Maximum Principal Stress Axis Research Articles

稠密余震観測データを用いて推定した２００４年新潟県中越地震震源域の応力場

We have investigated a stress field in the source region of the 2004 Mid-Niigata Prefecture (Niigata-Chuetsu) earthquake (M=6.8) by using P-wave polarity data obtained by a dense temporal seismic observation. The earthquake took place in central Japan near the eastern border of the Niigata-Kobe Tectonic Zone characterized by E-W contraction. The purpose of this study is to get insight into the stress heterogeneities in the source area including quite complex fault system with two parallel, westerly dipping fault planes and one conjugate plane. About two-thirds of well-constrained 514 focal mechanisms are reverse-fault type with WNW-ESE trending P-axes, which is consistent with the regional stress field in the area. Minor strike-slip events are distributed in the northern and central part of aftershock zone. Ten normal-fault events that are rare in the northeastern Japan are scattered mostly outside the major earthquake faults and occurred in a short period, suggesting that they reflect local stress field. Stress tensors derived from the focal mechanisms exhibit the compressional stress field characterized by nearly horizontal maximum principal stress that is consistent with the average direction of P-axes in the whole aftershock zone. Horizontal rotation of maximum principal stress axis is evident from WNW-ESE in the northeastern part of aftershock zone to E-W in the southwestern part. The direction of intermediate principle stress axis also changes from NE-SW in the northeastern part to NNE-SSW in the southwestern part. The fold axes in the area show similar strike variation. An interesting feature of stress distribution found in this study is the change in dip angle of the maximum principle stress axes. The axes through the aftershock zone dip gently to WNW, but those for earthquake clusters of mainshock fault dip steeply in a similar direction. Judging from the stress ratio, the horizontal compression is dominant near the mainshock hypocenter while the state in the surrounding areas is relatively close to isotropic stress. Though the difference in dip angle may not significant by considering the estimation error of stress axes, the combined anomalies with the stress ratio suggest that the mainshock occurred in an area of local stress inhomogeneity. We further examined the spatial distribution of earthquakes that have inconsistent focal mechanisms with the estimated stress tensors. Such earthquakes are distributed in the northeastern edge of the aftershock zone, in the southern edge of easterly dipping fault plane, and in a shallow earthquake cluster that is located between the faults of mainshock and the largest aftershock. The location of these events indicates small-scale stress heterogeneities in the source area superposed on the larger-scale stress variation mentioned above.

Interseismic crustal deformation in the Taiwan plate boundary zone revealed by GPS observations, seismicity, and earthquake focal mechanisms

We use GPS-derived surface velocities, seismicity, as well as estimates of earthquake focal mechanisms from the time period before the 1999 Chi-Chi earthquake to evaluate spatial variations of surface strain rate and crustal stress regime in the Taiwan plate boundary zone. We estimate strain rates with a new but simple approach that solves for surface velocity on a rectangular grid while accounting for the distance between observations and each grid node and the impact of a spatially variable density of observations. This approach provides stable and interpretable strain-rate estimates. In addition, we perform a stress tensor inversion using earthquake focal mechanisms determined by P waves first-motion polarities. Our estimates of the principal orientations of two-dimensional surface strain rate tensor generally agree with the inferred orientations of the stress axes. This agreement suggests that a large scale variation of stress orientations from the surface to the base of the crust is insignificant and the predicted faulting style is consistent with stress buildup during the interseismic loading. We find that the geometric configuration of the Chinese continental margin alone cannot fully explain the distribution of maximum contraction and compressive axes in Taiwan. Distribution of seismicity and focal mechanisms before and after the Chi-Chi mainshock suggest that the maximum principal stress axis is vertically-oriented in the Central Range; in contrast to the horizontal maximum principal stress axis in western Taiwan and the Longitudinal Valley. Extension in the Central Range reflects the consequence of exhumation and crustal thickening.

