Spatial distribution and energy release of nonvolcanic tremor at Parkfield, California

Nonvolcanic tremors (NVTs) are observed in transition zones between freely slipping andlocked sections of faults and normally occur below the seismogenic zone. Based on NVT recordings in theParkfield region of the San Andreas Fault, we provide a novel approach to assess the energy release of theseevents and assign magnitudes (Me) that are compatible with size estimates of small earthquakes in the sameregion. To assess the energy magnitude of a detected tremor, we refine the estimate of its duration andperform a spectral analysis that accounts for local attenuation. For the 218 NVTs that we were able to process,we resolveMevalues in the range of 0.67 to 0.84. For events, which we could not process using the spectralanalysis technique, we propose a statistical model to estimateMevalues using observable characteristics,such as peak amplitude, spectral velocity at the source corner frequency, and duration. We furthermoreprovide seismic moment and moment magnitude estimates and calculate stress drops in a range of 3–10 kPa.As a result of our spectral analyses, wefind strong indications regarding the ongoing debate aboutpotential NVT location hypotheses: the Parkfield NVTs have a higher probability to be located in the proposedthree-dimensional cloud-like cluster than in any other suggested location distributi

Generic dependence of the frequency-size distribution of earthquakes on depth and its relation to the strength profile of the crust

[1] We explore the idea that the relative size distribution of earthquakes, quantified using the so-called b-value, is negatively correlated with differential stress. Because the maximum possible differential stress increases linearly in the brittle upper crust, we expect to find a decrease of b with depth. We test this expectation for seven continental areas around the world, each of which is described by a regional earthquake catalog. We find a monotonic decrease in b-value between 5 and 15 km depth. The decrease stops near the brittle-ductile transition. We specifically focus on the high-quality catalogs of earthquakes in California to perform a sensitivity test with respect to depth uncertainty; we also estimate the probability-depth gradient for the occurrence of a target magnitude event and study the behavior of b with depth in near- and off-fault zones. We also translate the observed b-depth gradients into b-differential stress gradients. Our findings suggest that b-values are negatively correlated with differential stress and hus have the potential to act as stress meters in the Earth\u27s crust

Size distribution of Parkfield's microearthquakes reflects changes in surface creep rate

The nucleation area of the series of M6 events in Parkfield has been shown to be characterized by low b-values throughout the seismic cycle. Since low b-values represent high differential stresses, the asperity structure seems to be always stably stressed and even unaffected by the latest main shock in 2004. However, because fault loading rates and applied shear stress vary with time, some degree of temporal variability of the b-value within stable blocks is to be expected. We discuss in this study adequate techniques and uncertainty treatment for a detailed analysis of the temporal evolution of b-values. We show that the derived signal for the Parkfield asperity correlates with changes in surface creep, suggesting a sensitive time resolution of the b-value stress meter, and confirming near-critical loading conditions within the Parkfield asperit

The frequency-size scaling of non-volcanic tremors beneath the San Andreas Fault at Parkfield: Possible implications for seismic energy release

© 2019 Elsevier B.V. We analyse the frequency-size-distribution of non-volcanic tremors observed along the Parkfield section of the San Andreas Fault. We suggest that these non-volcanic tremors follow a power-law scaling typical of scale-invariant, stick slip tectonic earthquakes, but with an unusually high scaling exponent of more than 2.0 and a systematic depth-dependency. While each individual non-volcanic tremor releases only a minuscule amount of energy and slip, this is more than compensated by their sheer numbers. Consequently, the integrated contribution of this largely ‘invisible’ seismicity (non-volcanic tremors and nano-earthquakes) is non-negligible and could potentially account in selected patches along the San Andreas fault for up to 100% of the plate motion

Simultaneous Dependence of the Earthquake-Size Distribution on Faulting Style and Depth

We analyze two high-quality Southern Californian earthquake catalogues, one with focal mechanisms, to statistically model and test for dependencies of the earthquake-size distribution, the b values, on both faulting style and depth. In our null hypothesis, b is assumed constant. We then develop and calibrate one model based only on faulting style, another based only on depth dependence and two models that assume a simultaneous dependence on both parameters. We develop a new maximum-likelihood estimator corrected for the degrees of freedom to assess models' performances. Our results show that all models significantly reject the null hypothesis. The best performing is the one that simultaneously takes account of depth and faulting style. Our results suggest that differential stress variations in the Earth's crust systematically influence b values and that this variability should be considered for contemporary seismic hazard studies

Can We Map Asperities Using b-Values?

Can We Map Asperities Using b-Values

Can We Map Asperities Using b-Values?

Can We Map Asperities Using b-Values?Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy Institute of Statistical Mathematics (ISM), Tokyo, Japan Swiss Seismological Service, Institute of Geophysics (ETH), Zürich, SwitzerlandUnpublishedErice, Italyope