Martha K. Savage

Lower crustal anisotropy or dipping boundaries? Effects on receiver functions and a case study in New Zealand

We examine the effects on receiver functions of transverse anisotropy and of dipping isotropic boundaries. Splitting of the Moho Ps phase predicts the anisotropy from a postulated 10-km-thick layer of highly anisotropic crustal material only when other phases can be well isolated and when allowance is made for the rotation of the incident energy out of the plane of energy propagation expected for isotropic models. We examine azimuthal variations of the synthetic radial and transverse receiver functions. For both transversely anisotropic layers and dipping isotropic boundaries, radial receiver functions are symmetric and transverse receiver functions are antisymmetric about a given back azimuth (the horizontal projection of the axis of symmetry or the dip direction). Differences include the following: (1) Transverse energy from dipping boundaries arrives at the time of the initial P phase no matter at what depth the dipping boundaries occur, while transverse energy arrives at the time of the initial P phase for anisotropic material only when the anisotropy is at the surface. (2) Transversely anisotropic systems with horizontal symmetry axes have waveforms with 180° periodicity as a function of back azimuth, while dipping symmetry axes or dipping boundaries just have 360° periodicity. A consequence of the symmetry is that stacking the initial P and Ps phases from transverse receiver functions for back azimuths that are 180° apart tends to decrease the arrivals from shallowly dipping isotropic boundaries and doubles the arrivals from anisotropic layers with horizontal symmetry axes. We examine receiver functions for station SNZO in New Zealand, above a subducting plate dipping at 15° beneath the station. Opposite polarities of transverse receiver functions separated by 180° in back azimuth suggest that either dipping isotropic layers or dipping axes of symmetry of anisotropic layers are present. The 15° dip of the subduction zone is insufficient to explain the energy on the transverse receiver functions by the isotropic structures previously determined from refraction and earthquake travel time inversion. However, modifying the isotropic models for the region by anisotropy determined via shear wave splitting measurements and refraction studies, allows many of the features of both the radial and transverse receiver functions to be explained. A relatively highly anisotropic layer about 7 km thick at the base of the crust with an axis of symmetry dipping at 20° - 40° may be caused by metamorphosed oceanic crust.

Mantle deformation and tectonics: constraints from seismic anisotropy in the western United States

Abstract We have examined shear-wave splitting in teleseismic shear waves (SKS, SKKS, S) from 15 stations in the western United States, based on analysis of 123 records from 67 events. The varied past and present tectonic styles in this region (subduction, transform faulting, extension, and stable domains) are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory. Fast polarization azimuths (φ) vary from E-W to nearly N-S and time delays (δt) range from being barely detectable (less than 0.6 s) to 1.6 s. Most stations yielded consistent measurements independent of station-event geometry. The exceptions were stations situated very close to the San Andreas Fault, which yielded well-constrained but inconsistent splitting parameters. These have been successfully modeled by two anisotropic layers with different horizontal symmetry axes. The upper layer has a fast direction parallel to the fault and δt of about 1 s. The lower layer, with δt from 0.6 to 0.9 s, is oriented E-W near the San Francisco Bay Area and NE-SW in the Mojave Desert. Other measurements of E-W fast φ are observed as far east as western Nevada, with large delay times of 1.3–1.5 s, but disappear to the north. Stations in the northwestern Basin and Range have values of φ oriented at about + 70° with delay times ranging from 0.7 to 1.2 s. φ varies in other regions of the Basin and Range, from − 70° in eastern Nevada to + 20° in the transition zone between the Colorado Plateau and the Basin and Range, with δt of 1.0 s. Measurements are geographically coherent, and in many cases changes correspond to geologic or tectonic boundaries. The E-W fast feature extends across several tectonic provinces, but disappears near the southern boundary of the Gorda plate, and may therefore be related to the dynamics of plate interactions. Regional variations in the fast directions of anisotropy within the Basin and Range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent. Fast polarization directions within the transition zone between the Basin and Range and Colorado Plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries. The size of the delay times suggests that in many of the areas studied, either the mantle is almost twice as anisotropic as we estimate, or the anisotropy, and therefore mantle deformation, extends into the sublithospheric mantle. We used the two-layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the North American Plate, and found the best estimate of contribution from a uniform layer is 0.1 s of δt, although a contribution of up to 0.7 s cannot be ruled out. A value as low as 0.1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer, or that the layer thickness is of the order of 10 km.

