Makoto Yamano

Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust

For coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault. In this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature. We have developed a transient thermal model for the Nankai subduction zone of southwest Japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from (1) seismicity and tsunami data for two great subduction earthquakes, (2) the coseismic faulting extent of these events estimated from geodetic deformation data, and (3) the interseismic locked zone determined from interseismic geodetic data. The Nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures. It has earthquake, tsunami, and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain. On the Nankaido margin off Shikoku Island, the thermal model indicates that a temperature of 350°C (taken to be the limit for seismic initiation from laboratory and field data) is reached on the subduction thrust fault 150 km from the trench. A transition zone into which rupture may extend with decreasing offset (taken to be 450°C) extends an additional 45 km downdip. The thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 Nankaido Ms = 8.2 earthquake off Shikoku Island and with the downdip extent of the present locked zone. In the region of the 1944 Tonankai Ms = 8.2 earthquake to the northeast, the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent. The seismogenic-locked zone from earthquake and geodetic data is also narrower. Thus our analysis of the southwest Japan margin indicates that all three constraints on the downdip extent of the seismogenic zone, thermal, coseismic and interseismic geodetic data, are in general agreement. The study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature. The thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip. The subducting plate age and thus heat flow, and the thickness of the insulating sediments on the incoming plate, are also very important to the thermal regime and thus to the seismogenic width. The relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as Cascadia where there have been no great historical events.

Venting of carbon dioxide-rich fluid and hydrate formation in mid-okinawa trough backarc basin.

Carbon dioxide-rich fluid bubbles, containing approximately 86 percent CO2, 3 percent H2S, and 11 percent residual gas (CH4 + H2), were observed to emerge from the sea floor at 1335- to 1550-m depth in the JADE hydrothermal field, mid-Okinawa Trough. Upon contact with seawater at 3.8�C, gas hydrate immediately formed on the surface of the bubbles and these hydrates coalesced to form pipes standing on the sediments. Chemical composition and carbon, sulfur, and helium isotopic ratios indicate that the CO2-rich fluid was derived from the same magmatic source as dissolved gases in 320�C hydrothermal solution emitted from a nearby black smoker chimney. The CO2-rich fluid phase may be separated by subsurface boiling of hydrothermal solutions or by leaching of CO2-rich fluid inclusion during posteruption interaction between pore water and volcanogenic sediments.

Estimates of heat flow derived from gas hydrates

A gas hydrate phase boundary in marine sediments is often observed as an anomalous reflector in seismic profiling data. From the depth of the reflectors, heat-flow values can be estimated using the phase relation of the gas hydrate system and inferred density and thermal conductivity of sediments. This method is applied to the data in the Nankai Trough, around Central America, and along the Blake Outer Ridge. The estimated values, consistent with those measured by conventional means, suggest that derivation of heat flow by this method is feasible. On the landward slope of the Nankai Trough we find that heat flow increases downslope toward the floor of the trough.

Sediment deformation and hydrogeology of the Nankai Trough accretionary prism: Synthesis of shipboard results of ODP Leg 131

The main objective of Leg 131 was to provide data on the deformational processes and associated hydrogeology of the Nankai prism toe. Drilling succeeded, for the first time in the history of ocean drilling, in penetrating the complete sedimentary sequence to basaltic basement, reaching 1327 mbsf (metres below seafloor) with good core recovery (55%). Excellent correlation of the lithology and structure, including the frontal thrust and the decollement, with seismic reflection images was also determined. Bedding dips, faults and shear bands analyzed in the cores confirm the pattern of deformation to be mainly due to NW-SE shortening, as expected from the plate tectonic convergence vector. Below the decollement, no significant deformation features were observed, indicating that the decollement is a sharp discontinuity in stress transmission. Physical properties data show major discontinuities at the decollement, notably an increase in porosity below the later. This may indicate excess pore pressure in the subducted section and decollement zone. A less marked increase in porosity below the frontal thrust may reflect the youthfulness of this feature. Attempts to make downhole measurements were severely hampered by unstable hole conditions, but useful constraints have been placed on the thermal regime, and some calibration of laboratory physical properties toin-situ conditions has been provided, andin-situ stress and pore pressure were measured in the uppermost sediments. Evidence of channelized fluid flows is inconclusive. No sharp geochemical signatures or unequivocal geochemical anomalies indicative of channelized fluid flow were found. Thermal measurements are not significantly different from those predicted by a purely conductive heat flow model. A signature of low chloride pore water near the decollement may partly be related to smectite diagenesis but may also be due to episodic fluid flow events. We conclude that dewatering probably occurred dominantly through diffuse flow throughout the accreted sediments at this site.

Thermal regime of the Southwest Japan subduction zone: effects of age history of the subducting plate

Abstract During a mid-Miocene (at about 15 Ma) tectonic reorganization, an ocean spreading ridge perpendicular to the strike of the Nankai Trough subduction zone stopped spreading. Since that time there has been subduction of the cooling fossil spreading ridge. In the present study, we have developed a time-dependent thermal subduction model for this region using the finite-element method. There are good seismological and geological constraints for the plate geometry and for the subduction history of the past 15 Ma. Boundary conditions are specified so that the oceanic plate subducting at the Nankai Trough becomes cooler as it gets older. The model results agree with the present heat-flow trend that decreases landward, and explain the paleothermal regime of high mid-Miocene temperatures inferred from land geological studies. The tectonics of the region prior to 15 Ma has some uncertainties. Assuming subduction of an active spreading ridge or an 80-Ma-old lithosphere prior to 15 Ma give the same results for the present thermal regime of the seaward portion of the forearc but different results for the most landward region (> 250 km). The thermal history of the forearc since 15 Ma can be summarized as a rapid warming period as a consequence of ridge subduction, followed by a cooling trend to the present as a result of the aging of the subducting plate. The results illustrate the thermal consequences of one type of ridge subduction. They also demonstrate that the thermal regime of a subduction zone depends critically on the age history of the subducting oceanic lithosphere, especially if it is young, as well as such parameters as the subducting plate dip angle and thickness of insulating sediments on the incoming oceanic crust. This dependence is especially important when the thermal regime is used to constrain the seismogenic behaviour of the subduction thrust fault.

