Laboratory observations of slow earthquakes and the spectrum of tectonic fault slip modes

Slow earthquakes represent an important conundrum in earthquake physics. While regular earthquakes are catastrophic events with rupture velocities governed by elastic wave speed, the processes that underlie slow fault slip phenomena, including recent discoveries of tremor, slow-slip and low-frequency earthquakes, are less understood. Theoretical models and sparse laboratory observations have provided insights, but the physics of slow fault rupture remain enigmatic. Here we report on laboratory observations that illuminate the mechanics of slow-slip phenomena. We show that a spectrum of slow-slip behaviours arises near the threshold between stable and unstable failure, and is governed by frictional dynamics via the interplay of fault frictional properties, effective normal stress and the elastic stiffness of the surrounding material. This generalizable frictional mechanism may act in concert with other hypothesized processes that damp dynamic ruptures, and is consistent with the broad range of geologic environments where slow earthquakes are observed

Competition between preslip and deviatoric stress modulates precursors for laboratory earthquakes

Abstract Variations in elastic wave velocity and amplitude prior to failure have been documented in laboratory experiments as well as in a limited number of crustal earthquakes. These variations have generally been attributed to fault zone healing, changes in crack density, or pore fluid effects modulated dilatation or fault slip. However, the relationships between amplitude and velocity variations during the seismic cycle, and the underlying mechanisms of precursors to failure remain poorly understood. Here, we perform frictional shear experiments and measure the evolution of elastic wave velocity and amplitude throughout the laboratory seismic cycle. We find that elastic amplitudes and velocities undergo clear preseismic variations prior to fault failure. While preseismic amplitude reduction occurs early in the interseismic period, wave speed reduces later, just prior to failure. We perform a complementary set of stress oscillation experiments to quantify the response of seismic amplitudes and velocities to variations in the stress tensor. Taken together, our results indicate that preseismic amplitude variations are primarily controlled by fault slip rate and acceleration. On the other hand, elastic velocity responds to a combination of fault preslip which reduces seismic wavespeed and increasing stress in the wallrock, which increases wavespeed. Our data show that precursory changes in seismic wave speed may be more common than previously thought because they are masked by changes in wallrock stress. These results underscore the importance of continuous and long-term time-lapse monitoring of crustal faults for seismic hazard assessment and potential precursors to failure

Effects of normal stress variation on the strength and stability of creeping faults

Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): B03406, doi:10.1029/2003JB002824.A central problem in studies of fault interaction and earthquake triggering is that of quantifying changes in frictional strength and the constitutive response caused by dynamic stressing. We imposed normal stress vibrations on creeping laboratory shear zones to investigate the process of dynamic weakening and the conditions under which resonant frictional behavior occurs. Layers of quartz powder were sheared at room temperature in a double-direct shear geometry at normal stress sigma barn = 25–200 MPa, vibration amplitude A = 0.1–10 MPa, period T = 0.1–200 s, and loading rate V = 1–1000 μm/s. Frictional response varied systematically with A, T, and V. Small-amplitude, short-period vibrations had no effect on frictional strength, but large-amplitude, short-period vibrations reduced shear zone strength by about 1%. Intermediate periods caused phase lags between shear strength and imposed vibrations. During long-period vibrations, frictional strength varied sinusoidally, in phase with vibrations and with an amplitude consistent with a constant coefficient of friction. Our data show that friction exhibits a critical vibration period, as predicted by theory. At long periods, the Dieterich (aging) friction law, with the Linker and Dieterich modification to describe step changes in normal stress, provides a good fit to our experimental results for all A and V. At short periods, however, theory predicts more dynamic weakening than we observed experimentally, suggesting that existing rate and state friction laws do not account for the full physics of our laboratory experiments. Our data show that normal-force vibrations can weaken and potentially destabilize steadily creeping fault zones.This research was supported by NSF grant EAR 01-96570 and USGS grant 02HQGR0156, and M.B. was supported by a NSF Graduate Research Fellowship

