The role of gravity in normal and reverse faulting earthquakes

Shear stress levels on reverse faults are anticipated to be several times higher than on normal faults with the same pore pressure ratio. In addition, ruptures on normal faults release gravitational potential energy, whereas earthquakes on reverse faults expend work in uplifting rocks. In this study, I investigate the significance of these differences for earthquake cycles and I question whether the source of energy driving earthquakes is the same on reverse and normal faults. Based on the assumption that normal and reverse faults have the same background frictional properties and pore pressure states, I use numerical simulations with a two‐dimensional dynamic elastic‐plastic model to show that due to stress differences, earthquakes on reverse faults tend to occur less frequently, produce more coseismic slip and stress drop and involve higher slip rates than ruptures on normal faults with equivalent dimensions. The analysis also shows differences in the energy changes associated with earthquake cycles on reverse and normal faults. However, the earthquakes on both fault types result from abrupt release of elastic strain energy, which proceed and essentially drive variations in gravitational potential energy. Thus, ruptures on both normal and reverse faults are consistent with elastic rebound theory.

We have evaluated the stress–strain behavior of calcite precipitated and mechanically twinned in small-offset strike-slip, normal and thrust faults of a variety of ages and from a variety of tectonic settings (n = 3001 twin measurements, 63 strain analyses from 18 field sites). Five strike-slip faults with syn-faulting, horizontally striated calcite (rake = 0°) were studied and we report the orientations of the contemporaneous stress–strain field associated with each fault: intrusion of the Marathon Large Igneous Province mafic dikes (~ 2.1 Ga in Archean crust, Minnesota, USA); post-Keweenaw rift (1.1 Ga) faulting (Island Lake fault, central Ontario, Canada); subduction associated with metamorphic core complex formation (Cretaceous, China); subduction (Cretaceous to Miocene, Italy), and continental extension (recently active Furnace Creek fault, Death Valley, California, USA). Seven normal faults with synfaulting, dip-slip striated calcite were studied and are from the following tectonic settings: a normal fault slip surface in an Ordovician Piedmont fold, Appalachian’s; paleo-subduction associated with Cretaceous metamorphic core complex formation (China, 3 sites); the paleo-extensional Atlantic margin (~ 55 Ma, Ireland, 1 site with a U–Pb calcite age); continental extension (1 active site, Mojave desert); a transcurrent margin (Jamaica, 1 active fault site), and subduction [2 active faults along the Eur-African margin in Italy (with calcite U–Th disequilibria ages) and Crete, respectively]. Six thrust fault examples are all from convergent orogenic settings: the basal thrust of the Penokean (1850 Ma) fold-and-thrust belt; the Penokean orogen foreland in Mesabi Range banded iron formation folds; an offset breccia body in the Permian Gondwanide belt, Ellsworth Mountains, Antarctica; the frontal thrust of the Gondwanide Cape belt, South Africa; the Paleocene frontal Prospect thrust, Sevier belt, Wyoming, and an Alpine foreland back-thrust, Lulworth Cove, U.K. For each strike-slip fault system the twinning shortening strain is horizontal and at an angle of 0°–60° to the respective fault plane (dextral or sinistral) although in the majority of cases the shortening axis is parallel to fault strike (13 of 23 results). In each normal fault example, dip-slip kinematic striations dominate the faulted surface yet the orientation of the maximum principal compressive stress (σ1) and shortening strain axis (e1) are not 45° to the fault plane as predicted but are sub-horizontal and either strike-parallel (25 of 35 results) or strike-normal (10 of 35 results). Thrust faults preserve shortening strain axes parallel to the dip-slip kinematic direction, within the fault plane (plane strain) and not at 45° to the principal plane (5 of 5 results). None of the fault stress–strain field results reported here support the Andersonian or Mohr–Coulomb criteria for stress–strain relations predicted along faults.

The role of tectonic horizontal stresses in activating deep-seated gravitational slope deformations (DSGSD) has rarely been studied. We applied a new numerical technique for determining the present-day stress states within a large DSGSD, situated near the Periadriatic Line fault (PAL) in the Eastern Alps. The stress states were calculated from three-dimensional displacements of the sinistral Obir fault conjugated to the dextral PAL fault, which also forms the upper DSGSD detachment plane. The analysed fault displacements occurred in two distinct activity phases associated with the elastic rebound along (i) the Obir fault in the summer of 2014, and (ii) the core of the PAL fault in the winter of 2014/2015. These periods were synchronous with the periods of increased local seismicity. The results brought an insight into a possible causative link between horizontal tectonic stresses and DSGSDs activation. We observed, that transient dextral transpressions and dextral transtensions opposing to the general fault kinematics associated to the elastic rebound had a potential to destabilize the DSGSD at the micrometre level. The applied stress-tensor calculation, although restricted only to shallow near-surface conditions, is a reliable method that can reveal stress states almost in the real-time.