Memory function assessment under propofol anaesthesia

Izmir metropolitan area is one of the biggest cities of Turkey in terms of population and industrial density. Amount of recordings and quality of located events increased since the new local seismic array, IzmirNET, was installed in 2008. Present study addresses to elucidate seismotectonics of Izmir vicinity by a new data set and reveal first results of seismological analysis obtained for 4.5 years.930 small earthquakes are well located within less than 1.0 km horizontal error (RMS ≤ 0.15 s) over a total of 1421 recorded events. Data were processed by using SEISAN Earthquake Analysis Software implemented at SeismoLab of Dokuz Eylul University. The Orhanli–Tuzla Fault Zone and Izmir faults are capable of producing a destructive earthquake. Events show clear clusters around the Outer Bay of Izmir Gulf and Sigacik Bay. We also detected some dense clusters Sigacik Bay to the South, and in the outer bay of Izmir Gulf along NW–SE direction.We investigated kinematics of active faults around the Izmir and Foca areas by using 41 focal mechanism solutions from different algorithms. Foca is located 60 km in the NW direction of Izmir. Mechanisms generally resulted in pure normal or dominant normal faulting with minor strike-slip components in agreement with the kinematics of the other fault zones in Western Turkey. A stress tensor study using selected fault plane solution indicates that the study area is under extension regime in an N–S direction with 71° plunge for maximum compressive principal stress axis (S1), and closer to horizontals for intermediate (S2) and minimum compressive principal stress axis (S3).

Application of seismic interferometry to extract P- and S-wave propagation and observation of shear-wave splitting from noise data at Cold Lake, Alberta, Canada

We extract downward-propagating P- and S-waves from industrial noise generated by human and/or machine activity at the surface propagating down a borehole at Cold Lake, Alberta, Canada, and measure shear-wave splitting from these data. The continuous seismic data are recorded at eight sensors along a downhole well during steam injection into a 420–470-m-deep oil reservoir. We crosscorrelate the waveforms observed at the top sensor and other sensors to extract estimates of the direct P- and S-wave components of the Green’s function that account for wave propagation between sensors. Fast high-frequency and slow low-frequency signals propagating vertically from the surface to the bottom are found for the vertical and horizontal components of the wave motion, which are identified with P- and S-waves, respectively. The fastest S-wave polarized in the east-northeast–west-southwest direction is about 1.9% faster than the slowest S-wave polarized in the northwest-southeast direction. The direction of polarization of the fast S-wave is rotated clockwise by [Formula: see text] from the maximum principal stress axis as estimated from the regional stress field. This study demonstrates the useful application of seismic interferometry to field data to determine structural parameters, which are P- and S-wave velocities and a shear-wave-splitting coefficient, with high accuracy.

Estimation of the stress field in the region of the 2000 Western Tottori Earthquake: Using numerous aftershock focal mechanisms

We estimated the regional stress field before and after the 2000 Western Tottori Earthquake (Mw = 6.6) from numerous focal mechanisms. To constrain the stress field prior to the main shock, we compared the spatial distribution of the static stress changes generated by the main shock with the stress field following the main shock. In the northern and central parts of the aftershock region, it is inferred that the deviatoric stress magnitude prior to the main shock was too large to be affected by the static stress changes. The direction of the maximum principal stress axis prior to the main shock is consistent with the tectonic stress field. On the other hand, in the southern part of the aftershock area, in and around the region where the main shock slip was large, it is found that the stress field is spatially heterogeneous after the main shock because of the large magnitude of the static stress changes. Around the southern edge of the fault, the spatial distribution of the static stress changes is consistent with that of the P axis azimuths of the aftershocks, and it is inferred that the deviatoric stress magnitude prior to the main shock was small enough to be affected by the static stress changes (5 MPa). The strength of the preexisting aftershock fault planes in the southern edge might be exceptionally weak. It is found that the stress field prior to the main shock was inhomogeneous on scales smaller than the length of the main shock fault.

