Location of Non-volcanic Tremors along the Cascadia Subduction Zone Using the Source- Scanning Algorithm
Due to the nature of Episodic Tremor and Slip (ETS) events, a long-term study and continuous seismic and geodetic data are required for a detailed study. Here we focus on tremors that occur along the Cascadia subduction zone between southern Vancouver Island and Northern California during slow slip events in two full-life cycles starting February 2003. The origin times and hypocenters of all tremors are estimated using the Source-Scanning Algorithm (SSA) of Kao (2004). We processed more than 200 days of continuously recorded seismic data from the US roughly the same amount of information extracted from the Canadian seismograms by compiling tremor catalogs provided by the Geological Survey of Canada (GSC) or by direct analysis of the waveforms. The majority of the well-located tremors in southern Vancouver Island, the Canada-US border region and northern Washington occur at a depth which ranges from 20 km to 40 km. In central and southern Washington, the depth of the well-located events gradually decreases with a westward shift of the epicenters towards the coast. Also both temporally and spatially it seems that tremors occur in locations with absent or sparse seismicity. In this study we will examine the geographical variability of ETS events as well as hypocentral migration rates and segmentation.
The Seismic Signature of High pore-fluid Pressure Within Subducting Oceanic Crust Using Receiver Functions
Episodic tremor and slip (ETS) events in subduction zones occur in the general vicinity of the plate boundary, downdip of the locked zone. In developing an understanding of the ETS phenomenon it is important to relate the spatial occurrence of non-volcanic tremor (NVT) to the principal structural elements within the subduction complex. In northern Cascadia we document the evidence for high pore-fluid pressure within the subducted oceanic crust based on measurements of Poisson's ratios and velocity contrasts using stacked receiver functions. High pore pressure at depths <40 km and evidence for the release of water into the mantle wedge downdip points to a transition from a low-permeability barrier that traps fluids within the oceanic crust, to a high-permeability interface at depths corresponding to eclogitization of oceanic crust. This transition occurs in the vicinity of the wedge corner, where ETS occurrence is observed. We suggest that ETS events are related to transient micro-fracturing of the plate interface by volume changes across the boundary due to continuous dehydration reactions (eclogitization) of the oceanic crust, and hydration (serpentinization) of the mantle wedge. The recurrence of ETS at regular intervals are possibly explained by episodic pore-fluid pressure build-up and fluid release across the plate boundary. We will explore the possibility of recovering temporal variations in velocity structure caused by ETS using receiver functions.
Adjoint Tomography of the Crust and Upper Mantle in the Japan Subduction Zone
The adjoint tomography technique is an effective tool for using 3-D models as initial models and refining them by iteratively minimizing the misfit between synthetics and data. In this study, we use this technique to obtain a more detailed 3-D image of the descending slab in the Japan Subduction Zone and its neighboring regions. We have very dense station coverage of our study area with a total of 845 stations, including Hi-net (more than 600 stations), F-net and Global Seismographic Network (GSN) stations. We use Zhao et al's (1994) 3-D slab model embedded in Lebedev and Nolet's (2003) regional model as the initial model in the tomographic inversion and calculate synthetics for each event. According to finite-frequency theory, the sensitive region along the ray path is given by a 3-D 'banana-doughnut' kernel, and the overall spatial distribution of the sum of all available event-station kernels determines the resolvable volume in the inversion. Using the automated windowing code FLEXWIN, we select a set of 206 events. We processed the data and synthetics using two types of bandpass filters: 6--30 s for all the records and 24--120 s for F-net and GSN records. For the first iteration, the frequency-dependent traveltime misfit measurements between synthetics and data are made in 44,709 windows for the period range of 24-120 s and 119,376 windows for the period range of 6-30 s. The combined adjoint sources are thus constructed based on these traveltime misfit measurements. Given the adjoint sources, we use the adjoint spectral-element method to calculate banana-doughnut kernels for P, S and surface waves for the selected records. The weighted sums of the banana-doughnut kernels for all event-station pairs, with weights determined by the traveltime measurements, are used to construct misfit kernels. These gradients are then used in a non-linear conjugate gradient algorithm to further improve the existing 3-D models. We are currently at the first iteration of the 3-D models. The preliminary results indicate that seismic velocities of the Pacific slab need to be faster to reduce the misfit; the observed small-scale features requiring slower seismic velocities might be related to mantle wedge melts, but this needs further investigation in subsequent iterations.
Mountain Building across the Northeastern Margin of the Tibetan Plateau
The high-elevation Altyn Tagh Range at the northern edge of the Tibetan plateau is a unique place where mountain building correlates with large-scale strike-slip motion. Diverse geologic and geophysical observations so far have not resulted in a conclusive tectonic model for the uplift of the mountain range. Crustal models derived from deep seismic profiles can inform this debate. Here we present the detailed compressional-wave seismic velocity structure of the crust and upper-most mantle beneath the Altyn Tagh Range and its adjacent basins. North of the Cherchen fault, beneath the east-central Tarim basin, we find a 48- km thick platform-type crust with a high-velocity lower-crustal layer. In contrast, south the Cherchen fault, beneath the Tibetan plateau, the crust has a crustal thickness of 50-58 km but lacks a high-velocity lower- crustal layer. The high topography (~3 km) of the Altyn Tagh Range is supported by a wedge-shaped region of 7.6-7.8 km/s that we interpret as a low-density zone of crust-mantle mix. Previous studies within the India-Asia collision zone have assumed that an offset at the base of the crust beneath major strike slip fault is in general thought to indicate strike-slip motion at a whole-lithospheric scale. Based on tomographic imaging it is suggest that the Altyn Tagh fault forms a lithospheric-scale boundary where crustal deformation within the Altyn Tagh Range is backstopped and results in a step at the base of the crust. In contrast, our results show a smooth crust-mantle boundary between the Altyn Tagh Range and Qaidam basin. This could indicate that the Altyn Tagh fault is limited to the upper crust, as previously suggested. As a consequence, our crustal model would allow not just lower-crustal deformation but actual channel flow of the lower crust between the Altyn Tagh Range and the Qaidam basin. Lower crustal flow has been proposed as the dominant mechanisms for the formation of the northeastern Tibetan Plateau. The model of the crust and upper-most mantle derived from our data supports a tectonic model for mountain building at the NE margin of the Tibetan plateau that involves (1) the underthrusting of the Tarim lower crust and mantle beneath the Altyn Tagh Range, and (2) upper-crustal thrust faulting and lower-crustal pure-shear thickening beneath the Altyn Tagh Range.
