SCEC Award Number 15003 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Nonlinear attenuation of strong seismic waves and ambient intact rock and tectonic stress as fragile geological features
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 6e, 6b, 2d SCEC Groups GMP, CS, Geology
Report Due Date 03/15/2016 Date Report Submitted 03/02/2016
Project Abstract
We continued to investigate the amplitude of past and future strong seismic waves. Sufficiently strong surface waves impose dynamic strain that brings the shallow (upper few 100 m) subsurface into frictional failure. The dynamic strain is nearly constant with depth and proportional to peak ground velocity; the dynamic stress is proportional to strain times the shear modulus. In the simple situation of constant rock density, constant coefficient of friction, and surface water table, the failure stress increases linearly with depth. Stiff rocks crack under the imposed strain, which reduces their shear modulus Eventually, the rock self-organizes so that the shear modulus increases linearly with depth, so that failure is barely reached during typical strong shaking. Conversely, stronger future dynamic strains will the subsurface into frictional failure nonlinearly attenuating the seismic waves. The effect on shear modulus is predictable but more complicated if the water table is not at the surface and the coefficient of friction is not constant. High-resolution well logs near Parkfield and San Jose provide data that is consistent with these predictions. Ambient tectonic stresses should relax during nonlinear failure, accommodating long-term tectonic deformation. Shallow distributed tectonic
deformation occurs beneath frequently shaken Whittier Narrows. We began to investigate the analogous effect of strong near-fault seismic waves on near-fault deformation at seismogenic depths. Heat-flow data constrain the stress on thrust faults near Parkfield and the fault-normal stress on the San Andreas. Extreme stresses associated with crack-tip rupture and enhanced near-fault tectonics occur within 10-100 m of the fault plane.
Intellectual Merit Inferring the past and future amplitudes of strong seismic shaking is central to the SECC project. We infer past shaking from strong Love waves and past near-field velocity pulses (expressed as peak ground velocity) from the shear modulus as a function of depth within the sedimentary basins of Greater Los Angeles and Greater San Jose. Crack failure, damage, and nonlinear attenuation occur when dynamic stress exceeds frictional strength. Conversely, the persistence of stiff intact rock and ambient tectonic stress at shallow depths is a fragile geological feature. Strong waves from San Andreas events funnel through Whittier Narrows. We have warned that pumping the water table down to a few hundred meters depth in this region would allow much stronger Love waves from San Andreas events to impinge on Downtown Los Angeles. Fortunately, the current hydrological practice is to recharge ground water at Whittier Narrows, thereby maintaining the water table at shallow pre-industrial levels.
The relaxation of tectonic stress during strong shaking produces anelastic strains. These strains accommodate the shallow tectonic deformation that is associated with thrust faults at greater depths. In general, strong seismic waves modulate tectonics. We published an outreach paper on the analogous tidal process on the Saturn moon Enceladus. Strong tides cause repeated frictional failure in the shallow ice. With regard to the SCEC purview, strong seismic waves from rupture tips in major earthquakes likely modulate near-fault tectonics. These waves weaken the rock in friction within tens to hundreds of the main fault.
Broader Impacts Our work on slow seismically modulated landslides, sackungen, is analogous to nonlinear failure within the tow of the accretion wedge during great earthquakes. This distal part of the wedge slips seaward with anelastic strain occurring within the wedge. There is a net downhill movement of wedge material driven by gravity. The large displacements in the wedge generate huge tsunamis as in the Tohoku earthquake. A similar process may be relevant to the tow of the wedge in Nepal.
We continue our interest in extreme seismic waves associated with large asteroid impacts. We plan fieldwork to look for rock damage with Peter Olds and his undergraduates at no cost to SCEC with rocks in Colorado that formed at the time of the end Cretaceous impact. There are barite deposits associated with the sudden release of fresh groundwater from aquifers through cracks into shallow marine environments.
Exemplary Figure none submitted