Deformation-induced gravity changes in volcanic regions

Gravity changes (corrected for subsidence) of up to -1 000 (±300) μGal have occurred in the [Formula: see text] area of the production bore field at Wairakei, and smaller decreases extend over a [Formula: see text] surrounding area. The largest part of these decreases occurred during the 1960s; since then the net gravity change for the whole field has been zero, indicating mass flow equilibrium. The principal causes of gravity change have been deep liquid pressure drawdown which resulted in formation of a steam zone, subsequent saturation changes in the steam zone, liquid temperature decline, and groundwater level changes. Gravity models suggest saturation of the steam zone was 0.7 (±0.1) in 1962 and decreased to 0.6 by 1972. Gravity increases in the northern and eastern bore field since the early 1970s are attributed to cool water invading the steam zone.

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Delineating a Volcanic Aquifer Using Groundwater-induced Gravity Changes in the Tatun Volcano Group, Taiwan

10.21203/rs.3.rs-839361/v1 ◽

2021 ◽

Author(s):

Tzuyi Lien ◽

Emmy Tsui-Yu Chang ◽

cheinway Hwang ◽

Ching-Chung Cheng ◽

Kin-Fai Lam ◽

...

Keyword(s):

Active Volcano ◽

Volcanic Hazards ◽

Phreatic Eruption ◽

Volcanic Aquifer ◽

Induced Gravity ◽

Gravity Station ◽

Additional Information ◽

Gravity Changes ◽

Superconducting Gravity ◽

The Media

Abstract The Tatun Volcanic Group (TVG) is an active volcano that could cause volcanic hazards in northern Taiwan. The latest phreatic eruption of the TVG occurred some 6000 years ago. Understanding the state of groundwater around the TVG can be a crucial step towards effectively assessing the risk of phreatic explosion by providing information about the sources of groundwater and the media it flows. We measured gravity changes at a superconducting gravity station and several groundwater-sensitive sites to examine the way the groundwater altered the gravity values around the TVG. Groundwater-induced gravity changes are simulated by two hydrological models (A and B). Both models show coherent seasonal variations in groundwater level and gravity value in the center of the TVG (Chintiengang). This coherence indicates inter-connected porous media for free groundwater flows below Chintiengang. However, inconsistencies between the modeled and observed gravity changes occurred in the eastern part of the TVG, suggesting here highly heterogeneous formations with fractures and barriers may exist below Chihsinshan and Dayoukeng. The gravity consistencies and inconsistencies between the observations and the models are used to delineate a volcanic aquifer, which can provide additional information for assessing the probability of a potential phreatic eruption over the TVG.

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