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3
Ground-Shaking Hazard
This chapter examines the selection of scenario earthquakes. As
discussed in Working Paper C, use of scenario earthquakes is not
the only way to address loss estimation, but it is the most common
method. There are two general approaches to evaluating scenario
earthquakes that are commonly referred to as deterministic and prom
abilistic methods, although elements of judgment and uncertainty are
present in both.
DETERMINISTIC METHODS
In this method, one or more earthquakes are postulated with-
out explicit consideration of the probability that those events will
occur. The most common form of this method is use of the largest
earthquake known to have occurred in a region, and this event is
termed the historical maximum earthquake. This approach is based
on a premise that is geologically sound as well as intuitively convinc-
ing: if an earthquake has occurred once, it can occur again. Usually
this approach is acceptable to both the governmental users of Toss
estimates and the general public.
Once a decision to adopt this basic approach has been made,
various questions must be answered in order to establish a scenario
20
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earthquake. For example, will it be assumed that the same earth-
quake reoccurs with the same extent, location, and type of fault-
ing? The distribution of ground-shaking intensities outward from
the earthquake may have been recorded, and can then be used di-
rectly. If this distribution was not thoroughly recorded, it will be
necessary to use attenuation relationships (derived from analysis of
data from many different earthquakes) to calculate some or all of
this distribution. Alternatively, it may be decided that a different
location should be considered, perhaps closer to the region being
studied. In this case, use of attenuation relationships to calculate
intensities is essential.
If there are multiple faults near the region being studied, it will
generally be desirable to consider separately the historical maximum
earthquake for each fault. This is because each of these several
earthquakes may produce the largest losses in some portion of the
region.
In some studies, two levels of earthquakes have been used: the
historical maximum earthquake and a smaller earthquake chosen by
judgment. The smaller earthquake has often been taken to have a
magnitude one unit less than the historical maximum earthquake.
This practice has been adopted when planning for a response to
several levels of disaster is deemed desirable, or when a repetition
in the near future of a large historical maximum earthquake lacks
credibility.
There are also instances where earth scientists present convinc-
ing evidence that an earthquake larger or closer than the historical
maximum event should be considered. This may happen when there
is geological evidence of earthquakes more severe than those that
have occurred in historic time.
The proper characteristics of the scenario earthquake for use in
planning how to respond to a validated earthquake prediction would
be the predicted earthquake's magnitude, location, or other avail-
able seismological information accompanying the prediction. Ex-
cept for the greater potential for controversy concerning predicted
earthquakes, the other aspects of loss estimation are the same for
nonpredicted scenario earthquakes.
It is clear that this deterministic approach involves some judg-
ment and uncertainty. Even in the most seismic regions of the coun-
try, no one knows when the next major earthquake will occur or just
what it will be like; almost certainly there will be surprises. There is
no clear definition of the largest possible earthquake some expert
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can always envision a larger event and even if there were a welI-
defined max~rnum earthquake, it is not obvious that this immense
earthquake is the proper basis for hazard reduction planning. As one
moves away from use of the actual historical maximum earthquake,
and as use of attenuation relationships comes into play, uncertainty
increases. As stated earlier, it is desirable that at least a rough in-
dication of the probability of occurrence be attached to ah scenario
earthquakes, if only to convey to users and the public some indication
of the likelihood of such an event.
PROBABILISTIC METHODS
As just noted, there are two situations where attempts to use the
historical maximum earthquake run into difficulties. At one extreme
is the situation where a very large earthquake has occurred within
recorded history, but it is thought unlikely that it will reoccur soon
and in the same locale. The other extreme is the situation where it is
thought relatively likely that there can be an earthquake larger than
the historical maximum earthquake. ("Historical" merely refers to
a brief sample of the geologic timespan, up to about 400 years in
the eastern United States and 200 years in the West, and some
earthquakes that occur only once every several centuries are unlikely
to be included.) For such situations, it would be useful to have a
systematic method for selecting the scenario earthquakes that meet
the criteria of being plausible but damaging.
Probabilistic hazard analysis offers this possibility, and is dis-
cussed in Working Paper C and in a report of the National Research
Council (1987~. The elements of this method are sketched in Figure
3-1. Information is required concerning: the location of potential
sources (such as known faults) of earthquakes, the probability that
different magnitudes will occur within or along each source, and
the attenuation of intensity away from the source, including uncer-
tainty in the attenuation relation. This information Is then formally
combined to produce a ground-shaking versus hazard curve (Figure
3-1D), giving the probability that any ground-motion level will be
exceeded. An exceedance probability is selected and the associated
ground-motion level (target level) is found from the hazard curve.
