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iPlex Lunch - spring-2013

Fracture Dissolution and Deformation Under Reservoir Conditions

April 24, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Jean E. Elkhoury - HSSEAS
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Geometrical alteration in fractures, caused by mineral dissolution, changes the contact area between fracture surfaces and affects their mechanical strength. It is difficult to determine the influence of dissolution on fracture porosity and permeability given the competition between fracture opening due to dissolution and fracture closer caused by mechanical stresses and deformations. Here, I will present results from detailed laboratory scale experiments on fractured rock samples and simulations that explore the small-scale physics of dissolution and deformation of fractures under reservoir conditions. To first order, the evolution of the dissolution process, in both reservoir and cap rocks, depends on the dimensionless Damkohler number Da (ratio of advection to reaction time scales). In the regime of small Da (large flow velocities), the influence of the mechanical stresses is stronger on the cap rock. Comparison of the simulated dissolved aperture fields with measured aperture fields after flow-through shows strong qualitative agreement. We use our reactive-transport model to explore the role of reaction and flow rates on dissolution and the resulting alteration of porosity and permeability beyond our experimental conditions. Our results emphasize the importance of fracture length-scales and the coupled response of hydro-chemo-mechanical disequilibrium in fractured reservoirs under reservoir flow and stress conditions critical in the up-scaling to field scale.

Spatial and temporal evolution of fault slip on the longitudinal valley fault, Taiwan

May 8, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Marion Thomas - Caltech
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The Longitudinal Valley Fault (LVF) in Eastern Taiwan is an exceptional example of a fault with high slip rate that produces both seismic and aseismic slip. This particular fault accounts for more than a third of the 9 cm/yr oblique convergence rate across Taiwan. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of fault slip near the surface whereas a fraction of the slip is also seismic since this fault has produced large earthquakes (Mw>6.8) in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF including campaign-mode GPS measurements, times-series of daily solutions for continuous GPS stations (cGPS), accelerometric records of the 2003 Chenkung earthquake, and levelling data. To enhance the spatial resolution provided by these data we complement them with inSAR measurements produced from a series of ALOS images processed with the permanent scatter technique. The data, which cover the entire LVF and span the period from 1994 to 2010 are inverted for the temporal evolution of fault slip a depth using the Principal Component Analysis base Inversion Method (PCAIM). The technique allows the joint inversion of these diverse data, thus taking the advantage of the spatial resolution given by the inSAR data and the temporal resolution afforded by the cGPS data. We find that 1- seismic slip during the 2003 Chengkung earthquake occurred on a fault patch which had remained locked to a certain degree in the interseismic period; 2- the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface; 3- that aseismic afterslip occurred around the ruptured area. The study allows estimating the fault slip budget (the fraction of aseismic and seismic slip) over the seismogenic depth range, and placing constraints on the fault frictional properties and their variations with space.

Active tectonics of east Central Tibet; a Quaternary slip-rate study of the Beng Co fault

May 15, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Jame Hollingsworth - CNRS, Nice
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Wildfire-induced sediment yield and debris-flow initiation in steep bedrock landscapes

May 22, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Michael Lamb - Caltech
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Catastrophic debris flows are common following wildfire in steep, bedrock landscapes. However, we lack mechanistic, predictive models for sediment transport on bedrock hillslopes, debris-flow initiation and the role of wildfire in influencing these processes. I will present recent field, experimental and modeling work that shows that sediment transport on steep, rocky hillslopes is different than on soil-mantled slopes, and is controlled by transient sediment storage by vegetation. Upon incineration of vegetation dams, hillslope sediment is rapidly transported to channels by gravity even in the absence of rainfall. Flume experiments on fluvial sediment transport in steep channels show that grain stability is enhanced on steep slopes due to wake turbulence and particle emergence. At even steeper slopes, we find that debris flows can initiate in-channel prior to any fluvial transport, and this process is sensitive to perturbations in bed-sediment size. Consequently, I will argue that debris flows may be more common following wildfire in bedrock landscapes due to incineration of vegetation dams, rapid loading of channels with relatively fine particles, and the transition from fluvial sediment entrainment to mass failure.

On the relationship between forearc deformation, friction properties, pore pressure, and megathrust

May 29, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Nadaya Cubas - Caltech
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Determining the spatio-temporal variations of frictional properties is a key issue in seismotectonics, since these properties are thought to determine the seismic potential of a subduction as well as the deformation style of a continental margin. In this presentation, we propose to compare frictional properties of the regions of the 2010 Mw 8.8 Maule earthquake and the 2011 Mw 9.0 Tohoku-Oki earthquake. To retrieve these properties, two kinds of mechanical analyses are conducted. The first one relies on the critical taper theory and yields the effective basal friction on the subduction interface, internal friction, and internal pore fluid pressure. The second is based on the limit analysis approach that allows constraining variations of frictional properties based on the location and style of forearc faulting. For the Maule area, we first show that the rupture area of the earthquake coincides with the mechanically stable part of the wedge. In the surrounding area, the wedge is critical, consistent with various evidence for active deformation. This is in particular true for the Arauco peninsula area, which seems to have stopped the Maule earthquake's rupture to the South. This observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area. In the frontal aseismic zone, we found a long-term hydrostatic pore pressure within the wedge and an intermediate effective friction along the megathrust (? ~ 0.3) probably due to the presence of clays. In the rupture area, a low effective dynamic friction (? < 0.14) is found that probably reflects strong dynamic weakening. On the contrary, the frontal wedge of the Tohoku-Oki area is characterized by a long-term high internal pore pressure and a low effective friction along the megathrust (? ~ 0.1). Moreover, the earthquake activated a landward normal fault downdip of the patch of maximum slip. From the modelling of this splay fault with limit analysis, we show that the frontal wedge was submitted to a strong increase of pore pressure during the earthquake. The difference of properties of the frontal wedge of these two regions might actually reflect differences in permeability. A lower permeability would enhance dynamic weakening and allow a frontal propagation of the rupture. This hypothesis is confronted with 2D earthquake sequence model. We then study whether the low dynamic friction in the seismogenic zone is due to an intrinsically lower static friction and/or chronically elevated pore pressure, or due to a dynamic weakening process induced by thermal pressurization.

How do subduction zones flip

June 5, 2013
noon - 1 p.m.
Geology 1707

Presented By:

  • Johann-Christoph von Hagke - Catltech
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Seminar Description coming soon.