Crustal Stress Field Research Articles

Microseismicity and stress in the vicinity of the Alpine Fault, central Southern Alps, New Zealand

We investigate present‐day microseismicity associated with the central Alpine Fault and the zone of active deformation and uplift in the central Southern Alps. Using 14 months of data, robust hypocenter locations have been obtained for ∼1800 earthquakes of magnitudes between −0.3 and 4.2. We derived a magnitude scale with a frequency‐dependent attenuation factor, γ(f) = γ0f, where γ0 = 1.89 ± 0.02 × 10−3 s/km, that enables magnitudes to be calculated consistently for earthquakes of different sizes and frequency contents. The maximum depth of the seismicity varies systematically with distance from the Alpine Fault, from 10 ± 2 km near the fault to 8 ± 2 km within 20 km and 15 ± 2 km further southeast. This distribution correlates with lateral variations in crustal resistivity: earthquake hypocenters are concentrated in areas of strong resistivity gradients and restricted to depths of resistivities >100 Ωm. Rocks at greater depth are too hot, too fluid‐saturated, or too weak to produce detectable earthquakes. Focal mechanism solutions computed for 211 earthquakes (ML > 0.44) exhibit predominantly strike‐slip mechanisms. We obtain a maximum horizontal compressive stress direction of 115 ± 10° from focal mechanism inversion. This azimuth is consistent with findings from elsewhere in the central and northern South Island, and indicates a uniform crustal stress field despite pronounced variations in crustal structure and topographic relief. Our stress estimates suggest that the Alpine Fault (with a mean strike of 055°) is poorly oriented in an Andersonian sense but that individual thrust and strike‐slip segments of the fault's surface trace have close to optimal orientations.

Geomagnetic variations possibly associated with the Pisco earthquake on 15 August 2007, Peru

On 15 August 2007, Pisco earthquake (magnitude 8.0) hit the central coast of Peru near the MAGDAS Ancon (ANC) station. Geomagnetic data from ANC and other reference stations have been analyzed to detect any signature related to this great earthquake. Results indicate the presence of annual geomagnetic variations in the vertical component at ANC and Huancayo (HUA) stations (in the vicinity of the epicenter of Pisco earthquake). These variations have a quasi-sinusoidal waveform with amplitudes of about 10 and 5nT for ANC and HUA stations respectively. They appeared clearly during the period preceding the onset of the Pisco earthquake especially at ANC station. By using HUA, Eusebio (EUS) and Kourou (KOU) as reference stations in the vicinity and away from the epicenter of Pisco earthquake, a clear disappearance of the diurnal variation of the vertical component was observed at ANC station during the day of earthquake. Moreover, the Pisco earthquake and another earthquake (on 29 March 2008) near ANC station were found to occur concurrently with the depressions in the polarization ratio (Z/H) of Pc 3 (10–45s) amplitude. Such anomalous variations appear to be a result of changes in the crustal stress field and the lithospheric conductivity in the studied region.

Splitting parameter yield (SPY): A program for semiautomatic analysis of shear-wave splitting

SPY is a Matlab algorithm that analyzes seismic waveforms in a semiautomatic way, providing estimates of the two observables of the anisotropy: the shear-wave splitting parameters. We chose to exploit those computational processes that require less intervention by the user, gaining objectivity and reliability as a result. The algorithm joins the covariance matrix and the cross-correlation techniques, and all the computation steps are interspersed by several automatic checks intended to verify the reliability of the yields. The resulting semiautomation generates two new advantages in the field of anisotropy studies: handling a huge amount of data at the same time, and comparing different yields. From this perspective, SPY has been developed in the Matlab environment, which is widespread, versatile, and user-friendly. Our intention is to provide the scientific community with a new monitoring tool for tracking the temporal variations of the crustal stress field.

