Yingjie Yang

Seismic evidence for widespread western-US deep-crustal deformation caused by extension

Laboratory experiments have established that many of the materials comprising the Earth are strongly anisotropic in terms of seismic-wave speeds. Observations of azimuthal and radial anisotropy in the upper mantle are attributed to the lattice-preferred orientation of olivine caused by the shear strains associated with deformation, and provide some of the most direct evidence for deformation and flow within the Earth’s interior. Although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes, large-scale observations have been limited to regions of particularly thick crust. Here we show that observations from ambient noise tomography in the western United States reveal strong deep (middle to lower)-crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the Cenozoic Era (since ∼65 Myr ago). The coincidence of crustal radial anisotropy with the extensional provinces of the western United States suggests that the radial anisotropy results from the lattice-preferred orientation of anisotropic crustal minerals caused by extensional deformation. These observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension.

Rayleigh wave phase velocity maps of Tibet and the surrounding regions from ambient seismic noise tomography

Ambient noise tomography is applied to the significant data resources now available across Tibet and surrounding regions to produce Rayleigh wave phase speed maps at periods between 6 and 50 s. Data resources include the permanent Federation of Digital Seismographic Networks, five temporary U.S. Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) experiments in and around Tibet, and Chinese provincial networks surrounding Tibet from 2003 to 2009, totaling ∼600 stations and ∼150,000 interstation paths. With such a heterogeneous data set, data quality control is of utmost importance. We apply conservative data quality control criteria to accept between ∼5000 and ∼45,000 measurements as a function of period, which produce a lateral resolution between 100 and 200 km across most of the Tibetan Plateau and adjacent regions to the east. Misfits to the accepted measurements among PASSCAL stations and among Chinese stations are similar, with a standard deviation of ∼1.7 s, which indicates that the final dispersion measurements from Chinese and PASSCAL stations are of similar quality. Phase velocities across the Tibetan Plateau are lower, on average, than those in the surrounding nonbasin regions. Phase velocities in northern Tibet are lower than those in southern Tibet, perhaps implying different spatial and temporal variations in the way the high elevations of the plateau are created and maintained. At short periods ( 20 s), very high velocities are imaged in the Tarim Basin, the Ordos Block, and the Sichuan Basin. These phase velocity dispersion maps provide information needed to construct a 3-D shear velocity model of the crust across the Tibetan Plateau and surrounding regions.

A synoptic view of the distribution and connectivity of the mid-crustal low velocity zone beneath Tibet

[1] Based on 1–2 years of continuous observations of seismic ambient noise data obtained at more than 600 stations in and around Tibet, Rayleigh wave phase velocity maps are constructed from 10 s to 60 s period. A 3-D Vsv model of the crust and uppermost mantle is derived from these maps. The 3-D model exhibits significant apparently inter-connected low shear velocity features across most of the Tibetan middle crust at depths between 20 and 40 km. These low velocity zones (LVZs) do not conform to surface faults and, significantly, are most prominent near the periphery of Tibet. The observations support the internal deformation model in which strain is dispersed in the deeper crust into broad ductile shear zones, rather than being localized horizontally near the edges of rigid blocks. The prominent LVZs are coincident with strong mid-crustal radial anisotropy in western and central Tibet and probably result at least partially from anisotropic minerals aligned by deformation, which mitigates the need to invoke partial melt to explain the observations. Irrespective of their cause in partial melt or mineral alignment, mid-crustal LVZs reflect deformation and their amplification near the periphery of Tibet provides new information about the mode of deformation across Tibet.

Attenuation in the upper mantle beneath Southern California: Physical state of the lithosphere and asthenosphere

[1] We invert phase and amplitude data of Rayleigh waves for attenuation (Q � 1 ) and shear wave velocities beneath southern California using teleseismic sources recorded by the TriNet/USArray network. Fundamental mode surface wave studies from 25 to 143 s period allow us to constrain the vertical variation of shear quality factor Qm in the upper mantle. We use 2-D sensitivity kernels for surface waves based on single-scattering (Born) approximation to account for the effects of scattering on amplitude. A onedimensional shear velocity model reveals a pronounced low velocity zone (LVZ) from � 80 km to � 200 km underlying a high velocity lid. Qm shows a similar pattern; large Qm at depths shallower than 80 km and much smaller Qm at depths greater than 100 km. Models that attribute the variations of attenuation and shear velocities with depth solely to temperature and pressure effects predict too low Qm values if they match the shear velocities. Alternative models considering the presence of partial melt can explain the observed very low Vs velocities in the asthenosphere. Partial melt in the asthenosphere could be generated due to decompression and the reduced solidus for damp mantle when the asthenosphere rose to fill the space left by the subducted Farallon plate.

