Seismotectonic of a Locked Subduction Patch: The Southern Hikurangi Margin

<p>Subduction zones produce the largest earthquakes on the planet, where rupture along the plate interface can result in the release of stress over large areas, with up to tens of meters of slip extending from below the surface to the trench. The regional stress field is a primary control on the faulting process, ergo understanding the regional stress field leads to a better understanding of the current and future faulting in the area. Abundant new seismic and continuous Global Positioning System (cGPS) data in the southern North and northern South Island, New Zealand, make it possible to characterize stress and strain parameters throughout the southern Hikurangi subduction zone. Stress orientations calculated within the subducting plate, the overriding Australian plate, and due to gravitational forces reveal that stress throughout the subducting system varies across the southern North Island. Margin parallel motion is being accommodated by shear deformation west of theWairarapa fault, whereas margin perpendicular motion is being accommodated east of theWairarapa fault. Stress parameters within the double Benioff zone (DBZ) were characterized in term of two bands of seismicity. In the deep region of the DBZ, inversion the upper band of seismicity shows down-dip tension, while the lower band shows compression. Tension in the upper band and compression in the lower band is consistent with bending stresses. In the shallow region of the DBZ, the inversion of both the upper and lower bands seismicity showed tension; this is indicative of slab pull. Shear-wave splitting of stacked waveforms of local earthquakes recorded on 291 three-component stations showed an average fast azimuth of N-S to NNE-SSW, west of theWairarapa fault. A fast azimuth orientation of N-S to NNE-SSW is sub-parallel to the local major faults. This indicates that the observed anisotropy west of theWairarapa fault is structurally derived. East of the Wairarapa fault, within the Wairarapa Basin, the average fast azimuth orientation isNNW-SSE. Because the fast azimuth orientation showed no dependence on station-earthquake distance, depth, or back azimuth and is perpendicular to major local faults; it has been interpreted as being reflective of the SHmax orientation. cGPS daily solutions for long-term and inter-slow slip events (inter-SSE) time periods show distinctly differing regions of shear strain rate in the southern North Island and northern South Island. Compression and positive (clockwise) rotation in the southern North and northern South Island was observed using both datasets. Inter-SSE time periods resulted in lower magnitude strain parameters than those calculated during time periods including SSEs. These datasets shows that strain parameters change on time scales of SSEs (< 10 years).</p>

Seismotectonic of a Locked Subduction Patch: The Southern Hikurangi Margin

<p>Subduction zones produce the largest earthquakes on the planet, where rupture along the plate interface can result in the release of stress over large areas, with up to tens of meters of slip extending from below the surface to the trench. The regional stress field is a primary control on the faulting process, ergo understanding the regional stress field leads to a better understanding of the current and future faulting in the area. Abundant new seismic and continuous Global Positioning System (cGPS) data in the southern North and northern South Island, New Zealand, make it possible to characterize stress and strain parameters throughout the southern Hikurangi subduction zone. Stress orientations calculated within the subducting plate, the overriding Australian plate, and due to gravitational forces reveal that stress throughout the subducting system varies across the southern North Island. Margin parallel motion is being accommodated by shear deformation west of theWairarapa fault, whereas margin perpendicular motion is being accommodated east of theWairarapa fault. Stress parameters within the double Benioff zone (DBZ) were characterized in term of two bands of seismicity. In the deep region of the DBZ, inversion the upper band of seismicity shows down-dip tension, while the lower band shows compression. Tension in the upper band and compression in the lower band is consistent with bending stresses. In the shallow region of the DBZ, the inversion of both the upper and lower bands seismicity showed tension; this is indicative of slab pull. Shear-wave splitting of stacked waveforms of local earthquakes recorded on 291 three-component stations showed an average fast azimuth of N-S to NNE-SSW, west of theWairarapa fault. A fast azimuth orientation of N-S to NNE-SSW is sub-parallel to the local major faults. This indicates that the observed anisotropy west of theWairarapa fault is structurally derived. East of the Wairarapa fault, within the Wairarapa Basin, the average fast azimuth orientation isNNW-SSE. Because the fast azimuth orientation showed no dependence on station-earthquake distance, depth, or back azimuth and is perpendicular to major local faults; it has been interpreted as being reflective of the SHmax orientation. cGPS daily solutions for long-term and inter-slow slip events (inter-SSE) time periods show distinctly differing regions of shear strain rate in the southern North Island and northern South Island. Compression and positive (clockwise) rotation in the southern North and northern South Island was observed using both datasets. Inter-SSE time periods resulted in lower magnitude strain parameters than those calculated during time periods including SSEs. These datasets shows that strain parameters change on time scales of SSEs (< 10 years).</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

