Nodal Planes Of Focal Mechanisms Research Articles

Spatially Varying Stress State in the Central U.S. From Bayesian Inversion of Focal Mechanism and In Situ Maximum Horizontal Stress Orientation Data

AbstractThe crustal stress state in the Midcontinent of the United States has been a focus of research for many years due to anomalously high rates of seismicity in the region. This interest is recently renewed because of ongoing wastewater injection and CO2 sequestration in the Illinois Basin and its potential to induce seismicity. When fluid is injected, pore pressure increases, decreasing effective normal stress and increasing the potential for faults optimally oriented with respect to the direction of maximum compressive stress to slip. In order to better manage this risk, we present a study on the spatial distribution of principal stress orientations in the Illinois Basin and surrounding region using focal mechanism solutions and in situ measured maximum horizontal stress orientation indicators from borehole breakout, core fracture, overcoring, hydraulic fracture, and strain gauge measurements. To invert these data for the deviatoric stress tensor, we extend the damped‐least stress inversion of Hardebeck and Michael (2006, https://doi:10.1029/2005JB004144) with the mixed linear/nonlinear Bayesian inversion scheme of Fukuda and Johnson (2010, https://doi:10.1111/j.1365‐246X.2010.04564.x). The stress inversion provides an objective means to estimate nonlinear parameters including the spatial smoothing parameter, unknown data uncertainties, and selection of focal mechanism nodal planes. Results indicate a systematic rotation of the maximum horizontal stress orientation (SHmax) across a 1,000‐km width of the Midcontinent. SHmax rotates from ~N60°E to ~E/W orientation across the southern Illinois Basin. The principal stress orientations are largely consistent with the observed pattern of strike‐slip faulting and reverse faulting near the New Madrid and Wabash Valley Seismic Zones.

Crustal seismicity associated to rpid surface uplift at Laguna del Maule Volcanic Complex, Southern Volcanic Zone of the Andes

Laguna del Maule Volcanic Complex (LMVC, Southern Andes of Chile) has been experiencing large rates (ca. 30cm/yr) of surface uplift as detected since 2008 by satellite geodetic measurements. Previous works have modeled the source of this deformation as an inflating rectangular sub-horizontal sill underlying LMVC at 5km depth, which is supposedly related to an active process of magmatic replenishment of a shallow silicic reservoir. However little is known about the tectonic context on which this activity is taking place, particularly its relation with crustal seismicity that could help understanding and monitoring the current deformation process. Here we present the first detailed characterization of the seismic activity taking place at LMVC and integrate it with structural data acquired in the field in order to illuminate the possible connection between the ongoing process of surface uplift and the activation of crustal faults. Our main finding is the recognition of repetitive volcano-tectonic (VT) seismic swarms that occur periodically between 2011 and 2014 near the SW corner of the sill modeled by InSAR studies. A cross-correlation analysis of the waveforms recorded for these VT events allows identifying three different seismic families. Families F1 and F3 share some common features in the stacked waveform and its locations, which markedly differ from those of family F2. Swarms belonging to this later family are more energetic and its energy was increasing since 2011 to a peak in January 2013, which coincide with maximum vertical velocities detected by local GPS stations. This points to a common process relating both phenomena. The location of VT seismic swarms roughly coincides with the intersection of a NE-SW lineament with a WNW-ESE lineament. The former shows clear field evidences of dextral strike-slip that are fully consistent with one nodal plane of focal mechanism for well-recorded F2 events. The conjugate nodal plane of these focal mechanisms could coincide with the WNW-ESE lineament, for which our field reconnaissance suggests a dominant normal motion. Events belonging to families F1 and F3 are also dominantly strike-slip but with some mixture with thrust and normal components. Our results, in conjunction with results of previous authors, suggest a complex mechanical interaction between the arrangement of crustal faults forming the structural framework on which the magmatic plumbing system of LMVC is emplaced and the inflating source at depth.

