Shear Wave Splitting Delay Times Research Articles

Seismic Anisotropy of Mafic Blueschists: EBSD‐Based Constraints From the Exhumed Rock Record

AbstractSeismic anisotropy constitutes a useful tool for imaging the structure along the plate interface in subduction zones, but the seismic properties of mafic blueschists, a common rock type in subduction zones, remain poorly constrained. We applied the technique of electron backscatter diffraction (EBSD) based petrofabric analysis to calculate the seismic anisotropies of 14 naturally deformed mafic blueschists at dry, ambient conditions. The ductilely deformed blueschists were collected from terranes with inferred peak P‐T conditions applicable to subducting slabs at or near the plate interface in active subduction zones. Epidote blueschists display the greatest P wave anisotropy range (AVp ∼7%–20%), while lawsonite blueschist AVp ranges from ∼2% to 10%. S wave anisotropies generate shear wave splitting delay times up to ∼0.1 s over a thickness of 5 km. AVp magnitude increases with glaucophane abundance (from areal EBSD measurements), decreases with increasing epidote or lawsonite abundance, and is enhanced by glaucophane crystallographic preferred orientation (CPO) strength. Two‐phase rock recipe models provide further evidence of the primary role of glaucophane, epidote, and lawsonite in generating blueschist seismic anisotropy. The symmetry of P wave velocity patterns reflects the deformation‐induced CPO type in glaucophane—an effect previously observed for hornblende on amphibolite P wave anisotropy. The distinctive seismic properties that distinguish blueschist from other subduction zone rock types and the strong correlation between anisotropy magnitude/symmetry and glaucophane CPO suggest that seismic anisotropy may be a useful tool in mapping the extent and deformation of blueschists along the interface, and the blueschist‐eclogite transition in active subduction zones.

Pre‐Alpine Fault Fabrics in Mantle Xenoliths From East Otago, South Island, New Zealand

AbstractWe present a quantitative microstructural and seismic property analysis of harzburgitic and lherzolitic xenoliths from East Otago, New Zealand. Olivine crystallographic preferred orientations (CPOs) are dominated by orthorhombic CPOs with a tight maxima of [100] orthogonal to (010). Orthopyroxene showed the [001] axes aligned (sub)parallel to the [100] axes of olivine and the (100) plane subparallel to the (010) plane of olivine. Misorientations of olivine subgrain boundaries indicate that dislocations with [100] slip vectors are common and CPOs are interpreted as olivine [100] representing the flow direction and (010) the shear plane. Olivine grains are coarse with lobate grain boundaries, indicative of grain boundary migration at high temperatures. Olivine contains numerous subgrains whilst orthopyroxene is mostly strain‐free and it is possible that orthopyroxene crystallized from melt during deformation. Clinopyroxene lamellae in orthopyroxene, relate to decompressional cooling prior to Alpine Fault movement. Seismic properties derived from the measured CPOs, indicate P‐wave and S‐wave anisotropies at mantle conditions of 4%–10% and 3%–9%, respectively. Calculated shear wave splitting delay times of 0.1–0.9 s, for the current lithospheric mantle thickness, fit seismological observations, suggesting that the xenoliths may be representative of the lithospheric mantle that lies below East Otago today. To explain the seismic data, olivine [100] needs to be subhorizontal and parallel to NNW‐SSE fast polarization directions with [010] subhorizontal and perpendicular to these directions. This is consistent with the East Otago lithospheric mantle containing a fossil vertical shear zone with NNW‐SSE strike slip motion.

Strong seismic anisotropy in the deep upper mantle beneath the Cascadia backarc: Constraints from probabilistic finite-frequency SKS splitting intensity tomography

The volcanically active High Lava Plains (HLP) region is a striking tectonomagmatic feature of eastern Oregon, in the backarc of the Cascadia subduction zone. It features young (<12 Ma), bimodal volcanic activity; the rhyolitic volcanism has a spatiotemporal trend that is oblique to that expected from the absolute motion of the North American plate. Several models have been proposed to explain the tectonic evolution of Cascadia backarc and the relationships between upper mantle processes and volcanic activity; however, consensus remains elusive. Because seismic anisotropy in the upper mantle reflects processes such as mantle flow and partial melting, constraints on anisotropic structure can shed light on the connections between mantle dynamics and tectonomagmatic activity in the Cascadia backarc. Anisotropy is often constrained via SKS splitting measurements; however, their interpretation is typically ambiguous because they lack depth resolution. Here we present new constraints on upper mantle anisotropy beneath the HLP region from probabilistic finite-frequency SKS splitting intensity tomography, which provides both lateral and depth constraints on anisotropic structure. Our technique is based on a Markov chain Monte Carlo approach to searching parameter space, and we use finite-frequency sensitivity kernels to relate model perturbations to splitting intensity observations. We use data from broadband stations of the dense High Lava Plains experiment, which provide good resolution of upper mantle anisotropic structure, as demonstrated via resolution tests. We find evidence for particularly strong seismic anisotropy in the deep upper mantle (200-400 km depth) beneath the Cascadia backarc, suggesting that flow in the deep upper mantle, rather than alignment of partial melt in the shallow mantle, provides the first-order control on shear wave splitting delay times. Our model provides additional support for the idea that mantle flow beneath the Cascadia backarc is controlled by rollback subduction. Our results suggest that anisotropy in the deep upper mantle may be more important to the interpretation of SKS splitting measurements in some settings than commonly appreciated, and provides an avenue for reconciling apparently contradictory constraints on anisotropic structure from surface waves and SKS splitting.