Earthquakes, Uplift, and Landscape Evolution in the NW Himalayas
The terrain between Main Mantle Thrust to Salt Range Thrust in the NW Himalayas has been characterized by surface and subsurface features with variable tectonic activity. These features show relatively variable tectonic activity, existence of blind faults and basement faulting. In the present study, we use seismological and remote sensing analysis backed by field observations to investigate the relationship between earthquakes, uplift, and landscape evolution. We use nonlinear analysis to understand the earthquake dynamics in relation to surface faults and blind faults. The fractal analysis of the seismicity in three subsurface features of the area is used to characterize the roughness of the faults' surface. We find a high fault surface roughness in the Indus Kohistan Seismic Zone (IKSZ). It is concluded that the area is in the process of being uplifted and landscape is evolving. This evolution is further investigated using a set of geomorphological analyses consisting of extracting a drainage network from digital elevation models (DEM). The extracted streams are analysed using to calculate geomorphic indices and relative uplift rates. These analyses were applied on Indus, Swat, Kabul, Kunhar, Kishanganga, Poonch, Jehlum, Swan and Kurram, Kabul Rivers and their associate tributaries. The analyses provide us with the spatial variation of relative uplift based upon specific streams. We found that the Hazara Kashmir Syntaxis and Nanga Parbat Haramosh Massif are subject to a relatively high uplift. It is observed that the neotectonic activities are linearizing the drainage network from meandering pattern. We analyse the complete drainage texture using fractal dimension and lacunarity analysis. The analysis of the fractal dimension (D) employing box counting methods is calculated with a moving window approach and the lower values of D demonstrate the effect of neotectonic activity. The locations with lower but similar D values are further differentiated using lacunarity analysis of the selected sites by measuring the spatial homogeneity between the empty spaces. By analysing different locations and features we are concluding that landscape evolution along Hazara Kashmir Syntaxis was rapid.
Subduction to Continental Delamination: Insights From Laboratory Experiments
The evolution of the lithosphere through subduction-collision and delamination and its surface/crustal response (topography/deformation) is investigated in this work. We present a series of lithosphere scale two dimensional (2-D) and three dimensional (3-D) laboratory experiments to better understand such processes. In these experiments, an idealized viscously deforming crust-mantle lithosphere-mantle system is configured with silicone putty (representing lithospheric mantle and upper crust) and glucose syrup (representing the upper mantle and lower crust). The initial focus was to investigate the physical development of delamination versus continental subduction without plate convergence. Experiments show that the delamination or continental subduction is strongly dependent on the density of the crust (both crust and mantle lithosphere subducts when crust has a higher density, instead of delamination), while in the investigated range, the viscosity of the weak layer does not have much influence on the process. In all the experiments, the topography is asymmetric with subsidence above the delaminating hinge due to the dynamic vertical pulling driven by the delaminating slab, and uplift above the delaminated region due to the buoyancy of asthenosphere. Our investigation on the oceanic subduction with a convergence rate of ~ 3cm/year plate velocity suggests that subduction -collision - delamination is well defined and at the end, the delaminating crust from the lithosphere is overthrusted on top of the overriding plate. Our results provide integrated insights on the Alpine-Himalayan type orogenies, in particular the neotectonic evolution of Eastern Anatolian plateau.
Geodynamics of Slab Retreat in the North-Central Mediterranean
As with a number of other orogenic regions, the northern Apennines--where the Italian peninsula is currently over-riding and consuming the Adriatic Sea plate--is undergoing an intriguing syn-convergent extension. One hypothesis is that the lithospheric extension may be related to mantle flow associated with retreat of the subducted slab. While it is possible to qualitatively envision such a flow regime, the mantle circulation in the region is likely complicated by the possibility that mantle material flows under and/or around the slab. The purpose of this work is to explore the potential three-dimensional flow patterns around a model retreating slab. Specifically, we try to determine how mantle material is moved away in front of the retreating slab and whether/when divergent flow occurs in the region behind the slab. Computational geodynamic experiments were conducted to model the 3D flow and we monitored the flow vector changes with variations to: boundary conditions on the solution space; geometry (width, depth, angle) of the slab; and rate of subduction. The modeling shows that flow beneath the slab is relatively subdued (even with a shallow slab); rather, mantle material shifts around the sides. Depending on the position of the slab relative to the solution space boundaries, the flow may become asymmetric to the slab and break into smaller scale flow bands around the retreating block. The results are interpreted with available seismic data that map shear wave anisotropy across a broad portion of the Adriatic, Apennines and Tyrrhenian Sea. These seismic data show patterns of SKS splits with directions that are trench-parallel to the east of the Apennine divide and trench-normal to the west of the divide and if SKS splitting directions are a proxy for the mantle flow direction, then this implies that certain models are more feasible than others.