(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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