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. 2018 Nov 6;8(1):16401.
doi: 10.1038/s41598-018-34596-0.

Seismic Detection of a Magma Reservoir beneath Turtle Island of Taiwan by S-Wave Shadows and Reflections

Cheng-Horng Lin  1   2   3   4 , Ya-Chuan Lai  5   6 , Min-Hung Shih  7   6 , Hsin-Chieh Pu  8 , Shiann-Jong Lee  7
Affiliations

Seismic Detection of a Magma Reservoir beneath Turtle Island of Taiwan by S-Wave Shadows and Reflections

Cheng-Horng Lin et al. Sci Rep. .

Abstract

Although surface geology, eruption information and clustering seismicity all suggest Turtle Island (Kueishantao) of northern Taiwan is an active volcano, there was no direct evidence to conclude that magma reservoirs exist beneath it. Even less evidence is available to determine their spatial configuration. If the magma reservoirs are filled by liquids and melt, S-waves are totally reflected and leave behind a shadow, like when passing through the Earth's outer core. We detect both these S-wave shadows and strong reflections from the surface using earthquakes at different depths and azimuths. These observations identify a km-scale molten-filled volume located beneath Turtle Island. The magmatic nature of the reservoir is supported by the onset of non-double-couple earthquakes with strong CLVD (Compensated Linear Vector Dipole) and ISO (Isotropic) components, which show a tensor crack compatible with some volume changes within the reservoir. Combining these results with two independent 3-D velocity models and aeromagnetic anomalies recorded in Taiwan, a partially-molten ~19% low-velocity volume is estimated in the mid-crust (13-23 km), with spatial uncertainties of ~3 km. The elongated direction approximately follows the strike of the Okinawa trough, indicating that the source of the magma reservoir might be a back-arc opening.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Figure 1
Figure 1
Seismicity and general tectonics in the NE Taiwan area, which is located at the westernmost end of the Okinawa trough. Background seismicity is divided into two groups. The shallow earthquakes in the upper crust are associated with the opening of the back-arc basin along the Okinawa trough, and the deep earthquakes are located within the Benioff zone of the subducted PSP (Philippine Sea Plate).
Figure 2
Figure 2
Cross-check of S-wave shadows in the NE Taiwan area. (a) Locations of seismic stations (triangles) and 3 earthquakes (circles). The color beams (red and yellow) from earthquakes to seismic stations indicate the ray-paths in which S-wave shadows are detected. Comparison between 3-component velocity seismograms recorded at seismic stations (b) without and (c) with S-waves from Event 2.
Figure 3
Figure 3
Background seismicity in and around Turtle Island in 2015. Colorful circles with different sizes show the distributions of earthquake depths and magnitudes, respectively. The magma reservoir in red is roughly delineated by a broken-line circle.
Figure 4
Figure 4
P-wave velocity profiles across Turtle Island (Latitude = 24.85°N) modified from (a) Wu et al., 2009 and (b) Huang et al., 2014. A low velocity zone (LVZ, dashed-circle) is consistently obtained from both tomographic images. Two CLVD earthquakes (white circles shown at the upper panel) indicate the vertical rising might be resulting from the horizontal compression (σ1: maximum stress) within LVZ while the horizontal migration was associated with the horizontal extension (σ3: maximum stress) around its top.
Figure 5
Figure 5
Comparison of S-wave arrivals between two seismic stations (IL10 and IL08). (a) S-wave arrivals at all of different frequency bands were clearly identified at IL08, while (b) they were hardly observed at IL10.
Figure 6
Figure 6
Schematic plot showing ray-paths of the S-wave shadows examined from three representative earthquakes (Events 2, 3, and 4 in Table 1). The magma reservoir is filled by molten sills and dikes in red and partial melting rocks in pink.
Figure 7
Figure 7
Schematic plot showing strong S-waves reflected from a magma reservoir. (a) Locations of 5 felt earthquakes (circles), seismic stations (triangles), a magma reservoir (red ellipse), and ray-paths (dashed-lines) in the Ilan area of Taiwan. (b) Three-component seismograms showing both S waves (S1 an d S2) recorded at Station IL08 and generated by Events 5–9.
Figure 8
Figure 8
Moment tensor inversion results of two non-double-couple earthquakes (events 11 and 12 in Table 1) beneath Turtle Island. The moment tensors are decomposed into DC (double-couple), CLVD (Compensated Linear Vector Dipole) and ISO (Isotropic) components.
See this image and copyright information in PMC

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