Tomography of the westernmost Ryukyu subduction zone and the serpentinization of the fore‐arc mantle

Abstract

[1] Three-dimensional tomographic images of the subducting Philippine Sea slab and the fore-arc mantle beneath NE Taiwan and the westernmost Ryukyu were generated in this study. More than 5600 events recorded simultaneously by seismic networks in Taiwan and Japan were relocated for the inversion for variations in V P , V S , and V P /V S . Analysis of the trade-off between the data variance reduction and model variance helps to determine the appropriate strictness for regularization to avoid either overinterpretatioin or underinterpretation of data. The regularization parameters were also chosen to ensure suppression of artificial V P /V S anomalies. The subducting slab is characterized by high V P , high V S , and intermediate to low V P /V S . Notable in the mantle wedge is the high V P /V S anomalies that abut the surface of the subducting slab at depths of 30-80 km. The previously identified positive V P /V S channel connecting the slab and the arc volcano interpreted to be a melt pathway is not reproduced in this study. We convert V S and V P /V S to perturbations of temperature and serpentinization of the mantle. The slab is cooled by 200°C-400°C relative to the mantle, in accord with the estimates from theoretical modeling of subduction. The serpentinization reaches $15%, or 2% water content, at 50 km depth in the fore-arc mantle. We interpret the peak serpentinization as hydrated by the water released from the basalt-eclogite metamorphic reaction in the oceanic crust of the subducting Philippine Sea Plate. The spatial limitation of the present seismic networks in this region with respect to subduction zone events hinders a full description of the pattern of melting in much of the mantle wedge. Resolution tests of the tomographic inversion provide a basis to guide our interpretation to better resolved regions.

Seismic structure of subducted oceanic crust near the slow-earthquake source region in the southern Ryukyu arc

“…Moreover, the thickness of the serpentinized layer estimated by McCormack et al (2013) is only 6 km, which is smaller than the node intervals (horizontal 10 km; vertical 7.5 km) used in this study; thus, the resolution is too low to distinguish a 6-kmthick layer within the surrounding mantle. Chou et al (2009) investigated the velocity structure of the mantle wedge above the subducted PHS in the TaiwanRyukyu junction zone (121.8°E) and showed that approximately 60% serpentinized mantle wedge lies on the PHS at a depth of 50 km. Thus, a serpentinized area is distributed in the mantle wedge beneath the southern Ryukyu arc.…”

Measuring and crust-correcting finite-frequency travel time residuals – application to southwestern Scandinavia

“…If the same inversion technique and model parameterization is used to invert for V P and V S , the resulting models can be compared quantitatively as in e.g. Chou et al (2009) and Hung et al (2011).…”

A data-adaptive, multiscale approach of finite-frequency, traveltime tomography with special reference toPandSwave data from central Tibet

“…[40] Theoretically, dln(V P /V S ) is simply the difference between dlnV P and dlnV S [e.g., Chou et al, 2009]. Although Figure 6.…”

“…[9] It is important to point out that although theoretically the relationship among dlnV P , dlnV S , and dln(V P /V S ) is simple [e.g., Chou et al, 2009], reliable estimates of dln(V P /V S ) requires a careful selection of data to ensure that only sourcereceiver pairs common to both P and S wave are being used. Subsequently, the entire process of inversion, along with additional resolution tests, is carried out anew.…”