Frequency-dependent anisotropy in Wellington, New Zealand

Shear-wave splitting measurements are made using teleseismic S, ScS and SKS waveforms recorded at the GDSN broadband station SNZO, situated in South Karori, New Zealand. The average S and SKS delay times are around 2 to 3 s, among the highest in the world. The average ScS delay time is significantly smaller, around 1 s. This discrepancy appears to be due to differences in dominant frequency. The most likely cause of frequency-dependent anisotropy is oriented heterogeneities with a scale-length much smaller than the wavelength. The fast polarizations range between 21° and 79°, with an average NE-SW direction which is sub-parallel to the trend of the local geologic structure and the strike of the Hikurangi subduction zone. Azimuthal variations in delay time, which cannot be explained by differences in period, may be due to a dipping axis of symmetry, or laterally varying anisotropy, or a more complicated symmetry system.

Shear‐wave splitting beneath western United States in relation to plate tectonics

We have examined shear wave splitting in teleseismic shear waves from 26 broadband stations in the western United States. Fast polarization directions (ϕ) and delay times (δt) show spatial variations that are coherent within geologic provinces. Stations located near the San Andreas fault show clear evidence for fault-parallel anisotropy in the crust and upper mantle (115–125 km thickness). This can be explained by the finite strain associated with the relative plate motion between the North American and Pacific plates. The lateral extent of this strain field is probably narrow to the west, because stations 55 km west of the San Andreas fault do not show fault-parallel anisotropy in southern California. Station LAC located 80 km east of the San Andreas fault shows large fault-parallel anisotropy. This suggests that the Pacific-North American plate boundary in the mantle might be displaced to the east in southern California. A deeper E-W oriented fast direction of anisotropy underlies the fault-parallel anisotropic layer in the vicinity of the San Andreas fault. An E-W fast feature is also present beneath the western Basin and Range and the foothills of the Sierra-Nevada, although local variations are present. The magnitude of delay times suggests that this feature resides in the asthenosphere. We interpret this feature as the asthenospheric flow in the slabless window left behind the Farallon plate. The flow-induced anisotropy may partially be frozen-in at shallow depths. Station ORV is located near the southern edge of the Gorda slab where no anisotropy is detected. The absence of anisotropy at this location could therefore mark a boundary between Farallon associated flow and regions where E-W oriented asthenospheric flow did not occur. The lack of evidence for NE-SW fast orientation within the Walker Lane Shear Belt of western Nevada suggests that this crustal feature does not extend into the mantle or that is not as well developed as that beneath the San Andreas fault. Stations located over the young subducting Gorda plate mark a change in the fast direction to nearly NE-SW. This direction aligns well with the maximum compressive stress direction in the overlying North American plate and the NE-SW directed internal shearing of the Gorda plate. The anisotropic thicknesses calculated from delay times suggest roughly double that expected for purely lithospheric contributions. This implies that the anisotropic thickness may include some of the asthenosphere. Alternatively, using a higher anisotropy of 8% can bring thicknesses in line with other measures of lithospheric thicknesses. The correspondence between the fast directions and the present plate tectonic deformations suggest that mapping upper mantle deformation through seismic anisotropy is a viable method, and that asthenospheric flow may be a significant contributor to seismic anisotropy.

Upper mantle anisotropy in the New Zealand Region

Shear-wave splitting parameters of fast polarization direction (Φ) and delay time (δt) are determined using data from the Southern Alps Passive Seismic Experiment (SAPSE), on the South Island of New Zealand and in the surrounding region. Our results clearly show that Φ are subparallel to trends of the Alpine and Marlborough Faults, and to the Pacific-Australian plate boundary. The δt values range from 0.6–2.2 s with an average value of 1.6 s; the largest values are from the central South Island. The main source of the observed shear-wave splitting is an anisotropic region between 40–400 km. The width of the zone is approximately 200 km. We attribute the coincidence of surface structural trends with the measured Φ, and the large δt values, to significant shear deformation in a 200 km thick zone along the plate boundary extending from the surface to deep within the upper mantle.

Seismic anisotropy from local earthquakes in the transition region from a subduction to a strike-slip plate boundary, New Zealand

Shear wave splitting is used to investigate anisotropy in the crust and upper mantle in a subduction zone (lower half of the North Island of New Zealand), and its transition to oblique transform faulting (Marlborough area, northern South Island). In Marlborough, delay times show almost no increase with depth, and it is most likely that the higher-frequency phases used in this study respond mainly to lithospheric anisotropy. In the central Marlborough Fault System (MFS), fast polarizations are subparallel to the faults. Anisotropy is attributed to the presence of metamorphosed schist (eclogite), of 30 ± 10 km thickness and located 50-80 km beneath the MFS. On the edges of the MFS, fast polarizations are parallel to the maximum compressive stress direction, consistent with crack-induced anisotropy in the crust. The shear zone, which is as wide as the island in the mantle as inferred from SKS phases, seems to occur in a narrower zone in the crust. In the lower half of the North Island, fast polarizations from events at all depths are oriented parallel to the strike of the Hikurangi subduction zone as well as to the faults. Polarisations are similar to those of SKS phases, which mainly sample the mantle. This suggests that the lithosphere and the upper mantle asthenosphere deform in a coherent strike-slip shear. We calculate 1.2 ± 0.3% velocity anisotropy in the first 200 km of the mantle from increasing delay times with depth. In order to match the SKS delay times, this result requires the presence of anisotropic material down to 580 ± 100-km depth, or a change in anisotropy with depth, or frequency dependent splitting.