Geothermal gradient and heat flow data in and around Japan (I): Appraisal of heat flow from geothermal gradient data

We published a CD-ROM “Geothermal Gradient and Heat Flow Data in and around Japan” (Tanaka et al., 2004a), which includes values of heat flow and geothermal gradient data. This compilation was intended to improve the understanding of variations in the thermal regime in and around Japan. Our current knowledge of the heat flow distribution can be increased by including information derived from geothermal gradient data contained in this compilation. In southern Kyushu, the pattern of heat flow is significantly modified by incorporation of estimates of heat flow from geothermal gradient data.

Deep sea bottom-simulating-reflectors: calibration of the base of the hydrate stability field as used for heat flow estimates *

Ocean Drilling Program and Deep Sea Drilling Project downhole data from three areas, the southwestern Japan Nankai margin, the continental slope off Peru, and the Blake-Bahama Outer Ridge, provide temperature calibrations for bottom simulating reflectors (BSR) that mark the base of a clathrate hydrate stability field. The inferred temperatures at BSRs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges. The borehole results provide information on which stability field is applicable for the BSRs and thus calibrate the heat flow estimates. While an ideal calibration has not been possible, the BSR temperatures at the three sites in the temperature range 25–27°C, have been estimated with uncertainties of ±0.7 to ±2.0°C. The temperatures correspond closely to the laboratory dissociation temperatures for pure water-pure methane hydrate at equivalent pressures. No laboratory data are available for seawater salinity and methane at equivalent pressures, but extrapolation from lower pressures gives temperatures 1–2°C lower, which is just significantly different. The data also could be explained by the stability curve for seawater salinity and methane with about 7% CO2, or with a small amount of higher hydrocarbons, but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane. The uncertainties in the temperature at the BSR should contribute no more than ±5% error in heat flow estimates from BSR depths if the pure water-methane stability field is used.

Nankai Trough: A hot trench?

Heat flow estimated from the gas hydrate layers on the landward slope of the Nankai Trough reveals that heat flow increases downslope toward the trench floor. This data plus six new heat flow values obtained by a conventional probe and two values available from DSDP drill holes give a fairly detailed heat flow distribution in the Nankai Trough area, when combined with the already existing data set. There appears to be a zonal pattern parallel to the trough axis, with a high heat flow zone on the floor of the trough that is quite anomalous for a subduction zone. It might be explained as a result of subduction of the hot portion of the Philippine Sea plate, i.e. the Shikoku Basin, and/or of more local effects such as heating due to intrusion of hot water from subducted sediments to shallow depth beneath the trough floor. Surface heat flow patterns landward of the trough were calculated for a simple thermal model of subduction. Perfect reproduction of the observed zonal pattern is difficult to achieve by the simple model, suggesting the necessity for further heat flow and other observations.

Active hydrothermal mounds in the Okinawa Trough backarc basin, Japan

Abstract Dive studies by the “SHINKAI 2000” were made in September, 1984 and July, 1986 in the central axial rift of the Middle Okinawa Trough, a possible active backarc basin in a nascent stage. Hydrothermally active mounds were discovered on a small knoll (27°34.4′N, 127°08.6′E) directly south of the area where a high heat flow (higher than 1 W/m2) had been observed previously. The largest mound, 5–6 m high and 15–25 m wide, has several peaks and ridges, along which yellow materials are exposed. Shimmering water was noted at several places on the yellow materials. The rest of the mound is thinly covered by brown materials. The temperature of the ejecting water was higher than that of the ambient seawater by 2–3°C, and a 40 cm long thermometer inserted into the mound recorded a temperature of 20–50°C. Analysis of the ejected water showed a methane content of about 200 nl/kg. It is possible that further dive studies will reveal larger scale hydrothermal activity in the area.

Detecting urbanization effects on surface and subsurface thermal environment — A case study of Osaka

Tremendous efforts have been devoted to improve our understanding of the anthropogenic effects on the atmospheric temperature change. In comparison, little has been done in the study of the human impacts on the subsurface thermal environment. The objective of this study is to analyze surface air temperature records and borehole subsurface temperature records for a better understanding of the urban heat island effects across the ground surface. The annual surface air temperature time series from six meteorological stations and six deep borehole temperature profiles of high qualities show that Osaka has been undergoing excess warming since late 19th century. The mean warming rate in Osaka surface air temperature is about 2.0 degrees C/100a over the period from 1883 to 2006, at least half of which can be attributed to the urban heat island effects. However, this surface air temperature warming is not as strong as the ground warming recorded in the subsurface temperature profiles. The surface temperature anomaly from the Osaka meteorological record can only account for part of the temperature anomaly recorded in the borehole temperature profiles. Surface air temperature is conventionally measured around 1.5 m above the ground; whereas borehole temperatures are measured from rocks in the subsurface. Heat conduction in the subsurface is much less efficient than the heat convection of the air above the ground surface. Therefore, the anthropogenic thermal impacts on the subsurface can be more persistent and profound than the impacts on the atmosphere. This study suggests that the surface air temperature records alone might underestimate the full extent of urban heat island effects on the subsurface environment.