Strong Aftershock Study Based on Coulomb Stress Triggering—A Case Study on the 2016 Ecuador Mw 7.8 Earthquake

The 2016 Ecuador M 7.8 earthquake ruptured the subduction zone boundary between the Nazca plate and the South America plate. This M 7.8 earthquake may have promoted failure in the surrounding crust, where six M ≥ 6 aftershocks occurred following this mainshock. These crustal ruptures were triggered by the high coulomb stress changes produced by the M 7.8 mainshock. Here, we investigate whether the six M ≥ 6 aftershocks are consistent with the positive coulomb stress region due to the mainshock. To explore the correlation between the mainshock and the aftershocks, we adopt a recently published high-quality finite fault model and focal mechanisms to study the coulomb stress triggers during the M 7.8 earthquake sequence. We compute the coulomb failure stress changes (ΔCFS) on both of the focal mechanism nodal planes. We compare the ΔCFS imparted by the M 7.8 mainshock on the subsequent aftershocks with the epicenter location of each aftershock. In addition, the shear stress, normal stress, and coulomb stress changes in the focal sources of each aftershock are also computed. Coulomb stress changes in the focal source for the six M ≥ 6 aftershocks are in the range of −2.17–7.564 bar. Only one computational result for the M 6.9 aftershock is negative; other results are positive. We found that the vast majority of the six M ≥ 6 aftershocks occurred in positive coulomb stress areas triggered by the M 7.8 mainshock. Our results suggest that the coulomb stress changes contributed to the development of the Ecuador M 7.8 earthquake sequence.

Study on Coulomb Stress Triggering of the April 2015 M7.8 Nepal Earthquake Sequence

In April 2015, a M7.8 earthquake occurred less than one month before a M7.3 earthquake near Kodari, Nepal. The Nepal earthquake sequences also include four larger (M > 6) aftershocks. To reveal the interrelation between the main shock and the aftershocks, we check the role of coseismic coulomb stress triggering on aftershocks that follow the M7.8 main shock. Based on the focal mechanisms of the aftershocks and source models of the main shock, the coulomb failure stress changes on both of the focal mechanism nodal planes are calculated. In addition, the coulomb stress changes on the focal sources of each aftershock are also calculated. A large proportion of the M > 6 aftershocks occurred in positive coulomb stress areas triggered by the M7.8 main shock. The secondary triggering effect of the M7.3 aftershock is also found in this paper. More specifically, the M7.3 aftershock promoted failure on the rupture plane of the M6.3 aftershock. Therefore, we may conclude that the majority of larger aftershocks, which accumulated positive coulomb stress changes during the sequence, were promoted or triggered by the main shock failure. It suggests that coulomb stress triggering contributed to the evolution of the Nepal M7.8 earthquake sequence.

The Aftershock Sequence of the 2011 Mineral, Virginia, Earthquake: Temporal and Spatial Distribution, Focal Mechanisms, Regional Stress, and the Role of Coulomb Stress Transfer

Abstract The aftershocks of the 23 August 2011 M w 5.7 Mineral, Virginia, earthquake were recorded by 36 temporary stations installed by several institutions. We located 3960 aftershocks from 25 August 2011 through 31 December 2011. A subset of 1666 aftershocks resolves details of the hypocenter distribution. We determined 393 focal mechanism solutions. Aftershocks near the mainshock define a previously recognized tabular cluster with orientation similar to a mainshock nodal plane; other aftershocks occurred 10–20 km to the northeast. A large percentage of the aftershocks occurred in regions of positive Coulomb static stress change, and ∼80% of the focal mechanism nodal planes were brought closer to failure. However, the aftershock distribution near the mainshock appears to have been influenced strongly by rupture directivity. Aftershocks at depths less than 4 km exhibit reverse mechanisms with north‐northwest‐trending nodal planes. Most focal mechanisms at depths greater than 6 km are similar to the mainshock, with north‐northeast‐trending nodal planes. A concentration of aftershocks in the 4–6 km depth range near the mainshock are mostly of reverse type but display a 90° range of nodal‐plane trend. Those events appear to outline the periphery of mainshock rupture, where positive Coulomb stress transfer is largest. The focal mechanisms of aftershocks at depths less than 4 km and those greater than 6 km, along with the mainshock, point to the possibility of a depth‐dependent stress field prior to the occurrence of the mainshock. Analysis of earthquake occurrence using a new magnitude scale () indicates a Gutenberg–Richer law b ‐value of 0.864 and an Omori law p ‐value of 1.085, indicative of a typical aftershock sequence. Online Material: Catalogs of aftershock location, magnitude, and focal mechanisms.