Finally, the scenario earthquake is defined as the most likely event
among those that produce ground motions more intense than the tar-
get level. The technology for this type of analysis is well advanced,
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A. Seismic Source i
(Earthquake locations in space
lead to a distribution of
epicentral distances f R (r ~ m)
,' Ruoture
'~'*~4
fR(r~ m)
Site Fault i
c f M (m)
Distance r
C. Ground motion estimation:
A ~ m, r
a,
cot
on
a
-
Distance
(log scale)
1 GAIm,r(a ~
~ ~ _
. ~ ~ ~
-
/ ~ on
~ ~ 0
~ _
B. Magnitude distribution and rate
of occurrence for Source i:
f M (m), hi
mO m max
Magnitude m
D. Probability analysis:
P[ A ~ a. in time t ] /t ~
Ski JJGA|m,r~a ~
f M (m) f R (r | m) dmdr
I
~ \\
\
Ground Motion Level a.
(log scale)
FIGURE 3-1 Graphs indicating probabilistic seismic hazard analysis steps.
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although there are often problems of statistically inadequate data for
evaluating parameters required by the theory.
One difficulty in the use of probabilistic ground-shaking hazard
analysis is in selection of the probability of exceedance to be used
for defining a scenario earthquake. There are no generally accepted
rules for this purpose. Some of the historical maximum earthquakes
used for earlier loss studies have annual probabilities of about .002,
which is equivalent to a mean recurrence interval of 500 years.
The panel rejects the notion of a single standard probability at
this time, but accepts that, in the absence of a suitable historical
maximum earthquake, a scenario earthquake with an annual proba-
bility in the range from .001 to .005 is reasonable for disaster response
and mitigation planning. Despite the lack of definite criteria, use of
probabilistic seismic hazard analysis offers the only rational means
for selecting scenario earthquakes for many parts of the country.
DESCRIBING INTENSITY OF GROUND MOTION
As noted earlier, there is no generally accepted, objective, quan-
titative scale for measuring the damaging effects of strong ground
motion. This is because different buildings, structures, or other facil-
ities respond in different degrees to various aspects (e.g., predominant
frequency, duration, and so on) of ground motion.
Most U.S. loss estimates have used MMI as a scale for the in-
tensity of ground shaking. This scale involves subjective evaluation
of the effects of ground shaking, and its use is subject to abuse and
misinterpretation. However, in most parts of the country the histor-
ical seismic record is known only in terms of MMI. Instrumentally
recorded strong-motion data are much more sparse.
While urging continued research to develop a satisfactory quan-
titative measure of ground-motion severity, the pane! accepts the
continued use of MMI as a basis for the usual loss estimate study.
One aspect of MM! that does require careful attention is the
meaning and use of intensities XI and XIl. The scaTe's criteria for
these levels emphasize observations of ground failure, some of which
may occur when other indicators of shaking severity imply a MM!
as low as VI. The use of high MMI values in a loss estimate requires
explicit explanation to avoid misunderstanding. Some on the panel
interpret the MMI scale as implying that intensity X represents
maximum possible ground shaking. Others feel that ground shaking
stronger than that associated with MMI X is possible, and there
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have been some instances in which loss estunators have used MM! XT
and XIT to represent increasingly strong ground shaking apart from
ground failures.
The panel recommends that MM! XT and XIT not be used to
indicate increased intensities of ground shaking. If this ~ nonetheless
done, it is essential that the interpretation of these intensity levels
be set forth very clearly, and an explicit statement of how the MM}
scale was interpreted should be included in any study where it is
used.
EFFECTS OF LOCAL SITE CONDITIONS
Local site conditions can have a great effect on earthquake losses.
Greater losses often occur because of ground failures, increased in-
tensity of shaking for some soil and topographic conditions, and
selective amplification of ground motion at the frequencies critical to
structural response. It is Report ant to take site effects into account
in a loss estimate. While geotechnical data collected at individual
construction sites can be very valuable in this effort, more general-
ized geologic mapping of districts and zones in a city or region is also
useful and can lead to refinements in seismic hazard analyses.
The essential requirement is to make clear whether the inten-
sity in a scenario earthquake applies to the ground as it is locally
found (i.e., no further correction for local soil conditions required)
or whether it applies to some standard ground condition and must
be further modified for actual local conditions. This is a matter
requiring good communication among seismologists, geologists, and
engineers.
Representative terms from entire chapter:
maximum earthquake
