重力を測ると地下の様子や火山のマグマが分かる/en

#0017 Science & nature

Fact rank

A

Explanation by a primary source (GSI) agrees with a university report on repeated gravity measurements at Sakurajima

Wow rank

A

Surprising idea: seeing the invisible underground through weight

Usefulness

—

A fact worth knowing for its own sake

Verification: Supported by evidence

Posted by: — ・ Verified: 2026-10-29 ・ Method: AI + human ・ On Wikipedia?: Already on Wikipedia

AI verification reports the evidence it found; it does not guarantee that a claim is true.

The Geospatial Information Authority of Japan measures gravity across the country to maintain the national reference, and the values are used to explore for resources and to estimate active faults; measuring repeatedly at the same spot can even track the movement of magma in a volcano.

Why

If there is heavy rock or a cavity underground, gravity directly above it changes slightly.

Evidence

Primary Explanation by GSI. Gravity also changes with the density structure underground; the results are used for resource exploration and estimating active faults. Measuring continuously at the same spot also lets you monitor things like the movement of magma link

Primary Report by the Disaster Prevention Research Institute, Kyoto University, "Repeated relative gravity measurements at Sakurajima volcano (FY2021)". Gravity observation is effective for detecting changes in underground mass with volcanic activity; an increase of up to about 0.2 milligal was seen in the active period from the 1970s to the early 1990s link

Notes and limits

The changes seen at volcanoes are small, from a few microgal to 0.2 milligal (a few hundred-millionths to about one five-millionth of gravity), and rain and groundwater also change gravity, so those effects are removed before reading. The main purpose of GSI's gravity survey is the national gravity reference and accurate elevations; volcano monitoring is one of the uses. That dense rock changes gravity is also in the Japanese Wikipedia article on gravity anomalies.

Derivations

Derived from: 体重計の数字は緯度が高いほど大きく出る

Derived facts: 足元の岩の重さで重力は僅かに変わる