Lateral interaction between metamorphic core complexes and less-extended, tilt-block domains: the Alpujarras strike-slip transfer fault zone (Betics, SE Spain)

The Alpujarras area in southeastern Spain exhibits one of the scant documented examples of extension related strike-slip faults bordering core complexes in the world. Faults of the Alpujarras fault zone define a regional-scale complex ENE-striking transfer zone that marks the boundary among the Sierra Nevada elongated dome, a highly-extended, constricted core-complex, and a less-extended domain formed by large-scale tilt-blocks. Detailed mapping and structural analysis show that the Alpujarras fault zone is an integral part of the WSW-directed normal fault systems that thinned the Betic hinterland during the middle Miocene to Recent time. Fault patterns and palaeostress analysis both indicate that dextral movement along the strike-slip faults was induced by a local stress field with a sub-horizontal E–W- to ESE–WNW-trending maximum principal stress axis, which is synchronous with the regional stress field driving the normal fault systems. Palaeostress analysis also indicates subsequent variations in the stress field with a sub-horizontal NW–SE- to N–S-trending maximum principal stress axis, thus producing both the tectonic inversion of the northern fault of the Alpujarras system and shortening of the unloaded extensional detachment footwall. A simplified kinematic model for the tectonic evolution of the Alpujarras area from the middle Miocene to Recent emphasizes the kinematic coupling of normal faults and strike-slip transfer zones in the extensional process.

Formation and deformation of the Derecske Trough, Pannonian Basin: Insights from analog modeling

We present the results of a fault reactivation study that used the analog modeling technique. The modeled tectonic processes represent the formation and subsequent deformation of the Derecske Trough and have implications for the evolution of the entire Pannonian basin system. Structural inversion of former thrusts led to normal faulting during the Miocene and corresponds to the formation, whereas subsequent reactivation of these faults in a strike-slip manner during the late Miocene to Quaternary represents neotectonic deformation of the basin. Modeling results show that reactivation of thrust faults in a transtensional manner can occur in a broad range of dip angles, and of angular differences between the maximum principal stress axis and the strike of the fault. The reactivation pattern of strike-slip zones in sandbox models critically depends on the orientation of the fault system with respect to the confining stress field, on the material property of the sand, and on the presence of a lubricant layer, representing a potential detachment horizon at the base of the models. The shear zones at the border zones of the Derecske pull-apart basin are prone to further reactivation, regarding the present-day stress field in the area. These inferences are confirmed by the recent seismic activity of the area, and point to the key importance of fault reactivation studies during seismic hazard assessment.

From ductile to brittle: Evolution and localization of deformation below a crustal detachment (Tinos, Cyclades, Greece)

The Cycladic Oligo‐Miocene detachment of Tinos island is an example of a flat‐lying extensional shear zone evolving into a low‐angle brittle detachment. A clear continuum of extensional strain from ductile to brittle regime is observed in the footwall. The main brittle structures marking extension are shallow‐ and steeply dipping normal faults associated with subvertical extensional joints and veins. The earliest brittle structures are low‐angle normal faults which commonly superimpose on, and reactivate, earlier (precursory) ductile shear bands, but newly formed low‐angle normal faults could also be observed. Low‐angle normal faults are cut by late steeply dipping normal faults. The inversion of fault slip data collected within, and away from, the main detachment zone shows that the direction of the minimum stress axis is strictly parallel to the NE‐SW stretching lineation and that the maximum principal stress axis remained subvertical during the whole brittle evolution, in agreement with the subvertical attitude of veins throughout the island. The high angle of σ1 to the main detachment suggests that the detachment was weak. This observation, together with the presence of a thick layer of cataclasites below the main detachment and the kinematic continuum from ductile to brittle, leads us to propose a kinematic model for the formation of the detachment. Boudinage at the crustal scale induces formation, near the brittle‐ductile transition, of ductile shear zones near the edges of boudins. Shear zones are progressively exhumed and replaced by shallow‐dipping cataclastic shear zones when they reached the brittle field. Most of the displacement is achieved through cataclastic flow in the upper crust and only the last increment of strain gives rise to the formation of brittle faults. The formation of the low‐angle brittle detachment is thus “prepared” by the ductile shear zone and the cataclasites and favored by the circulation of surface‐derived fluids in the shear zone.