Temporal Changes in Seismic Anisotropy as a New Eruption Forecasting Tool

<p>The orientation of crustal anisotropy changed by ~80 degrees in association with the 1995/96 eruption of Mt. Ruapehu volcano, New Zealand. This change occurred with a confidence level of more than 99.9%, and affects an area with a radius of at least 5 km around the summit. It provides the basis for a new monitoring technique and possibly for future mid-term eruption forecasting at volcanoes. Three deployments of seismometers were conducted on Mt. Ruapehu in 1994, 1998 and 2002. The fast anisotropic direction was measured by a semi-automatic algorithm, using the method of shear wave splitting. Prior to the eruption, a strong trend for the fast anisotropic direction was found to be around NW-SE, which is approximately perpendicular to the regional main stress direction. This deployment was followed by a moderate phreatomagmatic eruption in 1995/96, which ejected material with an overall volume of around 0.02-0.05 km3. Splitting results from a deployment after the eruption (1998) suggested that the fast anisotropic direction for deep earthquakes (>55 km) has changed by around 80 degrees, becoming parallel to the regional stress field. Shallow earthquakes (<35 km) also show this behaviour, but with more scatter of the fast directions. Another deployment (2002) covered the exact station locations of both the 1994 and the 1998 deployments and indicates further changes. Fast directions of deep events remain rotated by 80 degrees compared to the pre-eruption direction, whereas a realignment of the shallow events towards the pre-eruption direction is observed. The interpretation is that prior to the eruption, a pressurised magma dike system overprinted the regional stress field, generating a local stress field and therefore altering the fast anisotropic direction via preferred crack alignment. Numerical modelling suggests that the stress drop during the eruption was sufficient to change the local stress direction back to the regional trend, which was then observed in the 1998 experiment. A refilling and pressurising magma dike system is responsible for the newly observed realignment of the fast directions for the shallow events, but is not yet strong enough to rotate the deeper events with their longer delay times and lower frequencies. These effects provide a new method for volcano monitoring at Mt. Ruapehu and possibly at other volcanoes on Earth. They might, after further work, serve as a tool for eruption forecasting at Mt. Ruapehu or elsewhere. It is therefore proposed that changes in anisotropy around other volcanoes be investigated.</p>

Two key switches in regional stress field during multi-stage deformation in the Carboniferous−Triassic southernmost Altaids (Beishan, NW China): Response to orocline-related roll-back processes

The orogenic architecture of the Altaids of Central Asia was created by multiple large-scale slab roll-back and oroclinal bending. However, no regional structural deformation related to roll-back processes has been described. In this paper, we report a structural study of the Beishan orogenic collage in the southernmost Altaids, which is located in the southern wing of the Tuva-Mongol Orocline. Our new field mapping and structural analysis integrated with an electron backscatter diffraction study, paleontology, U-Pb dating, 39Ar-40Ar dating, together with published isotopic ages enables us to construct a detailed deformation-time sequence: During D1 times many thrusts were propagated northwards. In D2 there was ductile sinistral shearing at 336−326 Ma. In D3 times there was top-to-W/WNW ductile thrusting at 303−289 Ma. Two phases of folding were defined as D4 and D5. Three stages of extensional events (E1−E3) separately occurred during D1−D5. Two switches of the regional stress field were identified in the Carboniferous to Early Permian (D1-E1-D2-D3-E2) and Late Permian to Early Triassic (D4-E3-D5). These two switches in the stress field were associated with formation of bimodal volcanic rocks, and an extensional interarc basin with deposition of Permian-Triassic sediments, which can be related to two stages of roll-back of the subduction zone on the Paleo-Asian oceanic margin. We demonstrate for the first time that two key stress field switches were responses to the formation of the Tuva-Mongol Orocline.

Stress-field orientation and crustal deformation in the Vienna Basin region (Alpine-Pannonian-Carpathian junction)

&lt;p&gt;Gaining insight into the regional stress field and deformation in the crust is challenging. As we cannot measure these directly, we rely on proxy measurements and numerical modelling to infer their orientation. For the Alpine-Pannonian-Carpathian junction, only a limited number of studies exist that provide such insights. They are based on either the interpretation of sparse and point-wise measurements of local stress-field orientations or on numerical modelling that aims to satisfy tectonic and geological constraints.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We infer seismic azimuthal anisotropy that relates to the orientation of the regional stress-field and crustal deformation from ambient-noise-derived Rayleigh waves in the region. This approach provides a spatially broad and independent measurement that complements previous studies. We use Rayleigh-wave group-velocity residuals after isotropic inversion at 5s and 20s center period, which are sensitive to crustal structure at different depths. They allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths (5s), fast orientations in the region are N/S to NNE/SSW, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress field. At greater depths (20s), fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the expected direction of aligned crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.&lt;/p&gt;

Regional stress field computation along the West Svalbard margin (Vestnesa ridge): Effect of the glacial isostatic adjustment.

