Tectonic Stress Accumulation Research Articles

Dispersion attenuation structure of the crust around the Weifang-Jiashan segment of the Tanlu fault zone in North China

On the condition that the velocity ratio of compressive wave to shear wave is stable during the propagating process of seismic wave, this paper develops an estimation method for medium quality factor based on single station’s travel time difference between direct S and P waves and the first period signal of direct P wave. 8 774 high SNR wave data altogether recorded by 75 stations are analyzed. The results show that: (1) under the normal regional stress field, the quality factors QmSP in the stable tectonic area are higher than that in the active tectonic area around the Weifang-Jiashan section and its surrounding area in the Tanlu fault zone; (2) in the Juxian-Tancheng section the seismic wave attenuation is relatively quick, and the media is relatively broken, suggesting no tectonic stress accumulation; (3) the Xinyi-Sihong section is currently locking and in accumulating elastic strain energy stage, which has the deep environment and conditions of strong earthquake generation similar to those of the 1668 M8.5 Tancheng strong earthquake.

Structural setting and geochemistry of Devonian dikes in the Kola Peninsula

The structural setting and geochemistry of the Devonian dikes in the Kola Peninsula are considered. The alkaline dikes are controlled by rift- and drift-related structural elements. The first type of structures is exemplified by the Khibiny-Kontozero Fault Zone, which is regarded as a propagation zone of the East Barents Rift. The second type comprises Early Precambrian structural elements reactivated during Devonian drift of the East European Plate. Alkaline dikes occur in the ring structures at the ends of rifts and in the accommodation zones where the polarity of the main fault plane changes. These are the sites of accumulation and abrupt relaxation of tectonic stresses. Despite the significant differences in the petrography and chemistry of alkaline dikes, all of them are enriched in REE, apparently due to the elevated concentrations of these elements in the fluids. The dolerite dikes at the Murmansk Coast are located in the outer zone of the vast, concentrically zoned plateau basalt province with the East Barents Trough as its center. According to geophysical data, basaltic flows are suggested within this trough. The dolerite dikes increase in number in the inland propagation zone of the East Barents Trough extending in the Baltic Shield. The alkaline igneous rocks of the Kola Peninsula were formed under local compression at the periphery of the plateau basalt province.

Ocean loading effects on stress at near shore plate boundary fault systems

Changes in eustatic sea level since the Last Glacial Maximum create a differential load across coastlines globally. The resulting plate bending in response to this load alters the state of stress within the lithosphere within a half flexural wavelength of the coast. We calculate the perturbation to the total stress tensor due to ocean loading in coastal regions. Our stress calculation is fully 3‐D and makes use of a semianalytic model to efficiently calculate stresses within a thick elastic plate overlying a viscoelastic or fluid half‐space. The 3‐D stress perturbation is resolved into normal and shear stresses on plate boundary fault planes of known orientation so that Coulomb stress perturbations can be calculated. In the absence of complete paleoseismic indicators that span the time since the Last Glacial Maximum, we investigate the possibility that the seismic cycle of coastal plate boundary faults was affected by stress perturbations due to the change in sea level. Coulomb stress on onshore transform faults, such as the San Andreas and Alpine faults, is increased by up to 1–1.5 MPa, respectively, promoting failure primarily through a reduction in normal stress. These stress perturbations may perceptibly alter the seismic cycle of major plate boundary faults, but such effects are more likely to be observed on nearby secondary faults with a lower tectonic stress accumulation rate. In the specific instance of rapid sea level rise at the Black Sea, the seismic cycle of the nearby North Anatolian fault was likely significantly advanced.

Tectonomagnetic study in the seismoactive area of Narmada-Son lineament, central India: Preliminary results on repeat field observations

Repeated measurements of the total geomagnetic field on the five profiles have revealed a picture of stress-induced tectonomagnetic effect in the form of secular variation of the total geomagnetic field in the tectonically and seismically active area of Jabalpur and adjoining areas of the Narmada-Son lineament (NSL), central India. For this experiment, a reference base station was established within the study area at Jabalpur. Using proton precession magnetometers with a sensitivity of 0.1 nT, simultaneous measurements of total geomagnetic field were made annually at the base and all field stations. Five cycles of repeated observations have been performed between 2003 and 2007. For data analysis, a difference method has been applied and the residuals have been calculated as secular variations of the total geomagnetic field with values ranging from ±0.1nT to about ±14.6nT/yr over the different stations. The anomalies in secular variation of the total geomagnetic field may be related to anomalous accumulation of tectonic stresses and tensions on the deep fault zones and crustal blocks due to recent geodynamic processes and active geological inhomogeneities in the NSL.

