Current Stress Regime Research Articles

Seismicity and seismoactive faults of Cuba

The first catalogue of active regional faults of Cuba is presented. The seismotectonic map of Cuba is a base for studying the seismicity in this region. Of the 30 faults studied, only twelve are active. The main seismotectonic structure is the Bartlett-Cayman fault system, which borders the eastern and southeastern seismotectonic units in this region. Approximately 70% of Cuban seismicity is concentrated here. The Cauto-Nipe, Cochinos and Nortecubana faults border other seismotectonic units. The Nortecubana fault is the only one associated with a tsunami. All the faults are segmented. The faults described are related to the current tectonic stress regime of the Northern Caribbean. All the available information (maps, sections and profiles, photos, geological and neotectonic data on seismicity and focal mechanisms) is supported by a GIS.

Detailed imaging of the fault planes of the 2004 Niigata–Chuetsu, central Japan, earthquake sequence by double-difference tomography

Imaging of the hypocentral faults of the Niigata–Chuetsu earthquake sequence was performed by double-difference tomography using data obtained by a temporary aftershock observation network densely deployed in the hypocentral region. Two parallel “unfavorly oriented” fault planes steeply inclined to the WNW are revealed, that of the main shock and that of the largest aftershock, as well as a zone of velocity change between the high-velocity footwall and the low-velocity hanging wall for both P- and S-waves. This suggests that the main shock and the largest aftershock are caused by the reactivation of normal faults in the Miocene under the current compressional stress regime. Parts of the fault planes are also found to be located within the low-velocity zone. This low-velocity zone continues to the lower crust, which is imaged by an inversion from the regional data, and would correspond with highly fractured zone around the fault plane with high fluid pressure, which reactivated the “unfavorly oriented” fault and triggered the present earthquake. Large coseismic slip area appears to extend to the northeastern portion with relatively high velocity. This suggests that the asperity (large coseismic slip area) of the main shock possibly correspond with a higher velocity region along the fault plane.

Fault Reactivation Potential During CO2 Injection in the Gippsland Basin, Australia

The risk of fault reactivation in the Gippsland Basin was calculated using the FAST (Fault Analysis Seal Technology) technique, which determines fault reactivation risk by estimating the increase in pore pressure required to cause reactivation within the present-day stress field. The stress regime in the Gippsland Basin is on the boundary between strike-slip and reverse faulting: maximum horizontal stress (~ 40.5 MPa/km) > vertical stress (21 MPa/km) ~ minimum horizontal stress (20 MPa/km). Pore pressure is hydrostatic above the Campanian Volcanics of the Golden Beach Subgroup. The NW-SE maximum horizontal stress orientation (139°N) determined herein is broadly consistent with previous estimates, and verifies a NW-SE maximum horizontal stress orientation in the Gippsland Basin. Fault reactivation risk in the Gippsland Basin was calculated using two fault strength scenarios; cohesionless faults (C = 0; μ = 0.65) and healed faults (C = 5.4; μ = 0.78). The orientations of faults with relatively high and relatively low reactivation potential are almost identical for healed and cohesionless fault strength scenarios. High-angle faults striking NE-SW are unlikely to reactivate in the current stress regime. High-angle faults oriented SSE-NNW and ENE-WSW have the highest fault reactivation risk. Additionally, low-angle faults (thrust faults) striking NE-SW have a relatively high risk of reactivation. The highest reactivation risk for optimally oriented faults corresponds to an estimated pore pressure increase (Delta- P) of 3.8 MPa (~548 psi) for cohesionless faults and 15.6 MPa (~2262 psi) for healed faults. The absolute values of pore pressure increase obtained from fault reactivation analysis presented in this paper are subject to large errors because of uncertainties in the geomechanical model (in situ stress and rock strength data). In particular, the maximum horizontal stress magnitude and fault strength data are poorly constrained. Therefore, fault reactivation analysis cannot be used to directly measure the maximum allowable pore pressure increase within a reservoir. We argue that fault reactivation analysis of this type can only be used for assessing the relative risk of fault reactivation and not to determine the maximum allowable pore pressure increase a fault can withstand prior to reactivation.

The Olmsted fault zone, southernmost Illinois: A key to understanding seismic hazard in the northern new Madrid seismic zone

