Mud volcanoes are structures formed as a result of the emissions on a land surface or the sea floor of argillaceous material, which is composed of erupting remobilized mud, petroliferous or magmatic gases, and high-salinity water. Recently, large constructions have been planned deep underground besed on the expectation of geological stability. Therefore, it is important to study the origin of erupted mud and groundwater and the depths from which they ascend when evaluating long-term stability. Three active mud volcanoes and a passive mud volcano are found in the Tertiary Shiiya Formation distributed in Tokamachi City, southern part of Niigata Prefecture. Detailed descriptions of the mud volcanoes are provided by Shinya and Tanaka (2005). However, the origin of erupted mud and the formation mechanism of abnormal pore water pressure have not yet been identified. The authors measured the oxygen and hydrogen isotopic ratio of groundwater and vitrinite reflectance of coal fragments separated from erupted mud of an active mud volcano to investigate the origin of erupted mud, particularly the depth of the origin, and the formation mechanism of abnormal pore water pressure. As a result, δ18O and δD values of erupted water are 1.2‰, -5‰ respectively, showing good agreement with those of the Nanatani Formation distributed at a depth of 3400 m in depth in the studied area. Vitrinite reflectance (Ro) shows a bimodal distribution (i.e., 0.3-1.2% and 1.5-1.8%). Ro value of coal fragments sampled from the Shiiya Formation at the outcrop in the studied area are 0.3-0.45%. High Ro (1.5-1.8%) values of coal fragments are obtained in core samples at a depth of 4000 m in the Gimyo SK-1 oil well, which was excavated 2 km NW from the mud volcano. As a result of an investigation of erupted materials at the mud volcano, they were found to have originated at depths of from 3400 m to 4000 m in the studied area. Geothermal temperature of underground at depth of 3400 m to 4000 m in the in the studied area is estimated to be about 120°C to 150°C. Estimated temperature is high enough to cause diagenetic transition from smectite to illite. Transition from smectite to illite results in the release of a large volume of pore water into the sediment. It is concluded that dehydration due to mineral transition might be the major reason for abnormal pore water pressure formation at depths of 3500 m to 4000 m in the study area.
Mud volcanoes are defined as conical geometric high mounds composed of erupted mud that originates deep underground, and are induced by abnormal pore water pressure. It is well known that a serious problem occurred due to swelling mudstone during tunnel construction work in the Tertiary sedimentary basin in Japan. The swelling mudstone is partly caused by the activity of mud volcanoes. However, it has not been demonstrated in the field yet. Boreholes with a depth of 120 m were drilled at an altitude of 329 m inside a depression in the Kamou area, Tokamachi City, Niigata Prefecture where two active mud volcanoes and an inactive mud volcano are distributed and the Nabetachiyama Tunnel with swelling mudstone is located. Geologic structure and geochemical properties of groundwater around the mud volcanoes were studied by core logging and geochemical analysis of pore water squeezed from cores. Humus soil occurs from ground surface to a depth of 2.1 m. Erupted deposits composed of mudstone fragments and clay are located in the interval from 2.1 m to 5.8 m in depth. Intact massive mudstone is located from 5.8 m to about 50 m in depth. Mud breccia composed of mudstone fragments and scaly soft clay is located from fifty meters in depth to the bottom of a borehole. Mud breccia is classified into two types based on its textural and structural characteristics. The distribution of each type of mud breccia suggests that mud breccia was formed by hydro-fracturing caused by the activity of a mud volcano mainly at depths greater than fifty meters. As a result of an XRD analysis of clay minerals and XRF analysis of rock, Ca-type smectite is found to be dominant and CaO content is rich from the ground surface to 55 m in depth, whereas Na-type smectite is dominant and Na2O content is rich at depths greater than 50 m. Groundwater squeezed from cores greater than fifty meters in depth is highly saline. Therefore, the cation exchange from Ca2+ to Na+ occurred due to the highly saline groundwater. In summary, saline pressurized groundwater ascended with hydro-fracturing to a depth of fifty meters and replaced the fresh groundwater. Variation of stable isotope ratios of pore water with depth supports the assumption above.