Tectonophysical interpretation of earthquake focal mechanisms of the Zagros system

Structural-paragenetic and kinematic methods of tectonophysics are applied to study earthquake focal mechanisms of the Zagros system. Nodal planes of focal mechanisms are identified as L-, L′- and R-, R′-shears by the first method, whereby coordinates of principal stress axes P, T and N (i.e. in tectonophysics, σ1, σ3 and σ2, if σ1 ≥ σ2 ≥ σ3) are defined. ‘Working’ nodal planes corresponding to activated ruptures are revealed. Axes of the main normal stresses are combined into local groups on the basis of the kinematic identity of planes of seismogenic ruptures (Figure 2). The second method is applied to construct stereograms of the main axes P, T and N, to construct and interpret stereograms of vectors of seismogenic shifts (Figure 3), and to more clearly define coordinates of principal axes σ1, σ3 и σ2. As evidenced by their comparison, coordinates of the principal axes obtained by the two tectonophysical methods are well coincident (see Figure 2). Five groups of seismogenesis are distinguished; they differ in combination of deformation regimes and kinematic conditions. Locations are determined of the areas wherein earthquake foci of similar parameters are located. This means that seismogenic zones are distinguished; structural and kinematic characteristics of such zones are determined by parameters of stereographic models of corresponding types of seismogenesis (Figures 4 and 5). It is established that the region is dominated by shear and upthrust deformation regimes due to regional submeridional compression and SW-NE compression (see Figures 4 and 5). Submeridional subhorizontal compression is explained by the northward movement of the Arabian plate, and SW-NE compression is explained by divergent processes within the limits of the Red Sea rift. The time pattern of the seismogenic processes from 1979 to 2001 shows that submeridional compression and SW-NE compression are associated with different deep mechanisms. Processes of SE-NW compression, which are observed in the northern part of the Arabian plate, are caused by its interaction with the Eastern Black Sea microplate.

Intraplate earthquakes, regional stress and fault mechanics in the Central and Eastern U.S. and Southeastern Canada

Utilizing 75 high quality individual earthquake focal plane mechanisms and 10 formal stress inversions we investigate the consistency of regional stress orientations in the central and eastern United States and southeastern Canada, the variation of relative stress magnitudes across the region and the compatibility of slip on optimally-oriented nodal planes with frictional faulting theory. To map faulting styles and relative stress magnitudes across the region of study, we utilize the high quality focal plane mechanisms to calculate the AΦ parameter (following Angelier, 1979; Simpson, 1997) that ranges from 0 (uniform horizontal extension with SV >>SHmax=Shmin) to 1.5 (strike–slip faulting with SHmax>SV>Shmin) to 3 (uniform horizontal compression with SHmax=Shmin>SV). We find that horizontal stresses become increasingly more compressive with respect to the vertical stress from the south-central United States (characterized predominantly by strike–slip focal mechanisms) toward the northeastern U.S. and southeastern Canada (predominantly thrust mechanisms). In a manner similar to the study by M.L. Zoback (1992a), which used a much smaller data set, we utilize the Mohr–Coulomb criterion to calculate the difference in orientation between the theoretically-optimal orientation of a fault plane (for various coefficients of friction, μ) and the focal mechanism nodal planes assuming that pore pressure in the brittle crust is hydrostatic. For the 75 focal plane mechanisms utilized in our study, the preferred (better fitting) nodal planes deviate on average only 7° in strike and dip from the theoretically-optimal planes for μ=0.6. As such minor differences could represent small variations in the stress field (or uncertainties in the focal plane mechanisms), we conclude that nearly all earthquakes in the study region slip in a manner compatible with shear failure on pre-existing faults in the local stress field.

