A Strong Seismic Reflector within the Mantle Wedge above the Ryukyu Subduction of Northern Taiwan

2019 ◽  
Vol 91 (1) ◽  
pp. 310-316
Author(s):  
Cheng‐Horng Lin ◽  
Min‐Hung Shih ◽  
Ya‐Chuan Lai
Keyword(s):  
Opposite Direction ◽  
Mantle Wedge ◽  
Seismic Velocity ◽  
Low Velocity ◽  
P Waves ◽  
Deep Earthquake ◽  
Subduction System ◽  
Subducting Slab ◽  
Seismic Reflector ◽  
Northern Taiwan

Abstract Major structures within the mantle wedge are often revealed from seismic velocity anomalies, such as low‐velocity zones at magma reservoirs, partial melting regions, or the upwelling asthenosphere. However, no significant seismic boundaries have been reported in the shallow mantle wedge beneath volcanic arcs. Here, we present evidence for a strong seismic reflector dipping in the opposite direction of the subducting slab in the mantle wedge beneath northern Taiwan in the western end of the Ryukyu subduction system. We find that two unambiguous P waves generated by a deep earthquake (ML 5.1) at a depth of 132.5 km were clearly recorded by the dense seismic array (Formosa Array), composed of 140 broadband seismic stations with a station spacing of approximately 5 km in northern Taiwan. Forward modeling using both raytracing and travel times shows that a seismic reflector exists beneath the Tatun volcano group (TVG) around depths of 80–110 km. The reflector dips in the opposite direction of the subducting slab and is unlikely to be associated with mantle wedge corner flow. Instead, it probably belonged to parts of possible structures such as the asthenospheric flow, the mantle diapir, or other undiscovered structures above the subducting slab. No matter what the seismic boundary is exactly, it might be associated with the active volcanism in the TVG. The detailed geometry and mechanism of the seismic boundary in the mantle wedge will be obtained as the Formosa Array collects more seismic data in the near future.

Deep Structure and Active Tectonics of the South Aegean Volcanic Arc

Elements ◽  
2019 ◽  
Vol 15 (3) ◽  
pp. 153-158 ◽  
Author(s):  
Costas B. Papazachos
Keyword(s):  
Deep Structure ◽  
Active Tectonics ◽  
Aegean Sea ◽  
Low Velocity ◽  
Volcanic Arc ◽  
High Attenuation ◽  
Global Positioning ◽  
Back Arc ◽  
The Impact ◽  
Subducting Slab

The seismotectonic setting of the Aegean Sea, based on information from seismicity, neotectonics and global positioning system studies, is characterized by a sharp transition from a compressional outer arc to a complex back-arc, with an approximate north–south extension along the volcanic arc. Seismicity and 3-D tomography studies reveal the geometry of the subducting slab and image the low-velocity/high-attenuation mantle wedge at depths of 50–80 km beneath the volcanic arc where magma is generated. The 1956 Amorgos M7.5 earthquake and the impact from its seismic shaking and landslide-triggered tsunamis are discussed in the context of the regional seismotectonic setting.

Plume-modified mantle flow in the northern Basin and Range and southern Cascadia back-arc region since ca. 12 Ma

Geology ◽  
2019 ◽  
Vol 47 (8) ◽  
pp. 695-699 ◽  
Author(s):  
Victor E. Camp
Keyword(s):  
United States ◽  
Seismic Velocity ◽  
Flow Line ◽  
Basin And Range ◽  
Mantle Flow ◽  
Low Density ◽  
Low Velocity ◽  
Northern Basin ◽  
High Lava Plains ◽  
Back Arc

AbstractBimodal volcanism and rhyolite migration along the High Lava Plains in central Oregon (United States) lie above a broader feature defined by low seismic velocity in the upper mantle that emanates from the Yellowstone hotspot (northwest United States) and extends westward across the northern Basin and Range. It was emplaced by a westward current, driven in part by rapid buoyancy-driven flow across the east-west cratonic boundary of North America. Geothermometry studies and geochemical considerations suggest that the low-velocity feature may be composed of moderately hot, low-density mantle derived from the Yellowstone plume but diluted by thermomechanical erosion and entrainment of colder mantle lithosphere. Finger-like conduits of plume-modified mantle beneath Quaternary eruption sites delineate flow-line channels that have developed across the broader mantle structure since 2 Ma. These channels have allowed low-density mantle to accumulate against the Cascades arc, thus providing a heated mantle source for mafic magmatism in the Newberry (Oregon) and Medicine Lake (California) volcanic fields.

