Estimating temporal changes in seismic velocity using a Markov chain Monte Carlo approach

We present a new method for estimating time-series of relative seismic velocity changes (dv/v) within the Earth. Our approach is a Markov chain Monte Carlo (MCMC) technique that seeks to construct the full posterior probability distribution of the dv/v variations. Our method provides a robust, computationally efficient way to compute dv/v time-series that can incorporate information about measurement uncertainty, and any prior constraints that may be available. We demonstrate the method with a synthetic experiment, and then apply the MCMC algorithm to three data examples. In the first two examples we reproduce dv/v time-series associated with the response to the 2010 M 7.2 El Mayor-Cucapah earthquake at two sites in southern California, that have been studied in previous literature. In the San Jacinto fault zone environment we reproduce the dv/v signature of a deep creep slip sequence triggered by the El Mayor-Cucapah event, that is superimposed on a strong seasonal signal. At the Salton Sea Geothermal Field we corroborate the previously observed drop-and-recovery in seismic velocity caused by ground shaking related to the El Mayor-Cucapah event. In a third, new example we compute a month long velocity change time-series at hourly resolution at Pinion Flat, California. We observe a low amplitude variation in seismic velocity with a dominant frequency of 1 cycle per day, as well as a second transient signal with a frequency of 1.93 cycles per day. We attribute the 1-d periodicity in the dv/v variation to the combined effects of the diurnal tide and solar heating. The frequency of the signal at 1.93 cycles per day matches that of the lunar (semi-diurnal) tide. Analysis of the uncertainties in the Pinion Flat time-series shows that the error contains a signal with a frequency of 1 cycle per day. We attribute this variation to seismic noise produced by freight trains operating within the Coachella Valley. By demonstrating the applicability of the MCMC method in these examples, we show that it is well suited to tackle problems involving large data volumes that are typically associated with modern seismic experiments.Peer reviewe

Dilatancy controlled spatiotemporal slip evolution of a sealed fault with spatial variations of the pore pressure

A range of observations suggest the formation and maintenance of sealed and hence overpressured compartments in fluid-infiltrated fault zones. It is assumed that hydromechanical properties of regions with variable pore pressure states control the fault's stability and thus its characteristic response, that is, seismic or aseismic slip accumulation. We investigate in a systematic parameter space study the effects of spatial variations in pore pressure on spatiotemporal slip evolution along a hydraulically isolated fault plane. The 3-D continuum model is governed by rate-and-state friction and constitutive laws for porosity reduction. We show that the model response is sensitive to the degree of overpressurization and the efficiency of dilatant hardening mechanisms. Low pore pressures and small dilatancy effects result in unstable response types, whereas high pore pressures and large dilatant effects lead to stable and aseismic creep. Regions with an unstable response are shown to support most of the stresses accumulated during interseismic periods. Accelerated slip nucleates preferably in regions of low pore pressure. Statistical properties of model seismicity produce a wide range of event sizes for moderate and large earthquakes, in the case where dilatant mechanisms are inefficient. In case of efficient slip rate controlled porosity increase, less instabilities grow into large earthquakes. Final slip maps demonstrate the applicability of the chosen method to model seismicity controlled by frictional and hydraulic processes on a planar fault plane. The evolution of governing variables that depend on the pore pressure environment provide a conceptual basis for the interpretation of observed response characteristic

Seismicity in a model governed by competing frictional weakening and healing mechanisms

Observations from laboratory, field and numerical work spanning a wide range of space and time scales suggest a strain dependent progressive evolution of material properties that control the stability of earthquake faults. The associated weakening mechanisms are counterbalanced by a variety of restrengthening mechanisms. The efficiency of the healing processes depends on local material properties and on rheologic, temperature, and hydraulic conditions. We investigate the relative effects of these competing non-linear feedbacks on seismogenesis in the context of evolving frictional properties, using a mechanical earthquake model that is governed by slip weakening friction. Weakening and strengthening mechanisms are parametrized by the evolution of the frictional control variable—the slip weakening rate R—using empirical relationships obtained from laboratory experiments. In our model, weakening depends on the slip of an earthquake and tends to increase R, following the behaviour of real and simulated frictional interfaces. Healing causes R to decrease and depends on the time passed since the last slip. Results from models with these competing feedbacks are compared with simulations using non-evolving friction. Compared to fixed R conditions, evolving properties result in a significantly increased variability in the system dynamics. We find that for a given set of weakening parameters the resulting seismicity patterns are sensitive to details of the restrengthening process, such as the healing rate b and a lower cutoff time, tc , up to which no significant change in the friction parameter is observed. For relatively large and small cutoff times, the statistics are typical of fixed large and small R values, respectively. However, a wide range of intermediate values leads to significant fluctuations in the internal energy levels. The frequency-size statistics of earthquake occurrence show corresponding non-stationary characteristics on time scales over which negligible fluctuations are observed in the fixed-R case. The progressive evolution implies that -— except for extreme weakening and healing rates -— faults and fault networks possibly are not well characterized by steady states on typical catalogue time scales, thus highlighting the essential role of memory and history dependence in seismogenesis. The results suggest that an extrapolation to future seismicity occurrence based on temporally limited data may be misleading due to variability in seismicity patterns associated with competing mechanisms that affect fault stability

Near-surface structure of the North Anatolian Fault zone from Rayleigh and Love wave tomography using ambient seismic noise

