ALBERT

Description: We discuss an innovation in traveltime tomography that combines wavelet-based, multiscale parameterization and finite-frequency theory to solve two outstanding issues that inevitably arise from uneven source station distributions and from the three-dimensional (3-D) nature of wavefront healing: how to objectively address the intrinsically multiscale nature of data coverage while simultaneously maintain model resolution at each scale level. We apply the new, integrated methodology to investigate 3-D variations of P and S wave speeds (δlnVP and δlnVS) beneath the Himalayan-Tibetan orogen. In particular, we are able to constrain variations in the Poisson's ratio via δln(VP/VS). The formulation is naturally data adaptive, resolving features at each scale only if the required data converge is available. The very first, long-wavelength feature that emerges is a clear anomaly of high δlnV that extends over more than 500 km beyond the northern edge of the Lhasa terrane at places. Farther northward, a strong negative anomaly underlies the region where recent volcanism occurs in northern Tibet. Regions of negative δln(VP/VS) delineate a slab-like, subhorizontal feature concentrated between depths of ∼100–250 km. Such characteristics are consistent with the notion that chemically refractory, and therefore buoyant, mantle lithosphere of the Indian shield (“Greater India”) has advanced subhorizontally northward far beyond the surficial Bangong-Nujiang suture. In the crust, two isolated regions of low δlnV, each extending to depths near 100 km, occur along the Lunggar and the Yadong-Gulu active rifts in southern Tibet. Deep penetrating rifts imply that only a limited amount of horizontal displacement is being accommodated on subvertical structures.

Description: Understanding the spatial variation of anisotropy in the upper mantle is important for characterizing the lithospheric deformation and mantle flow dynamics. In this study, we apply a full-wave approach to image the upper-mantle anisotropy in Southern California using 5954 SKS splitting data. Three-dimensional sensitivity kernels combined with a wavelet-based model parameterization are adopted in a multiscale inversion. Spatial resolution lengths are estimated based on a statistical resolution matrix approach, showing a finest resolution length of ~25 km in regions with densely-distributed stations. The anisotropic model displays structural fabric in relation to surface geologic features such as the Salton Trough, the Transverse Ranges and the San Andreas Fault. The depth variation of anisotropy does not suggest a lithosphere-asthenosphere decoupling. At long-wavelengths, the fast directions of anisotropy are aligned with the absolute plate motion inside the Pacific and North American plates.

Description: An improved inversion technique is needed to effectively separate the frequency dependence of the source from the intrinsic attenuation of the medium. We developed a cluster-event method (CEM) in which clusters of nearby events, instead of individual events, pair with stations to form the basis for measurements of Q value and corner frequency (fc). We assume that the raypaths from one cluster to a station share an identical Q while each event in the same cluster is allowed for only one fc in the inversion process. This approach largely reduces the degrees of freedom to achieve a robust inversion. We use an optimization algorithm of simulated annealing to solve the nonlinear inverse problem. The CEM was applied to events at 70–150 km depths in the Japan subduction zone recorded by F-net. We show that the method proposed here leads to better constraints on both source parameters and attenuation. The resultant Q's in the mantle wedge increase from lower than 300 beneath the arc and back-arc to greater than 600 in the fore-arc region. The fc's satisfy a self-similar scaling relationship with seismic moment of M0 ∝ fc−3 with a best fit stress drop of 21.9 ± 6.9 MPa in Madariaga's form. This contrasts to the stress drop of 1.4 ± 1.1 MPa for a global data set composed of prior measurements for crustal events. The results of this study agree with results from previous studies, except with an upward deviation due to higher corner frequencies and stress drops.

Description: To detect temporal changes of elastic properties associated with the 2006 Mw 6.1 Taitung earthquake in southeast Taiwan, whereby the collision between the Luzon arc riding on the Philippine Sea plate and the Asian margin is taking place, we construct the Green's functions from auto- and cross-correlation function (ACF and CCF) of continuous ambient noise between stations. Time lapse changes in the retrieved coda arrivals are estimated for monitoring spatiotemporal variations of seismic velocities around the ruptured fault zones. To the south of the main shock epicenter where the earthquake may have ruptured along two perpendicularly-intersecting fault planes resulting in intense coseismic slip and widely-dispersed aftershocks, the ACFs (2–8 Hz) at nearby stations reveal a large, sudden velocity drop of over 1% after the event occurrence. The CCFs (0.1–0.9 Hz) also show moderate reduction about 0.1% for the interstation paths traveling through the southeast quadrant of the focal sphere which has experienced the maximum peak ground acceleration and dilatational strain change. The intense seismic shaking combined with the rupture-induced damage near the junction of the two faults is the most plausible cause for such localized, but significant coseismic velocity reduction. The response of groundwater levels to precipitation is in-phase correlated with velocity variations over 3 years of investigation. Unlike the postseismic surface displacements gradually relaxed, the velocity remained slow until October 2006 due to the typhoon-induced heavy rains between May and September.

