温泉科学（J. Hot Spring Sci.），62，135‑143（2012）

原著

Gravity Anomaly and Subsurface Structure in
the Kaga Hot Springs Area
Akihiro SAWADA1）*, Hiromi BESSYO2）, Yoshihiro HIRAMATSU1）,
Yoshiteru KONO2） and Muneyoshi FURUMOTO3）(Received May 9, 2012, Accepted Jul. 2, 2012)

加賀温泉郷地域の重力異常と地下構造
澤田明宏1)*，別所宏美2)，平松良浩1)，河野芳輝2) ，古本宗充3)

要旨

石川県南部の温泉群がある地域（加賀温泉郷）の地下構造を調べることを目的として重力探査を行った．これまでに得られていた重力測定値と合わせて，当該地域における精密なブーゲー重力異常分布図を作成し，逆解法により地下構造を推定した．加賀温泉郷の付近は相対的に 15 mgal 程度負の重力異常となっており，基盤の大きな陥没を示している．負の重力異常域
は直径が 10 km 強のほぼ円形をしている．基盤の最大の陥没量は 2.6 km と見積もられる．付近の岩石が中新世の火山岩であることから，この構造は埋没したカルデラである可能性がある．加賀温泉郷の内，片山津，粟津，そして山代温泉はこのカルデラ状構造の内側にあり，山代温泉はその縁付近にある．これらの温泉の熱源は中新世の火山体という可能性がある．
キーワード：重力異常，基盤構造，埋没カルデラ，加賀温泉郷

Abstract
To study the structure beneath the Kaga hot spring area of southern Ishikawa prefecture,a gravity survey was conducted in and around the area. Combining these data with thoseobtained previously, we obtained a high-resolution Bouguer gravity anomaly map of the study
1)

School of Natural System, College of Science and Engineering, Kanazawa University, Kakuma-machi,
Kanazawa-shi, Ishikawa Prefecture 920‑1192, Japan. 1）金沢大学理工研究域自然システム学系 〒920‑1192 金沢市角間町．*Corresponding author：E-mail sawada@hakusan.s.kanazawa-u.ac.jp, TEL 076‑
264‑6513, FAX 076‑264‑6545.
2）
Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawashi, Ishikawa Prefecture 920‑1192, Japan. 2）金沢大学大学院自然科学研究科 〒920‑1192 金沢市角間町．
3）
Earthquake and Volcanology Research Center, Graduate School of Environmental Studies, Nagoya
University, D2‑2(510) Furo-cho, Chikusa-ku, Nagoya-shi, Aichi Prefecture 464‑8601, Japan. 3）名古屋大
学大学院環境学研究科附属地震火山研究センター 〒464‑8601 名古屋市千種区不老町 D2‑2（510）．
Deceased
故人

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澤田明宏，別所宏美，平松良浩，河野芳輝，古本宗充

温泉科学

region. We then estimated the structure from the map using an inversion method. The
relative gravity anomaly of the area is negative by about 15 mgal, suggesting that thebasement beneath the area is deeply depressed. The region with the negative gravity
anomaly has a nearly circular shape with diameter greater than 10 km. The maximum valueof the basement depression was estimated as 2.6 km using inversion analysis. Because
younger layers overlying the granitic basement are mainly Miocene volcanic rocks, thepossibility exists that the circular structure is a buried Miocene caldera volcano. The Katayamazu, Yamashiro, and Awazu hot springs are inside the caldera-like structure, and the
Yamanaka hot spring appears near its margin. It is probable that the hot springs heatsource is a buried volcanic ediﬁce that was created during the Miocene.
Key words : Gravity anomaly, Basement structure, Buried caldera, Kaga hot spring area

1． Introduction
The Japanese islands have many hot springs that are regarded as formed by activities of
volcanoes and faults (e.g., Nishimura