A normal-faulting seismic sequence triggered by the 2011 off the Pacific coast of Tohoku Earthquake: Wholesale stress regime changes in the upper plate

We here show relocated hypocenters within a seismic sequence with normal faults in the northern part of Ibaraki Prefecture, triggered after the M 9.0 2011 off the Pacific coast of Tohoku Earthquake. Depth-sections of the hypocenters from the center of the northern Ibaraki region to the north show an earthquake alignment dipping westwards at 40° to 50° at depths shallower than 10 km. On the other hand, hypocenters from the center to the south show a cross-cutting geometry consisting of conjugate westward- and eastward-dipping planes at the same depths. The dip angles of the hypocenter alignments are roughly consistent with the nodal planes of focal mechanisms of large normal earthquakes, and exhibit optimal-orientations in terms of the frictional failure criterion. Furthermore, comparison of the focal mechanisms recorded before and after the 2011 Tohoku Earthquake suggests that the stress field abruptly changed from horizontal compression to extension in the study area. The most plausible explanation of the drastic stress changes is a significant reduction in trench-normal compressive stress compared with reduction in trench-parallel stress accompanying large horizontal extensional deformation within the overlying plate.

Performance of regional distance centroid moment tensor inversion applied to the 2004 mid-Niigata prefecture earthquake, Japan

SUMMARY We develop a centroid moment tensor (CMT) inversion method by using a densely distributed regional seismic network in order to calculate the centroid locations and MT solutions of regional earthquakes. Centroid location and time are obtained using a grid search algorithm with the new CMT inversion method. This method is applied to the earthquakes that occurred in the focal area of the 2004 mid-Niigata prefecture earthquake, central Japan. The centroid locations and focal mechanisms of the main shock and aftershocks with moment magnitudes greater than 3.4 are effectively constrained. We assess the reliability of the CMT inversion method by comparing its results with accurate hypocentres calculated with a local temporary seismic network. The accuracy of the centroid locations in the horizontal and vertical directions is approximately less than 4 km. The MT solutions are consistent with the P-wave first-motion polarities. By using the nodal planes of focal mechanisms and the centroid locations of many earthquakes, we successfully imaged the major faults distributed on a vertical cross-section across the main shock fault. Further, we evaluate the validity of the application to ‘offshore earthquakes’ by using the aftershocks that occur in the landward area, wherein the seismic network does not cover the focal area but is located in a one-sided region of the focal area. These results show that the focal mechanism and depth are very stable, although the accuracy of the horizontal distribution is reduced to half.

On correlation between seismic activities and focal mechanisms of subsequent strong earthquakes

On the basis of the results from spatial-temporal scanning of moderate seismic activities prior to 63 strong earthquakes occurred since 1966, we investigate in this paper the characteristics of anomalous evolution of moderate seismic activities prior to strong earthquakes with different focal mechanisms in different tectonic blocks, as well as the correlation between principal seismic activities and nodal planes of focal mechanisms. The research results indicate that the moderate seismic activities prior to strong earthquakes display two significant stages of stress concentration and stress weakening, which are consistent to the evolvement of local tectonic stress field. The first stage, defined as medium-term stage, is characterized by seismic belts, seismic gaps and centralized activities; The second stage, defined as short-term stage, is predominated by continuous quiescence. For the earthquakes occurred on the strike-slip and normal faults, the two stages are relatively evident; while for the earthquakes occurred on the thrust faults, there are not characteristic images and seismic quiescence in both stages. The spatial distribution of moderate shock belts before strong earthquakes show a good coherence with that of the two nodal planes of focal mechanisms, while the spatial distribution of seismic gaps enclosed by moderate shocks display a poor coherence with the nodal planes.

Fault Orientation in the Eastern Tennessee Seismic Zone: A Study Using the Double-Difference Earthquake Location Algorithm

We use the double-difference earthquake location algorithm to relocate approximately 1,000 earthquakes in the eastern Tennessee seismic zone (ETSZ). We examine the earthquake hypocenter relocations in an effort to resolve fault orientations and thereby gain insights into the tectonics of the seismic zone. The analysis involves visual comparison of three-dimensional perspective plots of the hypocenter relocations oriented accord ing to focal mechanism nodal planes derived from events within several clusters of earthquakes. Relocations in the central, most seismically active, portion of the ETSZ indicate a diffuse west-striking, north-dipping zone of hypocenters. The orientation of this zone of hypocen ters is consistent with structure imaged by a previous seismic reflection profile that shows a reflective mid-to-upper crust with apparent dips of approximately 35° to the north. Relocation of a smaller cluster of earthquakes near the Tennessee-North Carolina border suggests a steeply dipping northwest trending fault in that area.

