Gravity Map Series 27

Gravity map of Okayama district (Bouguer anomalies) 1:200,000

MURATA Yasuaki, KOMAZAWA Masao, MAKINO Masahiko, SATOH Hideyuki, NAWA Kazunari, JOSHIMA Masato, KISIMOTO Kiyoyuki, OKUMA Shigeo, SHICHI Ryuichi, KOMURO Hiroaki, NISHIMURA Kiichi, AKAMATSU Junpei
published on August 7, 2009

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Gravity Map of Okayama District (Bouguer Anomalies)

A new gravity map of Okayama district has been published. Compiled gravity data within the map area are 13,860 stations including newly added 1,136 gravity stations on land. All the measured gravity data were referred to the Japan Gravity Standardization Net 1975 (JGSN75) and normal gravity values were calculated according to the Geodetic Reference system 1980 (GRS80). Bouguer, terrain and other corrections were applied, following the standard procedure of gravity data processing at the Geological Survey of Japan, AIST (SPECG 1988)(GSJ Gravity Survey Group,1989). The Bouguer and terrain corrections were carried out to remove an effect of bounded spherical crust and an effect of actual topographic undulation relative to the spherical surface, respectively, within a distance range of 60 km. The density value used for both Bouguer and terrain corrections is 2.3 g/cm³ (shown on the map as assumed density). Bouguer anomaly maps which have been corrected by densities of 2.00 g/cm³ and 2.67 g/cm³ are shown in Fig. 1 and Fig. 2, respectively. Three additional maps, i.e. regional Bouguer anomalies continued upward 2 km (Fig. 3),gravity residual anomalies with the removal of the 2 km upward-continuation (Fig. 4) and horizontal derivation (Fig. 5) are also produced to enhance the features described in the following.
The Bouguer anomaly maps and filtered gravity maps revealed following features:
(1) This area is mainly covered by gentle slope positive Bouguer anomalies.
(2) Short-wavelength and high Bouguer anomalies are very good match with Paleozoic rocks such as Permian Maizuru group.
(3) Steep gravity change over 45 mGal exists between the Shimane peninsula and the Shinji-ko lake and Naka-umi sea.
(4) Low gravity anomaly about 5-10 mGal corresponds to the Daisen volcano and Hiruzen volcano.
(5) Gravity changes along and/or across active faults or the earthquake fault of the 2000 Tottoriken-Seibu earthquake are not clearly defined.

Fig. 1 Gravity Map (Bouguer Anomalies) (Assumed Density: 2.0 g/cm³)
Contour interval is 1 mGal. Dashed lines indicate negative values. H and L indicate high and low gravity anomalies, respectively. Thick solid lines indicate active faults. Thick dotted lines indicate earthquake faults of the 2000 Tottori-ken Seibu earthquake by Fusejima et al. (2001).

Fig. 2 Gravity Map (Bouguer Anomalies) (Assumed Density: 2.67 g/cm³）
Contour interval is 1 mGal. Lines specification is the same as Fig. 1.

Fig. 3 Regional Bouguer anomalies continued upward 2 km.
Contour interval is 1 mGal. Gray areas indicate negative values. Thick short solid lines indicate active faults. Thick dotted lines indicate earthquake faults of the 2000 Tottori-ken Seibu earthquake by Fusejima et al. (2001).

Fig. 4 Residual Bouguer anomalies with removable of the 2 km upward-continuetion.
Contour interval is 1 mGal. Gray areas and lines denote the same as in Fig. 3.

Fig. 5 Horizontal gradients of the Bouguer anomaly map shown in Fig. 2.
Contour interval is 1 mGal/km. Gray areas indicate values more than 3 mGal/km. Lines denote the same as in Fig. 3.