**4.3. Anomaly modeling**

Middle Miocene volcanic rocks, collectively named the Nijo Group, are exposed in a hilly province upon the eastern margin of the Osaka Basin. It has been studied from stratigraphic (e.g., [27]) and paleomagnetic (e.g., [28]) points of view, and regarded as a volcanic product formed just after the clockwise rotation of southwest Japan related to the opening of the Japan Sea.

We assumed that conspicuous gravity and geomagnetic anomalies in the southern Osaka Plain are caused by a buried middle Miocene volcano, and estimated its three-dimensional structure based on Talwani's methods [29,30]. Figure 14 summarizes the modeling parameters. We assumed that the body of the volcano is magnetized parallel to the Earth's axial dipole field because the majority of the Nijo Group acquired its thermoremanent magnetization during the normal polarity Chron after the Miocene clockwise rotation of southwest Japan. As shown in Figure 14, gravity and geomagnetic anomalies are successfully simulated on the assumption of a conical volcano on the base of the sedimentary basin. In the Osaka Basin, there are isolated positive Bouguer anomalies associated with magnetic signatures from place to place. Com‐ bined anomaly modeling is a useful tool to estimate the origin of subsurface units in advance of a comprehensive interpretation of seismic data.

**Figure 14.** Three-dimensional gravity and geomagnetic modeling of subsurface structure in the southern Osaka Plain. See Figure 10 for mapped area