Static Stress Transferred by the 1999 Chi-Chi, Taiwan, Earthquake: Effects on the Stability of the Surrounding Fault Systems and Aftershock Triggering with a 3D Fault-Slip Model

We investigated the effect of changes in static stress caused by the 1999 M L 7.3 Chi-Chi earthquake on the surrounding fault systems in Taiwan. We examined this problem from two perspectives. First, we applied a statistical test for the hypothesis of static-stress triggering by comparing the distributions of the Coulomb failure stress changes on the focal mechanism nodal planes both before and after the mainshock. Since preshocks could obviously not have been affected by the mainshock, the deviation of these distributions determined the significance of static-stress triggering. Second, we calculated the Coulomb failure stress changes on the Chukou and Meishan faults in specified slip directions to evaluate the stability of these two active faults after the Chi-Chi earthquake. The static-stress changes were obtained by applying the theory of dislocation in an elastic half-space to a 3D rupture model derived from the surface displacements associated with the Chi-Chi earthquake. Our results showed that the distributions of the Coulomb failure stress changes for events preceding and following the Chi-Chi earthquake were significantly different, yet consistent with the hypothesis of static-stress triggering. We also found that both the Chukou and the Meishan faults were in stress shadows at depths shallower than 10 km. However, because a number of aftershocks fell in these stress shadows, it is possible that other mechanisms, such as dynamic stress, were responsible for triggering these events. Manuscript received 6 November 2000.

A new statistical test for static stress triggering: Application to the 1987 Superstition Hills earthquake sequence

Over the past several years, many investigators have argued that static stress changes caused by large earthquakes influence the spatial and temporal distributions of subsequent regional seismicity, with earthquakes occurring preferentially in areas of stress increase and reduced seismicity where stress decreases. Some workers have developed quantitative methods to test for the existence of such static stress triggering, but no firm consensus has yet been reached as to the significance of these effects. We have developed a new test for static stress triggering in which we compute the change in Coulomb stress on the focal mechanism nodal planes of a set of events spanning the occurrence of a large earthquake. We compare the statistical distributions of these stress changes for events before and after the mainshock to decide if we can reject the hypothesis that these distributions are the same. Such rejection would be evidence for stress triggering. We have applied this test to the November 24, 1987, Elmore Ranch/Superstition Hills earthquake sequence and find that those post‐mainshock events that experienced stress increases of at least 0.01–0.03 MPa (0.1–0.3 bar) or that occurred from 1.4 to 2.8 years after the mainshocks are consistent with having been triggered by mainshock‐generated static stress changes.

On the origin of diverse aftershock mechanisms following the 1989 Loma Prieta earthquake

SUMMARY We test the hypothesis that the origin at the diverse suite of aftershock mechanisms Following the 1989 M 7.1 Loma Prieta, California, earthquake is related to the post-main-shock static stress field We use a 3-D boundary-element algorithm to calculate static stresses combined with a Coulomb failure criterion to calculate conjugate failure planes at aftershock locations. The post-main-shock static stress field is taken as the of a sum of a pre-existing stress field and changes in stress due to the heterogeneous slip across the Loma Prieta rupture plane. The background stress field is assumed to be either a simple shear parallel to the regional trend of the San Andreas fault or approximately fault-normal compression A suite of synthetic aftershock mechanisms from the conjugate failure planes is generated and quantitatively compared (allowing for uncertainties in both mechanism parameters and earthquake locations) to well-constrained mechanisms reported in the US Geological Survey Northern California Seismic Network catalogue. We also compare calculated rakes with those observed by resolving the calculated stress tensor onto observed focal mechanism nodal planes, assuming either plane to be a likely rupture plane. Various permutations of the assumed background stress field, fractional coefficients of aftershock fault planer methods of comparisons, etc. explain between 52 and 92 per cent of the aftershock mechanisms. We can explain a similar proportion of mechanisms, however, by comparing, a randomly reordered catalogue with the various suites of synthetic aftershocks. The inability to duplicate aftershock mechanisms reliably on a one-to-one basis is probably a function of the combined uncertainties in models of main-shock slip distribution, the background stress field, and aftershock locations. In particular, we show theoretically that any specific main-shock slip distribution and a reasonable background stress field are able lo generate a highly variable suite of failure planes such that quite different aftershock mechanisms may be expected to occur within a kilometre of less of each other. This scale of variability is less than the probable location error of aftershock earthquakes in the Loma Prieta region. We successfully duplicate a measure of the variability in the mechanisms of the entire suite of aftershocks. If static stress changes are responsible for the generation of aftershock mechanisms, we are able to place quantitative constraints on the level of stress that must have existed in the upper crust prior to the Loma Prieta rupture. This stress level appears to be too low to generate the average dip across the main-shock rupture plane. Possible reasons for this result range from incorrect initial assumptions of homogeneity in the background stress field, friction and fault geometry to driving stresses that arise from deeper in the crust or upper mantle. Alternatively, aftershock focal mechanisms may be determined by processes other than, or in addition to, static stress changes, such as pore-pressure changes or dynamic stresses.