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Supplemental Material: Three-dimensional finite-element modeling of Coulomb stress changes on normal and thrust faults caused by pore fluid pressure changes and postseismic viscoelastic relaxation

Recent global positioning system (GPS) records of surface deformation caused by earthquakes on intracontinental dip-slip faults revealed in unprecedented detail a significant strike-slip component near the fault tips that is markedly different for thrust and normal faults. In the hanging wall of the thrust fault ruptured during the A.D. 2003 Chengkung (Taiwan) earthquake, a divergent displacement pattern was recorded. In contrast, a convergent slip pattern was observed in the hanging wall of the normal fault that produced the 2009 L’Aquila (Italy) earthquake. Such convergent slip patterns are also evident in field records of cumulative fault slip from central Italy, which underlines the coseismic origin of cumulative displacement patterns. Here we use three-dimensional numerical modeling to demonstrate that the observed fault-parallel motions are a characteristic feature of the coseismic slip pattern on normal and thrust faults. Modeled slip vectors converge toward the center of normal faults, whereas they diverge for thrust faults; this causes contrasting fault-parallel displacements at the model surface. Our model also predicts divergent movements in normal fault footwalls, which were recorded for the first time during the L’Aquila earthquake. During the postseismic phase, viscous flow in the lower crust induces fault-parallel surface displacements, which have the same direction as the coseismic displacements but are distributed over a larger area that extends far beyond the fault tips. Therefore, detecting this signal requires GPS stations in the prolongation of the fault strike. Postseismic velocities vary over several orders of magnitude depending on the lower crustal viscosity and may reach tens of millimeters per year for low viscosities. Our study establishes the link between coseismic and cumulative slip patterns on normal and thrust faults and emphasizes that understanding fault-parallel slip components and associated surface displacements is essential for inferring regional deformation patterns from space-geodetic and fault-slip data.

Specialized mapping of crustal fault zones. Part 1: Basic theoretical concepts and principles

Normal fault origin and evolution, a hot research topic in rock mechanics, geology and seismology, not only plays an important role in effective and safe petroleum production but also is helpful to understand related shallow-focus earthquake. This study proposed a method to determine normal fault origin and evolution based on strain energy. Following similarity principle, we designed and conducted 3 physical simulation experiments using normal faults at Ordos basin, China, and acquired dip angle, displacement, and strain energy via velocity profile recorded by high-resolution Particle Image Velocimetry (PIV). Fault origin and evolution stages were identified using strain energy and elastic rebound theory. The density of strain energy was determined using surfer 12 to describe spatial variation and internal structure of fault zone. Strain energy release rate and fractured zone derived from fault growth were obtained through calculating scale and central position of strain energy with material point method. Earthquake generated by the releasing of strain energy was classified with the Richter magnitude scale and was further verified with Bath’s law. Results show that normal fault is originated from one point and grows by connecting and merging multiple points. Releasing energy from several centralized strain energy regions is responsible for the unstable fault line and related strata. Small-scale strain energy releasing from multiple regions indicates the formation and evolution of normal faults since energy releasing was typically along with the growth of normal faults. However, low energy release rate does not represent weak fault activity, whereas increasing rate is a good indicator of fault stabilizing. The determination of normal fault origin and evolution suggests that the variation of central position results in both tension and shear fractures in fracture zone, which demonstrates the valid application of strain energy in normal fault. Residual strain energy mainly distributes in the hanging wall after the formation of normal fault; this is why aftershock of shallow-focus earthquake commonly occurs in the hanging wall. Therefore, this study can provide insight into shallow-focus earthquake associated with normal fault.

Changes in the volumes of ice caps considerably alter the stress state of the lithosphere by generating a transient signal that is added to the tectonic background stress field. These stress field changes, in turn, affect crustal deformation and in particular the slip behavior of existing faults. Here we use three‐dimensional finite element models to investigate how arrays of normal and thrust faults near a growing and subsequently melting ice cap are influenced in their slip evolution. The results show that regardless of fault dip, both types of faults experience a decrease in their slip rate during ice cap advance and an increase in their slip rate during ice cap retreat if they are located beneath the ice cap. In contrast, faults outside the ice cap that are loaded on their footwall or hanging wall only show the opposite pattern: their slip rate increases during glacial loading and decreases during subsequent unloading. If the load is located along strike of the fault; that is, at one of its tips, the slip behavior of normal and thrust faults is different: The normal fault shows a slip rate increase during unloading, the thrust fault during loading. Our results explain the location and timing of deglaciation‐induced paleoearthquakes in Scandinavia and the contrasting slip histories reported from normal faults in the Basin and Range Province, which are located at different positions relative to the former Yellowstone ice cap. More generally, our findings imply that a uniform slip behavior of faults in formerly glaciated regions should not be expected.