In Tokamachi City, Niigata Prefecture, where Tertiary sedimentary rocks are distributed, excavation of the Mt. Nabetachiyama tunnel took a long time to complete due to the presence of a swelling mudstone zone. This swelling zone is distributed at a depth of 180m under the mud volcanoes. Geological and geographical surveys as well as electromagnetic surveys applying the Controlled Source Audio-frequency Magneto-Telluric Method (CSAMT), were carried out to clarify the geological structure under the mud volcanoes and the relationship between geographical features and the swelling zone. The laser scanner survey revealed that the geological structure around the topographic depression is intensely disturbed and the geological survey showed that the mud volcanoes and eruption points of groundwater and natural gas are located mainly along the outside edge of the geologically disturbed zone. This suggests that the area was active in the past due to the uplift and eruption of groundwater. The extremely low resistivity zone (ELR) at a depth of 400m with a diameter of 500m was detected below the geologically disturbed zone by the CSAMT survey. The ELR is estimated to correspond to a mud chamber filled with saline groundwater and mud, based on laboratory tests measuring the electrical properties of several rock samples obtained from the survey area. The low resistivity zone (LR) was also detected; it continues from the ELR to the mud volcano at the ground surface, indicating that saline groundwater, mud, and natural gas may be ascending via a concentric path to the mud volcanoes. On the basis of all the results obtained, it is interpreted that the swelling zone in the tunnel corresponds to the path of mobilized mud and saline groundwater in the fractured mudstone with abnormal pore water pressure.
We investigated the geological structure below the Matsudai-Murono mud volcano, Tokamachi city, Niigata prefecture, Japan, using vertical distributions of S-wave velocities, estimated from surface wave inversion. From the surface to a depth of several meters, the S-wave velocity first increases, then decreases, and finally increases again. Below a depth of 10 m, there are two areas with high and low velocities in the horizontal direction. We found that the subsurface low-velocity layer is a mud layer produced by an eruption, and the deep low-velocity zone is a mudstone zone, some parts of which are transformed into soft clay by muddy water that drifts upward. The planar distribution of the S-wave velocity indicates that the deep low-velocity zone may be subdivided. The zone in which the velocity is particularly high corresponds to the area without surface uplift indicated by GPS; hence, this area is not influenced by the mud volcano. This study shows that it is possible to conduct subsurface geophysical investigations to clarify the eruption mechanism of the mud volcano.
The geology and geochemistry of mud volcanoes in Taiwan was investigated to elucidate the relationship between their distribution and geological structure and the mechanism of ascending fluid migration from deep underground regions caused by abnormal pore water pressure. A detailed geological survey was carried out to describe the geological structure and the stratigraphy of mud volcanoes in the Pliocene Gutingkeng Formation in the Hsiaokunshui area. Groups of several to tens of mud volcanoes are distributed along the anticline axis within an area of 400 m in diameter. Mud volcanoes are classified into three types on the basis of differences in their morphological features (pudding type, crater type and pool type) corresponding to three types of erupted groundwater having different viscosities. As a result of geochemical studies on groundwater that erupted from mud volcanoes, it is shown that the geochemistry of groundwater that erupted from mud volcanoes distributed along the anticline, such as the Hsiaokunshui mud volcanoes, is characterized by lower δ18O ratios and high concentrations of soluble ions compared to those distributed along the Chishan Fault. Also, it is concluded that pressurized groundwater diluted by water produced during the dehydration of clay minerals ascended through the Chishan Fault and along the Hsiaokunshui anticline. On the other hand, on the basis of high δ18O ratios, it is suggested that the groundwater of mud volcanoes along the Chishan Fault was quickly expelled from underground regions deeper than those along the Hsiaokunshui anticline. Also, the groundwater of mud volcanoes along the Hsiaokunshui anticline ascended through a variety of paths from the mud chamber to the ground surface, and consequently various types of mud volcanoes were formed on the ground surface.
