SCEC Award Number 14001 View PDF
Proposal Category Individual Proposal (Integration and Theory)
Proposal Title Nonlinear attenuation of basin waves in Greater Los Angeles and ambient intact rock and regolith as fragile geological features
Name Organization
Norman Sleep Stanford University
Other Participants
SCEC Priorities 6e, 1b, 6b SCEC Groups GMP, EEII, Geology
Report Due Date 03/15/2015 Date Report Submitted N/A
Project Abstract
Strong Love waves propagate from the San Andreas Fault into Greater Los Angeles. Three-dimensional numerical calculations from the SCEC community indicate that these waves funnel through Whittier Narrows with high amplitudes. Nonlinear behavior depends on the shallow ambient tectonic stress. Conversely, we found that shallow ambient tectonic stress is a potential fragile geological feature. Sufficient strong shaking tends to relax ambient stress and also reduce the shear modulus by producing cracks. Over several strong events, the shear modulus will tend to self-organize so that it is linear with depth. The shallow rock barely becomes nonlinear under the typical dynamic strain impose by strong events. Dynamic stress and strain are proportional to the peak ground velocity (PGV). We infer past PGV from the gradient of shear modulus with depth. As an example of both processes, the regional tectonic stress (fault-normal tectonic compression) appears to be relaxed in the upper 1 km within the Pilot Hole at Parkfield. The shear modulus increases approximately linearly with depth. These observations are compatible with occasional (few 1000 year) participation of the Parkfield segment in strong events with ca. 5 m slip and ca. 2 m/s particle velocity. Furthermore, the relaxation of ambient stress couples tectonics with strong seismic waves. The anelastic strain in each episode of stress relaxation accommodates the long-term tectonic strain. This process has relevance to near-fault strain at crustal depths. We examined field examples of nonlinear attenuation including the suppression of S-waves by Rayleigh waves in the Tohoku event.
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 (expressed as peak ground velocity) from the shear wave velocity as a function of depth within the sedimentary basins of Greater Los Angeles. 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 are a fragile geological feature. Strong waves from San Andreas events funnel through Whittier Narrows. We expect that these waves have relaxed shallow ambient tectonic stress beneath this region. We examined the analogous process for Rayleigh waves with regard to the Tohoku earthquake. In general, our results aid in formulating, extending, and understanding nonlinear numerical calculations of strong seismic waves.

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. With regard to the SCEC purview, strong seismic waves from rupture tips in major earthquakes likely modulate near-fault tectonics. The waves weaken the rock in friction within a kilometer of the main fault.

We searched for examples that demonstrated nonlinear behavior of seismic waves. Strong low-frequency Rayleigh waves appear to have suppressed high-frequency S-waves. The tensile cycle of strong high frequency P-waves likely suppressed high frequency S-waves. The stresses ahead of the rupture tip of the 1992 Landers earthquake likely brought the uppermost few hundred meters of granite into frictional failure. This process suppressed high-frequency S-waves.
Broader Impacts We concentrated on the societal aspect of SCEC finding new classes of fragile geological features that allow inference of past peak ground velocity. We also studied nonlinear attenuation of surface waves that allows us to infer the maximum future peak ground velocity. We have obtained examples where dynamic stresses from long-period waves suppress high-frequency S-waves.

Our recognition of the modulation of tectonics by strong seismic waves likely has application to near-fault tectonics at crustal depths. This topic falls with priorities 1b and 6b.

We have continued with outreach to the planetary community. We submitted paper on the tidal processes within the Saturn moon Enceladus. Strong tides cause repeated frictional failure in the shallow ice and should relax tectonic stresses. The process is analogous to the modulation of tectonics by strong seismic waves.
Exemplary Figure Figure 2: Seismic velocity data near the Pilot Hole as a function of depth. Well log data in granite after [Boness and Zoback, 20904] and sandstone log after [Ryberg et al., 2012]. Seismic survey data inversions: blue dashed and green dotted “R” curves are two estimates by Ryberg et al. [2012] and purple dashed “Z” curve is estimate by Zhang et al. [2009]. The tracked line is the square root of depth calibrated with the sandstone data expected from low-cycle fatigue at constant dynamic strain. The shallow seismic velocity at the nearby UPSAR array (box U) [Fischer et al., 2008] is shown, as the inversions did not have good shallow resolution. From Sleep [2014].
Boness, N. L., and M. D. Zoback (2004), Stress-induced seismic velocity anisotropy and physical properties in the SAFOD Pilot Hole in Parkfield, CA. Geophys. Res. Lett., 31, L15S17, doi:10.1029/2003GL019020.
Ryberg, T., J. A. Hole, G. S. Fuis, M. J. Rymer, F. Bleibinhaus, D. Stromeyer and K. Bauer (2012) Tomographic Vp and Vs structure of the California Central Coast Ranges, in the vicinity of SAFOD, from controlled-source seismic data. Geophys. J. Int., 190, 1341–1360.
Zhang, H., C. Thurber, and P. Bedrosian (2009) Joint inversion for Vp, Vs, and Vp/Vs at SAFOD, Parkfield, California. Geochem. Geophys. Geosyst., 10, Q11002, doi:10.1029/2009GC002709.
Fischer, A. D., Z. G. Peng, and C. G. Sammis, (2008b) Dynamic triggering of high-frequency bursts by strong motions during the 2004 Parkfield earthquake sequence. Geophys. Res. Lett., 35, L12305.
Sleep, N. H. (2014), Ambient tectonic stress as fragile geological feature, Geochem. Geophys. Geosyst., 15, 3628–3644, doi:10.1002/2014GC005426.