Abstract: Many active faults in extensional and contractional tectonic settings experienced a slip rate increase after the last glacial period when the volume of nearby glaciers and lakes decreased. This post-glacial slip rate increase is caused by transient stresses that are generated by the unloading-induced rebound and superimposed on the tectonic background stress field. As the latter is different for normal and thrust faults, the response to loading and unloading should depend on the fault type. Here we use finite-element models including a fault in rheologically layered lithosphere to explore the conditions under which both normal and thrust faults experience a post-glacial slip rate increase. The results show that a post-glacial slip rate increase occurs on normal faults if the lower crust is stronger than the lithospheric mantle, whereas thrust faults accelerate if the lower crust is weaker than the lithospheric mantle. These findings imply that the response of faults to mass fluctuations on the Earth's surface may provide constraints on the rheological stratification of the lithosphere. We use our results to make predictions on the viscosity structure of the Basin-and-Range Province and northern Scandinavia, where palaeoseismological data document a pronounced increase in seismicity as a result of post-glacial unloading and rebound.

Previous studies that made use of basin models have shown that the normal geological evolution of the Paris basin does not generate the observed, albeit weak, excess pressures in some shale layers of the basin. Other processes that may have created the overpressures, currently neglected in such models, are investigated here. Terms accounting for osmotic effects and tectonic stress changes are successively added to the diffusivity equation. The effect of changes in outcrop boundary conditions is also calculated with a pseudo‐two‐dimensional analytical solution. These solutions are applied to the Callovo‐Oxfordian shale formation in the eastern part of the Paris basin, France. It is shown that a long‐term transient osmotic effect starting in the Tertiary could explain in part the observed excess pressures in the Callovo‐Oxfordian shale assuming effective diffusion coefficients of 1–5 × 10−12 m2 s−1 in line with the measurements and a pore radius b around 20 Å for the shales. However, because of the uncertainty on the value of the shale pore radius, additional head measurements and osmotic experiments on samples should be made to fully establish the possibility of an osmotic process. Our study also shows that recent changes in hydrodynamic boundary conditions could also explain excess pressure distribution in this shale layer. It is plausible that a combination of the two processes could best explain the distribution and intensity of the “overpressures.” Tectonic stress changes do not appear to be important; it is shown that for such processes, to maintain high pressures, strong and recent increase in tectonic compressive stress would be required.

<p>Despite natural faults are variably oriented to the Earth's surface and to the local stress field, the mechanics of fault reactivation and slip under variable loading paths (sensu Sibson, 1993) is still poorly understood. Nonetheless, different loading paths commonly occur in natural faults, from load-strengthening when the increase in shear stress is coupled with an increase in normal stress (e.g., reverse faults in absence of the fluid pressure increase) to load-weakening when the increase in shear stress is coupled with a decrease in normal stress (e.g., normal faults). According to the Mohr-Coulomb theory, the reactivation of pre-existing faults is only influenced by the fault orientation to the stress field, the fault friction, and the principal stresses magnitude. Therefore, the stress path the fault experienced is often neglected when evaluating the potential for reactivation. Yet, in natural faults characterized by thick, incohesive fault zone and highly fractured damage zone, the loading path could not be ruled out. Here we propose a laboratory approach aimed at reproducing the typical tectonic loading paths for reverse and normal faults. We performed triaxial saw-cut experiments, simulating the reactivation of well-oriented (i.e., 30° to the maximum principal stress) and misoriented (i.e., 50° to the maximum principal stress), normal and reverse gouge-bearing faults under dry and water-saturated conditions. We find that load-strengthening versus load-weakening path results in clearly different hydro-mechanical behavior. Particularly, prior to reactivation, reverse faults undergo <em>compaction</em> even at differential stresses well below the value required for reactivation. Contrarily, normal faults experience <em>dilation</em>, most of which occurs only near the differential stress values required for reactivation. Moreover, when reactivating at comparable normal stress, normal faults (load-weakening path) are more prone to slip seismically than reverse fault (load-strengthening path). Indeed, the higher mean stress that normal fault experienced before reactivation compacts more efficiently the gouge layer, thus increasing the fault stiffness and favoring seismic slip. This contrasting fault zone compaction and dilation prior to reactivation may occur in different natural tectonic settings, affecting the fault hydro-mechanical behavior. Thus, to take into account the loading path the fault experienced is fundamental in evaluating both natural and induced fault reactivation and the related seismic risk assessment.</p>

