Periods Of Seismic Activity Research Articles

The study of recent crustal movements from extensometric data

Extensometric observations provide valuable data on recent crustal movements because of the high sensitivity of geophysical instruments and their ability to produce observation series continuous in time. The problem is whether the tectonic component of the movement can be determined from extensometric data. In the present paper an attempt is made to identify tectonic deformations on the basis of observation series of a 20-year period conducted at one of the extensometric stations (Talgar). Preparatory studies are made to find the nature of seasonal variations in the deformation time changes. The correlation of these variations with changes in the amount of atmospheric precipitation, and also the general periodicity in the deformation curve and in the atmospheric precipitation curve, enable us to eliminate the deformation caused by atmospheric precipitation from the deformation curve. The residual deformation curve reveals variations coinciding in time with the periods of higher seismic activity in the region of observations. Harmonics are derived from the deformation curve and their periods coincide with the periods of recurrent local earthquakes of energetic class 13–14.

Stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle

An elementary mechanical model of a coupled subduction zone is presented. It is used to study the transfer of extensional stresses along the underthrusting plate, and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate, during the earthquake cycle. The model assumes an elastic plate which deforms one dimensionally, has shear drag interactions (treated approximately by the Elsasser procedure) with a Maxwellian viscoelastic mantle, and is subject to steady gravitational sinking forces. Great thrust earthquakes are simulated as sudden stress‐relieving displacements at the contact zone between the slab and overriding plate. The model allows analysis of the space‐ and time‐variable stresses in the main thrust zone, the adjacent oceanic plate, and the subducting slab over each earthquake cycle. Stress variations leading to outer‐rise events are discussed, as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone, based on simulations of periodic great earthquakes. The results of simulated slow progression of deformation (slip) from below into the thrust contact zone are also presented. Such preslip, occurring over a time scale that is small in comparison with the whole seismic cycle (e.g., of the order of a tenth or so), is shown to transiently slow the rate of buildup of, or even relieve, extensional stresses in the slab. In the absence of preslip those stresses build up steadily in time during the latter part of the cycle. The results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle. Correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world. The observations include tensional outer‐rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle, normal (tensional) intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones, and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab.

The study of recent crustal movements from extensometric data

Abstract Extensometric observations provide valuable data on recent crustal movements because of the high sensitivity of geophysical instruments and their ability to produce observation series continuous in time. The problem is whether the tectonic component of the movement can be determined from extensometric data. In the present paper an attempt is made to identify tectonic deformations on the basis of observation series of a 20-year period conducted at one of the extensometric stations (Talgar). Preparatory studies are made to find the nature of seasonal variations in the deformation time changes. The correlation of these variations with changes in the amount of atmospheric precipitation, and also the general periodicity in the deformation curve and in the atmospheric precipitation curve, enable us to eliminate the deformation caused by atmospheric precipitation from the deformation curve. The residual deformation curve reveals variations coinciding in time with the periods of higher seismic activity in the region of observations. Harmonics are derived from the deformation curve and their periods coincide with the periods of recurrent local earthquakes of energetic class 13–14.

Geochemical characteristics of underground fluids in some active fault zones in China

This paper presents a study of tectonic activity in China by means of fluid‐geochemical methods. It is observed that deep fluids with characteristic chemical and isotopic compositions (high values of He and H2 and very low value of tritium) have migrated upward at some active faults. Many fluid‐geochemical anomalies were observed during periods of seismic activity, suggesting the usefulness of fluid‐geochemical methods for earthquake prediction. To properly select monitoring springs (or wells) and chemical components, local geological conditions have to be fully considered.

Large‐volume rhyolite ash flow eruptions and submarine caldera collapse in the Lower Mesozoic Sierra Nevada, California