A high-resolution seismic reflection survey was conducted over submarine mounds on continental slope ridges in passive margin offshore southwestern Taiwan during the NT07-05 cruise in March 2007. The continental slope ridges were formed by an erosion system in which submarine canyons incise the continental slope and control ridge formations. The seismic survey revealed the geologic structure of continental slope ridges where there are target locations, site F and site G, of submarine mounds. Site F corresponds to the southern peak of the Formosa Ridge, where sedimentary strata are generally flat lying. On the Formosa Ridge, methane hydrate BSR (bottom simulating reflector) was widely observed, but it is chopped off just below site F. There is also a vertical narrow reflection blanking between the cut of the BSR and site F on the seafloor. SeaBat multibeam echo sounder detected a gas plume rising from the mound of site F. ROV Hyper-Dolphin discovered very large and dense chemosynthetic communities at the top of the mound. Site G is also a small submarine mound in another continental slope ridge. The mound is thought to be a mud volcano. There is no clear BSR, but there are high-amplitude beds below the mound. The high-amplitude beds peak just below site G, and there is also a vertical, high amplitude zone between the peak and the submarine mound of site G. Thus, the geologic structures in sites F and G have morphologically similar characteristics. It can be considered that the BSR in the Formosa Ridge should act as a good cap for trapping gassy fluid, and the narrow blanking just below site F is a fluid conduit where strata have been disturbed. On the other hand, in site G, the high-amplitude beds acts in the same way as BSR in site F, and the vertical high-amplitude zone just below site G is considered also to be the mark of a conduit from the peak of the high-amplitude beds to the submarine mound, where remnant gas components in the conduit may have emphasized partial acoustic impedance, although the Hyper-Dolphin could not find any present activity on the mound. We found the cold seep site at the top of site F in the Formosa Ridge. By integrating bathymetric characters and seismic reflection data, we can have a better understanding of the fluid circulation system in the cold seep site. In site F, the BSR that forms a trap as a gas reservoir is shallower than the canyon floors on either side of the ridge. We suggest that this reservoir configuration may enable a thermal pump system in the ridge and the surrounding cold seawater to enter the fluid system, and form a notable fluid circulation and unusual chemosynthetic communities.
Submarine mud volcanoes are remarkable geological features on the seafloor, which are probably formed by mud breccia extruded from sub-seafloor sediment layers to the seafloor. Most of such volcanoes are found near the continental margin. The driving force of mud volcanism is thought to be unusually high pressure within the deep sedimentary layer and the release of that high pressure. It is important to know the origins of fluids in a mud volcano, because the production of low-density fluid and/or gas production in the deep sedimentary layer has been assumed to be one of the most probable sources of the pressure. Therefore, geochemical studies of pore fluids have been done at various mud volcanoes to identify the fluid origin. These studies revealed common chemical characteristics of the fluids, indicating the effects of dehydration of clay minerals. Also, the fluids contain hydrocarbon gases derived from thermocatalyte decomposition of sedimentary organic matter. These characteristics suggest that the mud volcano fluids must originate at a depth in the sedimentary layer greater than 2 km. In some mud volcano fields in the active continental margin, it is proposed that fluid in the mud volcano has migrated through faults from greater depths than the original depth of extruded sediments. Such fluid migration may be another source of high pressure in sedimentary layers.
Many surface oil and gas seepages including small mud volcanoes are found in the Higashi-Kubiki area, Niigata Prefecture. The purpose of this paper is to demonstrate the geochemical characteristics of these seepages and to discuss their source rocks and migration processes. The geochemical characteristics of oils from the Kamou mud volcano show they derive from mature source rocks in the Lower Teradomari Formation. The gases collected from seepages including mud volcanoes are of a thermogenic origin and are slightly biodegraded based on their geochemical characteristics. The maturity of the gases inferred from their carbon isotopic compositions indicates that they were generated in the Lower Teradomari Formation which is more deeply buried than the source rocks of Kamou oil. Most of them probably migrated upward slowly in formations based on their high C1/ (C2 + C3) ratios, and were biodegraded near the surface. Our head-space gas analysis of two shallow boreholes shows that the head-space gas analysis is a useful tool for understanding the vertical distribution of absorbed gases, and the migration and alteration process of gases in a shallow subsurface.