<p>The Epiligurian wedge-top basins of the Italian Northern Apennines fold and-thrust belt have been thoroughly investigated in the past from a sedimentological and paleogeographic perspective leading to the identification of several regional unconformities and the appreciation of their significance to track down the complex evolution of the accretionary wedge. Among these, a major Burdigalian unconformity has been recognised as a key regional element marking an abrupt shift from a deep marine (pre-Burdigalian) to platform (post-Burdigalian) environment during the progressive uplift of the accretionary wedge. We integrate these studies by providing a solid structural framework wherein to set this evolution. We investigated the pattern and the kinematics of the deformation structures deforming the Epiligurian Units both in the pre- and post-Burdigalian sequences exposed in the Emilia-Romagna Region of the Northern Apennines. Field investigations were integrated with the remote sensing of lineaments mapped at the regional scale to unravel the significance of the Burdigalian unconformity during the thickening and later dismantling of the Northern Apennines wedge. Fieldwork data document the existence of different structures and lineament trends affecting the pre- and post-Burdigalian sedimentary sequences. For example, the lowermost units of the pre-Burdigalian sequence are affected by top-to-the SE, NE-SW-striking reverse faults defined by planar slip surfaces associated with thin clastic damage zones. These reverse faults are cut across by scattered normal faults accommodating centimetric to decimetric throws and associated with clusters of disaggregation deformation bands. The post-Burdigalian succession, instead, is affected by more systematic trends of both reverse and normal faults. The reverse faults are oriented either NE-SW or WNW-ESE, with a general NW or NE tectonic transport, respectively. The crosscutting normal faults strike from NW-SE to NE-SW and are associated with extension-oriented NE-SW and NW-SE, respectively. Normal faults are locally decorated by calcite slickenfibres and syn-kinematic calcite veins, documenting structurally controlled circulation of mineralising paleofluids. All the structures affecting both the pre- and post-Burdigalian sequences are linked to a tectonic evolution encompassing syn-orogenic compression and post-orogenic extension, with the latter accompanied by local instabilities during overall thinning of the transiently supercritical wedge. To assess the significance of our results on a regional scale, a remote sensing analysis of tectonic and morphological lineaments was performed by systematically mapping lineaments within a study area of 200 km² at an observation scale varying from 1:50.000 to 1:5.000. Statistical analysis of open-access dataset focused on reverse and normal faults, confirming the significant lineament orientation variations indicated by field data. NE-SW striking normal and reverse faults define the pre-Burdigalian dataset, whereas NE-SW-striking normal faults and NW-SE-striking compressional structures define the post-Burdigalian dataset. Preliminary results from the combination of field and remote sensing made it possible to not only differentiate tectonic and morphological elements and to identify the preferential trend of deformation structures, but to also conclude that the polyphasic tectonic evolution of the Epiligurian Units during the NE-verging accretion of the Northern Apennines wedge accommodated significant changes in stress field orientation and faulting regime in the pre-and post-Burdigalian period.</p><p> </p><p> </p>

ABSTRACT: Indonesia, as one of the oil and gas producing countries, has potential shale gas and tight oil resources in some of its basins. However, careful assessment of the geomechanical setting is required because most Indonesian basins lie on active tectonic margins, in contrast to most shale play in the U.S, Canada, and China which lie in stable cratonic settings with normal or strike-slip fault regimes. This study aims to analyze the in-situ stress in the potential shale resources of the Baong, Belumai, and Bampo formations in the North Sumatra Basin, Indonesia, and discuss its implication to hydraulic fracturing strategies. Analyses were conducted on the regional and borehole scales. On the regional scale, the earthquake focal mechanism and surface geological features indicate a reverse fault (RF) environment with the maximum horizontal stress (SHmax) in the northeast-southwest (NE-SW) direction. At the borehole scale, analyses were carried out using wireline logs, drilling reports, leak-off test, and hydraulic fracturing data from a pilot hole in the North Sumatra Basin. Borehole stress analysis suggests a near isotropic stress condition in the RF/SS (strike-slip) faulting regime due to overpressure within the Baong formation. On the other hand, the Belumai and Bampo formations show an anisotropic stress state with a normal pressure interval in the NF (normal fault)/SS faulting regime. These conditions lead to a lower minimum principal stress in the Belumai and Bampo formations which suggests that these formations cannot act as a fracture barrier for hydraulic fracturing in the Baong formation. In addition, considering the RF stress state in the Baong formation and the bedding planes that dip at about 30°, horizontal hydraulic fractures can form propagating in all azimuth directions without any obvious fracture barrier preventing them from penetrating into adjacent formations. Meanwhile, hydraulic fracturing in the Belumai formation will create vertical fractures due to the SS/NF faulting regime that will propagate until it reaches the Baong Formation acting as a fracture barrier. Our study suggests that overpressured formations in this region whose prevailing faulting regime is RF are not favorable for hydraulic fracturing operations.