Expedition 344 summary

Abstract

Expedition 344 summaryProc. IODP | Volume 344 2 from velocity-strengthening to velocity-weakening friction, and shear becomes localized. The onset of seismogenic behavior is correlated with the intersection of the 100°-150°C isotherm and the subduction thrust (Hyndman et al., 1997;Oleskevich et al., 1999). With increasing depth down the subduction thrust, the frictional characteristics undergo a second transition either due to the juxtaposition with the forearc mantle or because the rocks are heated to 350°-450°C and can no longer store elastic stresses needed for rupture. Transitional regions between the three zones have conditional stability and can host rupture but are generally not thought to be regions where large earthquakes initiate.Although this three-zone two-dimensional view of the subduction thrust provides a reasonable framework, it is simplistic. Rupture models for large subduction earthquakes suggest significant fault plane heterogeneity in slip and moment release that in three dimensions is characterized as patchiness (Bilek and Lay, 2002). Additionally, we now know the transition zone from stable to unstable sliding is not simple but hosts a range of fault behaviors that includes creep events, strain transients, slow and silent earthquakes, and low-frequency earthquakes (Peng and Gomberg, 2010;Beroza and Ide, 2011;Ide, 2012).Fundamentally unknown are the processes that change fault behavior from stable sliding to stick-slip behavior. Understanding these processes is important for understanding earthquakes, the mechanics of slip, and rupture dynamics. For a fault to undergo unstable slip, fault rocks must have the ability to store elastic strain, be velocity weakening, and have sufficient stiffness. Hypotheses for mechanisms leading to the transition between stable and unstable slip invoke temperature, pressure, and strain-activated processes that lead to downdip changes in the mechanical properties of rocks. These transitions are also sensitive to fault zone composition, lithology, fabric, and fluid pressures.The composition of the material in the fault zone and its contrast with the surrounding wall rock play a key role in rock frictional behavior. The frictional state of the incoming sediment changes progressively with increasing temperature and pressure as it travels downdip. Important lithologic factors influencing friction are composition, fabric, texture, and cementation of rocks, as well as fluid pore pressure (Bernabé et al., 1992;Moore and Saffer, 2001;Beeler, 2007;Marone and Saffer, 2007;Collettini et al., 2009). For example, fault rocks with high phyllosilicate content are generally weaker than rocks with low phyllosilicate content (Ikari et al., 2011). Sediment properties including porosity, permeability, consolidation state, and alteration history also exert a strong influence on fault zone behavior. At erosive margins, where the plate boundary cuts into the overriding plate, the composition and strength of the upper plate is also important (McCaffrey, 1993).Field observations and la...

Data report: gas isotope and conversion ratio of headspace and void gas, IODP Expedition 344

“…The HS analysis results show that methane is a major component and ethane a minor component at all sites (see the "Expedition 344 summary" chapter [Harris et al, 2013a]) ( Table T1; Fig. F1).…”

“…The HS samples were analyzed onboard by an Agilent 6890 Series II gas chromatograph (GC3) (see the "Methods" chapter [Harris et al, 2013b]). The VG samples were analyzed onshore by an Agilent 7890A gas chromatograph with a flame ionization detector and a thermal conductivity detector at Korea Institute of Geoscience and Mineral Resources (KIGAM).…”

“…F1; Table T1). This interval contains a sulfate methane transition zone (see the "Expedition 344 summary" chapter [Harris et al, 2013a]). The δ…”

“…F3). Carbon stable isotopes of organic matter are between -25.0‰ and -23.5‰ at Site U1412 and between -26.0‰ and -23.6‰ at Site U1413 (see the "Expedition 344 summary" chapter [Harris et al, 2013a]). Estimated conversion ratios vary between 21.8% and 30.7% at Site U1412 and between 26.4% and 30.2% at Site U1413, slightly decreasing with depth (Fig.…”

“…The Costa Rica Seismogenesis Project, Program A Stage 2, drilled at five sites located offshore of the Osa Peninsula near the incoming Cocos Ridge to study the processes controlling earthquakes in an erosional subduction zone (see the "Expedition 344 summary" chapter [Harris et al, 2013a]). Among the five sites, Sites U1380, U1412, and U1413 were studied to investigate deep fluid transportation.…”