Description: SUMMARY The relation between the complex geological history of the western margin of the North American plate and the processes in the mantle is still not fully documented and understood. Several pre-USArray local seismic studies showed how the characteristics of key geological features such as the Colorado Plateau and the Yellowstone Snake River Plains are linked to their deep mantle structure. Recent body-wave models based on the deployment of the high density, large aperture USArray have provided far more details on the mantle structure while surface-wave tomography (ballistic waves and noise correlations) informs us on the shallow structure. Here we combine constraints from these two data sets to image and study the link between the geology of the western United States, the shallow structure of the Earth and the convective processes in mantle. Our multiphase DNA10-S model provides new constraints on the extent of the Archean lithosphere imaged as a large, deeply rooted fast body that encompasses the stable Great Plains and a large portion of the Northern and Central Rocky Mountains. Widespread slow anomalies are found in the lower crust and upper mantle, suggesting that low-density rocks isostatically sustain part of the high topography of the western United States. The Yellowstone anomaly is imaged as a large slow body rising from the lower mantle, intruding the overlying lithosphere and controlling locally the seismicity and the topography. The large E–W extent of the USArray used in this study allows imaging the ‘slab graveyard’, a sequence of Farallon fragments aligned with the currently subducting Juan de Fuca Slab, north of the Mendocino Triple Junction. The lithospheric root of the Colorado Plateau has apparently been weakened and partly removed through dripping. The distribution of the slower regions around the Colorado Plateau and other rigid blocks follows closely the trend of Cenozoic volcanic fields and ancient lithospheric sutures, suggesting that the later exert a control on the locus of magmato-tectonic activity today. The DNA velocity models are available for download and slicing at http://dna.berkeley.edu .

Description: Taking advantage of a unique opportunity provided by a dense array of coastal short-period seismic stations and the diverse bathymetry around Taiwan, we examine how the long-range coherent ambient noises are influenced by surrounding ocean settings using the cross-correlation functions (CCFs) between pairs of stations. The effective energy of the CCFs derived from three components of short-period seismometer data falls within the frequency range of the short period secondary microseism (SPSM). The spatial variations mapped from the amplitude asymmetry of CCFs and source migration images evidently demonstrate that the SPSM strengths are closely linked to the drastic changes in offshore ocean characteristics and result in much stronger SPSM in the shallow and narrow Taiwan Strait than in deep open seas of eastern Taiwan. The temporal variations of the CCF strengths exhibit very good correlations with the wind speeds and wave heights, explicitly indicating the observed SPSM is dominated by local sources generated from wind-driven ocean waves around offshore Taiwan.

Description: The India-Eurasia collision and the decratonization of the North China Craton have drawn much attention from the scientific community. Here we provide the first large-scale S wave velocity model for China (CH11-S) based on constraints from both teleseismic surface and body waves. We take advantage of the recent deployment of the 140 permanent stations of the Chinese Digital Seismic Network and temporary network deployments to resolve both the lithospheric and deeper mantle structure. Slow velocities are widespread in the crust and upper mantle. Deeply rooted fast anomalies are located beneath the stable Yangtze Craton and the western (Ordos) block of the North China Craton. An upper mantle fast anomaly is observed beneath the eastern block of the North China Craton and could represent thermally eroded or delaminated Precambrian lithosphere. Another flat and fast feature appears beneath the Tibetan Plateau from 50 to 250 km depth. This may represent the Indian slab stalled in the mantle due to its buoyancy or a lithospheric instability triggered by the India-Eurasia collision. A large fast anomaly apparently stagnant in the transition zone is observed beneath the Yangtze Craton and may play a role in the stability of this block. In contrast, on both sides of the South China Block, active and reactivated areas coincide with oceanic slab material that has already sunk into the lower mantle and that may have enhanced tectonic activity by forcing convection. Finally, the upper mantle beneath Tibet seems almost completely surrounded by adjacent high-velocity and presumably strong blocks.