Crustal and upper mantle structure and the deep seismogenic environment in the source regions of the Lushan earthquake and the Wenchuan earthquake

Following the Mw7.9 Wenchuan earthquake, the Mw6.6 Lushan earthquake is another devastating earthquake that struck the Longmenshan Fault Zone (LFZ) and caused severe damages. In this study, we collected continuous broadband ambient noise seismic data and earthquake event data from Chinese provincial digital seismic network, and then utilized ambient noise tomography method and receiver function method to obtain high resolution shear wave velocity structure, crustal thickness, and Poisson ratio in the earthquake source region and its surroundings. Based on the tomography images and the receiver function results, we further analyzed the deep seismogenic environment of the LFZ and its neighborhood. We reveal three main findings: (1) There is big contrast of the shear wave velocities across the LFZ. (2) Both the Lushan earthquake and the Wenchuan earthquake occurred in the regions where crustal shear wave velocity and crustal thickness change dramatically. The rupture faults and the aftershock zones are also concentrated in the areas where the lateral gradients of crustal seismic wave speed and crustal thickness change significantly, and the focal depths of the earthquakes are concentrated in the transitional depths where shear wave velocities change dramatically from laterally uniform to laterally non-uniform. (3) The Wenchuan earthquake and its aftershocks occurred in low Poisson ratio region, while the Lushan earthquake sequences are located in high Poisson ratio zone. We proposed that the effect of the dramatic lateral variation of shear wave velocity, and the gravity potential energy differences caused by the big contrast in the topography and the crustal thickness across the LFZ may constitute the seismogenic environment for the strong earthquakes in the LFZ, and the Poisson ratio difference between the rocks in the south and north segments of the Longmenshan Fault zone may explain the 5 years delay of the occurrence of the Lushan earthquake than the Wenchuan earthquake.

Improving Epicentral and Magnitude Estimation of Earthquakes from T Phases by Considering the Excitation Function

A standard technique for locating events with T phases is to pick the peak energy of T phases as the arrival time, then proceed as if it was an unscattered phase originating at the epicenter. The peak energy arrival time, however, can shift to different parts of the wave train due to incoherent scattering. We show that a 50% reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the log of the entire envelope of the T phase. We test the stability of this approach by comparing relative event locations based on this method with those determined by cross-correlating waveforms of Rayleigh and Love surface waves recorded teleseismically using a swarm of earthquakes at the northern end of the Easter microplate as an example. Relative event locations show that there is no systematic bias in T-phase locations. The T-phase location and detection can be extended to much smaller events than are detectable by surface waves. We estimate T-phase maximum amplitudes of about 50 events within the swarm from the amplitude of the fitted functions rather than directly from the seismograms. Twenty-four of these events are large enough to estimate their magnitudes by surface- wave analysis. The empirical analysis shows the log of T-phase maximum amplitude and surface-wave magnitude (MS) exhibit a relatively uniform linear relationship with much less scatter than in previously published studies of T-phase amplitude versus magnitude. The reduction in scatter is due to both the stability of the functional estimate of amplitude and the use of a swarm of earthquakes with similar depth, mechanism, and source-receiver geometry. The observed T-phase coda for shallow earthquakes can be synthesized using a simple model of multiple-reverberation seafloor scattering. The results show that scattering of energy at a rough seafloor from multiple reverberations in the water column is significant in T-phase generation even where the water depths are relatively uniform.

Crustal structure determined from ambient noise tomography near the magmatic centers of the Coso region, southeastern California

We apply seismic ambient noise tomography to image and investigate the shallow shear velocity structure beneath the Coso geothermal field and surrounding areas. Data from a PASSCAL experiment operated within the Coso geothermal field between 1998 and 2000 and surrounding broadband stations from the Southern California Seismic Network are acquired and processed. Daily cross correlations of ambient noise between all pairs of stations that overlapped in time of deployment were calculated and then stacked over the duration of deployment. Phase velocities of Rayleigh waves between 3 and 10 s periods are measured from the resulting cross correlations. Depending on the period, between about 300 and 600 reliable phase velocity measurements are inverted for phase velocity maps from 3 to 10 s periods, which in turn are inverted for a 3-D shear velocity model beneath the region. The resulting 3-D model reveals features throughout the region that correlate with surface geology. Beneath the Coso geothermal area shear velocities are generally depressed, a prominent low-velocity anomaly is resolved clearly within the top 2 km, no significant anomaly is seen below about 14 km depth, and a weakly resolved anomaly is observed between 6 and 12 km depth. The anomaly in the top 2 km probably results from geothermal alteration in the shallow subsurface, no magmatic body is imaged beneath 14 km depth, but the shear velocity anomaly between 6 and 12 km may be attributable to partial melt. The thickness and amplitude of the magma body trade off in the inversion and are ill determined. Low velocities in the regions surrounding Coso at depths near 7 km underlie areas with Miocene to recent volcanism, suggesting that some magmatic processing of the crust could be focused near this depth.