Solubility of magnetite-hematite assemblages in slab-derived saline fluids

2020 ◽  
Author(s):  
Carla Tiraboschi ◽  
Carmen Sanchez-Valle
Keyword(s):  
Subduction Zones ◽  
Microprobe Analysis ◽  
Mantle Wedge ◽  
Sedimentary Cover ◽  
Low Pressure ◽  
Oceanic Lithosphere ◽  
Major Elements ◽  
Piston Cylinder ◽  
Water Saturated ◽  
Subducting Slab

<p>In subduction zones, aqueous fluids derived from devolatilization processes of the oceanic lithosphere and its sedimentary cover, are major vectors of mass transfer from the slab to the mantle wedge and contribute to the recycling of elements and to their geochemical cycles. In this setting, assessing the mobility of redox sensitive elements, such as iron, can provide useful insights on the oxygen fugacity conditions of slab-derived fluid. However, the amount of iron mobilized by deep aqueous fluids and melts, is still poorly constrained.</p><p>We experimentally investigate the solubility of magnetite-hematite assemblages in water-saturated haplogranitic liquids, which represent the felsic melt produced by subducted eclogites. Experiments were conducted at 1 GPa and temperature ranging from 700 to 900 °C employing a piston cylinder apparatus. Single gold capsules were loaded with natural hematite, magnetite and synthetic haplogranite (Na<sub>0.56</sub>K<sub>0.38</sub>Al<sub>0.95</sub>Si<sub>5.19</sub>O<sub>12.2</sub>). Two sets of experiments were conducted: one with H<sub>2</sub>O-only fluids and the second one adding a 1.5 m H<sub>2</sub>O–NaCl solution. The capsule was kept frozen during welding to ensure no water loss. After quench, the presence of H<sub>2</sub>O in the quenched haplogranite glass was checked by Raman spectroscopy, while major elements were determined by microprobe analysis.</p><p>Preliminary results indicate that a significant amount of Fe is released from magnetite and hematite in hydrous melts, even at relatively low-pressure conditions. At 1 GPa the FeO<sub>tot</sub> quenched in the haplogranite glass ranges from 0.60 wt% at 700 °C, to 1.87 wt% at 900 °C. In the presence of NaCl, we observed an increase in the amount of iron quenched in the glass (e.g., at 800 °C from 1.04 wt% to 1.56 wt% of FeO<sub>tot</sub>). Our results suggest that hydrous melts can effectively mobilize iron even at low-pressure conditions and represent a valid agent for the cycling of iron from the subducting slab to the mantle wedge.</p>

MODEL STUDY OF RADIATION FROM AN EXPLOSION IN A CIRCULAR DISK

Geophysics ◽  
1969 ◽  
Vol 34 (2) ◽  
pp. 213-221 ◽  
Author(s):  
Dhari S. Bahjat ◽  
Carl Kisslinger
Keyword(s):  
Solid Medium ◽  
Circular Disk ◽  
Peak Frequency ◽  
Approximate Theory ◽  
Low Velocity ◽  
P Waves ◽  
Compressional Waves ◽  
Model Study ◽  
Disk Material ◽  
Solid Circular Cylinder

The effect of the medium immediately surrounding the shot on the properties of the primary compressional waves has been studied by means of two‐dimensional models. Circular disks of Plexiglas and Styrofoam of various diameters were inserted in large sheets of aluminum and of Plexiglas, respectively. Charges were fired in the center of the disk. An approximate theory for radiation from a simple harmonic line source of P waves on the axis of a solid circular cylinder embedded in an infinite homogeneous solid medium has been developed. The transfer function of the disk has been calculated from the observed spectra in order to compare it with the frequency response calculated from theory. The results show that for these cases of a low‐velocity disk a peak frequency exists which decreases with increasing diameter. The maximum peak frequency is that for a shot in an infinite sheet of the high‐velocity material, and the minimum peak frequency is that for a shot in a sheet of the low‐velocity disk material. This minimum peak frequency is approached rapidly as the disk diameter increases. The geometric effect of radiation from the circular inhomogeneity is important only over a narrow range of cavity diameters for which the resonance associated with the cavity happens to fall near the peak of the spectrum of the explosion‐generated wave.

