Page 252 - Global Tectonics

238 CHAPTER 8

as well as along their boundaries, causing the surface Published estimates of locking depths for the San
velocities to deviate from the “rigid plate” requirement Andreas Fault typically range from 0 to 25 km. However,
of plate tectonics. locking depths are not known a priori and, therefore,
In the case of southern California, the incorporation must be inferred on the basis of seismicity, long-term
of block rotations and the small-scale displacement dis- geologic slip rates, deformation patterns at the surface,
continuities associated with creep on and near major or inferences about the rheology of the lithosphere.

faults has provided a relatively good fi t to the available Locking depths that fall significantly below the pre-
geodetic data (Becker et al., 2005; McCaffrey, 2005; dicted depth of the brittle–ductile transition (8–15 km)
Meade & Hager, 2005). A common way of evaluating for a typical geotherm, or below the seismogenic layer,

the fit of the models involves the calculation of residual usually require some sort of explanation. In some cases,
velocities, which represent the difference between the slow slip rates on the faults have been used to infer
modeled and observed values. An example of one of relatively deep locking depths for some segments of the
these comparisons is shown in Fig. 8.19d. In this applica- San Andreas Fault (Meade & Hager, 2005; Titus et al.,
tion, crustal blocks were chosen to minimize the resid- 2005). These and other studies illustrate how the choice
uals, while still conforming to known boundary of locking depth is directly related to inferences about
conditions, such as the orientation of fault traces and slip rates on or near major faults.
the sense of slip on them. The comparison shows Other reasons why geodetic and geologic slip rates
that despite the improvement over some continuous commonly differ may include inherent biases during
models there are still areas of mismatch. In the Eastern sampling or changes in the behavior of faults over time.
California Shear Zone, for example, Meade & Hager This latter possibility is especially important when the
(2005) found that slip rates estimated using geodetic effects of long-term, permanent strains are considered
data and the results of block models are almost twice (Jackson, 2004). Meade & Hager (2005) concluded that
−1
as fast as the 2 mm a geologic estimates (Beanland & the differences between their calculated slip rates and
Clark, 1994) for the past 10,000 years. A similar discrep- geologic slip rates on faults might be explained by the
ancy occurs on the San Jacinto Fault. In addition, the time-dependent behavior of the fault system. In this
modeled slip rates on the San Bernadino segment of the interpretation, the San Bernadino segment of the San
San Andreas Fault are much slower than geologically Andreas Fault is less active now than it has been in the
determined rates for the past 14,000 years. Finding ways past. By contrast, the San Jacinto Fault and faults in the
to explain and minimize these mismatches remains an Eastern California Shear Zone are relatively more active
important area of research. now compared with geologic estimates, possibly due to
One possible explanation of why geodetic and geo- the effects of earthquake clusters. This possibility high-
logic rates commonly mismatch lies with the mechani- lights the importance of combining geologic, geodetic,
cal behavior of large faults and the vertical extent of and seismologic information to better understand the
brittle faulting within the lithosphere. Because slip on a relationship between the short- and long-term (perma-
fault plane near the surface is controlled by its frictional nent) behaviors of faults.
properties (Section 2.1.5), there is a tendency for faults By incorporating elements of permanent deforma-
to become stuck or locked for certain periods of time tion into block rotation models, McCaffrey (2005) found
(Section 8.5.2). This locking may result in elastic strain that the largest blocks in the southwestern US, includ-
rates that are evident in short-term geodetic data but ing the Sierra Nevada–Great Valley and the eastern
not in the long-term record of permanent displace- Basin and Range Province, show approximately rigid
ments (McCaffrey, 2005). To address this problem, inves- behavior after all nonpermanent (elastic) strain has
tigators utilize the concept of the elastic locking depth been removed from the data. Most of the blocks rotate

(Savage & Burford, 1973). This depth is defined as the about vertical axes at approximately the same rate as

level below which there is a transition from localized the Pacific plate (relative to North America), suggesting
elastic strain accumulations on a fault plane to distrib- that, locally, rotation rates are communicated from

uted aseismic flow. The value of the parameter is related block to block. This and several other properties of the
directly to the mechanical strength of the fault and the model support a plate tectonic-style description of
geometry of deformation at the surface. Strong faults deformation in the western USA, where the rotating
and wide zones of surface deformation correspond to blocks behave like microplates. Nevertheless, the
deeper locking depths. problem of determining the mechanisms of the defor-