Seismic anisotropy beneath the lower half of the North Island, New Zealand

Teleseismic ScS and SKS events recorded on nine broadband seismograph stations have been used to investigate seismic anisotropy beneath the lower half of the North Island, New Zealand. This area lies above the Hikurangi subduction zone, and the array provides ray paths which sample the mantle both above and below the slab. Shear wave splitting measurements give similar fast polarizations and delay times at each station. The average SKS fast polarization is approximately NE-SW, subparallel to the strike of subduction and the major geological features, with an average SKS delay time of 1.6±0.1 s. This lack of variation in splitting parameters suggests that similar fast polarizations are found in both the mantle wedge and the subslab mantle. The anisotropy in the lithospheric portion of the mantle wedge is most likely caused by the preferred orientation of olivine due to the shear deformation associated with oblique convergence. Any anisotropy in the slab is probably due to fossil mineral alignment. Anisotropy in the asthenosphere is most likely caused by the preferred orientation of olivine due to asthenospheric flow. The similar NE-SW fast polarizations found in the asthenosphere both above and below the slab suggest that the mantle flow is in a trench-parallel direction in both regions.

Passive seismic imaging using microearthquakes

We have developed a method of processing seismic signals generated by microearthquakes to image local subsurface structure beneath a recording station. This technique uses the autocorrelation of the vertically traveling earthquake signals to generate pseudoreflection seismograms that can be interpreted for subsurface structure. Processed pseudoreflection data, from microearthquakes recorded in the island of Hawaii, show consistent reflectivity patterns that are interpreted as near-surface horizontal features. Forward modeling of the pseudoreflection data results in a P-wave velocity model that shows reasonable agreement with the velocity model derived from a refraction study in the region. Usable signal-to-noise ratio is obtained down to 2 s. A shear-wave velocity model was also generated by applying this technique to horizontal component data.

Shear wave splitting across the Rocky Mountain Front

We search for evidence of seismic anisotropy by determining shear-wave splitting parameters using teleseismic phases from stations placed across the Rocky Mountain Front. Three features are striking: 1) fast polarization orientations are consistent within small geographic regions, but vary rapidly across the network, 2) a large number of high quality “null” measurements indicate that little transverse anisotropy with horizontal symmetry axis is present, and 3) stations with well-constrained but inconsistent parameters for rays from different sources imply that a single layer of anisotropic material with a horizontal symmetry axis is an inadequate parameterization. The general pattern suggests fast axes pointing toward (or away from) a central region of little anisotropy. A model of asthenospheric flow converging on or diverging from the central uplifted region is postulated. Within the southern Rocky Mountains, parameters are similar to those in the nearby northern Rio Grande Rift, suggesting that a similar mechanism causes the anisotropy in both regions.

Extreme hydrothermal conditions at an active plate-bounding fault

Temperature and fluid pressure conditions control rock deformation and mineralization on geological faults, and hence the distribution of earthquakes. Typical intraplate continental crust has hydrostatic fluid pressure and a near-surface thermal gradient of 31 ± 15 degrees Celsius per kilometre. At temperatures above 300–450 degrees Celsius, usually found at depths greater than 10–15 kilometres, the intra-crystalline plasticity of quartz and feldspar relieves stress by aseismic creep and earthquakes are infrequent. Hydrothermal conditions control the stability of mineral phases and hence frictional–mechanical processes associated with earthquake rupture cycles, but there are few temperature and fluid pressure data from active plate-bounding faults. Here we report results from a borehole drilled into the upper part of the Alpine Fault, which is late in its cycle of stress accumulation and expected to rupture in a magnitude 8 earthquake in the coming decades. The borehole (depth 893 metres) revealed a pore fluid pressure gradient exceeding 9 ± 1 per cent above hydrostatic levels and an average geothermal gradient of 125 ± 55 degrees Celsius per kilometre within the hanging wall of the fault. These extreme hydrothermal conditions result from rapid fault movement, which transports rock and heat from depth, and topographically driven fluid movement that concentrates heat into valleys. Shear heating may occur within the fault but is not required to explain our observations. Our data and models show that highly anomalous fluid pressure and temperature gradients in the upper part of the seismogenic zone can be created by positive feedbacks between processes of fault slip, rock fracturing and alteration, and landscape development at plate-bounding faults.