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

Spatial organization of foreshocks as a tool to forecast large earthquakes

An increase in the number of smaller magnitude events, retrospectively named foreshocks, is often observed before large earthquakes. We show that the linear density probability of earthquakes occurring before and after small or intermediate mainshocks displays a symmetrical behavior, indicating that the size of the area fractured during the mainshock is encoded in the foreshock spatial organization. This observation can be used to discriminate spatial clustering due to foreshocks from the one induced by aftershocks and is implemented in an alarm-based model to forecast m > 6 earthquakes. A retrospective study of the last 19 years Southern California catalog shows that the daily occurrence probability presents isolated peaks closely located in time and space to the epicenters of five of the six m > 6 earthquakes. We find daily probabilities as high as 25% (in cells of size 0.04 × 0.04deg(2)), with significant probability gains with respect to standard models

Scoring and Testing Procedures Devoted to Probabilistic Seismic Hazard Assessment

This review addresses long-term (tens of years) seismic ground-motion forecasting (seismic hazard assessment) in the presence of alternative computational models (the so-called epistemic uncertainty affecting hazard estimates). We review the different approaches that have been proposed to manage epistemic uncertainty in the context of probabilistic seismic hazard assessment (PSHA). Ex-ante procedures (based on the combination of expert judgments about inherent characteristics of the PSHA model) and ex-post approaches (based on empirical comparison of model outcomes and observations) should not be considered as mutually exclusive alternatives but can be combined in a coherent Bayesian view. Therefore, we propose a procedure that allows a better exploitation of available PSHA models to obtain comprehensive estimates, which account for both epistemic and aleatory uncertainty. We also discuss the respective roles of empirical ex-post scoring and testing of alternative models concurring in the development of comprehensive hazard maps. In order to show how the proposed procedure may work, we also present a tentative application to the Italian area. In particular, four PSHA models are evaluated ex-post against macroseismic effects actually observed in a large set of Italian municipalities during the time span 1957-2006. This analysis shows that, when the whole Italian area is considered, all the models provide estimates that do not agree with the observations. However, two of them provide results that are compatible with observations, when a subregion of Italy (Apulia Region) is considered. By focusing on this area, we computed a comprehensive hazard curve for a single locality in order to show the feasibility of the proposed procedure