Stress field in the 2008 Iwate-Miyagi earthquake (M7.2) area

We determined the focal mechanism solutions (FMS) of 191 crustal earthquakes as well as the stress tensor in the source area of the 2008 Iwate-Miyagi earthquake (2008 IMEQ, M7.2) that occurred in the central portion of northeast (NE) Japan. The FMS and the stress tensors were determined by using both 1-D and 3-D velocity models, which exhibit almost the same results. The differences caused by the use of 1-D and 3-D models can be neglected when compared with the differences due to the different methods, which indicates that the FMS and the stress tensor determined with a 1-D model are accurate enough to study the crustal stress field in the study region. The obtained P axis (σ1) trends WNW-ESE subhorizontally, and the T axis (σ3) is oriented subvertically in a NNE-SSW belt perpendicular to σ1. The σ1 orientation is consistent with the motion of the Pacific plate relative to NE Japan, which indicates that the plate boundary forces dominate the intraplate stress regime. Both temporal and spatial variations of the stress field in the IMEQ source area are detected, which may be induced by the stress rotation accompanying the main shock and its aftershocks. The seismogenic faults in the study area are estimated to be very weak, which argues against the concept of strong crust. The faults may be weakened by the high-temperature magma and the fluids in the lower crust and uppermost mantle that intrude upward into the shallower crust.

An experimental study of fracture initiation mechanisms during hydraulic fracturing

The mechanism of fracture initiation is the basic issue for hydraulic fracture technology. Because of the huge differences in fracture initiation mechanisms for different reservoirs, some successful fracturing techniques applied to porosity reservoirs are ineffectual for fractured reservoirs. Laboratory tests using a process simulation device were performed to confirm the characteristics of fracture initiation and propagation in different reservoirs. The influences of crustal stress field, confining pressure, and natural fractures on the fracture initiation and propagation are discussed. Experimental results demonstrate that stress concentration around the hole would significantly increase the fracture pressure of the rock. At the same time, natural fractures in the borehole wall would eliminate the stress concentration, which leads to a decrease in the fracture initiation pressure.

Aftershocks are well aligned with the background stress field, contradicting the hypothesis of highly heterogeneous crustal stress

It has been proposed that the crustal stress field contains small‐length‐scale heterogeneity of much larger amplitude than the uniform background stress. This model predicts that earthquake focal mechanisms should reflect the loading stress rather than the uniform background stress. So, if the heterogeneous stress hypothesis is correct, focal mechanisms before and after a large earthquake should align with the tectonic loading and the earthquake‐induced static stress perturbation, respectively. However, I show that the off‐fault triggered aftershocks of the 1992 M7.3 Landers, California, earthquake align with the same stress field as the pre‐Landers mechanisms. The aftershocks occurred on faults that were well oriented for failure in the pre‐Landers stress field and then loaded by the Landers‐induced static stress change. Aftershocks in regions experiencing a 0.05 to 5 MPa coseismic differential stress change align with the modeled Landers‐induced static stress change, implying that they were triggered by the stress perturbation. Contrary to the heterogeneous stress hypothesis, these triggered aftershocks are also well aligned with the pre‐Landers stress field obtained from inverting the pre‐Landers focal mechanisms. Therefore, the inverted pre‐Landers stress must represent the persistent background stress field. Earthquake focal mechanisms provide an unbiased sample of the spatially coherent background stress field, which is large relative to any small‐scale stress heterogeneity. The counterexample provided by the Landers earthquake is strong evidence that the heterogeneous stress model is not widely applicable.

Seismicity, seismogenic structures, and crustal stress fields in the greater Rome area (central Italy)

Instrumental seismicity of the greater Rome area is analyzed using the most recent seismological database. The aim of this study is to identify potential seismogenic structures responsible for the moderate seismicity that characterizes this sector, where no surface faulting is expected on the basis of the absence of strong local earthquakes, as evidenced by the 2000‐year‐long historical record. Nevertheless, in light of the great value and the high vulnerability of the architectural and monumental patrimony of Rome, even the occurrence of moderate events imposes a careful assessment of the hazard, linked to the presence of active faults that are undetectable by means of classic geological field investigation. Seismic data here analyzed belong to the period 1997–2008 and show local magnitude (ML) ranging from 1.5 to 4.7. To better constrain the hypocentral depths, we computed the Vp/Vs ratio using a modified Wadati method and the minimum 1‐D velocity model that approximates the seismic structure of the study area. Moreover, we applied a double‐difference hypocentral determination technique to improve earthquake locations. Most of the located seismicity appears to be concentrated within seven main clusters. Spatial distribution of the seismicity allowed us to identify several seismogenic structures, corresponding to previously undetected or only supposed active faults. The stress tensor obtained by inverting the focal mechanisms computed in this study is in agreement with the NE–SW extensional tectonic stress active in the inner chain of the Apennines, although there is evidence of some local stress field heterogeneities.