Seismicity, shear failure and modes of deformation in deep subduction zones

Planes of shear failure in deep seismic zones are identified as being parallel to focal mechanism nodal planes of deep focus earthquakes. We relocate deep seismicity reported in the International Seismological Centre (ISC) catalogue for earthquakes deeper than 400 km in the Honshu, Bonin, Marianas, Java, Banda, and South America subduction zones using the joint hypocentral determination method. Recent events with Harvard centroid-moment tensor (CMT) solutions and nearby seismicity are viewed from the direction of the CMT null axis, comparing the two orthogonal nodal planes with the seismicity pattern. Planar features of seismicity parallel to CMT nodal planes are found in each deep seismic zone, and are identified as planes of shear failure. The sense of displacement on these planes is one of resistance to deeper penetration. Honshu and Bonin show imbrication and horizontal displacement of the deepest part of the seismogenic slab, while Marianas and Brazil show conjugate shear failure. Banda and Argentina show patterns of vertical and horizontal shear planes. For the Western Pacific slabs we find significant lateral displacements towards the south of the deepest part of the subduction flow. Tonga has been analyzed previously, showing the same pattern as identified here. In all Benioff zones, planes of shear failure are observed which act to shorten the slabs and displace matter horizontally, normal to the slab; moreover, differential faulting acts to rotate the slab of subducted lithosphere to a more horizontal posture, indicating that slabs experience resistance to penetration of the 670 km discontinuity.

An application of the slip model of brittle deformation to focal mechanism analysis in three different plate tectonic situations

The “slip model” of brittle deformation ( Reches, 1983) imposes a set of constraints on the angular relationships between faults, their striations on slickensides and the type and orientation of the deformation ellipsoid. In focal mechanisms of earthquakes, these relationships can be used to determine which of the two nodal planes is the fault plane, and then to obtain the strain ellipsoid of regional tectonic deformation controlling seismicity. In this approach it is convenient to use pitch/dip diagrams ( De Vicente, 1988) and P/T diagrams ( Angelier and Mechler, 1977), both developed for fault population analysis. In this paper three regions in different plate-tectonic situations are analysed using these methods. These regions are a zone of intracontinental seismicity in the eastern Peruvian Andes, a zone of continental compressional deformation in the Iranian Plateau, and the Fenwei Graben of China. Nodal planes of focal mechanisms, published in different papers about seismicity in these regions, have been used, the following conclusions being reached. In the analysed zone of the Peruvian Andes, three regional deformation types appear, causing fault movements that range from slightly reverse strike-slip to reverse strike-slip. In addition, exclusively at depths in excess of 18 km, there is a deformation of an uniaxial shortening type. In all of these deformations the horizontal shortening axis is oriented from N87° to N93°. In the Iranian Plateau two horizontal shortening directions appear, N151° and N61°, even showing faults that imply an uniaxial deformation ellipsoid the shortening axis of which is at N61°. In the Fenwei Graben the ellipsoid of regional deformation has a stretching principal axis at N145°/150° and a shortening axis at N55°/60°, the strike-slip movement along faults being as important as the normal one.