&lt;p&gt;Release of greenhouse gasses is of major concern when it comes to climate change. Large amount of those gases are released through faults and fractures at the ocean floor, forming pockmarks at the surface. Understanding the formation of pockmarks and the fracture - fault network underlying them, is thus of first importance to apprehend the dynamics of gas seepages. We suggest that such fractures are closely related to the regional stress field and thus control by the combination of large scale tectonic processes, sedimentation - erosion mechanism and reactivation of inherited structures in the underlying basement.&lt;/p&gt;&lt;p&gt;The present study focus on the calculation of the regional stress field along Vestnesa ridge, a key location for methane seepage and pockmarks study. This area is located in a tectonically active region, boarded in the west by the Atlantic ridge and two major transform faults. In addition, deglaciation since the last glacial maximum (LGM), has induced a rebound of the lithosphere which also affects the stress field of the area including Fennoscandia, Svalbard and Greenland. However, it is difficult to estimate the effect of post-glacial rebound on the regional stress field, especially in a zone where the stress is mostly dominated by the effect of the Atlantic ridge push. To assess this problem, we built a time-dependent mechanical model of an elastic crust and viscoelastic mantle underlying the area of interest. We apply an ice cover on the surface of the model that varies according to the time-dependent ice-thickness model of Patton et al., 2016; 2017. The model runs for 50 000 yrs which includes 1) a glaciation phase till the last glacial maximum (LGM) at about -16000 yrs and 2), a deglaciation phase from the last LGM up to present time.&lt;/p&gt;&lt;p&gt;Preliminary results show that the amplitude of the stress change resulting from glacial adjustment, can be of the order of -2 MPa to 2 MPa along Vestnesa ridge. Moreover, the orientation of the maximum horizontal stress (&amp;#963;H) is modified according to the geometry and evolution of the ice cover, just as to the topography of the region affected by the lithospheric adjustment.&lt;/p&gt;

Do pre-existing basement structures influence the geometry and growth of normal faults and rifts?

&lt;p&gt;Rifts often evolve on a template of crystalline basement that may contain strong lithological and mechanical heterogeneities related to complex pre-rift tectonic histories. Numerous studies argue that reactivation of such pre-existing structures can influence the geometry and evolution of normal faults and rift physiography. However, in many cases: (i) it is unclear where, if at all, structures at the rift margin continue along-strike below the rift axis; and (ii) the precise geometric and kinematic relationship between pre-existing structures and newly formed normal faults is not well understood. These uncertainties reflect the fact that: (i) potential field data are typically of low-resolution, and thus cannot resolve the detailed morphology of shallow fault networks; (ii) field data cannot provide an accurate 3D image of intra-basement structures and the overlying rift; and (iii) seismic reflection data typically do not image deeply buried intra-basement structures. Understanding the kinematic as well as geometric relationship between intra-basement structures and rift-related fault networks is important for understanding plate motions and for undertaking stress inversions, given that paleo-extension directions (and sigma 3) are, in many rifted provinces, typically thought to lie normal to the dominant fault strike.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;We here tackle these problems using subsurface data from the Taranaki Basin, offshore New Zealand, and the northern North Sea, offshore west Norway. Our data provide excellent imaging of shallowly buried intra-basement structures, as well as cover-hosted normal faults and their associated pre- and syn-growth strata. We identify a range of intra-basement structures, both extensional and contractional,, and a range of geometric and kinematic interactions between intra-basement structures and cover normal faults. For example, some of the normal faults are physically connected to intra-basement structures oriented oblique to the regional extension direction. It is notable that, even in cases, intra-basement structures were apparently not extensionally reactivated during the later rift phase. Displacement maxima on cover faults occur at 100-200 m above the crystalline basement-cover interface, suggesting the former did not form due to simple extensional reactivation and upward propagation of pre-existing structures; rather, &amp;#8216;passive&amp;#8217; basement structures somehow perturbed the regional stress field, leading to the development of normal faults whose strikes mimic those of the underlying pre-existing basement structures. Cover normal faults can also display a range of complex geometries related to the linkage of numerous, originally separate slip surfaces, and upward-bifurcation of strongly segmented fault systems. We also show that the timing of physical linkage between basement and cover structures can be recorded in the geometry of related growth strata, which document the switch from non-rotational to rotational faulting.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Our analyses show that km-scale, intra-basement structures can control the nucleation and development of newly formed, rift-related normal faults, most likely due to a local perturbation of the regional stress field. Because of this, simply inverting fault strike for causal extension direction may be incorrect, especially in provinces where pre-existing, intra-basement structures occur. We also show that a detailed kinematic analysis is key to deciphering the temporal as well as the geometric relationships between structures developed at multiple structural levels.&lt;/p&gt;