Lateral and Vertical Heterogeneity in the Lithospheric Mantle at the Northern Margin of the Pannonian Basin Reconstructed From Peridotite Xenolith Microstructures

AbstractThis study analyzes the microstructures and deformational characteristics of spinel peridotite xenoliths from the Nógrád‐Gömör Volcanic Field (NGVF), located on the northern margin of a young extensional basin presently affected by compression. The xenoliths show a wide range of microstructures, bearing the imprints of heterogeneous deformation and variable degrees of subsequent annealing. Olivine crystal preferred orientations (CPOs) have dominantly [010]‐fiber and orthorhombic patterns. Orthopyroxene CPOs indicate coeval deformation with olivine. Olivine J indices correlate positively with equilibration temperatures, suggesting that the CPO strength increases with depth. In contrast, the intensity of intragranular deformation in olivine varies as a function of the sampling locality. We interpret the microstructures and CPO patterns as recording deformation by dislocation creep in a transpressional regime, which is consistent with recent tectonic evolution in the Carpathian‐Pannonian region due to the convergence between the Adria microplate and the European platform. Postkinematic annealing is probably linked to percolation of metasomatism by mafic melts through the upper mantle of the NGVF prior to the eruption of the host alkali basalt. Elevated equilibration temperatures in xenoliths from the central part of the volcanic field are interpreted to be associated with the last metasomatic event, which only shortly preceded the ascent of the host magma. Despite well‐developed olivine CPOs in the xenoliths, which imply a strong seismic anisotropy, the lithospheric mantle alone cannot account for the shear wave splitting delay times measured in the NGVF, indicating that deformation in both the lithosphere and the asthenosphere contributes to the observed shear wave splitting.

Fluid‐Enhanced Annealing in the Subcontinental Lithospheric Mantle Beneath the Westernmost Margin of the Carpathian‐Pannonian Extensional Basin System

AbstractMantle xenoliths from the Styrian Basin Volcanic Field (Western Pannonian Basin, Austria) are mostly coarse granular amphibole‐bearing spinel lherzolites with microstructures attesting for extensive annealing. Olivine and pyroxene CPO (crystal‐preferred orientation) preserve nevertheless the record of coeval deformation during a preannealing tectonic event. Olivine shows transitional CPO symmetry from [010]‐fiber to orthogonal type. In most samples with [010]‐fiber olivine CPO symmetry, the [001] axes of the pyroxenes are also dispersed in the foliation plane. This CPO patterns are consistent with lithospheric deformation accommodated by dislocation creep in a transpressional tectonic regime. The lithospheric mantle deformed most probably during the transpressional phase after the Penninic slab breakoff in the Eastern Alps. The calculated seismic properties of the xenoliths indicate that a significant portion of shear wave splitting delay times in the Styrian Basin (0.5 s out of approximately 1.3 s) may originate in a highly annealed subcontinental lithospheric mantle. Hydroxyl content in olivine is correlated to the degree of annealing, with higher concentrations in the more annealed textures. Based on the correlation between microstructures and hydroxyl content in olivine, we propose that annealing was triggered by percolation of hydrous fluids/melts in the shallow subcontinental lithospheric mantle. A possible source of these fluids/melts is the dehydration of the subducted Penninic slab beneath the Styrian Basin. The studied xenoliths did not record the latest large‐scale geodynamic events in the region—the Miocene extension then tectonic inversion of the Pannonian Basin.