Seismic structure of basalt flows from surface seismic data, borehole measurements, and synthetic seismogram modeling

Geophysics ◽  
2001 ◽  
Vol 66 (6) ◽  
pp. 1925-1936 ◽  
Author(s):  
Moritz M. Fliedner ◽  
Robert S. White
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Synthetic Seismogram ◽  
P Wave ◽  
Wide Angle ◽  
Seismic Velocities ◽  
Seismic Structure ◽  
Low Velocity ◽  
Amplitude Variation With Offset ◽  
Velocity Gradients

We use the wide‐angle wavefield to constrain estimates of the seismic velocity and thickness of basalt flows overlying sediments. Wide angle means the seismic wavefield recorded at offsets beyond the emergence of the direct wave. This wide‐angle wavefield contains arrivals that are returned from within and below the basalt flows, including the diving wave through the basalts as the first arrival and P‐wave reflections from the base of the basalts and from subbasalt structures. The velocity structure of basalt flows can be determined to first order from traveltime information by ray tracing the basalt turning rays and the wide‐angle base‐basalt reflection. This can be refined by using the amplitude variation with offset (AVO) of the basalt diving wave. Synthetic seismogram models with varying flow thicknesses and velocity gradients demonstrate the sensitivity to the velocity structure of the basalt diving wave and of reflections from the base of the basalt layer and below. The diving‐wave amplitudes of the models containing velocity gradients show a local amplitude minimum followed by a maximum at a greater range if the basalt thickness exceeds one wavelength and beyond that an exponential amplitude decay. The offset at which the maximum occurs can be used to determine the basalt thickness. The velocity gradient within the basalt can be determined from the slope of the exponential amplitude decay. The amplitudes of subbasalt reflections can be used to determine seismic velocities of the overburden and the impedance contrast at the reflector. Combining wide‐angle traveltimes and amplitudes of the basalt diving wave and subbasalt reflections enables us to obtain a more detailed velocity profile than is possible with the NMO velocities of small‐offset reflections. This paper concentrates on the subbasalt problem, but the results are more generally applicable to situations where high‐velocity bodies overlie a low‐velocity target, such as subsalt structures.

Seismic finite‐difference modeling with spatially varying time steps

Geophysics ◽  
2000 ◽  
Vol 65 (4) ◽  
pp. 1290-1293 ◽  
Author(s):  
Ekkehart Tessmer
Keyword(s):  
Finite Difference ◽  
Seismic Velocity ◽  
Finite Difference Methods ◽  
Low Velocity ◽  
Step Size ◽  
Time Step ◽  
Time Step Size ◽  
Large Velocity ◽  
Numerical Grid ◽  
Spatially Varying

Numerical seismic modeling by finite‐difference methods usually works with a global time‐step size. Because of stability considerations, the time‐step size is determined essentially by the highest seismic velocity, i.e., the higher the highest velocity, the smaller the time step needs to be. Therefore, if large velocity contrasts exist within the numerical grid, domains of low velocity are oversampled temporally. Using different time‐step sizes in different parts of the numerical grid can reduce computational costs considerably.

Rapid map and inversion of P-SV waves

Geophysics ◽  
1991 ◽  
Vol 56 (6) ◽  
pp. 859-862 ◽  
Author(s):  
Robert R. Stewart
Keyword(s):  
Spatial Resolution ◽  
Seismic Data ◽  
P Wave ◽  
Rock Properties ◽  
Low Velocity ◽  
P Waves ◽  
Near Surface ◽  
Seismic Profiling ◽  
Vertical Seismic Profiling ◽  
And Inversion

Multicomponent seismic recordings are currently being analyzed in an attempt to improve conventional P‐wave sections and to find and use rock properties associated with shear waves (e.g. Dohr, 1985; Danbom and Dominico, 1986). Mode‐converted (P-SV) waves hold a special interest for several reasons: They are generated by conventional P‐wave sources and have only a one‐way travel path as a shear wave through the typically low velocity and attenuative near surface. For a given frequency, they will have a shorter wavelength than the original P wave, and thus offer higher spatial resolution; this has been observed in several vertical seismic profiling (VSP) cases (e.g., Geis et al., 1990). However, for surface seismic data, converted waves are often found to be of lower frequency than P-P waves (e.g., Eaton et al., 1991).

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