Interplate seismogenic zones along the Kuril–Japan trench inferred from GPS data inversion

Stress accumulation between earthquakes results from slip that is insufficient to fully accommodate plate movement. An inverse analysis of GPS data from the Kuril–Japan trench reveals a trench-parallel belt of stress accumulation with six peaks in the depth range of 10–40 km, suggesting potential source regions for future earthquakes. In the subduction zones around Japan, where four plates interact with one another, large earthquakes have occurred repeatedly1. These interplate earthquakes are part of the process of tectonic stress accumulation and release that is driven by relative plate motion2,3,4. Stress accumulation between earthquakes results from slip deficit (slip that is insufficient to fully accommodate plate movement). For the prediction of large earthquakes, it is therefore important to monitor the distribution of slip deficit on plate interfaces. Here we apply an inversion method based on Bayesian modelling (using direct and indirect prior information on the magnitude and distribution of fault slip5) to horizontal and vertical velocities from global positioning system data. For the seismically calm period between 1996 and 2000, we obtain a precise distribution of slip-deficit rates on the interface between the North American and Pacific plates around Japan, which reveals a trench-parallel belt of slip deficit with six peaks in the depth range of 10–40 km. These peaks agree with the source regions of past large interplate earthquakes along the Kuril–Japan trench. We conclude that the slip-deficit zones identified with our method are potential source regions of large earthquakes.

The Athens, Greece, Earthquake (Ms 5.9) of 7 September 1999: An Event Triggered by the Izmit, Turkey, 17 August 1999 Earthquake?

The Athens, Greece, earthquake ( M w 5.9) of 7 September 1999 occurred in a region of low seismicity at a distance of ca. 620 km from the epicenter of and only 21 days after the large ( M w 7.5) Izmit, Marmara Sea, earthquake of 17 August 1999. The possibility that the Izmit event triggered the Athens one is examined on the basis of three independent approaches. First, a stochastic test was used, showing that the probability for a strong shock to occur randomly in any 21-day time interval (1) in the region of Athens (radius ∼30 km) and (2) in the broad region of Athens (radius ∼70 km) is of the order of only 5.75 × 10-5 and 2.3 × 10-3, respectively. Second, a model that considers an earthquake triggered by another earthquake as an instability to transient slip was used to calculate the distant shear stress increase at the Athens earthquake fault as a result of the superimposed unrelaxed (coseismic) and relaxed stress change caused by the Izmit earthquake because of a deep aseismic viscous slip. The total stress accumulation within about 1 month of the Izmit earthquake was found to be of the order of 4.2 × 10-3 bars, which is rather significant in that it nearly doubles the yearly mean rate of tectonic stress accumulation in the region. Finally, a time-to-failure analysis showed that the process of small (4.0 ≥ M L ≥ 2.0) earthquake generation, at a distance of about one source dimension (∼30 km) from the Athens earthquake epicenter, started accelerating very slowly from the beginning of 1994. However, only immediately after the Izmit earthquake, the process culminated with short-term foreshock activity. The independent results shown previously favor the conclusion that the Athens earthquake was likely advanced in time possibly because of the stress changes generated by the Izmit large shock. Manuscript received 28 July 2000.