We use observations of surface waves in the ambient noise field recorded at a dense seismic array to image the North Anatolian Fault zone (NAFZ) in the region of the 1999 magnitude 7.6 Izmit earthquake in western Turkey. The NAFZ is a major strike-slip fault system extending ∼1200&thinsp;km across northern Turkey that poses a high level of seismic hazard, particularly to the city of Istanbul. We obtain maps of phase velocity variation using surface wave tomography applied to Rayleigh and Love waves and construct high-resolution images of S-wave velocity in the upper 10&thinsp;km of a 70&thinsp;×&thinsp;30&thinsp;km region around Lake Sapanca. We observe low S-wave velocities (&lt;2.5&thinsp;km&thinsp;s−1) associated with the Adapazari and Pamukova sedimentary basins, as well as the northern branch of the NAFZ. In the Armutlu Block, between the two major branches of the NAFZ, we image higher velocities (&gt;3.2&thinsp;km&thinsp;s−1) associated with a shallow crystalline basement. We measure azimuthal anisotropy in our phase velocity observations, with the fast direction seeming to align with the strike of the fault at periods shorter than 4&thinsp;s. At longer periods up to 10&thinsp;s, the fast direction aligns with the direction of maximum extension for the region (∼45∘). The signatures of both the northern and southern branches of the NAFZ are clearly associated with strong gradients in seismic velocity that also denote the boundaries of major tectonic units. Our results support the conclusion that the development of the NAFZ has exploited this pre-existing contrast in physical properties.</p

Statistical properties of seismicity of fault zones at different evolutionary stages

We perform a systematic parameter space study of the seismic response of a large fault with different levels of heterogeneity, using a 3-D elastic framework within the continuum limit. The fault is governed by rate-and-state friction and simulations are performed for model realizations with frictional and large scale properties characterized by different ranges of size scales. We use a number of seismicity and stress functions to characterize different types of seismic responses and test the correlation between hypocenter locations and the employed distributions of model parameters. The simulated hypocenters are found to correlate significantly with small L values of the rate-and-state friction. The final sizes of earthquakes are correlated with physical properties at their nucleation sites. The obtained stacked scaling relations are overall self-similar and have good correspondence with properties of natural earthquake

On the temporal stability of the coda of ambient noise correlations

We analyze the sensitivity of cross correlations to the anisotropy of the incident field in the context of seismic ambient noise monitoring of small velocity changes. Numerical simulations of elastic waves are performed in a 2D scattering plate with a focus on the comparative character of the direct and coda waves in the cross-correlation. We show that coda waves reconstructed from cross-correlations are far more robust than direct waves in the presence of azimuthal anisotropy of the incident field. We observe similar behavior with real data recorded on Erebus volcano, where a database of impulsive icequakes is used to simulate an anisotropic source field. We propose a simplified approach to evaluate the sensitivity of scattered waves to the anisotropy of the incoming noise field. We rely on previous results obtained for direct waves and on intrinsic properties of scattered waves to predict the errors produced by strong source anisotropy with numerical experiments. These results also yield realistic values for monitoring the accuracy to be expected with real data at crustal scales. Our analysis shows that high-precision noise-based monitoring could be performed with coda waves in the correlation functions, even in the presence of variations in the azimuthal distribution of the ambient noise field

In situ observations of velocity changes in response to tidal deformation from analysis of the high-frequency ambient wavefield

We report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity S_θ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low-velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of S_θ≈5 × 10^3 and a stress sensitivity of 0.5 MPa^(−1). The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here

Defining “Science-based Targets”

The term “science-based targets” has gained recent popularity. It is used to refer both to overall science-based targets (established through intergovernmental treaties), and to their disaggregation into specific science-based targets (determining contributions of individual actors). Biophysical achievability, measurability, and underpinning rationale are requirements for considering a target to be “science-based”

Statistical properties of seismicity of fault zones at different evolutionary stages

We perform a systematic parameter space study of the seismic response of a large fault with different levels of heterogeneity, using a 3-D elastic framework within the continuum limit. The fault is governed by rate-and-state friction and simulations are performed for model realizations with frictional and large scale properties characterized by different ranges of size scales. We use a number of seismicity and stress functions to characterize different types of seismic responses and test the correlation between hypocenter locations and the employed distributions of model parameters. The simulated hypocenters are found to correlate significantly with small L values of the rate-and-state friction. The final sizes of earthquakes are correlated with physical properties at their nucleation sites. The obtained stacked scaling relations are overall self-similar and have good correspondence with properties of natural earthquakes

Anatomy of the high-frequency ambient seismic wave field at the TCDP borehole.

International audienceThe Taiwan Chelungpu-fault Drilling Project (TCDP) installed a vertical seismic array between 950 and 1270 m depth in an active thrust fault environment. In this paper we analyze continuous noise records of the TCDP array between 1 and 16 Hz. We apply multiple array processing and noise correlation techniques to study the noise source process, properties of the propagation medium, and the ambient seismic wave field. Diurnal amplitude and slowness patterns suggest that noise is generated by cultural activity. The vicinity of the recording site to the excitation region, indicated by a narrow azimuthal distribution of propagation directions, leads to a predominant ballistic propagation regime. This is evident from the compatibility of the data with an incident plane wave model, polarized direct arrivals of noise correlation functions, and the asymmetric arrival shape. Evidence for contributions from scattering comes from equilibrated earthquake coda energy ratios, the frequency dependent randomization of propagation directions, and the existence of correlation coda waves. We conclude that the ballistic and scattered propagation regime coexist, where the first regime dominates the records, but the second is weaker yet not negligible. Consequently, the wave field is not equipartitioned. Correlation signal-to-noise ratios indicate a frequency dependent noise intensity. Iterations of the correlation procedure enhance the signature of the scattered regime. Discrepancies between phase velocities estimated from correlation functions and in-situ measurements are associated with the array geometry and its relative orientation to the predominant energy flux. The stability of correlation functions suggests their applicability in future monitoring efforts