Bottom Simulating Reflector and Gas Seepage in Okinawa Trough： Evidence of Gas Hydrate in an Active Back-Arc Basin

To look for gas hydrate, 22 multi-channel and 3 single-channel seismic lines on the East China Sea (ECS) shelf slope and at the bottom of the Okinawa Trough were examined. It was found that there was indeed bottom simulating reflector (BSR) occurrence, but it is very rare. Besides several BSRs, a gas seepage was also found. As shown by the data, both the BSR and gas seepage are all related with local geological structures, such as mud diapir, anticline, and fault-controlled graben-like structure. However, similar structural "anomalies" are quite common in the tectonically very active Okinawa Trough region, but very few of them have developed BSR or gas seepage. The article points out that the main reason is probably the low concentration of organic carbon of the sediment in this area. It was speculated that the rare occurrence of gas hydrates in this region is governed by structure-controlled fluid flow. Numerous faults and fractures form a network of high-permeability channels in the sediment and highly fractured igneous basement to allow fluid circulation and ventilation. Fluid flow in this tectonic environment is driven primarily by thermal buoyancy and takes place on a wide range of spatial scales. The fluid flow may play two roles to facilitate hydrate formation:to help gather enough methane into a small area and to modulate the thermal regime.     

An Overview of the 28 October 2012 M<Subscript>w</Subscript> 7.7 Earthquake in Haida Gwaii,Canada: A Tsunamigenic Thrust Event Along a Predominantly Strike-Slip Margin

The boundary between the Pacific and North America plates along Canada’s west coast is one of the most seismically active regions of Canada, and is where Canada’s two largest instrumentally recorded earthquakes have occurred. Although this is a predominantly strike-slip transform fault boundary, there is a component of oblique convergence between the Pacific and North America plates off Haida Gwaii. The 2012 M_w 7.7 Haida Gwaii earthquake was a thrust event that generated a tsunami with significant run up of over 7 m in several inlets on the west coast of Moresby Island (several over 6 m, with a maximum of 13 m). Damage from this earthquake and tsunami was minor due to the lack of population and vulnerable structures on this coast.     

The Nootka Fault Zone — a new plate boundary off western Canada

Summary. Relative motion across a boundary between the main Juan de Fuca plate and its northern extension, the Explorer plate, had earlier been suggested from sea-floor magnetic anomaly analysis and from earthquakes recorded on the western Canada land seismic network. The location of the boundary, called the Nootka fault zone, and the motion across it have been examined through seismic reflection profiles, accurate location of earthquakes with an array of ocean bottom seismometers and through analysis of magnetic, gravity and bathymetric data. The fault zone extends from a ridge-fault—fault triple point at the northern end of the Juan de Fuca ridge to a fault—trench—trench triple junction at the margin off north-central Vancouver Island. The active portion of the fault zone is about 20 km wide, and has produced extensive disturbance in the 0.5 to 1 km of overlying sediments. Magnetic anomaly analysis suggests present left-lateral strike slip motion of about 3 cm/yr, with convergence at the margin being more rapid to the south than to the north of the fault zone. Because of rapidly changing spreading parameters on the Explorer and Juan de Fuca ridges over the past 5 Myr the Nootka fault zone has had a very complex history.     

Biocurtain Design Using Reactive Transport Models

Bioremediation is an attractive alternative to traditional remediation methods for a variety of ground water contaminants. However, widespread implementation of bioremediation is currently limited by the complexity of the dynamic chemical and biological processes that need to be understood and incorporated into the design approach. Reactive transport models provide a powerful tool to simulate these complex interactions and, thus, can be used to improve and guide the design of bioremediation systems. We present a remediation design approach for intermittently stimulated biodegradation using multicomponent reactive transport models, parameterized using a series of nondimensional Damkohler numbers. Designs were based on either (1) a target aqueous phase concentration at the exit of the treatment system, or (2) the total contaminant mass fraction removed from a region of interest. The equation set used to develop this design approach is specific to the case of intermittent electron donor addition to drive cometabolic transformations. We illustrate the design procedure for a biocurtain that removes carbon tetrachloride. Our results for this case indicate that intermittent injection is significantly more efficient than strategies based on continuous pumping. Example design parameters include the length of the biologically active zone (i.e., biocurtain), the effective rate of degradation in this zone, and the interval between electron donor injection cycles. The presented dimensionless parametric approach can be used to design bench-scale column studies and should be helpful for scale-up to field-scale remediation systems.     

Analysis of recharge-induced geochemical change in a contaminated aquifer

Recharge events that deliver electron acceptors such as O2, NO3, SO4, and Fe3+ to anaerobic, contaminated aquifers are likely important for natural attenuation processes. However, the specific influence of recharge on (bio)geochemical processes in ground water systems is not well understood. The impact of a moderate-sized recharge event on ground water chemistry was evaluated at a shallow, sandy aquifer contaminated with waste fuels and chlorinated solvents. Multivariate statistical analyses coupled with three-dimensional visualization were used to analyze ground water chemistry data (including redox indicators, major ions, and physical parameters) to reveal associations between chemical parameters and to infer processes within the ground water plume. Factor analysis indicated that dominant chemical associations and their interpreted processes (anaerobic and aerobic microbial processes, mineral precipitation/dissolution, and temperature effects) did not change significantly after the spring recharge event of 2000. However, the relative importance of each of these processes within the plume changed. After the recharge event, the overall importance of aerobic processes increased from the fourth to the second most important factor, representing the variability within the data set. The anaerobic signatures became more complex, suggesting that zones with multiple terminal electron-accepting processes (TEAPs) likely occur in the same water mass. Three-dimensional visualization of well clusters showed that water samples with similar chemical associations occurred in distinct water masses within the aquifer. Water mass distinctions were not based on dominant TEAPs, suggesting that the recharge effects on TEAPs occurred primarily at the interface between infiltrating recharge water and the aquifer.