Complex states of stress during the normal faulting seismic cycle: Role of midcrustal postseismic creep

Numerical models are used to analyze coseismic and postseismic change in the state of stress near the lower tip of a normal fault directly below the brittle‐ductile transition. The models are compared with information obtained from the geological record of exhumed metamorphic rocks regarding the magnitude and the geometry of coseismic stress increase close to the lower fault termination and the mechanisms involved during postseismic stress relaxation. The numerical results predict a drop in differential and fault‐parallel shear stress in the upper crust and a stress increase in the lower crust due to the tapering off in fault slip. The coseismic deformation field in the crust causes significant deflection of the principal stresses from the horizontal and the vertical. During the postseismic period, the recovery of the crustal stress field remains incomplete, even for long recurrence intervals. Instead, the stress field evolves in self‐similar cycles, controlled by repeated earthquake activity. The numerical models show that postseismic viscous creep localized beneath the fault contributes to the recovery of all stress tensor components in the upper crust close to the fault. This result implies that a detailed understanding of the processes and conditions active during postseismic stress relaxation in the middle and lower crust is essential in order to estimate reloading rates in the upper crust surrounding the fault.

Spatial variation of the crustal stress field along the Ryukyu‐Taiwan‐Luzon convergent boundary

We applied an improved stress inversion method to a comprehensive data set of earthquake focal mechanisms to depict the pattern of crustal stress along the western convergent boundary of the Philippine Sea plate. Our results indicate that the crustal stress along the Ryukyu fore arc is segmented with boundaries at or near the places of seamount subduction, including the Tokara channel. An extensional stress regime is observed along the entire Ryukyu back arc, implying that back‐arc rifting may have extended northward to Kyushu. A triangular area near the southernmost terminus of the Ryukyu arc is characterized by a unique stress signature. The eastern boundary of this Ryukyu‐Taiwan Stress Transition coincides with the 123°E meridian where the Gagua ridge intercepts the Ryukyu trench; whereas its western boundary agrees remarkably well with the border between the postcollision and waning‐collision domains in northern Taiwan. The Taiwan collision zone is dominated by compression that rotates locally according to the structural configuration of the Lukang Magnetization High (LMH), suggesting that the LMH may be critical in controlling the local stress distribution. The stress signature of the Luzon arc–Taiwan collision reaches as far south as 19.5°N. The tectonic stress along the Manila trench–Luzon fore arc is dominated by a complex regime of extension that cannot be explained by simple plate bending or in‐slab membrane stress. Since this extensional regime is observed only south of ∼22°N, it probably marks the northern limit of the contemporary boundary between the subduction along the Manila trench and the collision in Taiwan.

Fault interaction and stresses along broad oceanic transform zone: Tjörnes Fracture Zone, north Iceland

[1] Transform motion along oceanic transforms generally occurs along narrow faults zones. Another class of oceanic transforms exists where the plate boundary is quite large (∼100 km) and includes several subparallel faults. Using a 2-D numerical modeling, we simulate the slip distribution and the crustal stress field geometry within such broad oceanic transforms (BOTs). We examine the possible configurations and evolution of such BOTs, where the plate boundary includes one, two, or three faults. Our experiments show that at any time during the development of the plate boundary, the plate motion is not distributed along each of the plate boundary faults but mainly occurs along a single master fault. The finite width of a BOT results from slip transfer through time with locking of early faults, not from a permanent distribution of deformation over a wide area. Because of fault interaction, the stress field geometry within the BOTs is more complex than that along classical oceanic transforms and includes stress deflections close to but also away from the major faults. Application of this modeling to the 100 km wide Tjornes Fracture Zone (TFZ) in North Iceland, a major BOT of the Mid-Atlantic Ridge that includes three main faults, suggests that the Dalvik Fault and the Husavik-Flatey Fault developed first, the Grismsey Fault being the latest active structure. Since initiation of the TFZ, the Husavik-Flatey Fault accommodated most of the plate motion and probably persists until now as the main plate structure.

