands, sandstones and mudstones,
some Upper Tertiary sandstones,
mudstones and limestone, some Lower
Tertiary mudstones and sandstones,
and Franciscan melange and
serpentinite
>
V
s
>
350ms
-1
D:
350 m s
-1
Soil type D includes some Quaternary
muds, sands, gravels and silts.
Significant amplification of shaking by
these soils is generally expected
>
V
s
>
200ms
-1
E:
200 m s
-1
Soil type E includes water-saturated mud
and artificial fill.
The strongest
amplification of shaking is expected
for this soil type
>
V
s
Source:
National Earthquake Hazards Reduction Program (NEHRP), USA.
as shown in the example in Figure 14.3. The MASW method uses an
(f-k) transform to calculate a phase velocity-frequency dispersion image
(Figure 14.4). The ReMi method uses a p-tau (also known as or slant-
stack) transformation to produce a slowness-frequency dispersion image
(Figure 14.5). Because ReMi sources can be in any direction, this trans-
form is applied for multiple directions through the geophone array and
summed.
ReMi and MASW dispersion curvces are obtained in different ways. For
ReMi data, there is uncertainty in the arrival azimuth of the Rayleigh wave