Geological deformation in the northern New Madrid seismic zone, near Olmsted, Illinois (USA), is analyzed using integrated compressional-wave (P) and horizontally polarized-wave (SH) seismic reflection and regional and dedicated borehole information. Seismic hazards are of special concern because of strategic facilities (e.g., lock and dam sites and chemical plants on the Ohio River near its confluence with the Mississippi River) and because of alluvial soils subject to high amplification of earthquake shock. We use an integrated approach starting with lower resolution, but deeper penetration, P-wave reflection profiles to identify displacement of Paleozoic bedrock. Higher resolution, but shallower penetration, SH-wave images show deformation that has propagated upward from bedrock faults into Pleistocene loess. We have mapped an intricate zone more than 8 km wide of high-angle faults in Mississippi embayment sediments localized over Paleozoic bedrock faults that trend north to northeast, parallel to the Ohio River. These faults align with the pattern of epicenters in the New Madrid seismic zone. Normal and reverse offsets along with positive flower structures imply a component of strike-slip; the current stress regime favors right-lateral slip on northeast-trending faults. The largest fault, the Olmsted fault, underwent principal displacement near the end of the Cretaceous Period 65 to 70 million years ago. Strata of this age (dated via fossil pollen) thicken greatly on the downthrown side of the Olmsted fault into a locally subsiding basin. Small offsets of Tertiary and Quaternary strata are evident on high-resolution SH-wave seismic profiles. Our results imply recent reactivation and possible future seismic activity in a critical area of the New Madrid seismic zone. This integrated approach provides a strategy for evaluating shallow seismic hazard-related targets for engineering concerns.

Resistivity and ground-penetrating radar images of fractures in a crystalline aquifer: a case study in Caiçara farm—NE Brazil

Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical–geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar—GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N–S and E–W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E–W direction are relatively open, as compared to fractures in the N–S direction. Probably, E–W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E–W compression and N–S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases: at a point near the recharge zone but where there is a low density of fractures, or at point far from the recharge zone but where there is a high density of fractures.

Resistivity and ground-penetrating radar images of fractures in a crystalline aquifer: a case study in Caiçara farm—NE Brazil

Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical–geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar—GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N–S and E–W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E–W direction are relatively open, as compared to fractures in the N–S direction. Probably, E–W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E–W compression and N–S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases: at a point near the recharge zone but where there is a low density of fractures, or at point far from the recharge zone but where there is a high density of fractures.

Stress balance above subduction: application to the Andes

The frequent changes in tectonic and morphological evolutions of active plate margins suggest that similar processes drive apparently antagonistic tectonics. We address the problem by a thin viscous sheet approach where the governing stresses are restricted to interplate traction, buoyancy and viscous stresses. Various cases are explored, from compression to extension, leading to dismiss corner-flow-driven models and to favor interplate traction-driven models. From the topography of the Andes, we determine the magnitude and profiles of the current stresses beneath the Andes at various locations. The total transmitted force is about 9×10 12 N m −1. Shear stresses at the boundaries of the deforming lithosphere range between 25 and 110 MPa and almost balance the buoyancy stresses due to crustal thickening, the remaining difference being the viscous stresses. The deviatoric stress is less than 10 MPa and the effective viscosity of the Andean lithosphere is as low as ∼3×10 21 Pa s. Maximum compression is located in the Eastern side of the Andes, matching the geological observations, but we find that the current stress regime is insufficient to explain the present-day elevation. This suggests that interplate traction was slightly stronger in the past when the Neogene plate convergence was faster, and this decrease in interplate traction implies that high and steep parts of the Andes cannot be sustained anymore. The shape of the Altiplano can be explained by a weak lateral variation of the viscosity, which allows the high plateau to be constructed in the characteristic time span of Andean building.

Stress balance above subduction: application to the Andes

The frequent changes in tectonic and morphological evolutions of active plate margins suggest that similar processes drive apparently antagonistic tectonics. We address the problem by a thin viscous sheet approach where the governing stresses are restricted to interplate traction, buoyancy and viscous stresses. Various cases are explored, from compression to extension, leading to dismiss corner-flow-driven models and to favor interplate traction-driven models. From the topography of the Andes, we determine the magnitude and profiles of the current stresses beneath the Andes at various locations. The total transmitted force is about 9 � 10 12 Nm � 1 . Shear stresses at the boundaries of the deforming lithosphere range between 25 and 110 MPa and almost balance the buoyancy stresses due to crustal thickening, the remaining difference being the viscous stresses. The deviatoric stress is less than 10 MPa and the effective viscosity of the Andean lithosphere is as low as f3 � 10 21 Pa s. Maximum compression is located in the Eastern side of the Andes, matching the geological observations, but we find that the current stress regime is insufficient to explain the present-day elevation. This suggests that interplate traction was slightly stronger in the past when the Neogene plate convergence was faster, and this decrease in interplate traction implies that high and steep parts of the Andes cannot be sustained anymore. The shape of the Altiplano can be explained by a weak lateral variation of the viscosity, which allows the high plateau to be constructed in the characteristic time span of Andean building. D 2004 Elsevier B.V. All rights reserved.

Quaternary reactivation of Tertiary faults in the southeastern Korean Peninsula: Age constraint by optically stimulated luminescence dating

Abstract Two groups of Quaternary faults occur in the southeastern Korean Peninsula. The first group is north‐northeast‐striking, high‐angle dextral strike–slip faults. The second group is north‐northeast‐striking, low‐angle reverse faults that represent the reactivation of the pre‐existing normal faults. Optically stimulated luminescence dating of Quaternary sediments cut by one of the reverse faults constrains the faulting age to post‐32 Ka. These faults seem to be capable of further slip under the current tectonic stress regime, as determined by recent earthquake events in northeast Asia. Therefore, the traditional concept that the southeastern Korean Peninsula is seismically stable should be reappraised.

