variable offsets during the 1857 earthquake and along-strike variations in recurrence intervals, to create synthetic histories many earthquake cycles long. Some of their histories mimick the actual historical and paleoseismic record of the fault, in that certain portions of the fault experience infrequent repeated offsets of several meters, whereas others experience frequent small offsets. Ward (48) has developed synthetic histories of large earthquakes along the Middle American subduction zone, using static dislocation theory and fault segments based upon the fault’s historical behavior. His models produce a variety of behaviors, including fault patches that are highly “characteristic” and patches that produce a variety of event sizes and rupture lengths. In all three of these attempts to model fault histories, the locations of rupture terminations and transitions from high- to low-slip patches are roughly stationary.
Rice and his colleagues (3, 49) have argued that theoretical models of smooth faults produce slip events that are highly regular in both space and time, if the numerical cell size one uses is small compared to the dimensions of the earthquake’s nucleation patch. An individual fault can be made to deviate from highly regular repetition of events only if it is allowed to change state between earthquakes, while it is not slipping (49). In these “aging” models, slip at a site varies by more than an order of magnitude, and slip-patch boundaries vary wildly with time. Even in this version of their smooth-fault models, however, power-law frequency-size earthquake populations of the G-R type fail to occur. From this, they conclude that observed G-R earthquake populations reflect geometrical irregularities along faults and the spectrum of fault sizes in a region. Wesnousky’s study (2) of paleoseismic and instrumental data for major faults in southern California supports this view that structures fail in large earthquakes that are grossly underpredicted by the G-R statistics of smaller earthquakes in the surrounding region.
Well-documented examples of repeated fault rupture are rare. But sparse available data support the view that smooth, individual fault patches fail during characteristic slip events. At sites where recent slip was small, previous events usually exhibit slip of the same magnitude. Likewise, where recent slip was meters, previous events usually exhibit slip of the same magnitude. Synthetic earthquake histories characterized by highly irregular slip during consecutive events do not appear to reflect reality.
Contrary to the “characteristic earthquake” model, patches commonly fail either singly or in tandem with one or more adjacent patches. Only along transition zones between slip patches does slip at a site appear to deviate much from event to event. Large seismic rupture in a transition region during one event may alternate with aseismic slip induced there by static stresses from coseismic slip on a neighboring patch.
The small differences in a patch’s slip function over two or more earthquake cycles indicate that slip is controlled principally by a physical property of the patch, not the length of the rupture or dynamic properties of the rupture, such as directivity.
Carrie Sieh assisted in drafting the figures. I deeply appreciate support during the past two decades by the National Earthquake Hazards Reduction Program, through the U.S. Geological Survey’s external grants program. My studies of the Landers earthquake were supported by California Institute of Technology’s Earthquake Research Affiliates and by the National Science Foundation/U.S. Geological Survey Southern California Earthquake Center. This paper is California Institute of Technology Division of Geological and Planetary Sciences contribution number 5588 and Southern California Earthquake Center contribution number 218.
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