The 2002 Molise earthquake sequence: What can we learn about the tectonics of southern Italy?

On October 31, 2002 a M L = 5.4 earthquake occurred in southern Italy, at the margin between the Apenninic thrust belt (to the west) and the Adriatic plate (to the east). In this area, neither historical event nor seismogenic fault is reported in the literature. In spite of its moderate magnitude, the earthquake caused severe damage in cities close to the epicenter and 27 people, out of a total of 29 casualties, were killed by the collapse of a primary school in S. Giuliano di Puglia. By inverting broadband regional waveforms, we computed moment tensor solutions for 15 events, as small as M L = 3.5 ( M w = 3.7). The obtained focal mechanisms show pure strike-slip geometry, mainly with focal planes oriented to NS (sinistral) and EW (dextral). In several solutions focal planes are rotated counterclockwise, in particular for later events, occurring west of the mainshock. From the relocated aftershock distribution, we found that the mainshock ruptured along an EW plane, and the fault mechanisms of some aftershocks were not consistent with the mainshock fault plane. The observed stress field, resulting from the stress tensor inversion, shows a maximum principal stress axis with an east–west trend (N83°W), whereas the minimum stress direction is almost N–S. Considering both the aftershock distribution and moment tensor solutions, it appears that several pre-existing faults were activated rather than a single planar fault associated with the mainshock. The finite fault analysis shows a very simple slip distribution with a slow rupture velocity of 1.1 km/s, that could explain the occurrence of a second mainshock about 30 h after. Finally, we attempt to interpret how the Molise sequence is related to the normal faulting system to the west (along the Apennines) and the dextral strike-slip Mattinata fault to the east.

Analysis on small-earthquake composite fault plane solutions from 1961 to 1999 in Xinfengjiang reservoir area

To understand the earthquake characteristics in Xinfengjiang (XFJ for short) reservoir area, we collected the small earthquakes occurred in the area from 1961 to 1999. We segmented this 40-year period, parted the research region and calculated the composite fault plane solution of each block, discussed the effect characteristics of stress field of water pressure using Mohr’s stress circle. The final result shows that the main rupture pattern was very different before and after the M=6.1 main shock, changing from strike slip to normal rupture. The maximum principal stress axes of composite fault plane solutions are characterized by synchronous change with water level.

Numerical simulations of deformation and dynamics of horizontally lying slabs

SUMMARY Recent results of seismic tomography have revealed that configurations of subducted slabs are varied; some lie horizontally on the boundary between the upper and the lower mantle, and others penetrate into the lower mantle. In this study, to understand the physical mechanism of deformation and dynamics of subducted slabs in the mantle, a 2-D numerical model was constructed. For this purpose, the slabs were treated so as to move freely in an incompressible fluid model by removing a guide right after its bending at a depth of 660 km and at a depth of 400 km in the upper mantle. Incorporating density and viscosity jumps and a Clapeyron slope for a γ → Pv + Mw phase transformation of olivine obtained from high-pressure and hightemperature experiments at a depth of 660 km into the model, we investigate temperature and velocity distributions, state of stress and viscosity distribution in relation to the formation of horizontally lying slabs at a depth of 660 km. When the slabs are treated to move freely just after their bending at the 660 km phase boundary, the slabs temporarily appear to lie horizontally on the phase boundary. However, the slabs penetrate the phase boundary and eventually sink into the lower mantle. The length of the horizontally lying portion of the slab becomes longer as descending velocity increases; the lengths are about 1500 km, 1200 km, and 800 km for subducting velocities of 8 cm yr −1 , 4 cm yr −1 ,a nd 2c m yr −1 , respectively. When a viscosity jump of 10 to 100 times the viscosity for the upper mantle is given at the 660 km phase boundary, the length of the horizontally lying portion of the slab becomes shorter because of friction between the slab and the underlying lower mantle. For all the models in which the guide was removed at a depth of 400 km, the slabs penetrate into the lower mantle without lying horizontally on the phase boundary. These results indicate that it is difficult for the slabs to bend at the γ → Pv + Mw phase boundary, and to form the configuration of the horizontally lying slabs even if the slabs descend with a small dip angle. The calculated maximum principal compressive stress axes are oriented horizontally within the horizontally lying portion of the slabs, which is inconsistent with seismologically observed vertical directions of the P-axes there. In order to explain the observed configurations and state of stress within the horizontally lying slabs, it would be necessary to investigate the effects of a complicated viscosity and composition structure in the mantle in more detail.

