Abstract
There are more than 7 million people living near the Tatun volcano group in northern Taiwan. For the safety of the Taipei metropolis, in particular, it has been debated for decades whether or not these volcanoes are active. Here I show evidence of a deep magma reservoir beneath the Taipei metropolis from both S-wave shadows and P-wave delays. The reservoir is probably composed of either a thin magma layer overlay or many molten sills within thick partially molten rocks. Assuming that 40% of the reservoir is partially molten, its total volume could be approximately 350 km3. The exact location and geometry of the magma reservoir will be obtained after dense seismic arrays are deployed in 2017–2020.
Highlights
The Tatun volcano group (TVG) is located near the administrated border between two large cities (Taipei, the capital of Taiwan, and New Taipei) in northern Taiwan (Fig. 1)
To examine the seismic waves propagating into a potential magma reservoir beneath the TVG, seismic data generated by deeper earthquakes in 2015 were collected in accordance with the earthquake catalog provided by the Central Weather Bureau in Taiwan
Since this earthquake was located at a depth of 219 km beneath northern Taiwan, the incidence angles of the seismic waves at the seismic stations in the TVG were very close to vertical (70–80°)
Summary
From the tectonic point of view, the TVG is located at the western end of the subduction system in the northern Taiwan area (Fig. 1), where the Philippine Sea plate (PSP) is subducting northward beneath the Eurasian plate. All deeper earthquakes (depth > 40 km) occur in a limited area east of 121.5°E. The subduction slab of the PSP plate, shown by seismicity, starts from the Hualien area of eastern Taiwan (approximately 24°N) and gradually increases its depth down to more than 250 km beneath northeastern Taiwan (Fig. 1b). The TVG is located on the overlying plate of the subduction system, geochemical characteristics of varying from low-K to calc-alkaline and. Shoshonitic compositions in the Northern Taiwan Volcanic Zone indicate that it might not be part of the typical volcanic arc induced directly by the subduction process[16]. The TVG volcanism might be the result of some degree of melting within an ascending region of asthenosphere mantle, due to the extensional collapse of the northern Taiwan mountain belt[17]
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