(2)
Maximum
earthquake
magnitudes.
that could significantly contribute to the seismic
Maximum magnitude is the physical limit of the size
hazard at a site.
Where earthquake sources are
of an earthquake that can be generated by an
defined as area sources, recurrence relationships are
earthquake source that is related to the dimensions of
usually developed on the basis of historical
the source or source segments. For seismic sources
seismicity. For sources defined as faults, however,
in the WUS, maximum magnitudes are usually
the available historical seismicity for the individual
estimated by assessing the largest dimension (e.g.,
fault is usually insufficient to characterize recurrence
area) of the source expected to rupture in a single
rates, particularly for larger earthquakes, and use is
event, and then using empirical relationships that
typically made of geologic data to supplement the
relate earthquake magnitude to rupture size.
An
historical records.
Geologic data include data on
example of a correlation between rupture area and
fault slip rates and data from paleo-seismic studies
earthquake moment magnitude is shown in Figure 3-
on the occurrence of large prehistoric earthquakes.
7. In the EUS, because the source dimensions are
typically unknown, there is a greater degree of
(b)
Earthquake recurrence curves are
uncertainty
as
to
the
maximum
earthquake
usually described by either a truncated exponential
magnitude.
Typically,
maximum
earthquake
recurrence model (Cornell and Vanmarke, 1969)
magnitudes in the EUS are estimated based on a
based on Gutenberg and Richter' (1954) recurrence
s
conservative interpretation of (or extrapolation
law, or a characteristic earthquake recurrence model
beyond) the historical seismicity on the source and
(Schwartz and Coppersmith, 1984; Youngs and
by analogies to similar geologic structures and
Coopersmith, 1985a, 1985b).
The exponential
tectonic regimes throughout the world. Johnston et
relationship
describes
a
rate
of
earthquake
al. (1994) present a methodology for assessing
occurrence
that
increases
exponentially
as
maximum earthquake magnitude in the EUS based
earthquake magnitude decreases. On the other hand,
on an analysis of worldwide data for similar stable
the characteristic relationship predicts that a
continental tectonic regions.
relatively greater number of earthquakes (compared
to the exponential relationship) will occur as
(3)
Recurrence relationships. Recurrence
"characteristic" magnitude events that are at or near
relationships
characterize
the
of
the maximum magnitude for the source.
A
occurrence of earthquakes of various sizes, from the
characteristic relationship is illustrated in Figure 3-
minimum magnitude of engineering significance to
8.
Characteristic and exponential forms of
the maximum magnitude estimated for the source.
recurrence relationships are compared in Figure 3-9.
Recurrence
relationships
are
illustrated
The exponential relationship is typically used for
schematically in diagram A of Figure 3-3 and 3-4.
seismic sources defined as areas, whereas both
exponential and characteristic earthquake models are
(a)
Earthquake recurrence relationships
used for individual fault sources. Detailed studies of
must be developed for each identified seismic source
earthquake recurrence in the Wasatch fault region,
3 - 27
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