Arc dynamics and tectonic history of Fiji based on stress and kinematic analysis of dikes and faults of the Tavua Volcano, Viti Levu Island, Fiji

Stress analysis of dikes and kinematic analysis of faults within the Tavua Volcano of Viti Levu, Fiji, indicate an apparent migration of the causative maximum principal stress axis (σ1) from NW toward north between ∼5.0 and <3.9 Ma. The dikes and faults are part of successive structural and magmatic events at the arc scale. We present data on the chronological succession of stress indicators in the Tavua Volcano, based on overprinting relationships and published geochronology, which also equate with a counterclockwise rotation of Fiji, assuming an approximately constant N‐S regional compressive stress orientation during this time. The latter interpretation is consistent with paleomagnetism studies that require substantial counterclockwise rotation of the Fiji Platform, since fragmentation of the formerly continuous Outer Melanesian Arc during the latest Miocene to earliest Pliocene. The stress data suggest that the Fiji platform has undergone an ∼50° rotation since ∼5 Ma. Given that rotation apparently stopped at 3 Ma, then particularly fast rotation of ∼25° Myr−1 occurred from ∼5 Ma to 3 Ma. Recent seismicity data indicate the platform is currently subject to N‐S compression. The history of late Miocene and Pliocene events in the region can be related to the consequences arising from collision of an island arc with an oceanic plateau.

Crustal stress field in southern California and its implications for fault mechanics

We present a new, high spatial resolution image of stress orientation in southern California based on the inversion of earthquake focal mechanisms. We use this image to study the mechanics of faulting in the plate boundary region. The stress field contains significant spatial heterogeneity, which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause. Temporal changes in the stress field are also observed, primarily related to major earthquakes. The observed 15° (±10°) rotation of the stress axes due to the 1992M7.3 Landers mainshock implies that the deviatoric stress magnitude in the crust is low, of the order of 10 MPa. This suggests that active faults in southern California are weak. The maximum principal stress axis near the San Andreas Fault is often at ∼50° to the fault strike, indicating that the shear stress on the fault is comparable to the deviatoric stress. The San Andreas in southern California may therefore be a weak fault in a low‐strength crust.

Rheology and seismotectonic regime in the northern central Mediterranean

The connection between thermal field and mechanical properties is analysed in the northern central Mediterranean region, extending from the Ligurian-Provençal basin to the Adriatic foredeep. As the thermal regime is still far from equilibrium in most of the tectonic units, transient thermal models are used. The temperature-depth distribution is estimated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic arc. In the Adriatic foredeep, the thermal effects of the recent thrust-faulting phase in the buried sectors of the northern Apennines are taken into account. The general context consists of convergence involving westward subduction of the Adriatic plate. This process caused anti-clockwise rotation of Corsica and Sardinia, which led to formation of the Ligurian marginal basin, and also resulted in crustal doubling and overthrusting in the northern Apennines and rifting in the northern Tyrrhenian. Seismic activity is focused in the internal and external zones of the Apenninic arc, where low surface heat flux is observed, and in the western margin of the Ligurian-Provençal basin. This is a consequence not only of lateral variations in the thermal field but also of the different tectonic settings. Regional extensional patterns in the shallow crust, with minimum principal stress axes trending N60°E and E-W, are observed in the northern and in the southern sectors of the Apenninic arc, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the arc, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip faults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axis oriented N120°E. Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, rheological and seismic data. There is clear evidence of the presence of the subducting slab of the Adriatic plate, corresponding to a thickening of the uppermost brittle layer. The crustal seismicity cut-off corresponds to temperatures of 320–390°C. A brittle layer of considerable thickness is present in the uppermost mantle beneath Variscan Corsica and the Adriatic foredeep, with estimated seismic cut-off temperature of about 550 ± 50°C.

