29. Distance across this aerial photograph is about 3 miles (5 km). The photograph shows where sand was vented through linear dikes in an area where no significant topographic depressions were within a few kilometers of many of the dikes; therefore, lateral spreading is unlikely as the origin of the linear dikes. These dikes, mainly parallel to one another, were probably formed by surface oscillations. Ground breakage from surface oscillations often develops far (kilometers) from any slopes, and dikes often form independent of geologic details of the cap or the local field setting.

What we saw on the slides 27, 28, and 29 is important for eliminating a nonseismic origin of the dikes in the region. Two nonseismic mechanisms must be considered as possibly causing dikes in any study area: nonseismic landsliding and artesian springs.

Nonseismic landslides can be eliminated in the two aerial photographs (slides 27 and 29) because the dikes have formed so far away from any significant slopes. Artesian conditions often can be eliminated for a similar reason, because artesian conditions typically require a special field setting such as that shown in the next slide.