Abstract
Abstract. The Tatun Volcano Group (TVG) is located in northern Taiwan near the capital Taipei. In this study we selected and analyzed almost four years (2004–2007) of its seismic activity. The seismic network established around TVG initially consisted of eight three-component seismic stations with this number increasing to twelve by 2007. Local seismicity mainly involved high frequency (HF) earthquakes occurring as isolated events or as part of spasmodic bursts. Mixed and low frequency (LF) events were observed during the same period but more rarely. During the analysis we estimated duration magnitudes for the HF earthquakes and used a probabilistic non-linear method to accurately locate all these events. The complex frequencies of LF events were also analyzed with the Sompi method indicating fluid compositions consistent with a misty or dusty gas. We juxtaposed these results with geochemical/temperature anomalies extracted from fumarole gas and rainfall levels covering a similar period. This comparison is interpreted in the context of a model proposed earlier for the volcano-hydrothermal system of TVG where fluids and magmatic gases ascend from a magma body that lies at around 7–8 km depth. Most HF earthquakes occur as a response to stresses induced by fluid circulation within a dense network of cracks pervading the upper crust at TVG. The largest (ML ~ 3.1) HF event that occurred on 24 April 2006 at a depth of 5–6 km had source characteristics compatible with that of a tensile crack. It was followed by an enrichment in magmatic components of the fumarole gases as well as a fumarole temperature increase, and provides evidence for ascending fluids from a magma body into the shallow hydrothermal system. This detailed analysis and previous physical volcanology observations at TVG suggest that the region is volcanically active and that measures to mitigate potential hazards have to be considered by the local authorities.
Highlights
The reliability of any volcano monitoring effort is dependent in part on an accurate understanding of the nature of background activity and eruptive history of the particular volcano
A model for the volcano-hydrothermal system in Tatun Volcano Group (TVG) has been previously proposed by Konstantinou et al (2007) based on available geological and seismicity observations (Fig. 11)
The depth of the magma body is inferred from geochemical and thermal observations to be 7–8 km, while the brittle zone extends from the surface down to about 5 km where after this depth seismogenesis ceases
Summary
The reliability of any volcano monitoring effort is dependent in part on an accurate understanding of the nature of background activity and eruptive history of the particular volcano. Geochemical monitoring of fumarolic gases and spring water could be expected to provide important information regarding changes in the magmatic-hydrothermal system. An understanding of the relation between background seismicity, fumarolic chemistry and previous volcanic eruptions can help to establish a baseline against which any future changes in the volcano-magmatic system can be evaluated. Among the signals that we have recorded are events caused by oscillations of fluid-filled cracks and we analyzed them using the Sompi method in order to infer possible fluid compositions These results are compared with temporal variations of several gas geochemistry indicators and fumarole temperatures published previously. This evidence includes mantlederived Helium in gases/spring waters, volcano-seismic signals similar to those observed in active volcanoes and radiocarbon dating of the latest eruption at about 6 ka. The highest (∼ 1120 m) and most well-preserved volcanic cone in TVG is Mt Chihsinshan which along with the nearby Tayiokeng area exhibit the strongest fumarolic activity and highest seismicity in TVG
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