Imaging lithospheric foundering in the structure of the Sierra Nevada

Tomographic studies of the mantle of southern California (USA) commonly found evidence for seismically high speed material, known as the Isabella anomaly, extending from near the base of the crust of the southwestern Sierra Nevada foothills into the asthenosphere. This anomaly has been interpreted to mark downwelling lithospheric material that had been removed from the southern Sierra Nevada. Using data from the Sierra Nevada EarthScope Project (SNEP) array, we investigate the lithosphere of the Sierra Nevada and surrounding region to better understand the process by which batholiths form dense lithospheric roots that become unstable and founder. Inverting phase velocities of fundamental mode Rayleigh waves for shear wave speeds provides observations of the distribution of high and low wave-speed anomalies, which correspond to portions of the batholith that formed an intact lithospheric root, and where seismically slower shallow asthenosphere marks areas where lithosphere has been removed. Our results corroborate previous observations that the southern Sierra Nevada has thin crust underlain by shallow asthenosphere. High shear wave velocity (Vs) material in the mantle beneath the southwestern foothills marks the location of the Isabella anomaly, to the east of which is a region of low Vs mantle where asthenosphere has risen to replace the delaminating root. Farther north, near the latitude of Long Valley, low velocities at shallow depths beneath the high elevations of the eastern Sierra indicate the presence of asthenosphere close to the base of the crust. Thicker high-speed material, however, underlies the western foothills of the Sierra Nevada at this latitude and dips to the east where it extends to depths of ∼100 km or more, giving it the appearance of a portion of lithosphere that has detached from the east but remains attached to the west as it is currently peeling off. The structure of the Sierra Nevada changes near the latitude of Lake Tahoe, where thinner lithosphere extends between depths of 40 and 80 km, but does not reach greater depths. It appears that the lithospheric material of the Sierra Nevada from latitudes close to Lake Tahoe, and continuing to the north, is not being removed, indicating a change between the structure and evolution of the southern and northern Sierra Nevada.

3-D multiobservable probabilistic inversion for the compositional and thermal structure of the lithosphere and upper mantle: III. Thermochemical tomography in the Western-Central U.S.

We apply a novel 3-D multiobservable probabilistic tomography method that we have recently developed and benchmarked, to directly image the thermochemical structure of the Colorado Plateau and surrounding areas by jointly inverting P wave and S wave teleseismic arrival times, Rayleigh wave dispersion data, Bouguer anomalies, satellite-derived gravity gradients, geoid height, absolute (local and dynamic) elevation, and surface heat flow data. The temperature and compositional structures recovered by our inversion reveal a high level of correlation between recent basaltic magmatism and zones of high temperature and low Mg# (i.e., refertilized mantle) in the lithosphere, consistent with independent geochemical data. However, the lithospheric mantle is overall characterized by a highly heterogeneous thermochemical structure, with only some features correlating well with either Proterozoic and/or Cenozoic crustal structures. This suggests that most of the present-day deep lithospheric architecture reflects the superposition of numerous geodynamic events of different scale and nature to those that created major crustal structures. This is consistent with the complex lithosphere-asthenosphere system that we image, which exhibits a variety of multiscale feedback mechanisms (e.g., small-scale convection, magmatic intrusion, delamination, etc.) driving surface processes. Our results also suggest that most of the present-day elevation in the Colorado Plateau and surrounding regions is the result of thermochemical buoyancy sources within the lithosphere, with dynamic effects (from sublithospheric mantle flow) contributing only locally up to ∼15–35%. ©2016. American Geophysical Union. All Rights Reserved.

Measurement of Rayleigh wave ellipticity and its application to the joint inversion of high‐resolution S wave velocity structure beneath northeast China

We present a new 3-D Swave velocity model of the northeast (NE) China from the joint inversion of the Rayleigh wave ellipticity and phase velocity at 8–40 s periods. Rayleigh wave ellipticity, or Rayleigh wave Z/H (vertical to horizontal) amplitude ratio, is extracted from both earthquake (10–40 s) and ambient noise data (8–25 s) recorded by the NorthEast China Extended SeiSmic Array with 127 stations. The estimated Z/H ratios from earthquake and ambient noise data show good consistency within the overlapped periods. The observed Z/H ratio shows a good spatial correlation with surface geology and is systematically low within the basins. We jointly invert the measured Z/H ratio and phase velocity dispersion data to obtain a refined 3-D S wave velocity model beneath the NE China. At shallow depth, the 3-D model is featured by strong low-velocity anomalies that are spatially well correlated with the Songliao, Sanjiang, and Erlian basins. The low-velocity anomaly beneath the Songliao basin extends to ~ 2–3 km deep in the south and ~5–6 km in the north. At lower crustal depths, we find a significant low-velocity anomaly beneath the Great Xing’an range that extends to the upper mantle in the south. Overall, the deep structures of the 3-D model are consistent with previous models, but the shallow structures show a much better spatial correlation with tectonic terranes. The difference in sedimentary structure between the southern and northern Songliao basin is likely caused by a mantle upwelling associated with the Pacific subduction.