NW Australian intraplate seismicity and stress regime

The interrelations between stress orientation, regional deformation, and seismic activity in the context of intraplate seismicity continue to pose difficult questions. Stress models of the Australian continent are largely based on nonseismic information. Our study of 26 microearthquakes in NW Australia shows that they are good indicators of the seismogenic stress field. By inverting focal mechanisms calculated from the events we obtained deviatoric stress tensors in full agreement with the known in‐situ stress field. The stress tensors corroborate the elastic modeling of the continent. The methods used here have widespread applicability in determining crustal stress field parameters in regions where in‐situ stress data are sparse or absent.

Global crustal stress pattern based on the World Stress Map database release 2008

The World Stress Map (WSM) project is a global compilation of information on the contemporary crustal stress field from a wide range of stress indicators. The WSM database release 2008 contains 21,750 stress data records that are quality-ranked using an updated and refined quality-ranking scheme. Almost 17,000 of these data records have A–C quality and are considered to record the orientation of maximum horizontal compressional stress S H to within ±25°. As this is almost a triplication of data records compared with the first WSM database release in 1992, we reinvestigate the spatial wave-length of the stress patterns with a statistical analysis on a global 0.5° grid. The resulting smoothed global stress map displays both; the mean S H orientation that follows from the maximum smoothing radius for which the standard deviation is <25° and a countour map that displays the wave-length of the stress pattern. This smoothed global map confirms that long wave-length stress patterns (>2000 km) exist for example in North America and NE Asia. These have been used in earlier analyses to conclude that the global stress pattern is primarily controlled by plate boundary forces that are transmitted into the intraplate region. However, our analysis reveals that rather short wave-length of the stress pattern <200 km are quite frequent too, particularly in western Europe, Alaska and the Aleutians, the southern Rocky Mountains, Basin and Range province, Scandinavia, Caucasus, most of the Himalayas and Indonesia. This implies that local stress sources such as density contrasts and active fault systems in some areas have high impact in comparison to plate boundary forces and control the regional stress pattern.

Crustal ductile flow and its contribution to tectonic stress in Southwest China

Knowledge of tectonic crustal stress in highland region can provide important insights into relative plate motion, mountain building and genesis of larger crustal earthquakes underneath tectonically active regions. Here we focus on the highland regions in Southwest (SW) China, including southeast Tibet and Sichuan and Yunnan provinces. To better understand the correlation between crustal stress and structural heterogeneities, three-dimensional (3-D) velocity (Vp, Vs) structures in the crust are imaged using a large number of P- and Pn-wave, and S- and Sn-wave arrival times simultaneously. A slow-velocity layer at depths of 16–26 km was clearly imaged in most regions of SW China, suggesting the presence of a ductile layer that may be associated with partial melting and/or aqueous fluids in the crust. The azimuths of the maximum compressional stresses estimated using 147 high-quality stress data revealed a characteristic regional stress field along the fault belts in the study region. The trajectories of the stress axes along the southeastern Tibetan plateau vary from NE–SW in southwest Tibet to E–W in southeast Tibet. Our study indicates that, apart from the influence from the collision between the Indian and Eurasian plates, the present-day crustal stress field along the tectonic zones in SW China is significantly affected by ductile deformation in the crust, suggesting that crustal structural heterogeneity plays an important role in the tectonic stress.

Seismic source zone characterization for the seismic hazard assessment project PEGASOS by the Expert Group 2 (EG1b)

