Fall 2016


September 30, 2016

Speaker: Elizabeth Bell, UCLA

Title: Was Early Earth Hellish or Habitable?

October 7, 2016

Speaker: Yves Guglielmi, Berkeley national lab

Title: Explorations of the dynamic hydromechanical behavior of fractures and faults

Insights from mesoscale field experiments in Underground Research Laboratories (URL)

Abstract: The appreciation of the sensitivity of fractures and fault zones to fluid-induced-deformations in the subsurface is a key question in predicting the reservoir/caprock system integrity around fluid manipulations with applications to reservoir leakage and induced seismicity. It is also a question of interest in understanding earthquakes source, and recently the hydraulic behavior of clay faults under a potential reactivation around nuclear underground depository sites. Fault and fractures dynamics studies face two key problems (1) the up-scaling of laboratory determined properties and constitutive laws to the reservoir/crustal scale which is not straightforward when considering faults and fractures heterogeneities, (2) the difficulties to control both the induced seismicity and the stimulated zone geometry when a fault is reactivated.

Since a decade, observations, experiments and instrument developments conducted in underground environments worldwide (mines, underground research laboratories - URL) allowed to bridge the scale gap between laboratory scale and large scale faults and fractures physics. New technologies coupling pressure-strain-seismic measurements were developed allowing to monitor the full displacement tensor during a borehole hydraulic test set across a fault zone. We conducted field academic experiments to characterize fractures and fault zones hydromechanical properties as a function of their multi-scale architecture, and to monitor their dynamic behavior during the earthquake nucleation process. We show experiments where different fault zones geologies under contrasted state of stresses were explored in three different underground research laboratories (IRSN-Tournemire-France, LSBB-Apt-France, Mt-Terri-Ste-Ursanne-Switzerland) where experimental conditions can be optimized. Experiments consisted in pressurizing intervals in different fault zone facies (core, fractured damage zone, etc.) to induce changes in effective stresses high enough to produce measurable fault movements. Shear and normal displacements respectively of 0.05 to 1.5 10-3m were measured at velocities of 0.1 to 10 micrometer per seconds.   

Key results of these experiments is to highlight how important the aseismic fault activation is compared to the induced seismicity, and a similarity in the way faults of contrasted geologies activate under effective stress variations. We show that about 80% of the fault kinematic moment is aseismic and discuss the complex associated fault friction coefficient variations by analyzing the way large slip surfaces are invaded by fluids, and through the comparison between laboratory and field estimated friction. We identify that the slip stability and the slip velocity are mainly controlled by the rate of the permeability/porosity increase, and discuss the conditions for slip nucleation leading to seismic instability. Two different hydromechanical fault responses are observed depending on the geological complexity within the fault zone: no permeability variation in the highly deformed fault core (called scaly facies) and a large non-linear factor-of-10-to-1000 permeability increase in the fractured damage zones of the fault.  Such high transient permeability thus locally occurs in the fault zone under conditions when most of the deformation remains reversible and small (micro-to-millistrains). Permeability recovery time may be very fast after the fault stimulation, occurring in a few tens of seconds. Interestingly, when there is a poroelastic pressurization in the damage zone, a >1 millimeter slip activates on the main fault plane located at the boundary between the low permeable fault core and the damage zone. This opens the possibility that fluid migration along fault zones may be decoupled from slip when the fault is not favorably oriented towards the Coulomb stress state.


October 14, 2016

Speaker: Howard Zebker, Stanford

Title: Studies of the Lakes of Titan from Cassini Altimeter and Radiometer Analysis

October 21, 2016

Speaker: Francis Nimmo, UCSC

Title: Geophysics of Pluto and Charon


October 28, 2016

Speaker: Paul Selvadurai, UC Berkeley

Title: Laboratory investigations into frictional strength heterogeneity and its influence on foreshock sequences

Abstract: Foreshock sequences have been observed to occur before large earthquakes. They may provide short-term information regarding the timing of the impending event. Our current understanding of stress states encouraging foreshock generation is limited. Geodetic data from Tohoku and north Chile sequences indicate that foreshocks occur on the frictionally locked section of the fault [Brodsky and Lay, 2014]. We show that foreshocks can be simulated on frictional interfaces in the laboratory and demonstrate physics that scale directly to large regional earthquakes.

We present findings on a PMMA-PMMA frictional interface in a direct shear configuration, where a catalogue of 68 foreshocks preceding 8 large rupture events along the interface were studied at four levels of normal load. A pressure sensitive film was used to locate, size and measure normal stress on individual asperity. Slow slip was observed to nucleate from the free edge and grew (at rates between 3 to 12 mm/s) into a ‘locked’ section of the fault and was fully dependent on the applied normal load. Within the ‘locked’ region, an increase in size and densities of asperities were present. As the nucleation zone grew into the locked region, foreshocks were recorded using absolutely calibrated acoustic emission sensor and were modeled using a double-couple source mechanism. The source radius, approximated using the Brune [1970] relationship, ranged from 0.21 to 1.09 mm – similar to film estimates for larger asperity radii in that region. Spatio-temporal distributions of foreshocks showed decreasing recurrence times (from ~ 10 s to 0.001 s) as they clustered towards the eventual main shock hypocenter. We observed a pronounced dependence between the magnitude of foreshocks in a sequence and the energy released during full rupture; suggesting that foreshock size may be an indicator for potential magnitude of the mainshock.

References:

[1] E. E. Brodsky and T. Lay. Recognizing foreshocks from the 1 April 2014 Chile earthquake. Science, 344(6185):700–702, 2014.

[2] J. N. Brune. Tectonic stress and spectra of seismic shear waves from earthquakes. Journal of Geophysical Research, 75(26):4997–5009, 1970.


November 4, 2016

Speaker: Robert Skoumal, USGS

Title: Induced seismicity in the Central and Eastern United States

November 18, 2016

Speaker: John Karcz, NASA Ames Research Center

Title: Science missions on Mars using privately designed and operated spacecraft

December 2, 2016

Speaker: Diego Melgar, UC Berkeley

Title: Fakequakes: From P-Waves to Static Offsets and Strong Shaking, Broadband Simulation for Hazards

December 9, 2016

Speaker: Sue Smrekar, JPL

Title: Venus and terrestrial planet evolution: Myths and Lessons