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.

Deformation and stress states along the central segmentof the Philippine Fault: implications to wrench fault tectonics

This paper discusses the space-time evolution of deformation and states of stress along the central segment of the left-lateral Philippine Fault from Bondoc Peninsula to Leyte Island. This segment is characterized by deformations and stress states which do not always conform to predictions of classical wrench tectonics. Directions of the maximum principal stress axis σ1 are, in some cases, oriented perpendicular to the fault trace, contradicting the classical fault strike σ1 angle of 30–45°. In other cases, it is the minimum axis σ3 that orients perpendicular to the fault or, in other cases, parallel to it. Rotations of σ1 directions as functions of time and space are observed all along the fault. More complex stress regimes are observed in areas where the Philippine Fault intersects with two other major left-lateral strike-slip faults, namely the Legaspi Lineament in the Ragay Gulf area and the Sibuyan Sea Fault in Masbate. These observations strongly underline the urgence of prudence and precaution in applying wrench fault tectonic principles in natural examples.

Neogene evolution of the Sorbas basin (SE Spain) determined by paleostress analysis

The Neogene subsidence history of the Sorbas basin, situated in the Internal Zone of the Betic Cordilleras of southeastern Spain, is strongly controlled by strike-slip deformation. The sedimentary sequence of the basin offers a good control on the timing of separate phases of fault activation. The southern border of the Sorbas basin is formed by a major east-west trending fault zone (the Gafarillos fault zone) which consists of intermixed slivers of pre-Neogene basement blocks and Neogene basin sediments. Fault patterns and paleostress analysis both indicate that major displacement along this fault zone was dextral with a N310E-oriented maximum principal stress axis. Strike-slip movement along the east-west-oriented faults ended during the earliest Messinian after a minimum of 6 Ma of activation. Paleostress analysis indicates a subsequent phase of strike-slip fault movement present in sites of up to Early Pliocene age. The maximum principal stress axis orientation is N350E for this phase. This deformation is synchronous with sinistral displacement along the Palomares fault zone situated directly to the east of the study area. Subsequently, our paleostress analysis indicates an extensional regime during the Late Pliocene to Quaternary. However, extensional tensors were calculated at a number of sites possibly representing local stress regimes coexisting with the main strike-slip deformation and Sorbas basin subsidence. Folding of Tortonian sediments points out that local compressional deformation took place as well during the Neogene basin history. Observed wide ranges for the ellipsoid shapes of the calculated tensors support the existence of lateral stress variations.

Seismicity and stress in the vicinity of Mount Spurr volcano, south central Alaska

Focal mechanism solutions and hypocenters for earthquakes near Mount Spurr volcano in south central Alaska reveal spatial perturbations in the regional stress field and in the maximum depth of seismicity. At the volcano, practically all the shocks during the dormant decade 1981–1991 have depths shallower than 5 km and are characterized by nearly pure normal slip. In contrast, earthquakes located outboard of the volcano mostly concentrate in the depth range 3–18 km and exhibit predominantly strike slip, oblique reverse slip, or reverse slip. The regional stress field is characterized by subhorizontal maximum principal stress directed N30°W, concordant with the direction of convergence between the North American and Pacific plates, whereas the maximum principal stress is probably more nearly vertical beneath the volcano. We suggest that the shoaling of seismicity beneath the volcano reflects localized elevation of the depth to the midcrustal transition from brittle failure to plastic flow. We attribute this localized elevation to relict magmatic heat associated with past volcanic eruptions. The available data are not sufficient to determine the cause of the suggested rotation of the maximum principal stress axis beneath the volcano; possible mechanisms include an increase in the vertical stress from the weight of the Mount Spurr massif and a decrease in the maximum horizontal stress associated with doming of the shallow crust from magmatic processes.

