Analysis of gross stratigraphy—the composition of the lithologic units and the deformation of beds—can indicate whether rates of uplift nearby were rapid or slow, whether volcanic arcs were nearby, or whether terranes that contributed clasts to the sediments had later moved away. At small scales, as in excavations dug across active faults, the sequence of sedimentation and faulting can be analyzed to determine how often large prehistoric earthquakes occurred. The patterns and rates of silting of rivers are extremely sensitive to tectonic changes.
Landforms, created by the competition between tectonic constructional and erosional destructional processes, contain abundant evidence of active tectonics. Classical geomorphology, however, has concentrated primarily on descriptions of landforms, and process and stages of erosion were stated in only generalized terms. In the past decade or so, quantitative, process-oriented geomorphology has been directed toward problems of active tectonics. Studies of marine terraces, river geomorphology, fault scarps, and eruptive volcanic features have demonstrated a rich and readily available source of information about active tectonics recorded in landforms. Coseismically uplifted marine terraces may provide evidence of prehistoric earthquakes. Direct measurement of fault-scarp morphology, for example, when analyzed according to an error-function solution of a diffusion model, can approximate the date of a great prehistoric earthquake. From such analysis, the pattern and recurrence interval of large earthquakes can be determined even where average recurrence intervals are measured in thousands of years. In summary, as more research is carried out on the effect of active tectonics on landforms, geomorphology can become one of the most powerful tools for evaluating active tectonics.
Clearly, tectonic processes are complex, often nonuniform, and the emphasis given here on predicting future tectonic activity introduces the important problem of how best to state the likelihood of future events. A variety of formal mathematical methods is currently available for expressing probability, but further study is warranted into techniques of conveying to both lay and technical audiences the likelihood and consequences of complex tectonic activity.
Applications of such techniques as described above have recently led to some new insights about active tectonics. The findings described below, furthermore, indicate some areas of research that are worthy of further attention.
Individual or groups of large prehistoric earthquakes can be clearly identified by geologic means, such as by microstratigraphy and microgeomorphology. The research efforts so directed are now termed paleoseismology. By such methods the long-term patterns and timing of some great earthquakes have been analyzed.
Coherence of signals among gravitational changes, lateral changes, and vertical changes confirm that tectonic changes on a time scale of months or years are real and are amenable to analysis. From such studies the normal dynamic behavior of the Earth’s crust may be determined, and the predictive value of unusual changes can be assessed.
A variety of active-tectonic realms of diverse sizes has been recognized and defined; each realm is characterized by distinctive patterns and rates of deformation. Such analysis provides a basis for zonation to assist in the reduction of geologic hazards.
Folding can be an important active-tectonic process. In the past, folding has been ignored to a large extent in active-tectonic studies even though a rich and voluminous mass of information about folding has been in existence for a long time in the classical tectonic literature. Further analysis will permit evaluation of the strain budget, that is, the distribution of strain, between faulting and folding.
Some well-known physical models such as the diffusion or heat-flow model have been applied successfully to the analysis of geomorphic processes.
Determining the distribution of microseismicity has proven to be one of the best