Quantification of the fracturing of the slab using a fractal approach

According to previous studies, the number of earthquakes associated to a fault length greater than L is given by N ≈ L −3/bc , where b and c are the slopes of Gutemberg's law and seismic moment-magnitude relationship, respectively. The quantification of the complexity of a fault is then given by a fractal dimension D = 3/bc. Application of such an analysis to the seismicity of subduction zones between 100 and 700 km depth provides a good description of the various earthquake groups, the fracturing, and the different mechanisms of failure along the slab. Our results show that the branching model of fracturing is generally valid except in subduction zones which present anomalies such as: (1) lateral distribution of stress and strain related to contortion of the slab or to curvature of the trench axis (e.g. eastern Indonesia), and (2) deep distortion of a slab which is anchored at depth (e.g. Tonga). Our results are in good agreement with previous studies which have shown that earthquakes at depths greater than 70–100 km cannot be attributed to the shear displacement between the descending slab and the adjacent mantle but rather indicate deformation within the slab, and that aftershocks of deep earthquakes do not occur preferentially on nodal planes of focal mechanisms of the main shocks.

Statistical methods for comparing directions to the orientations of focal mechanisms and Wadati-Benioff zones

Abstract We propose simple graphical methods for evaluating whether a set of direction vectors are clustered with respect to particular directions, axes, or planes. As examples of these techniques, we present data concerning the relationship between: (1) aftershock/initial event geometry and nodal planes of focal mechanisms; (2) aftershock/initial event geometry and the plane of the Wadati-Benioff zone; and (3) B axes of focal mechanisms and the plane of the Wadati-Benioff zone.

Aftershocks of deep earthquakes do not occur preferentially on nodal planes of focal mechanisms

Some earthquakes with focal depths exceeding 70 km possess one or more aftershocks which are well recorded by stations at teleseis-mic distances. Precise determination of relative vectors separating the aftershocks and ∼ 60 initial events finds the vector directions are distributed uniformly with respect to the nodal plane of main shock focal mechanisms. When focal mechanisms are available for both an initial event and an aftershock, the mechanisms are occasionally significantly different from one another. These results suggest that the failure process of deep earthquakes is not slip occurring along a simple, planar surface. Our results contradict several previous analyses of aftershocks because other investigators did not recognize that half of the focal sphere lies within 16° of one of the nodal planes.

Hydroselsmicity: A Hypothesis for The Role of Water in the Generation of Intraplate Seismicity

Abstract A new hypothesis termed Hydroseismicity that has hydrologic (diffusion of pore pressure transients from recharge areas of groundwater basins), geologic (rifted, fractured crust), and chemical (solubility of minerals) elements is proposed to explain the role of water in the generation of intraplate seismicity. Its basis is a spatial correlation in the southeastern U. S. between 1) seismogenic crustal volumes of high seismicity, 2) large gravity-driven river basins that can provide an adequate supply of water to the upper- and mid-crust, and 3) a permeable crust that is tectonically stressed close to failure. It is suggested that in crustal volumes with a combination of connected fractures and adequate groundwater, natural transient increases in hydraulic head in recharge areas of groundwater basins can be transmitted to depths of 10–20 km, and thereby trigger earthquakes, via a flow-path geometry that resembles except for scale the model familiar to groundwater hydrologists for near-surface flow. Possible trigger mechanisms for Hydroseismicity include small increases in fluid pressure at hypocentral depths caused by such transient increases, and hydrolytic weakening of minerals that leads to structural weakening. Implicit in the model is a diffuse distribution of epicenters (as is observed in the region) rather than concentrations along discrete geologie (faults) or geomorphic (rivers) elements. Open fractures imply fracture roughness, i.e., asperities under a higher stress that keep fractures open even in an ambient tectonic stress field. Intraplate earthquakes in a fractured crust prestressed to near-failure are thus postulated to be triggered by small transient increases in fluid pressure transmitted along preexisting fractures in a rock fabric weakened by stress corrosion of asperities. Abundant petrologic evidence is available to justify an assumption of fracture permeability to depths of 20 km near passive rifted margins. All four principal seismogenic volumes in the southeastern U. S. are within gravity-driven groundwater basins that can provide an abundant supply of water to the ernst, and that intersect known or suspected Eocambrian or Mesozoic rifted crust. The host basins have the largest surface recharge areas and contain rivers with the highest average stream gradients as measured from their headwaters to the Fall Line. Seismicity in the region is characterized by steeply dipping focal mechanism nodal planes and diffuse alignments and/or clusters of epicenters. These characteristics are compatible with a steep to vertical fracture fabric currently being reactivated by porc pressure diffusion from surface recharge of groundwater basins.