State of stress from focal mechanisms before and after the 1992 landers earthquake sequence

AbstractThe state of stress in the Eastern California Shear Zone (ECSZ) changed significantly because of the occurrence of the 1992 Mw 6.1 Joshua Tree and the MW 7.3 Landers earthquakes. To quantify this change, focal mechanisms from the 1975 Galway Lake sequence, the 1979 Homestead Valley sequence, background seismicity from 1981 to 1991, and the 1992 Landers sequence are inverted for the state of stress. In all cases, the intermediate principal stress axis (S2) remained vertical, and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes (S1 and S3) and small variations in the value of the relative stress magnitudes (ϕ). In general, the stress state in the ECSZ has S1 trending east of north and ϕ = 0.43 to 0.65, suggesting that the ECSZ is a moderate stress refractor and the style of faulting is transtensional.South of the Pinto Mountain fault, in the region of the 1992 Joshua Tree earthquake, the stress state determined from the 1981 to 1991 background seismicity changed on 23 April and again on 28 June 1992. In the central zone, S1 rotated from N14° ± 5°E to N28° ± 5°E on 23 April and back again to N16° ± 5°E on 28 June. Thus, the Landers mainshock in effect recharged some of the shear stress in the region of the Mw 6.1 Joshua Tree earthquke.Comparison of the state of stress before and after 28 June 1992, along the Landers mainshock rupture zone, showed that the mainshock changed the stress orientation. The S1 trend rotated 7° to 20° clockwise and became progressively more fault normal from south to north. Along the Emerson-Camp Rock faults, the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor. The complex distribution of P and T axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock.The San Bernardino Mountains region of the Mw 6.2 Big Bear earthquake has a distinctively different state of stress, as compared to the Landers region, with S1 trending N3° ± 5°W. This region did not show any significant change in the state of stress following the 1992 Mw 6.2 Big Bear sequence.

Orientation Analysis and Formation Ages of Fractures Filled with Clay Minerals (Clay Veins) in Hiroshima and Shimane Prefectures, Southwest Japan.

Fractures filled with clay minerals (clay veins) occur extensively in granitic andrhyolitic rocks throughout Hiroshima and Shimane Prefectures, southwest Japan. The main constituent clay minerals are mica clay mineral, smectite and kaolin mineral. Based on K-Ar dating of mica clay mineral, the formation age of the clay veins is almost the same or slightly predates the emplacement age of the host or surrounding granitic rocks. Therefore, it is suggested that most of the clay veins were formed immediately after solidification of the host or surrounding granitic rocks.The orientations of the clay veins in 13 specific regions in Hiroshima and Shimane Prefectures were systematically analyzed, and 20 K-Ar ages of constituent clay minerals (mainly mica clay mineral) within the veins of each region were determined, i.e. 31-080 Ma.The orientation analysis of fractures suggests that most of the fractures were formed under a regional stress field. The azimuth of maximum principal stress axis appears to have varied at least several times during period from the Cretaceous to Palaeogene in Hiroshima and Shimane Prefectures. The maximum compression stress directions vary from WNW-ESE to NE-SW, E-W, NW-SE or NNE-SSW, E-W and finally N-S or NWSE, over a ca. 50 million year period. Comparing these results with those obtained prebiously from dyke orientations, the azimuthes of maximum compression stress axis are breafly resemble.Thus, orientation analysis of clay veins is a useful technique to determine paleostress fields in granitic terrain.