地震発生における地球潮汐の影響 数値シミュレーションによるアプローチ

We conducted a numerical simulation to clarify the effects of the earth tide on earthquake occurrence. In the simulation, fault planes, having different initial stresses, are loaded by constantly increasing tectonic stress and cyclic stress due to the earth tide. Earthquakes are assumed to occur when the shear stress reached a certain threshold level. The result of the simulation indicates that the tidal effect on earthquake occurrence is strongly controlled by the ratio of stress change rate between earth tide and tectonic stress accumulation; earthquakes concentrate near phase angle 0° (maximum tidal stress) when the ratio is large, and they are distributed with a peak around-90° (maximum acceleration of tidal stress) when the ratio is small. This phase selectivity is very similar to the observations of tidetriggered earthquakes reported so far, and rock failure experiments under cyclic loading, suggesting that our approach may provide an important clue for clarifying the physical mechanism of tidal triggerings of earthquakes. Schuster's test has been widely used for detecting tidal effect on earthquake occurrence. However, it is shown by the numerical simulation that the result of a test strongly depends on the size of a data set when earthquakes have phase selectivity by nature, and that Schuster's p-value is not appropriate to represent the strength of tidal effects. Alternatively, we propose to use α, which is the amplitude of a sine curve fitted to the frequency distribution of earthquakes against phase angle, to evaluate the strength of tidal effects. We also emphasize that the effect of ocean loading is an important component of the earth tide, and cannot be neglected in a study of tidal triggerings of earthquakes.

Absence of earthquake correlation with Earth tides: An indication of high preseismic fault stress rate

Because the rate of stress change from the Earth tides exceeds that from tectonic stress accumulation, tidal triggering of earthquakes would be expected if the final hours of loading of the fault were at the tectonic rate and if rupture began soon after the achievement of a critical stress level. We analyze the tidal stresses and stress rates on the fault planes and at the times of 13,042 earthquakes which are so close to the San Andreas and Calaveras faults in California that we may take the fault plane to be known. We find that the stresses and stress rates from Earth tides at the times of earthquakes are distributed in the same way as tidal stresses and stress rates at random times. While the rate of earthquakes when the tidal stress promotes failure is 2% higher than when the stress does not, this difference in rate is not statistically significant. This lack of tidal triggering implies that preseismic stress rates in the nucleation zones of earthquakes are at least 0.15 bar/h just preceding seismic failure, much above the long‐term tectonic stress rate of 10−4 bar/h.

フィリピン海プレート北端部の沈み込み・衝突の3次元FEMモデル

The northernmost part of the Philippine Sea plate is thought to be subducting beneath the continental plate from the Sagami and Suruga troughs, and to be colliding with the continental plate in the northern end of the Izu peninsula. Four major interplate earthquakes (i. e., the 1703 Genroku, 1707 Hoei, 1854 Ansei-Tokai and 1923 Kanto earthquakes) have occurred along the Sagami and Suruga troughs since the beginning of the 18th century. The stress field observed in and around this subduction and collision zone indicates several features; radial pattern of the maximum principal stress axes and concentric pattern of the minimum principal stress axes. The model of the seismotectonics for the last two centuries is investigated on the basis of numerical simulations of the stress field by applying a 3-dimensional finite element method.The tectonic stress accumulation and release processes are incorporated by assuming that (1) the Philippine Sea plate is moving steadily against the continental plate at a velocity of 4 cm/yr in a direction of N30°W, and (2) the earthquake and the steady-state faulting occur across the sub-surfaces of the plate boundary surface between the Philippine Sea and continental plates. The distribution of elastic shear coupling strengths on the sub-surfaces, which determines the amount of faulting, is estimated as follows: the sub-surfaces along the Sagami and Suruga troughs are relatively weakly coupled, compared with the northwestern part of the plate boundary surface. The present seismotectonic model can approximately explain the features of the observed stress field. It can be explained that the characteristic distribution of horizontal tension axes is caused by the bending of the Philippine Sea plate associated with the subduction along the Sagami and Suruga troughs.

Interplate coupling in the Kanto district, central Japan, deduced from geodetic data inversion and its tectonic implications

