Hugo Benioff

Orogenesis and deep crustal structure -- additional evidence from seismology

Seismic evidence indicates that the principal orogenic structure responsible for each of the great linear and curvilinear mountain ranges and oceanic trenches is a complex reverse fault. A study of eight regions in which orogenic activity is in progress reveals that these great faults occur in two basic types, here designated oceanic and marginal. Oceanic faults, situated within the oceanic domain, extend from the surface to depths of 550 to 700 km. They exhibit an average dip of 61°. Their elastic strain-rebound characteristics show that these faults are composed of two separate mechanical units—a shallow component extending from the ocean bottom to a depth of roughly 60 km, and a deeper component extending to the 700km crustal boundary. The marginal faults situated along the continental margins occur in dual and triple forms. The dual faults comprise a shallow member extending from the surface to a depth of approximately 60 km and an intermediate member extending to a depth of 200 to 300 km. The average dip is 33°. The marginal triple form is similar to the dual down to the 300 km level. At this depth the dip changes abruptly to 60° to form a third component extending down to the 650± km crustal boundary. The elastic strain-rebound characteristics of the marginal faults indicate that the components of these structures also move as separate units, although in South America the two lower elements exhibit some evidence for mechanical coupling. In the continental domain the 300 km level thus represents a tectonic discontinuity not as yet revealed by seismic wave-propagation studies but which is apparently the lower boundary of the continents. Since the oceanic faults and the deep components of the marginal faults have the same average dip (61°) it may be assumed that both are fractures in a single, continuous mechanical structure subject to a single stress system. The different average dip (33°) of the marginal intermediate fault components suggests that they occur in a structure mechanically distinct from the deep oceanic and continental layer and that they are activated by a different stress system. A hypothesis offered for the origin of the volcanoes associated with the faults assumes that the source of volcanic energy is heat produced in the fault rocks by the inelastic components of the repeated to-and-fro strains involved in the generation of the sequences of earthquakes and aftershocks.

Seismic evidence for the fault origin of oceanic deeps

A method is described for determining the elastic-rebound strain increments associated with earthquakes occurring on a particular fault. For a given sequence of earthquakes, a graph of the accumulated increments so determined plotted against time represents the actual fault movement during the interval covered by the sequence. The method also provides a means for determining whether or not a chosen sequence of earthquakes represents movements of a single fault structure. In this way it becomes possible to discover faults which otherwise may escape detection. Evidence of this kind is offered which indicates that the Tonga-Kermadec and South American sequences of earthquakes originate on great faults which dip under the continental masses. The faults are approximately 2500 km. and 4500 km. in length respectively. Their transverse dimensions are approximately 900 km. each. They both extend to a depth of approximately 650 km.—more than one tenth of the radius of the earth. The oceanic deeps associated with these faults are surface expressions of the downwarping of their oceanic blocks. The upwarping of their continental blocks have produced islands in the Tonga-Kermadec region and the Andes Mountains in South America.

GLOBAL STRAIN ACCUMULATION AND RELEASE AS REVEALED BY GREAT EARTHQUAKES

The strain-rebound characteristic of the sequence of all great shallow earthquakes of magnitudes 8.0 and greater which have occurred since 1904 exhibits a saw-tooth shape with very nearly linear segments. The serration amplitudes and periods have been decreasing linearly with time since the beginning of the sequence. If a proposed interpretation is correct, the characteristic indicates the following conclusions: (1) Earthquakes in this magnitude range are not independent events, but are related in some form of world-wide stress system. (2) From 1908 (and presumably 1904) to 1950 the rate of total secular strain generation in the crustal layer in which these earthquakes originate was remarkably constant. (3) The strain was released in five active periods of decreasing lengths separated by quiescent intervals of very small or no activity during which crustal strain accumulated at a constant rate. (4) During the active periods the strain release proceeded at approximately twice the rate of secular strain generation. The strain-rebound characteristic of the sequence of all great earthquakes of intermediate depth (h = 70-300 km) exhibits no resemblance to that of the shallow sequence. The strain-rebound characteristic of the sequence of all great deep earthquakes (h ≥ 300 km) can be represented by the equation S = a + b log t indicating that activity at this depth has been falling off continuously since 1904. The different behavior of the three sequences is considered evidence for the existence of three layers in the earths crust having different relative movements in the tectonic sense. Heretofore evidence for layering in the earths crust has been derived from studies of the propagation of seismic waves and is thus limited to short-time mechanical properties (less than 5 minutes) only. The world strain-rebound characteristics presented herein provide the first evidence for layering on the basis of secular tectonic mechanical properties. From this point of view, the crust is made up of three distinct layers each with its own proper movement, the shallow 0-70 km, intermediate 70-300 km, and deep 300-680 km approximately.

FUSED-QUARTZ EXTENSOMETER FOR SECULAR, TIDAL, AND SEISMIC STRAINS

A description is given of two fused-quartz extensometers located in mountain tunnels at Dalton Canyon and Isabella in Southern California and designed for observing long-period seismic-wave strains, earth tidal strains, and secular strains. They consist essentially of instruments for measuring and recording variations in the separation of two piers by comparison with a length standard of fused-quartz tubing. The sensitivity for secular strains, denned as the least detectable strain increment, is approximately 10^(−7). For tidal and seismic-wave strains, the sensitivity is higher—a 1-mm deflection of the recorder represents a strain increment of 5.2 × 10^(−10). In both cases the maximum usable sensitivity is limited by ground-strain unrest or noise, generated by wind, barometric-pressure variations, temperature variations of the surface layers of the ground, and variations in ground-water saturation.

Earthquake Seismographs and Associated Instruments

Publisher Summary This chapter focuses on earthquake seismographs and associated instruments. Progress in seismograph instrument development in the past quarter century has been quite rapid. One of the first of the new instruments was the torsion seismograph invented by J. A. Anderson and developed jointly by him and H. O. Wood. Direct-recording pendulum seismographs, having periods greater than about 4 seconds, are severely limited as to their maximum useful magnification because of their high sensitivity to earth tilts in the case of the horizontal-component instruments, and to thermal response of the spring in the case of the vertical-component instruments. It is possible to eliminate these long-period drifts by means of a viscous coupling between the pendulum and the optical or mechanical recording elements. Such an instrument was first described by Arnold Romberg. One of the first effective modifications of the moving-conductor transducer (Galitzin) seismograph was made by B. Gutenberg. He shortened the period of the pendulum to 3 seconds (by tilting the base) and increased the magnetic field strength by decreasing the air gap clearance. This resulted in a frequency-response characteristic with higher magnifications for the short-period waves than the standard Galitzin or any other instrument in use at that time. This chapter discusses other such devices as: the variable reluctance electromagnetic pendulum seismograph, electrostatic transducer pendulum seismographs, carrier-current transducer seismographs, linear strain seismograph, remote recording seismographs, and various components present in a seismograph. This chapter ends with the discussion of various seismograph response characteristics.