Onset of strain localization in sheared glacial till

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Healing of simulated fault gouges aided by pressure solution: results from rock analogue experiments

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Acoustic energy release during the laboratory seismic cycle. Insights on laboratory earthquake precursors and prediction

Machine learning can predict the timing and magnitude of laboratory earthquakes using statistics of acoustic emissions. The evolution of acoustic energy is critical for lab earthquake prediction; however, the connections between acoustic energy and fault zone processes leading to failure are poorly understood. Here, we document in detail the temporal evolution of acoustic energy during the laboratory seismic cycle. We report on friction experiments for a range of shearing velocities, normal stresses, and granular particle sizes. Acoustic emission data are recorded continuously throughout shear using broadband piezo-ceramic sensors. The coseismic acoustic energy release scales directly with stress drop and is consistent with concepts of frictional contact mechanics and time-dependent fault healing. Experiments conducted with larger grains (10.5 μm) show that the temporal evolution of acoustic energy scales directly with fault slip rate. In particular, the acoustic energy is low when the fault is locked and increases to a maximum during coseismic failure. Data from traditional slide-hold-slide friction tests confirm that acoustic energy release is closely linked to fault slip rate. Furthermore, variations in the true contact area of fault zone particles play a key role in the generation of acoustic energy. Our data show that acoustic radiation is related primarily to breaking/sliding of frictional contact junctions, which suggests that machine learning-based laboratory earthquake prediction derives from frictional weakening processes that begin very early in the seismic cycle and well before macroscopic failure

HIV testing practices among men who have sex with men in Buenos Aires, Argentina

The objective of the study was to explore HIV-testing practices among men who have sex with men (MSM) in Buenos Aires, Argentina, in light of current international health guidelines that recommend frequent HIV testing for MSM who engage in high-risk behavior. Participants, who were recruited using respondent-driven sampling (RDS), were 500 mostly young, nongay-identified MSM of low socioeconomic status, high levels of unemployment, living mainly in the less-affluent areas surrounding Buenos Aires, and lacking health insurance. They provided blood samples for HIV testing and responded to a Computer Assisted Self Interview. Fifty-two percent had never been tested for HIV, and 20% had been tested only once; 17% were found to be HIV infected, of whom almost half were unaware of their status. Main reasons for never having tested previously were: not feeling at risk, fear of finding out results, and not knowing where to get tested. Among those previously tested, men had been tested a median of 2 times with their most recent test having occurred a median of 2.7 years prior to study enrollment. Of those who had not tested positive before entering the study, only 41% returned for their results. HIV testing was infrequent and insufficient for early detection of infection, entry into treatment, and protection of sexual partners. This was particularly the case among nongay-identified MSM. Testing campaigns should aim to help MSM become aware of their risk behavior, decrease fear of testing by explaining available treatment resources and decreasing the stigma associated with HIV, and by publicizing information about free and confidential testing locations. Rapid HIV testing should be made available to eliminate the need for a return visit and make results immediately available to individuals who are tested.Fil: Carballo Diéguez, Alex. Columbia University; Estados UnidosFil: Balán, Iván C.. Columbia University; Estados UnidosFil: Dolezal, Curtis. Columbia University; Estados UnidosFil: Pando, María de los Ángeles. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Microbiología. Centro Nacional de Referencia para el Sida; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas en Retrovirus y Sida. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas en Retrovirus y Sida; ArgentinaFil: Marone, Ruben. Nexo Asociación Civil; ArgentinaFil: Barreda, Victoria. Nexo Asociación Civil; ArgentinaFil: Ávila, María Mercedes. Universidad de Buenos Aires. Facultad de Medicina. Departamento de Microbiología. Centro Nacional de Referencia para el Sida; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Houssay. Instituto de Investigaciones Biomédicas en Retrovirus y Sida. Universidad de Buenos Aires. Facultad de Medicina. Instituto de Investigaciones Biomédicas en Retrovirus y Sida; Argentin