A comprehensive study of the seismic hazard related to the four NNP sites in NW Switzerland was performed within the project PEGASOS. To account for the epistemic uncertainties involved in the process of the characterization of seismic source zones in the frame of probabilistic seismic hazard assessments, four different expert teams have developed and defended their models in the frame of an intensive elicitation process. Here, the results of one out of four expert groups are presented. The model of this team is based first of all on considerations regarding the large scale tectonics in the context of the Alpine collision, and neotectonic constraints for defining seismic source zones. This leads to a large scale subdivision based on the structural ‘architectural’ considerations with little input from the present seismicity. Each of the eight large zones was characterized by the style of present-day faulting, fault orientation, and hypocentral depth distribution. A further subdivision of the larger zones is performed based on information provided by the seismicity patterns. 58 small source zones have been defined in this way, each of them characterized by the available tectonic constrains, as well as the pros and cons of different existing geologic views connected to them. Of special concern in this respect were the discussion regarding thin skinned vs. thick skinned tectonics, the tectonic origin of the 1356 Basel earthquake, the role of the Permo-Carboniferous graben structures, and finally the seismogenic orientation of faults with respect to the recent crustal stress field. The uncertainties connected to the delimitations of the small source zones have been handled in form of their regrouping, formalized by the logic tree technique. The maximum magnitudes were estimated as discretized probability distribution functions. After declustering the used ECOS earthquake catalogue and an analysis of data completeness as a function of time the parameters of the frequency-magnitude relations were derived within their uncertainties.

Characteristics of present-day active strain field of Chinese mainland

On the basis of the GPS data obtained from repeated measurements carried out in 2004 and 2007, the horizontal principal strain of the Chinese mainland is calculated, which shows that the direction of principal compressive strain axis of each subplate is basically consistent with the P-axis of focal mechanism solution and the principal compressive stress axis acquired by geological method. It indicates that the crustal tectonic stress field is relatively stable in regions in a long time. The principal compressive stress axes of Qinghai-Tibet and Xinjiang subplates in the western part of Chinese mainland direct to NS and NNE-SSW, which are controlled by the force from the collision of the Eurasia Plate and India Plate. The principal compressive strain axes of Heilongjiang and North China subplates in the eastern part direct to ENE-WSW, which shows that they are subject to the force from the collision and underthrust of the Eurasia Plate to the North America and Pacific plates. At the same time, they are also affected by the lateral force from Qinghai-Tibet and Xinjiang subplates. The principal compressive strain axis of South China plate is WNW-ESE, which reflects that it is affected by the force from the collision of Philippine Sea Plate and Eurasia Plate and it is also subject to the lateral force from Qinghai-Tibet subplate. It is apparent from the comparison between the principal compressive strain axes in the periods of 2004∼2007 and 2001∼2004 that the acting directions of principal compressive stress of subplates in both periods are basically consistent. However, there is certain difference between their directional concentrations of principal compressive stress axes. The surface strain rates of different tectonic units in both periods indicate that the events predominating by compressive variation decrease, while the events predominating by tensile change increase.

Recent seismicity and crustal stress field in the Lucanian Apennines and surrounding areas (Southern Italy): Seismotectonic implications

We analyzed the instrumental seismicity of Southern Italy in the area including the Lucanian Apennines and Bradano foredeep, making use of the most recent seismological data base available so far. P- and S-wave arrival times, recorded by the Italian National Seismic Network (RSNC) operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV), were re-picked along with those of the SAPTEX temporary array deployed in the region in the period 2001–2004. For some events located in the upper Val d'Agri, we also used data from the Eni-Agip oil company seismic network. We examined the seismicity occurred during the period between 2001 and 2006, considering 514 events with magnitudes M ≥ 2.0. We computed the V P / V S ratio obtaining a value of 1.83 and we carried out an analysis for the one-dimensional (1D) velocity model that approximates the seismic structure of the study area. Earthquakes were relocated and, for well- recorded events, we also computed 108 fault plane solutions. Finally, using 58 solutions, the most constrained, we computed regional stress field in the study area. Earthquake distribution shows three main seismic regions: the westernmost (Lucanian Apennines) characterized by high background seismicity, mostly with shallow hypocenters, the easternmost below the Bradano foredeep and the Murge with deeper and more scattered seismicity, and finally the more isolated and sparse seismicity localized in the Sila Range and in the offshore area along the northeastern Calabrian coast. Focal mechanisms computed in this work are in large part normal and strike-slip solutions and their tensional axes ( T-axes) have a generalized NE–SW orientation. The denser station coverage allowed us to improve hypocenters determination compared to those obtained by using only RSNC data, for a better characterization of the crustal and subcrustal seismicity in the study area.