The Lower Miocene Tanabe Group, exposed on the southern Kii Peninsula, is a thick pile of fore-arc basin sediments, which clino-unconformably covers the Paleogene Shimanto accretionary complex. Many mud diapirs and mud dykes intrude into the Tanabe Group. Several thick sequences of bedded breccia are found at Tanoi and Fukuro in Shirahama-cho, Wakayama Prefecture. In this paper, bedded breccias with shallow-marine sediments are described. The facies analysis shows that the bedded breccias are mud-volcanic deposits, and that two submarine mud volcanoes were involved in the southern Tanabe Group. The Tanoi sequences, which reach a thickness of 490 m, are mainly composed of sand-matrix, bedded breccia associated with mud-matrix, and bedded breccia. The bedded breccias range from 5 to 150 cm in thickness. They contain angular to sub-rounded clasts consisting of sandstone and mudstone from granule to cobble in size. The bedded breccia is matrix-supported with scattered clasts, which develop with inverse grading. It is considered to be a subaqueous debris flow deposit. The Fukuro sequences are mainly composed of mud-matrix, bedded breccias associated with clast-bearing sandstone of 1-15 cm thickness, which have turbidite-like sedimentary structures. The bedded breccias range from 5 to 150 cm in thickness. They contain angular to sub-rounded clasts consisting of mudstone and sandstone from granule to cobble in size. The bedded breccias are matrix-supported with scattered clasts, which develop with inverse grading. It is considered that the cause is a subaqueous debris flow deposit. The upper part of the clast-bearing sandstone is likely to have been reworked later by storm waves and tidal currents. The paleocurrent deduced from the sole marks of mud-matrix, bedded breccia flowed from northeast and east. The shallow-marine sediments develop wave ripple, planar cross-stratification, trough-type cross-stratification, chevron structure, off-shooting foreset, and hummocky cross-stratification, which indicate that the Fukuro mud-volcanic products were deposited at a depth near the lower limit of the storm wave base from the lower shoreface to the shelf. During the Early Miocene, submarine mud volcanism took place at Tanoi and Fukuro in the southern Tanabe Group. It is believed that the Tanoi mud volcano caused the Tanoi mud-volcanic deposits to erupt from the Tanoi mud diapir, and that the Fukuro mud volcano caused the Fukuro mud-volcanic deposits to erupt from the Migusagawa-Hirukawadani mud diapir.
The Nabetachiyama Tunnel 9116 m long was excavated in Tokamachi City, Niigata Prefecture and encountered the serious difficulties during excavation. In particular, a 600 m long section in the Matsudai area had experienced difficulties caused by swelling mudstone in the Tertiary Sugawa Formation. A 120 m bore hole long was excavated in the neighborhood of the section and geological and geochemical examinations of sampled cores were carried out to investigate the formation mechanism of the swelling rock mass. Mudstone distributed deeper than 50 m in the bore hole can be correlated to the tunnel troubled section geologically and geochemically. The section is assumed to be composed of mud breccia with mudstone fragments and clayey matrix, which is thought to be generated by hydro-fracturing of mudstone, showing weak strength due to large quantities of clay minerals. A gas pressure of 1.6 MPa thought to be caused by degassing of methane was measured during tunnel construction, which would increase the swelling properties. Mud breccia distributed deeper than 50 m contains a lot of Na-smectite formed in highly saline pore water ascending from deep underground. The result of slaking test showed that mud breccia filled with saline groundwater is characterized by quick slaking and swelling due to the marked contraction of Na-smectite when drying. In summary, the swelling rock mass distributed in the troubled section was formed by the weak rock strength caused by hydro-fracturing and high gaseous pressure generated by degassing. Furthermore, quick slaking caused by repeated wetting and drying was another reason for swelling during excavation.
