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

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.

Mantle wedge diapirs detected by a dense seismic array in Northern Taiwan

“…8 ). Similarly, the scattering obstacles near Turtle Island are just beneath another magma reservoir 28 . Although the obstacles are separated into two different depths, ~ 60 km and ~ 95 km, both are located just on top of the subducted slab.…”

“…Like many ordinary subduction systems, plenty of active volcanoes develop along the Ryukyu volcanic arc from Kyusyu, southern Japan, to northern Taiwan 18 , 19 . Taiwan is located just at the westernmost end of the Ryukyu volcanic arc 20 – 22 , where two active volcanoes have been identified 23 – 28 . One is the Tatun Volcano Group (TVG) nearby the Taipei metropolitan area, and the other is Kueishantao (or Turtle Island), named for its shape, offshore from the Ilan plain of northeastern Taiwan.…”

“…One is the Tatun Volcano Group (TVG) nearby the Taipei metropolitan area, and the other is Kueishantao (or Turtle Island), named for its shape, offshore from the Ilan plain of northeastern Taiwan. Despite the absence of eruption records in human history, both volcanoes have been recognized as active based on a variety of recent observations including Helium isotope analyses 25 , dating of volcanic ashes 26 , seismic detection of magma reservoirs 27 , 28 , and some other interesting seismic activities such as heartbeat-like seismicity 29 , VLP tremors 30 , infrasonic signals 31 , and many others 32 , 33 . Thus, more detailed investigation of both volcanoes becomes very important in Taiwan since potential volcanic impacts cannot be totally excluded in the future.…”

“…2 ). Despite the fact that magma reservoirs beneath both the TVG and Turtle Island have been detected by seismic observations of both S-wave shadows and P-wave delays from the dense seismic stations focused on two volcanic areas 27 , 28 , the exact geometry of magma reservoirs are still poorly obtained. The Formosa Array is designed to examine the existence of magma reservoirs and to improve seismic images beneath volcanic areas, with the installation of 146 broadband seismic stations in the northern Taiwan area.…”

Distribution and sexual dimorphism of the crab Xenograpsus testudinatus from the hydrothermal vent field of Kueishan Island, northeastern Taiwan

“…This island originated from volcanic eruptions about 7000 ± 700 years ago [4]. Due to the active magma beneath the earth's crust [5], Kueishan Island is defined as an active volcano in the Taiwan area [4,6]. Heated groundwater is ejected from the cracks in the seabed, creating a hydrothermal vent area in the east of the island [4,7].…”