2006). In and around the southern Kaga plain,

Ishikawa prefecture, there are many hot springs such as Awazu, Katayamazu, Yamashiro, andYamanaka. The area is therefore called the Kaga hot springs area (Fig. 1). These hot springs
have been known and used through the ages, for example Awazu hot spring, opened in 712, hasa long history of about 1,300 years. The salient feature of these hot springs is the outwelling ofabundant hot water. However, geological understandings of heat sources and underground pathsof hot waters have not been clariﬁed yet. No volcanoes and no major active faults are related tothe heat sources and the underground paths of the hot waters in and around the Kaga hot
springs area. To reveal them, it is important to obtain information related to the subsurface

Fig. 1 Topographic map of the Kaga hot springs area. Squares, triangles, solid lines andbroken lines respectively represent major towns, major hot springs, Quaternary active
faults (Research Group for Active Faults in Japan, 1991) and the prefecture boundary.

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加賀温泉郷地域の重力異常と地下構造

structure, from the surface to several kilometersʼ depth.
From the point of investigation of the subsurface structure, in addition to a geologicalsurvey of the surface, various geophysical and geochemical surveys are necessary. We can
expect that a gravity anomaly is a useful tool to provide information about the subsurface structure, especially for the basement structure, in the Kaga hot springs area. Herein, we present a
detailed gravity anomaly map based on a dense gravity survey and discuss the subsurface structure in and around the Kaga hot springs area and heat sources beneath the area.

2． Geological features in and around the Kaga hot springs area
Figure 2 presents a geological structure in and around the Kaga hot springs area (Geological
Survey of Japan, 2003). The basement in this area consists primarily of Hida metamorphic rocks(middle to late Palaeozoic), the Tedori Group (Mesozoic sedimentary rocks, middle Jurassic toearly Cretaceous), and early rhyolite pyroclastic rocks (Mesozoic volcanic rocks). The basement
rocks were covered with Miocene volcanic rocks (Tertiary volcanic rocks, about 15‑20 Ma).
These Miocene volcanic rocks are known as the Green Tuﬀ and consist of andesitic, dacitic, andrhyolitic volcanic/pyroclastic rocks that were not formed from recent volcanism but fromancient volcanism. The volcanic activities of this area are regarded as intense. It is possiblethat the heat source and the chemical components of the Kaga hot springs come from the bodyof the Miocene volcanic rocks (Fuji and Itakura, 1992). In the Hokuriku region, including thisstudy area, the Miocene volcanic rocks exist along the coast and extend from southwest to

Fig. 2 Geological map of the Kaga hot springs area (modiﬁed from
Geological Survey of Japan, AIST, 2003).

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澤田明宏，別所宏美，平松良浩，河野芳輝，古本宗充

温泉科学

northeast between Fukui prefecture and Toyama prefecture. The middle Miocene to Quaternary sediments are distributed between the Miocene volcanic rocks and the coast (Fig. 2). The
Kaga hot springs, excluding the Yamanaka hot spring, are on the plain or terraces formed
during the Quaternary.
Regarding the chemical components of hot springs, salt-water springs are found mainly near
the coast, for example the Katayamazu hot spring. However, sulfate springs are found mainly ininland areas. The Yamashiro and Awazu hot springs are sodium sulfate springs, and the Yamanaka hot spring, located in the most inland area, is a sodium sulfate and calcium sulfate spring(Fuji and Itakura, 1992).

3． Gravity survey
The gravity values in this study area were measured using a LaCoste-Lomberg gravimeterand another gravimeter (CG-3 ; Scintrex Ltd.). The gravity anomaly dataset includes measurement data obtained by Kanazawa University with 122 new measurement data measured in thisstudy and data measured by other institutes (Geospatial Information Authority of Japan, 2006,
Geological Survey of Japan, 2004, Gravity Research Group in Southwest Japan, 2001). The total
number of gravity measurement points used for this study was 3,628 (Fig. 3).
Most gravity values were measured on spot elevations marked on topographic maps. If an
appropriate spot elevation was not found, then we measured a gravity value on a point where
the elevation can be estimated easily, such as the intersection of a road and a contour line. The
error of the estimated height of the measurement points is less than a few meters in almost allcases. The errors of gravity values are therefore estimated as less than 1 mgal.