Lithospheric thickness and upper-mantle deformation beneath the NE Tibetan Plateau inferred fromSreceiver functions andSKSsplitting measurements

SUMMARY We present S receiver functions and SKS splitting measurements from the China Seismograph Network located in the Qinghai and Gansu provinces. Teleseismic data are used to interpret the structure of the lithosphere–asthenosphere boundary (LAB) and upper-mantle deformation beneath the northeastern Tibetan Plateau (NETP) in regions north of the east Kunlun Fault. Based on our observations, the LAB lies at a depth of 125–135 km beneath the northeastern Songpan–Ganzi block and the west Qinling orogen, between 145 and 175 km beneath the Kunlun and Qilian orogen, and deepens below the Qaidam Basin (175–190 km), Ordos Craton (170 km) and Alashan platform (200 km). The NETP is characterized by a nearly uniform fast NW–SE S-wave direction. These observations are different from those to the south of the Kunlun Fault where fast S directions are rotating clockwise from the inner plateau. The change in fast directions across the Kunlun Fault implies a sudden variation of upper-mantle deformation. Shear wave splitting delay times vary from 0.8 to 1.9 s. Data from beneath regions north of the Kunlun–Ayimaqin suture showed that delay time was positively correlated with lithospheric thickness with an increase of 0.7 s per 100 km. This indicates that the anisotropy may develop in the uppermost mantle, such as the lithosphere, beneath the NETP.

Shear wave splitting,vP/vS, and GPS during a time of enhanced activity at Aso caldera, Kyushu

[1] We performed shear wave splitting analyses and calculated vP/vS-ratios on local earthquakes recorded at seven stations around Aso volcano, Japan, between 2001 and 2008. The mean vP/vS of 1.61 ± 0.01 from all stations is among the lowest average values found anywhere in the world. A temporal change of shear wave splitting fast polarization is observed at one station. Additionally, spatial variations of shear wave splitting delay times and vP/vS-ratios show a correlation at three stations. We suggest that stress-controlled anisotropy dominates at these stations. GPS baselines and 2D strain reveal extension in the caldera in 2003, and compression before and after 2003. Strain shortening axes indicate either a variation in regional stress or a local stress source that overprints the regional field in 2003. Baseline changes correlate with periods of increased low frequency seismic activity located deeper than 15 km. Compaction and degassing of a previously reported sill at approximately 15 km depth could lead to constantly high pressure of volcanic gases over a wide area and explain the generally low vP/vS-ratios and the observed compression in the caldera. We propose that a small volume of magma intruded into the sill in 2003, caused extension at the surface, and triggered ash eruptions between 2003 and 2005. Associated localized migration of gases in overpressured cracks with aspect ratios potentially ∼0.5 in the upper crust with porosities ≥0.1 could be the source of changes in seismic anisotropy, and explain the slight variations observed in vP/vS.

Three‐dimensional shear‐wave splitting tomography in the Parkfield, California, region

We developed a three‐dimensional (3D) shear‐wave splitting tomography method to image the spatial anisotropy distribution by back projecting shear wave splitting delay times along ray paths derived from a 3D shear velocity model, assuming the delay times are accumulated along the ray paths. The local strength of the anisotropy is indicated by a parameter of anisotropy percentage, K. Using the shear‐wave splitting delay times for 575 earthquakes measured at PASO and HRSN stations, we imaged a detailed 3D anisotropy percentage model around the San Andreas Fault Observatory at Depth (SAFOD). The anisotropy percentage model shows strong heterogeneities, consistent with the strong spatial variations in both measured delay times and fast polarization directions. The San Andreas Fault (SAF) zone is highly anisotropic down to a depth of ∼4 km and then becomes less anisotropic at greater depths. Outside the fault zone, the highly anisotropic zone extends as deep as ∼7 km, consistent with the systematic depth dependence of the average time delays. To the southwest of the SAF, the Salinian granitic block shows relatively strong anisotropic anomalies that are presumably caused by aligned microcracks consistent with the direction of the regional maximum compressive horizontal stress. To the northeast of the fault zone, a strong anisotropic anomaly between depths ∼2 and ∼4 km corresponds to a serpentinite body sandwiched between Franciscan rocks.

Spatiotemporal variations of crustal anisotropy from similar events in aftershocks of the 1999M7.4 İzmit andM7.1 Düzce, Turkey, earthquake sequences

We analyse spatiotemporal variations of crustal anisotropy along the Karadere–Düzce branch of the North Anatolian Fault from similar earthquakes in the aftershock regions of the 1999 Mw7.4 İzmit and Mw7.1 Düzce earthquakes. The similar earthquake clusters are identified by performing cross-correlation on waveforms generated by ∼18 000 earthquakes. Depending on the applied similarity criterion, about 4–60 per cent of the events belong to similar event clusters. Splitting parameters averaged within each cluster show significant variations for slightly different ray paths, indicating strong spatial variations of crustal anisotropy in this area. We also find clear changes in the spatiotemporal seismicity patterns following the Düzce main shock. Large apparent co-seismic changes (up to 30 per cent) of shear wave splitting delay times are observed across the time of the Düzce main shock at stations near the epicentral region. However, the changes can be mostly explained by the spatial variations of ray paths due to the changing seismicity, rather than changes in the properties of the anisotropic medium. Splitting parameters measured within similar earthquake clusters indicate at most 2 per cent changes in delay times associated with the occurrence of the Düzce main shock. The results do not show systematic precursory changes before the Düzce main shock.