Understanding erosion processes is important to prevent natural disasters such as slope failure and bedrock erosion in immature sedimentary rocks. Pliocene–Pleistocene illite-rich, non-smectite mudstone of the Gutingkeng Formation is distributed over 250 km2 in southern Taiwan, forming badlands (locally called moon-world) with mud volcanoes nearby. These volcanoes erupt saline water and natural gas, and producing a Na+, Ca2+, Cl-, and SO42- rich unsaturated Popcorn crust, which covers the mudstone slope surfaces in the moon-world area. In the crust porewater, ion strength reaches about 10 mol/L; zeta potential on particle surfaces shows a highly positive voltage. Repulsion occurs between particles under this high voltage in the crust, which is rapidly slaked to form mud by heavy precipitation. The zone that is rapidly slaked by precipitation reaches 10-20 cm beneath the crust surface. Ion strength of porewater of fresh mudstone is 0.5 mol/L approaches 0 mV (range 0.2-0.5 mol/L, pH 4-6). The surface charge of particles decreases with the infiltration of precipitation into the crust and fresh rock, with a minus surface charge occurring with increased rain infiltration. This leads to many cracks forming on the surface of mudstone, which is different from the mechanism of rapid slaking. Evaporation from the 10-20 cm-thick zone between the crust and the underlying fresh mudstone would stop if water was not supplied from depth, which is supported by in-situ measurements of water evaporation in the field. These mudstones erode readily under high precipitation because of the repulsion caused by the high ion strength of porewater. High-salinity porewater including mudstone is distributed near the active mud volcanoes where saline water rises and there is a rapid uplift rate. Rapid slaking occurred with some elements in the concentrated crust and near the surface drying zone.
Fluctuations of subsurface temperature were measured and analyzed to evaluate the activity of a mud volcano in Matsudai district, Niigata prefecture, Japan. Fourteen thermocouples were installed in an inclined observation well and subsurface temperature was continuously measured for ten minutes. Some fluctuations of subsurface temperature over a period of a few days to several weeks were observed at some monitoring points along the well. These fluctuations may be caused by changes of flux and path of subsurface fluid flow. The results of this study suggest that measuring subsurface temperature is effective to monitor long-term changes of mud volcano activity.
High salinity Na-Cl-type geothermal waters have often been found in the anticlinal hilly terrains and landslide-prone areas of Niigata Prefecture by boring to depths of more than 1,000 meters. These fluid pressure gradients are much higher than the hydrostatic pressure gradient and approach the lithostatic pressure gradient with increasing depth. The geothermal waters in the Matsunoyama area, Tokamachi City, have the highest orifice temperature in Niigata Prefecture. Eight geothermal water wells in this area were drilled along the anticlinal axis of a nearby hilltop or higher breast of the Matsunoyama dome and ranged in depth from 170 to 1,170 meters and in temperature from 35 to 95°C. For example, the Takanoyu-1 geothermal water well is only 170 meters in depth but has an orifice temperature of 90°C. These waters show geyser action associated with methane gas, and typically have very high salinity with considerable amounts of chloride. Hydrogen and oxygen isotope values and chloride concentration of approximately 9,000mg/L suggest that the origin of the waters is altered fossil seawater trapped in organic-matter-bearing sedimentary rocks. The temperature and depth of the primary reservoir are estimated to be 139°C and 3,000 to 4,000 meters, respectively, using a Li-Mg geothermometer and the mean geothermal gradient of 30-40°C/km in the Niigata sedimentary basin. Na-Cl-type groundwaters emerged from several landslides located in the Higashi-kubiki and Naka-kubiki areas including the Matsunoyama area. Well loggings were carried out to profile the electric conductivity and hydrochemistry of groundwater in the Utsunomata landslide in the neighborhood of the Matsunoyama area. Profiles in the active landslide mass showed sharp increases of the electric conductivity and the NaCl components at around the depth of the sliding surface. In contrast, no significant variations of the profiles were recognized in the inactive landslide mass. Na-Cl-type groundwaters are formed by mixing deep Na-Cl-type geothermal waters with meteoric groundwaters. These phenomena suggest that the geothermal water injection into shallow aquifers in landslide mass generates a partially high pore water pressure around the sliding surface and causes landslides.
