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>

The search for hard-rock kappa (κ) in NGA-East: A semi-automated method for large, challenging datasets in stable continental regions

This article describes the work undertaken within the Next Generation Attenuation (NGA)-East project with the aim of estimating κ0 (the site-specific component of the high-frequency decay parameter, κ) for rock sites in Central and Eastern North America (CENA), using the project’s shallow crustal dataset. We introduce a methodology to address the numerous challenges in CENA: a large dataset in a low-seismicity stable continental region, with poor magnitude and distance coverage, undesirable recording sensor characteristics (low sampling rates leading to poor high-frequency resolution), high uncertainty in the regional stress drop, and lack of site-specific velocity characterization. We use two band-limited κ estimation approaches, the acceleration and displacement spectrum (AS and DS), applied above and below the source corner frequency ( fc), respectively. For band-limited approaches, the key requirement is an estimate of fc, which—apart from the event magnitude readily available in the flatfile—also heavily depends on the highly uncertain stress drop. By considering lower and upper bounds on regional stress drop, we propose a new method to quickly and automatically screen such very large datasets to identify all possible recordings for which band-limited κ approaches can be used. Combining them produces better-quantify estimates of κ and its epistemic uncertainties for this challenging dataset. The mean κ0 values combining the two methods are 13 ± 23 ms for horizontal ground motion.

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.

A time dependent model of elastic stress in the Central Apennines

&lt;p&gt;Seismicity in the Central Apennines is characterized by normal faulting with dip NE-SW near 45&amp;#176;. We show that if the stress at the hypocenter of the 2016 Norcia (Mw=6.5) and 2009 L&amp;#8217;Aquila (Mw=6.3 on the Paganica fault) earthquakes originated only from stress transfer from previous historical events occurred in 1315 and 1461 (L&amp;#8217;Aquila), 1703 (Montereale plain) and 1703 (Norcia/Valnerina), then the orientation of the principal stress axes would be inconsistent with the observed tensional regime. The additional contribution of a regional stress is thus required to properly align the principal stress axes to those of the moment tensor, but GNSS geodesy provides only stress rates. We empirically estimate a time multiplier for the regional stress rate, computed with a dense GNSS network, such that the principal stress axes resulting from the sum of the stress transferred by previous events and the regional stress rate multiplied by the empirical temporal scale are consistent with normal faulting, both at the L&amp;#8217;Aquila and Norcia hypocenters. Based on a Catalogue of 36 events of magnitude larger than 5.6 we estimate the total Coulomb stress at depths and along planes parallel to those of L&amp;#8217;Aquila and Norcia. We provide evidence of an asymmetry of the Coulomb stress leading to a stress concentration near the hypocenter of the two events just prior of the 2009 and 2016&amp;#160; earthquakes. This stress anomaly disappeared after the two events. Similar stress patterns are observed for earlier events which took place in 1461 at L&amp;#8217;Aquila, 1703 on the Montereale plain and in 1703 at Norcia/Valnerina. The 1997 sequence of Colfiorito exhibits a similar, anisotropic Coulomb stress pattern. Based on the Database of Individual Seismogenic Sources DISS 3.2.1 of INGV we identify as areas of maximum Coulomb stress at present (&gt;2016) the Gran Sasso , the Camerino and Sarnano areas and the area between the San Pio delle Camere, Tocco da Casauria and Sulmona faults.&lt;/p&gt;