Seismic anisotropy and mantle flow from the Great Basin to the Great Plains, western United States

Shear wave splitting and P, SKS, and S travel time residuals are calculated for teleseismic arrivals recorded on the Colorado Plateau‐Great Basin Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) portable broadband seismic deployment and for permanent stations in the western United States. Little shear wave splitting is observed for broadband recordings in the northern Colorado Plateau, the Rocky Mountains, or the central Great Basin. The transition between the Colorado Plateau and the Great Basin is marked by moderate shear wave splitting (1.0 s) and unusually late teleseismic phase arrivals. This suggests material with a higher content of mantle melt or volatiles than regions to either side. Splitting in the transition between the Colorado Plateau and Great Basin is part of a pattern of fast polarizations that align in a semicircle, surrounding a central Great Basin region of null (no splitting) measurements. Away from the California plate boundary, splitting to the north and south of our study region aligns roughly parallel to the absolute plate motion of the North American plate. No simple spatial relation of splitting with geological and geophysical features such as mountain ranges, velocity anomalies, gravity, magnetics, or heat flow is evident in most of the western United States. However, splitting in the Great Basin is compatible with asthenospheric flow. The smallest shear wave splitting delay times coincide with the Eureka Low in heat flow, also having low S velocity at 300 km depth. We suggest that the circumferential pattern of fast polarization directions ringing a central region of nulls in the Great Basin is caused by mantle flow, by the interaction of mantle up welling and the absolute motion of the North American plate.

Mantle variation within the Canadian Shield: Travel times from the portable broadband Archean‐Proterozoic Transect 1989

We report travel times from the Archean‐Proterozoic Transect 1989. This type of data set recorded by a transect of portable broadband instruments allows us to make inferences about mantle structure in the region between the Wyoming Craton and the Superior Province of the Canadian Shield. With station separations of 50 to 100 km and frequencies up to 5 Hz the resolution of lateral changes is increased by nearly an order of magnitude over previous studies to a scale that allows us to study the relation between velocity variation in the continental upper mantle, surface geology, tectonic features and age provinces. Travel times of direct and later phases are obtained from waveform matching. The values are corrected for crustal contribution and inverted for the vertical path upper mantle delay δtUM under each station as well as the azimuthal dependence of this quantity. The prominent feature in the upper mantle delays δtUM is the variation by at least 1.5 s for S, much of which occurs over a narrow zone of just a few hundred kilometers width. This suggests a major lateral upper mantle transition which does not coincide with the surface geological edge of the Canadian Shield but is located within the shield. This same transition is also observed in shear wave splitting delay times. Surprisingly, however, the P delays do not exhibit a corresponding variation. We address this apparent contradiction and show how it may be explained in conjunction with anisotropy in the subcontinental lithosphere. A simplified thermal model of the lithospheric transition zone, in which temperature controls the degree of crystallographic alignment and thus seismic anisotropy, predicts this phenomenon.

Quantitative measurements of shear wave polarizations at the Anza Seismic Network, southern California: Implications for shear wave splitting and earthquake prediction

We analyze shear wave polarizations from local earthquakes recorded by the Anza network in southern California, using an automated method which provides unbiased and quantitative measurements of the polarization and the duration of linear motion following the shear wave arrival (the linearity interval). Initial shear wave particle motions are strongly aligned at four stations, a feature that is not predicted by focal mechanisms. The particle motion alignment is most likely caused by shear wave splitting due to anisotropy beneath these stations, a result supported by the clear shear wave splitting seen in a borehole recording near one of the Anza stations. These results are consistent with an earlier analysis of these data by Peacock et al. [1988]. However, our analysis does not support claims by Crampin et al. [1990] that shear wave splitting delay times at station KNW exhibit temporal variations which can be correlated with the occurrence of the North Palm Springs earthquake (ML=5.6) of July 8, 1986. Automatically determined linearity intervals scatter widely from 0.02 to 0.15 s and exhibit no clear temporal trends. We find a correlation between earthquake moment and the linearity interval, possibly a result of longer effective source time functions for the larger events. The inability to identify a distinct slow quasi‐shear wave pulse for the vast majority of these events indicates that scattering strongly affects the particle motion, even in the very early shear wave coda. Analysis of earthquake clusters with similar waveforms recorded at KNW shows that seismic Green functions are stable throughout the observational period and that most linearity interval variation is due to source and ray path differences between events. If shear wave splitting is causing the observed delay times between horizontal components, the waveform stability for events in these clusters restricts any temporal changes in shear wave splitting delay times to less than 5–10%.