There have been many studies on the microbial ecology in methane-seep sites related to mud volcanoes. This microbial ecosystem is supported by chemosynthetic microorganisms which depend on not on sunlight but on high-flux methane and sulfate. In the subseafloor at the methane-seep sites, anoxic methane oxidation (AOM) occurs in the mixing zone of methane and sulfate. The primary microorganisms causing AOM are anaerobic methanotrophic archaea (ANME) and sulfate-reducing δ-proteobacteria (SRB). It is known that there are three groups of ANME (ANME-1, ANME-2, ANME-3) in deep-sea environments. These ANMEs are taxonomically related to known methanogens. ANME-2 and ANME-3 form symbiotic consortia with SRB, while ANME-1 does not form the symbiotic consortia. However, none of these microorganisms has been isolated yet. Therefore, details about ANMEs such as energy-metabolisms are still unknown. However, information about these microorganisms has recently been collected by the culture-independent analyses, such as metagenomic analysis. From these analyses, it has been proposed that ANME performed AOM by reverse-methanogenesis that proceeded in the opposite direction of known methanogenesis by methanogens. Hydrogen sulfide is produced through AOM. This supports other ecosystems at the sediment surface. At the sediment surface, H2S supports H2S-oxidizing microorganisms. There are also invertebrates including mussels, clams and tubeworms which harbor chemosynthetic bacteria in their bodies to gain organic compounds. Here, I introduce the activities of microorganisms and invertebrates living at methane-seep sites related to mud volcanoes.
The activity of onshore mud volcanoes may affect the stability of infrastructure and man-made underground structures through the eruption of huge volumes of mud and groundwater. For example, the LUSI mud volcano in Indonesia has erupted vast amounts of groundwater and mud since 2006, causing serious problems with respect to the ground surface and affecting the lives of local residents. The ascension of highly saline and pressurized groundwater from deep underground at mud volcanoes destroys man-made underground structures by hydro-fracturing and forms mud breccia, which adversely affects tunnel construction due to its swelling nature. Ascending groundwater increases the pore water pressure along the landslide plane and consequently triggers landslides as well as the activity of mud volcanoes. In addition, regional groundwater flow caused by ascending pressurized groundwater may result in the long-term stability of the groundwater. Furthermore, the migration of highly saline groundwater causes rapid weakening and erosion of sedimentary rock via the interaction of clay minerals and groundwater. It is assumed that the volume of methane, a potent greenhouse gas, released during eruptions of mud volcanoes is sufficiently large that it affects global warming. Because onshore areas support considerable human activity and the activity of the mud volcanoes can potentially cause many problems, investigation of the geological and hydrological phenomena related to mud volcanoes is warranted.
On February 25, 2009, damage was reported for the first time from the Shimekake landslide located in Yamagata Prefecture. Landslide activity has increased since April. The active landslide extends up the opposite bank of the Kariya River, and is 400 meters wide and 700 meters long. The authors have conducted field surveys several times since the beginning of April, and have begun displacement monitoring of the landslide using GPS. As a result, the main block of the north side of the Kariya River showed a direction of movement from south-southwest to south and the sub-block at the south side of the Kariya River showed a southeasterly movement. The rate of horizontal displacement was large at the lower side of the slope. It tended to accelerate after rainfall, and reached 15 cm/day in the sub-block. The existence of two or more slip surfaces is presumed in the sub-block based on the properties of cracks at the block boundary and the characteristics of movements of the landslide.