The Mineral King roof pendant in the southern Sierra Nevada, California, provides a view of a Triassic and early Jurassic submarine caldera complex which was tilted to vertical and intruded by middle Cretaceous granitoids. An estimated 90 km3 of rhyolitic and 10 km3 of andesitic volcanic rocks are interstratified with marine sandstones, siltstones, limestones, and conglomerates. Periods of volcanotectonic activity alternated with periods of marine sedimentation, Volcanic eruptions at Mineral King resulted in (1) deposition of widespread, very thick sheets of rhyolite ash flow tuff and (2) construction of small (1–8 km3) andesitic stratocones. Growth faults, fault talus aprons, slide blocks, slumps, and debris flows found in association with these volcanic deposits indicate that the eruptive phases represent periods of great seismic activity. Interuptive phases were periods of volcanic and tectonic quiescence during which sequences of shallow to deep marine sediments were deposited. Four different times at Mineral King, eruption of large volumes of rhyolite ash flow tuff resulted in catastrophic caldera collapse and ponding of single ash flow tuffs to thicknesses greater than 0.5 km. Only minimum volumes can be estimated for three of these intracaldera tuffs because they are truncated by younger granites. Volumes are estimated to be at least 25 km3 each, placing these tuffs in the lower size range of epicontinental ring structures. The fourth caldera, the Vandever Mountain caldera, is entirely preserved in the roof pendant. The vertically dipping strata of the Vandever Mountain caldera provide an unusual opportunity to examine a submarine caldera in cross section. Evidence for caldera subsidence during the eruption of the Vandever Mountain rhyolite ash flow tuff includes the following: (1) the great thickness of the ash flow tuff and the rapid accumulation of the tuff during progressive draw‐down of a compositionally zoned magma chamber, (2) high‐angle faults which pond the ash flow tuff and offset underlying but not overlying stratigraphy, and (3) megabreccia and slide blocks up to 0.5 km long that were derived from the caldera wall and floor and incorporated into the ash flow tuff as it filled the caldera. Disruption of the caldera floor was intense near the caldera margin but negligible along most of its length, owing to pistonlike subsidence of an intact cylinder of crust. Small‐volume ash flow eruptions occurred along an incipient, “leaky” ring fracture zone prior to the caldera‐forming eruption of the Vandever Mountain ash flow tuff. The Vandever Mountain caldera was not affected by resurgent doming or late stage rhyolite magmatism in the cross‐sectional view provided by the Mineral King roof pendant.

The North Anatolian transform fault: its age, offset and tectonic significance

The North Anatolian transform fault is a morphologically distinct and seismically active strike-slip fault which extends for about 1200 km from Karliova to the Gulf of Saros along the Black Sea mountains of N. Anatolia. It takes up the relative motion between the Black Sea and the Anatolian plates, thereby connecting the E. Anatolian convergent zone with the Hellenic Trench through the complex plate boundary zone of the Aegean. For most of its length, the transform has a typical strike-slip fault zone morphology, characterized by narrow ‘rift zone,’ offset, captured and dammed streams, sag ponds and other deformed morphological features. The fault zone is a broad region of extensively crushed country rock cut by a number of parallel and/or anastomosing strike-slip faults. The transform has periods of seismic activity the last of which, from 1939 to the present, is characterized by frequent 6 ≤ M≤7earthquakes; these are separated by quiet periods of about 150 years. The crust along the fault zone is thinner than normal. The transform probably originated some time between the Burdigalian and the Pliocene and has an offset of about 85 km. Whether the offset of the fault changes systematically along its strike is not known. The North Anatolian transform fault seems to have originated as a consequence of the Arabia-Anatolia collision durmg the late (?middle) Miocene, when the Anatolian Plate originated and was wedged out into the oceanic tract of the E. Mediterranean from the converging jaws of Arabia and Eurasia to prevent excessive crustal thickening in E. Anatolia. The westerly motion of Anatolia with respect to Eurasia and Africa caused a great change in the tectonic evolution of the eastern Mediterranean, giving rise to the Aegean extensional regime and to internal deformation of Anatolia.

SOME PROBLEMS IN THE PREDICTION OF THE NEMURO-OKI EARTHQUAKE

The latest results conceming the prediction of the June 17, 1973, Nemuro-oki earthquake (magnitude 7.4), off the Pacific coast of Hokkaido are discussed with special emphasis on the mode of strain release. This earthquake filled in the seismic gap that had been cited as a likely place for a future earthquake, but the preseismic land subsidence extending over the Eastern Hokkaido was not recovered. These facts suggest that the seismic slip at the plate boundary was deficent. Presently available evidence indicates that such a slip deficiency is unlikely to be taken up in the near future, by either aseismic faulting or occurrence of a great shock. More likely case proposed here is that much of the stored strain energy may remain unreleased during the interseismic quiescent period of seismic activity. In this case, it is suggested that the area of faulting and amount of slip differs greatly each time inter-plate earthquakes recur in at least a certain area, in view of the idea that plate motions at the subduction zone are not regular in size on a time scale of several hundreds of years or more.