Fig. 3 Distribution map of measurement points. Squares show new measurement
points, circles show points measured by Kanazawa University, and triangles
show points measured by other institutes.

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The gravity anomaly map used for this study portrays a distribution of the Bouguer anomaly
with terrain correction. We calculate the terrain correction using the method of Honda andKono (2005) with 50 m mesh digital elevation map data. The hills around the Kaga hot springs
consist of rhyolite pyroclastic rocks and lavas in the early Miocene (Kano

1999). The mean

3

density of Miocene felsic rocks is 2,440 kg/m . The mean density of the pyroclastic rocks is lessthan that of rhyolite rocks, of which the density was between 2,000 and 2,300 kg/m3 (Iwatani andKano, 2005). We therefore adopt the density of 2,300 kg/m3 for the Bouguer correction and terrain correction.

4． Remarkable features of the gravity anomaly
Figure 4 depicts a gravity anomaly map for a wide area around the Kaga hot springs. An
enlarged gravity anomaly map of the target area is presented in Fig. 5. A positive gravity
anomaly is apparent in the coastal area. A negative one extends to the inland area (Fig. 4). The
values of the gravity anomaly decrease southeastward. The negative gravity anomaly meansthat the average density of the crust is less than that of surrounding areas. The observed trend,
decreasing southeastward, is caused mainly by the crust thickening towards the central part ofHonshu (Zhao

1992 ; Iidaka

2003).

The most striking features of the distribution of the gravity anomaly in and around theKaga hot springs area is the existence of a negative gravity anomaly that cuts oﬀ the positive
gravity anomaly trending from northeast to southwest along the coast (Fig. 5). The negative
gravity anomaly is more than 10 km wide and extends to the southeast from the coast. This
negative gravity anomaly zone suggests that the depth of the basement rocks beneath this zone
is deeper than the surrounding area, i.e., this is a depression structure. From the contour line of
the gravity anomaly, this depression structure appears to consist of two near-circular depressions aligned in a northwest-southeast orientation (a buried caldera structure), or alternatively, a
depression zone extending in a northwest-southeast orientation (a graben structure). The mostnegative gravity anomaly is found not at the central part but at the northeastern side of the depression area, indicating that the basement becomes steeply deeper in the northeastern side.
Fault structures related to this depression structure have not been reported in the relevant
literature.

5． Inversion analysis of gravity anomaly
The Bouguer anomaly features suggest two possibilities of the subsurface structure : a buried
caldera structure and a graben structure. To examine these possibilities, we performed a threedimensional inversion analysis of gravity anomaly (Rama Rao1999) and estimated the lat-eral variation of the basement depth. Two layers were assumed in the inversion analysis so thatthe interface depth, the depth of the upper surface of the lower layer, is the ﬁtting parameter.
Geological information shows that the upper layer consists of rhyolite lavas and pyroclastic rocks
and the lower layer, which is the basement, consists of granite. We therefore set the upper layer
density as 2,300 kg/m3 and that of the lower layer as 2,670 kg/m3. In other words, the density139澤田明宏，別所宏美，平松良浩，河野芳輝，古本宗充

Fig. 4

温泉科学

Bouguer anomaly map. The assumed density is 2,300 kg/m3. The contour interval is 2 mgal.

Fig. 5 Detailed Bouguer anomaly map. The assumed density is 2,300 kg/m3. The contour interval is
2 mgal. Squares show major towns and inverted triangles show major hot springs (refer Fig. 1).

