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

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 QP/ QS 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).