FIGURE 11.11 Height change data cross the Pleito thrust fault, 1980–1983. Bench mark 1 is arbitrarily held fixed.

are necessary (Savage et al., 1979b). With such figures it is possible to achieve resolution approaching 0.5 µrad (Sylvester and Jackson, 1982).

Although there is a fairly voluminous literature on the origin of creep and related minor fault movements, sys

FIGURE 11.12 Site map of irregular leveling array across San Jacinto Fault near Anza, California.

tematic near-field geodetic measurements of these movements have proceeded only about two decades—now only beginning to be a significant length of time. Thus it is imperative that this time base of data be lengthened to obtain empirical data to answer and refine earlier answers to the following questions.

1. Why, with but the exception of the North Anatolian Fault in Turkey (Aytun, 1980), is creep restricted to the central and southern segments of the San Andreas Fault? Is it because the fault is relatively straight and parallel to the lithospheric plate motion, that in its central segment it juxtaposes relatively weak, serpentine-bearing rocks as several authors have maintained?

2. Why is vertical creep so uncommon? Is it simply because the search has not been sufficiently broad in scope and duration? Or are there more fundamental tectonic reasons?

3. How does creep relate to the earthquake mechanism? Is it a safety valve that periodically releases stress, thus precluding great earthquakes as many authors believe? Or is creep a form of long-term, preseismic slip for a truly great earthquake?