Dense seismic arrays deny a massive magma chamber beneath the Taipei metropolis, Taiwan

Yu-Lien Yeh,Strong Wen,Wei-Hau Wang

doi:10.1038/s41598-020-80051-4

Abstract

Several recent studies suggest that the Tatun Volcano Group (TVG) in the Taipei metropolis of Taiwan is still active with a mappable magma chamber beneath it. Here we report new seismic evidence from dense seismic arrays in northern Taiwan to refute the presence of a massive magma chamber. We investigated two near Taipei earthquakes with focal depths of ~ 140 km. We found that all the waveforms exhibited distinct S waves even when they traversed across the previously postulated magma chamber. Instead, the S-wave shadows found in the previous study may result from seismic waves traveling through a magma diapir above the subducting Philippine Sea Plate offshore northern Taiwan. Moreover, we found the P-wave delay increased with hypocentral distance when the seismic waves propagated through the footwall (west side) of the Shanchiao fault, regardless of whether they traversed across the postulated magma chamber. Our study results also indicate no abnormal attenuation when seismic rays traversed across the postulated magma chamber. Furthermore, the average {Q}_{P}/{Q}_{S} ratio around the TVG is less than 1, which implies that scattering attenuation is dominant. We conclude that a highly fractured rock body is beneath the TVG with a tiny fraction of magma instead of a massive magma chamber. Without sufficient magma supply, the TVG may stay dormant (except for small phreatic eruptions).

Highlights

The Tatun Volcano Group, comprising ~ 20 volcanoes, is situated in the Taipei metropolis of Taiwan above the western boundary of the subducting Philippine Sea Plate (Fig. 1a)
To test if the S-wave shadows originated from the postulated magma chamber, we examined the waveforms of 16 seismic rays that passed through the postulated magma chamber from EQ1 and EQ2 (Fig. 3)
We investigated other waveforms of these two earthquakes with the recording stations shown in Fig. 1, but still failed to find any S-wave shadow

Summary

Introduction

The Tatun Volcano Group, comprising ~ 20 volcanoes, is situated in the Taipei metropolis of Taiwan above the western boundary of the subducting Philippine Sea Plate (Fig. 1a). All these new findings imply that the TVG is being fed by an active magma chamber Such a magma chamber had never been identified by seismic tomographic studies until a recent report by Lin[10], who declared that a massive magma chamber with a size of 350 km[3] to 936 km[3] is embedded right beneath the Taipei metropolis at depths of about 28–34 km after studying four distant subduction events. He found S waves vanished and P waves lagged as the seismic waves of these events propagated through the postulated magma chamber. The unique foci of these two events allow us to investigate the physical properties beneath the TVG with a closer look via seismic waveforms, arrival times, and attenuation characteristics

Objectives

Results

Conclusion