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加賀温泉郷地域の重力異常と地下構造

diﬀerence between the two layers is 370 kg/m3.
We applied a band-pass ﬁlter of 8 km to 25 km to the Bouguer anomaly for the inversion
analysis to remove trends with long wavelength, which reﬂected much deeper structure than
the subsurface structure, on which we focus here, and to remove ﬂuctuations with short wavelength that might have been introduced by measurement errors and slight heterogeneities near
the surface (Fig. 6a). The blocks for the inversion are set as 2 km×2 km and the analyzed area
is covered with 19×21 blocks. We set the initial depth of the interface of 1 km that provides theminimum residual between the band-pass ﬁltered and the calculated gravity anomalies. In fact,
we tried various values as the initial depth and conﬁrmed that the depth of 1 km provides theminimum residual.
Figure 6c shows the optimum density structure model obtained using inversion analysis.
Figure 6b is the calculated gravity anomaly from the optimum model. We recognize that thecalculated gravity anomaly reproduces the band-pass ﬁltered gravity anomaly very well. Two
depression structures resembling ellipses are found beneath low anomalies in the ﬁltered gravity
anomaly. Based on the shape of these depression structures, we infer that the low-gravity anomalies are not caused by the graben structure but by the buried caldera structure. The depth ofthe larger one extends downward to 2.6 km. That of the smaller one extends downward to 2.0
km. The larger one is about 10 km. The smaller one is about 5 km. The Awazu, the Katayamazu,

Fig. 6 (a) Band pass ﬁltered gravity anomaly. (b) The calculated gravity
anomaly from the optimum basement structure model. The contour
interval is 1 mgal. (c) Optimum basement structure model. The contour
interval is 0.25 km. Squares denote major towns and triangles show
the major hot springs.

141澤田明宏，別所宏美，平松良浩，河野芳輝，古本宗充

温泉科学and the Yamashiro hot springs are located within the larger depression structure. Especially,
the Awazu hot spring is located near the deepest point. However, the Yamanaka hot spring islocated near the edge of the larger depression structure.

6． Implications for the origin of the Kaga hot springs
As presented in Fig. 5, the four hot springs in the Kaga hot springs area are located in ornear the large depressed area. The Katayamazu, Yamashiro, and Awazu hot springs are locatedrespectively near the northwestern, the southwestern, and the southeastern boundaries of thestructure. Because the most depressed part is in the eastern part of the structure, the Awazu
hot spring is located near the most depressed part. Moreover, although the Yamanaka hot
spring appears outside the circular structure, it is still located near its margin. These lines ofevidence suggest strongly that these hot springs have a causal link to the depression structure.
The most interesting feature of the depression structure of the basement is that it shows anearly circular boundary. As described above, the study area is covered mainly with Miocene
volcanic rocks, indicating the volcanic activity was extremely high in this region in the Miocene.
It is therefore likely that the circular structure beneath the surface is a remnant of a caldera
generated as a part of the volcanic activity in the relevant period.
If this is the case, we can propose another fascinating hypothesis : the heat source of the hotsprings is a high-temperature body in the ancient volcanic ediﬁce that has been buried up to thepresent. The ordinary interpretation of the heat source for the hot spring area is that the remnant heat of the Miocene volcanic formation might warm the underground water (Fuji and
Itakura, 1992). Discovery of the buried circular structure would cast new light on the questionof the heat source of the Kaga hot springs area.

7． Conclusions
We performed dense gravity measurements and compiled a database to ascertain details of
the gravity anomaly in the Kaga hot springs area. We produced a Bouguer anomaly map with
the Bouguer and the terrain corrections with density of 2,300 kg/m3. The Bouguer anomaly map
shows a negative gravity anomaly around the Kaga hot springs area. Inversion analysis using
the two-layer model revealed the basement structure beneath the Kaga hot springs area, particularly revealing depression structures resembling calderas of which the deepest point extends
downward to 2.6 km. Younger layers over the granitic basement are mainly Miocene volcanic
rocks. We therefore infer the possibility that the circular structure is a buried Miocene caldera
volcano. The Katayamazu, Yamashiro, and Awazu hot springs are inside the caldera-like structure, and the Yamanaka hot spring is apparently located near its margin. The possibility existsthat the heat source of the hot springs is a buried volcanic ediﬁce created during the Miocene.

Acknowledgments
We thank the Geospatial Information Authority of Japan, Geological Survey of Japan, and142

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The Gravity Research Group in Southwest Japan for providing gravity data.
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