Environmental Engineering Reference
In-Depth Information
the incidence of “felt” earthquakes. Seismograph stations have only been in operation for
about 100 years, and only since World War II has there been a substantial number of instal-
lations in many areas.
Relationships between intensity and magnitude are very general; therefore, conversion
of
I
from felt reports to
M
or
g
is at best, approximations. Recurrence predictions of
M
and
the time interval must be considered as broad approximations for most locations.
Attenuation
Attenuation, an important element in evaluating data from an existing event, depends on
many variables such as topography, geology, fault rupture length, and focal depth, and
can undergo extreme variation in a given area, as related to both distribution and areal
extent. Relationships have not been well defined and estimates must be considered only
as broad approximations for most areas.
Ground Conditions and Response
Fault Identification and Capability Evaluation
Faulting does not always extend to the surface; therefore, positive identification can be dif-
ficult. Estimation of capability has many uncertainties, and the knowledge of causes and
the basis for anticipation of activity are not well established. Faults, considered as dead,
and not studied thoroughly, can suddenly become active.
Relationships between fault rupture length and event magnitude are based on relatively
few data and restricted geographic areas. Relationships between percent of rupture length
and
M
, or between magnitude propagation (stress drop) and duration, have barely been
addressed. Relationships between fault displacement and
M
, especially for low-intensity
events, need study. It is not unusual to find structures in a risky condition with founda-
tions bearing on rock located over a fault in areas of low seismicity.
Ground Response
Relatively few events have been recorded on strong ground-motion instruments and the
greater majority of these are from the western United States, and a few other seismically
active countries, such as Japan. Foundation and topographic conditions for instrument
locations vary and require consideration when accelerograms are evaluated. Relationships
among peak and effective horizontal acceleration, vertical acceleration, frequency content,
and duration are not established for many conditions of geology and topography, includ-
ing effects of distance and foundation depth.
Effects of soil conditions have not been well-defined; although soil is generally consid-
ered to amplify rock excitation, cases of attenuation have been reported. Relationships
among soil type, depth, layering effects, bedrock boundary configuration, and response
characteristics of acceleration and frequency content are not well established.
Dynamic Soil Properties
Dynamic shear moduli and damping ratios can be measured with acceptable accuracy,
primarily in cyclic simple-shear testing, but basically only for those soils in which high-
quality undisturbed sampling is possible. Evaluations of other soils must rely upon esti-
mates of properties from various correlation procedures.
Prediction
It is still not possible to predict when an earthquake will occur, but we do know the gen-
eral location of where they are likely based on historical events. A better understanding of