A Detailed Microseismicity Study and Current Stress Regime in the Peninsular Ranges of Northern Baja California, Mexico: The Ojos Negros Region

The NW-trending San Miguel fault system is one of the most important seismogenic systems in northern Baja California, and the Ojos Negros region, comprising the Ojos Negros valley and bordering areas, is one of its most active regions. Within this region are found most of the mapped faults of the system: Ojos Negros, Tres Hermanos, most of San Miguel, and portions of the Vallecitos fault, which makes this a very important region from the points of view of intraplate tectonics and regional seismic hazard. A detailed microseismicity (0.2 ≤ M ≤ 4.0) survey of the Ojos Negros region, carried out in 1997 (one month, 13 Reftek stations recording at 200 samples/sec, plus two permanent RESNOM stations and other less sensitive instruments), yielded important results about the fault activity and the stress regime in the region. Our results are based on 278 hypocenters and 50 focal mechanisms selected from almost 2500 earthquakes recorded at a minimum of four stations. The selected database is comprised of good quality local events, for which the hypocentral depth can be reliably estimated. Locations and focal mechanisms were obtained using an improved velocity model ( Sierra97 ) for this part of the Peninsular Ranges. The hypocenters tend to cluster in space and time, with cluster interepicenter separations of the order of the location error (±1 km). The Ojos Negros valley (as defined by its sedimentary soil) is roughly covered by epicenters. The Tres Hermanos fault shows no significant seismicity, and the few earthquakes near (although not very close to) its southern third seem to be associated with seismicity that extends into the valley. Seismicity associated with the Ojos Negros fault consists almost exclusively of one large cluster. The San Miguel fault, the most active fault in the area, has epicenters within a 6–8 km wide band centered along its mapped trace. Most focal mechanisms are strike-slip with a minor normal component, while others are dominantly normal. The resulting pattern for the valley indicates a regional extensional regime with the average T axis in the ENE-WSW direction, and P axes distributed along an N-S strip with a slight inclination and concentrated near the poles.

State of stress and modern deformation of the Northern Basin and Range Province

Constraints on the current stress regime of the actively extending northern Basin and Range province are provided by deformation data (focal mechanisms and fault slip studies), hydraulic fracturing in situ stress measurements, borehole elongation (“breakouts”) analyses, and alignment of young volcanic vents. The integrated data indicate significant variations both in principal stress orientations and magnitudes. An approximately E‐W least principal stress direction appears to characterize both the eastern and western margins of the Basin and Range province, whereas in the active interior parts of the province extension occurs in response to a least principal stress oriented NW to N60°W. The contrast in stress orientations between the province boundaries and in the interior suggests that along the margins the least principal stress direction may be locally controlled by the generally northerly trending profound lithospheric discontinuities associated with these margins. Active deformation along the southeastern and western province margins is characterized by a combination of strike‐slip and normal faulting. Focal mechanisms along northeastern province margin (Wasatch front) and in central Nevada indicate a combination of normal and oblique‐normal faulting. Temporal, regional, and depth‐dependent variations in the relative magnitudes of the vertical and maximum horizontal stresses can explain much of the observed variations in deformation styles. However, some depth variation in faulting style inferred from focal mechanisms may be apparent and simply a function of the attitude of fault planes being reactivated. Evidence for significant temporal variation (or multiple cycles of variation) in relative stress magnitude comes from the Sierran front‐Basin and Range boundary region where recent earthquakes are predominantly strike slip, whereas the profound relative vertical relief across the Sierra frontal fault zone in the last 9–10 m.y. implies a normal faulting stress regime. Using the best data on stress orientation, relative stress magnitudes are constrained from slip vectors of major earthquakes and young fault displacements. Analysis of well‐constrained slip vectors in the Owens Valley, California, area indicate that large temporal variations in the magnitude of the approximately N‐S oriented maximum horizontal stress are required to explain dominantly dip‐slip and strike‐slip offsets on subparallel faults. Similar faulting relations are observed throughout much of the boundary zone between the Basin and Range‐Sierra Nevada (including the Walker Lane belt). Along the eastern province margin in the Wasatch front area in Utah, available data suggest that the maximum and minimum horizontal stresses may be approximately equal at depths of <4–5 km. Earthquake focal mechanisms in this area suggest more variability in relative magnitude of the two horizontal stresses with depth. Furthermore, superimposed sets of young fault striae along a segment of the Wasatch fault also indicate temporal variations of relative stress magnitudes. Sources of regional and temporal variations in the stress field may be linked to variable shear tractions applied to the base of the brittle crust related to intrusion, thermally induced flow, and the influence of the San Andreas plate boundary. Although difficult to date accurately, the fault slip data suggest that the temporal variations in relative magnitudes stress may occur on the time scale of both a single major earthquake cycle (1000–5000 years) and multiple earthquake cycles (10,000+ years).