Compressive stress field in the crust deduced from shear-wave anisotropy: an example in capital area of China

The rocks in the crust are pervaded by stress-aligned fluid-saturated microcracks, and the complex fault tectonics and stress control the configuration of the microcracks, however shear-wave splitting could indicate this kind of characteristics. In this paper, Capital Area Seismograph Network (CASN), the widest scope and highest density of regional seismograph network presently in China, is adopted to deduce the principal compressive stress field distribution pattern from polarizations of fast shear-waves, based on shear-wave splitting analysis. The principal compressive stress in capital area of China is at NE85.7°±41.0° in this study. Compared with the results of principal compressive stress field in North China obtained from other methods, the results in this study are reliable in the principal compressive stress field distribution in capital area. The results show that it is an effective way, although it is the first time to directly obtain crustal stress field from seismic anisotropy. It is effectively applied to the zones with dense seismograph stations.

The M w 3.1–4.7 earthquakes in the southern Baltic Sea and adjacent areas in 2000, 2001 and 2004

The area south and east of the Baltic Sea has very minor seismic activity. However, occasional events occur as illustrated by four events in recent years, which are analysed in this study: near Wittenburg, Germany, on May 19, 2000, M w = 3.1, near Rostock, Germany, on July 21, 2001, M w = 3.4 and in the Kaliningrad area, Russia, two events on September 21, 2004 with M w = 4.6 and 4.7. Locations, magnitudes (M L and M w) and focal mechanisms were determined for the two events in Germany. Synthetic modeling resulted in a well-confined focal depth for the Kaliningrad events. The inversion of macroseismic observations provided simultaneous solutions of the location, focal depth and epicentral intensity. The maximum horizontal compressive stress orientations obtained from focal mechanism solutions, approximately N–S for the two German events and NNW–SSE for the Kaliningrad events, show a good agreement with the regionally oriented crustal stress field.

Numerical manifold method and its application in the study of crustal movements in the Sichuan-Yunnan Area

The numerical manifold method (NMM) can calculate the movements and deformations of structures or materials. Both the finite element method (FEM) for continua and the discontinuous deformation analysis (DDA) for block systems are special cases of NMM. NMM has separate mathematical covers and physical meshes: the mathematical covers define only fine or rough approximations; as the real material boundary, the physical mesh defines the integration fields. The mathematical covers are triangle units; the physical mesh includes the fault boundaries, joints, blocks and interfaces of different crust zones on the basis of a geological tectonic background. Aiming at the complex problem of continuous and discontinuous deformation across the Chinese continent, the numerical manifold method (NMM) is brought in to study crustal movement of the Sichuan-Yunnan area. Based on the GPS velocity field in the Sichuan-Yunnan area, a crustal strain and stress field is simulated and analyzed. Moreover, results show that the NMM is a more suitable method than DDA in simulating the movement of the Sichuan-Yunnan area. Finally, a kind of mechanism of crustal motion in the Sichuan-Yunnan area is discussed in the paper.

Stress, strain, and mountain building in central Japan

The work described here has two key objectives: to investigate the geometric relationships between the principal directions of crustal stress and crustal strain rate in central Japan and to evaluate how crustal stresses are related to the rates of horizontal strain produced by different tectonic processes. Specifically, we consider the extent to which tectonic stress directions reflect transient deformation produced by interseismic subduction thrust locking. The axis of maximum horizontal compressive stress obtained from focal mechanism inversion agrees well with the axis of greatest contractional strain rate in central and southwest Japan only after the effects of interseismic strain accumulation on geodetic observations have been accounted for according to an elastic dislocation model of subduction thrust locking. The residual deformation, which is presumed to represent net upper plate deformation, is broadly confined to the area of pronounced topography in central Japan and is consistent with the deformation expected for horizontal motion of the Amurian plate with respect to northeastern Honshu. These observations suggest that part of the apparent discrepancy between crustal stress and strain rate directions reported by previous authors stems from a comparison of parameters representing processes occurring on different timescales. In this case, the strain rates associated with cyclic subduction zone locking are not reflected in the crustal stress field; conversely, long‐term horizontal motion between northeast and southwest Japan exerts a stronger influence on the crustal stress field and neotectonic mountain‐building processes in central Japan than do intermittent subduction zone earthquakes.