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Active Tectonics: Studies in Geophysics
A strategy for geodetic monitoring of active tectonics was concisely presented in Geodetic Monitoring of Tectonic Deformation—Toward a Strategy (1981), and a few highlights of that study serve to indicate the suggested types of problems, monitoring techniques, and evaluation procedures.
Questions exist, for example, about the relationship between the Chandler wobble of the Earth and earthquakes and whether major earthquakes cause a discernible change in the polar path. Up to a point, classical astronomical observations can be used to study such problems. However, improved geodetic space techniques such as very-long-baseline radio interferometry and satellite laser ranging are rapidly advancing our understanding of plate motions; such observational techniques are beginning to allow the direct measurement of present-day plate motion. Such studies can cast light on the rheological properties of the asthenosphere, its viscosity, and whether gross strain is rather more impulsive than continuous. Regional strain measurements are currently made by laser-ranging techniques in which measurements are infrequent. By employing two-color laser-ranging techniques, corrections for atmospheric conditions are made automatically in data processing and very frequent observations are practical. Small trilateration nets, level lines, and stretched-wire creepmeters spanning only a few tens of meters are used for studying localized deformation along faults. Tiltmeters, linear strain meters, and volumetric strain meters are among instruments currently used to study tectonic strain. The precision required is of the order of a few parts in ten million, but some measurements of the order of a few parts in a billion or smaller are desirable. Most instruments do not have, or barely have, such capabilities. Noise near the Earth’s surface, furthermore, exacerbates the problem of recognizing tectonic signals of such small size. Long-term stability of instruments is a special problem.
The historical record prior to modern instrumentation can provide important insight into longer-term rates and patterns of volcanism, earthquakes, and crustal deformation. In an area of active uplift in Iran, for example, a canal built 1700 yr ago has cut down about 5 m below its original bed, indicating an uplift rate of about 2 mm/yr. In western North American, the historical record of earthquakes is not even 200 years long, but in China it is almost 4000 yr long, although reasonably complete for only about 1000 yr. Similarly there is a historical record of volcanism that is useful as a measure of the frequency and characteristics of eruptions.
Because of low rates of tectonic strain accumulation in many places, the periods of time represented even by recorded history do not adequately sample longer-term trends and cycles. The historically most inactive sections of great faults, indeed, may be candidates for generating the largest future earthquakes. Geologic techniques provide the only means to sample sufficiently long time spans during which many rates and patterns of tectonic processes must be analyzed.
The stratigraphic record contains clear physical evidence of past events. In Japan, for example, bountiful evidence of complex volcanism is preserved by layer upon layer of volcanic ash, breccias, and lava flows. Furthermore, abundant tephra layers—each of which has a distinctive chemical and mineralogic characteristic and was deposited within a few days or weeks—over large areas provide distinctive time markers that permit many local and regional tectonic events to be analyzed in the context of a time frame.
Stratigraphic relations along active faults may reveal a sequence of successive offsets and deposition of sediments that permit the reconstruction of the history of faulting, the recurrence intervals, and size of faulting events. Sedimentary structures such as sand blows, clastic dikes, and deformed beds related to liquefaction may reveal the history of strong ground shaking and add to the evidence of prehistoric earthquakes. Repeated deposition of colluvial wedges along the bases of fault scarps creates unique stratigraphic relations from which an interpretation of the paleoseismological record can be derived.