Interplate coupling between the Philippine Sea (PHS), the North American (NAM), and the Pacific (PAC) plates in the Kanto district, central Japan, has been investigated through the inversion analysis of geodetic data using Akaike's Bayesian Information Criterion (ABIC). The data used for the analysis are annual rates of level changes (1972–1985) and horizontal length changes (1973–1990), which presumably represent average crustal movements during the interseismic period. Disregarding effects of steady plate subduction, we may ascribe the observed crustal movements to the effects of locking or faulting on some parts of the plate boundaries. In our model, the effect of locking is represented by back slip on the plate boundary, and that of faulting by forward slip. The results of the inversion analysis show the existence of a strongly coupled area on the southwestern, shallower part of the NAM-PHS plate boundary, which almost coincides with the faulting area of the 1923 Kanto earthquake (M 7.9), and a notable faulting area on the northeastern, deeper part of the PHS-PAC plate boundary. In the southwestern strongly coupled area, the rate of back slip reaches 3.5 cm/yr, and its direction is oriented N33°W, which is almost opposite to the direction of fault slip (S29°E) at the time of the 1923 event. This suggests that tectonic stress accumulation for the next large event is effectively proceeding there, and its recurrence time is roughly estimated as 245 years. The strongly coupled region is also recognized as corresponding to a recent seismic gap. The inverted forward-slip distribution on the PHS-PAC plate boundary, which has the average rate of 1.9 cm/yr and the average direction of N56°E, may be explained by aseismic northeastward slip in relation to the process to form a peculiar configuration of downward bend of the PHS plate beneath the Kanto district.

Rupture nucleation on unfavorably oriented faults

Abstract For typical values of rock friction, active faults may be defined as favorably oriented for frictional reactivation, unfavorably oriented , or severely misoriented , depending on their attitude in the prevailing stress field. Continued reactivation of unfavorably oriented faults requires fluid pressures ( P f ) elevated well above hydrostatic values to prevent the formation of new, more favorably oriented faults. In cases of severe misorientation , a necessary prefailure condition is that the least principal stress becomes effectively tensile, that is P f > σ 3 . As a consequence, low-stress hydrofracture dilatancy is likely to develop in regions of rupture nucleation on such faults prior to failure. Triggering of shear failure through the buildup of fluid pressure to meet this specific condition then becomes probable. Steep reverse faults require slightly supralithostatic fluid pressures for continued reactivation. Vein systems associated with ancient high-angle reverse faults demonstrate that this prefailure condition was met repeatedly. Failure episodes on such faults involve fault-valve action, where transient postseismic rupture permeability promotes upward fluid discharge from overpressured portions of the crust, followed by self-sealing and reaccumulation of fluid pressure. In compressional regimes, where steep reverse faults and severely misoriented strike-slip faults are currently active, there is direct evidence in several instances for fluid pressure levels approaching lithostatic values, and rupturing is occasionally followed by postseismic discharge. Where fault-valving occurs, recurrence intervals depend on strength reductions from increasing fluid pressure as well as on the rate of tectonic stress accumulation, and may be highly irregular.

Stress and subsidence resulting from subsurface fluid withdrawal in the epicentral region of the 1983 Coalinga earthquake : J Geophys ResV90, NB8, 10 July 1985, P6801–6816

The proximity of the May 2, 1983, Coalinga earthquake to active oil fields on Anticline Ridge led to speculation that the earthquake might have been triggered by oil field operations. Elsewhere, earthquakes have been associated with pore pressure increases resulting from fluid injection and also with subsidence resulting from fluid extraction. Simple calculations show that shales, which underlie the oil producing strata, hydraulically isolate the oil field from the earthquake focal region. The large volumes of fluid extracted from the oil fields caused a 50% decline in reservoir pressures from 1938 to 1983. These observations independently rule out substantial increases in pore pressure at focal depths due to fluid injection. A theoretical method, based on Biot's constitutive theory for fluid-infiltrated elastic media, is used to evaluate the change in stresses acting in the focal region resulting from fluid extraction in the overlying oil fields. As an independent check on the method, the subsidence of the earth's surface in response to fluid withdrawal is calculated and compared with measured elevation changes of Anticline Ridge. The producing horizons are taken to be horizontal permeable layers, bounded above and below by impermeable horizons. Strains within the producing layers are related to extraction-induced changes in pore fluid mass. Contraction of the producing layers causes the free surface to subside and strains the elastic surroundings. The calculated subsidence rate of Anticline Ridge between 1933 and 1972 is 3 mm/yr, in good agreement with the measured subsidence rate of 3.3±0.7 mm/yr. Calculated pore pressure changes in the deepest producing zone also compare well with observed changes in reservoir pressure. Although the shear stresses induced by extraction favor reverse slip on either the northeast or southwest dipping nodal plane, the induced normal stresses are compressive, inhibiting fault slip. The driving stress (shear stress minus frictional resistance) acting across the northeast dipping plane increased by 0.01 MPa (0.1 bar) between 4 and 9 km depth, weakly favoring slip, and decreased by half that amount at depths of 9 to 11 km, weakly inhibiting slip. The driving stress on the southwest dipping plane increased by 0.02 MPa (0.2 bar) at 10 km, slightly favoring slip. The sign and magnitude of the pore pressure and stress changes at hypocentral depths do not support the hypothesis that the earthquake was induced, although knowledge of the rate of tectonic stress accumulation is required to assess properly the significance of the extraction-induced stresses.

Stress and subsidence resulting from subsurface fluid withdrawal in the epicentral region of the 1983 Coalinga Earthquake

The proximity of the May 2, 1983, Coalinga earthquake to active oil fields on Anticline Ridge led to speculation that the earthquake might have been triggered by oil field operations. Elsewhere, earthquakes have been associated with pore pressure increases resulting from fluid injection and also with subsidence resulting from fluid extraction. Simple calculations show that shales, which underlie the oil producing strata, hydraulically isolate the oil field from the earthquake focal region. The large volumes of fluid extracted from the oil fields caused a 50% decline in reservoir pressures from 1938 to 1983. These observations independently rule out substantial increases in pore pressure at focal depths due to fluid injection. A theoretical method, based on Biot's constitutive theory for fluid‐infiltrated elastic media, is used to evaluate the change in stresses acting in the focal region resulting from fluid extraction in the overlying oil fields. As an independent check on the method, the subsidence of the earth's surface in response to fluid withdrawal is calculated and compared with measured elevation changes of Anticline Ridge. The producing horizons are taken to be horizontal permeable layers, bounded above and below by impermeable horizons. Strains within the producing layers are related to extraction‐induced changes in pore fluid mass. Contraction of the producing layers causes the free surface to subside and strains the elastic surroundings. The calculated subsidence rate of Anticline Ridge between 1933 and 1972 is 3 mm/yr, in good agreement with the measured subsidence rate of 3.3±0.7 mm/yr. Calculated pore pressure changes in the deepest producing zone also compare well with observed changes in reservoir pressure. Although the shear stresses induced by extraction favor reverse slip on either the northeast or southwest dipping nodal plane, the induced normal stresses are compressive, inhibiting fault slip. The driving stress (shear stress minus frictional resistance) acting across the northeast dipping plane increased by 0.01 MPa (0.1 bar) between 4 and 9 km depth, weakly favoring slip, and decreased by half that amount at depths of 9 to 11 km, weakly inhibiting slip. The driving stress on the southwest dipping plane increased by 0.02 MPa (0.2 bar) at 10 km, slightly favoring slip. The sign and magnitude of the pore pressure and stress changes at hypocentral depths do not support the hypothesis that the earthquake was induced, although knowledge of the rate of tectonic stress accumulation is required to assess properly the significance of the extraction‐induced stresses.

Seismicity in the Tsengwen reservoir area, Taiwan

abstractThe Tsengwen reservoir, with a maximum depth of 128 m, and a storage volume of 708 × 106 m3, is located over an active thrust fault, the Chuko Fault. The Chuko Fault was evidently the causative fault of a magnitude 634 (Pasadena, MS earthquake in 1964. Filling of the reservoir started in April 1973 and water reached the designed level in September of that same year; since then, the water level has undergone yearly cycles with 40 to 50 m amplitude.An earthquake swarm occurred near the dam in December 1972, before the filling of the reservoir, and microearthquakes in the area have been monitored for various periods since that time. Before filling, there were some very shallow events, with depths less than 2.5 km, but these disappeared shortly after the water level rose to the maximum. The majority of hypocenters after reservoir loading lie in a layer between depths of 2.5 and 8 km; the seismicity under the reservoir is noticeably lower than that in the surrounding area. There is no obvious correlation of seismicity with water level, based on available data. The seismicity in the Tsengwen area can be described as a response of the over-pressured and fractured sedimentary strata to the tectonic stress accumulation, modified by the loading effects of the reservoir.A ts/tp versus time study revealed anomalies, but these are not precursory to large earthquakes, as the duration of the anomalies would imply.