Distributed deformation and block rotation in 3D

The authors address how block rotation and complex distributed deformation in the Earth's shallow crust may be explained within a stationary regional stress field. Distributed deformation is characterized by domains of sub-parallel fault-bounded blocks. In response to the contemporaneous activity of neighboring domains some domains rotate, as suggested by both structural and paleomagnetic evidence. Rotations within domains are achieved through the contemporaneous slip and rotation of the faults and of the blocks they bound. Thus, in regions of distributed deformation, faults must remain active in spite of their poor orientation in the stress field. The authors developed a model that tracks the orientation of blocks and their bounding faults during rotation in a 3D stress field. In the model, the effective stress magnitudes of the principal stresses (sigma sub 1, sigma sub 2, and sigma sub 3) are controlled by the orientation of fault sets in each domain. Therefore, adjacent fault sets with differing orientations may be active and may display differing faulting styles, and a given set of faults may change its style of motion as it rotates within a stationary stress regime. The style of faulting predicted by the model depends on a dimensionless parameter phi = (sigma sub 2 - sigma sub 3)/(sigma sub 1 - sigma sub 3). Thus, the authors present a model for complex distributed deformation and complex offset history requiring neither geographical nor temporal changes in the stress regime. They apply the model to the Western Transverse Range domain of southern California. There, it is mechanically feasible for blocks and faults to have experienced up to 75 degrees of clockwise rotation in a phi = 0.1 strike-slip stress regime. The results of the model suggest that this domain may first have accommodated deformation along preexisting NNE-SSW faults, reactivated as normal faults. After rotation, these same faults became strike-slip in nature

Modelling earthquake rupture rates in fault systems for seismic hazard assessment: The Eastern Betics Shear Zone

Earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously. Consequently, modern fault-based probabilistic seismic hazard assessments (PSHA) need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties. We use SHERIFS, a recent approach of modelling annual rates of complex multi-fault ruptures, to obtain system-level magnitude-frequency distributions (MFDs) for the Eastern Betics Shear Zone (EBSZ, Spain) considering four fault rupture hypotheses. We then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies. The definition of the different rupture hypotheses was discussed within the frame of Fault2SHA ESC working group and critical fault input data is extracted from previous published studies. The four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics, with lengths varying from minimal fault sections to a rupture of nearly the whole system. The results suggest that multi-fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record. The method does not allow to completely discard any hypothesis, but it allows to weight the different models in a logic tree for seismic hazard assessment. The approach is revealed as a practical tool for obtaining fault-system MFDs and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed. This study aims to constitute a step forward in the consideration of complex multi-fault ruptures for future seismic hazard assessments in the region

Predominant-Period Site Classification for Response Spectra Prediction Equations in Italy

Abstract We propose a site classification scheme based on the predominant period of the site, as determined from the average horizontal-to-vertical (H/V) spectral ratios of ground motion. Our scheme extends Zhao et al. (2006) classifications by adding two classes, the most important of which is defined by flat H/V ratios with amplitudes less than 2. The proposed classification is investigated by using 5%-damped response spectra from Italian earthquake records. We select a dataset of 602 three-component analog and digital recordings from 120 earthquakes recorded at 214 seismic stations within an hypocentral distance of 200 km. Selected events are in the moment-magnitude range 4.0 ≤ Mw ≤ 6.8 and focal depths from a few kilometers to 46 km. We computed H/V ratios for these data and used these to classify each site into one of six classes. We then investigate the impact of this classification scheme on empirical ground-motion prediction equations by comparing its performance with that of the conventional rock/soil classification. Although the adopted approach results in a only a small reduction of overall standard deviation, the use of H/V spectral ratios in site classification does capture the signature of sites with flat frequency-response, well as deep and shallow soil profiles, characterized 2 C:\di_alessandro\site_classification_italy\final_version_of_paper\bssa-d-11-00084_di_alessandro_etal_final_revisions.doc by long-and short-period resonance, respectively; in addition, the classification scheme is relatively quick and inexpensive, which is an advantage over schemes based on measurements of shear-wave velocity

Which fault threatens me most? Bridging the gap between geologic data-providers and seismic risk practitioners

The aim of the Fault2SHA European Seismological Commission Working Group Central Apennines laboratory is to enhance the use of geological data in fault-based seismic hazard and risk assessment and to promote synergies between data providers (earthquake geologists), end users and decision-makers. Here we use the Fault2SHA Central Apennines Database where geologic data are provided in the form of characterized fault traces, grouped into faults and master faults, with individual slip rate estimates. The proposed methodology first derives slip rate profiles for each master fault. Master faults are then divided into distinct sections of length comparable to the seismogenic depth to allow consideration of variable slip rates along master faults and the exploration of multi-fault ruptures in the computations. The methodology further allows exploration of epistemic uncertainties documented in the database (e.g. master fault definition, slip rates) as well as additional parameters required to characterize the seismogenic potential of fault sources (e.g. 3D fault geometries). To illustrate the power of the methodology, in this paper we consider only one branch of the uncertainties affecting each step of the computation procedure. The resulting hazard and typological risk maps allow both data providers and end-users (1) to visualize the faults that threaten specific localities the most, (2) to appreciate the density of observations used for the computation of slip rate profiles, and (3) interrogate the degree of confidence on the fault parameters documented in the database (activity and location certainty). Finally, closing the loop, the methodology highlights priorities for future geological investigations in terms of where improvements in the density of data within the database would lead to the greatest decreases in epistemic uncertainties in the hazard and risk calculations. Key to this new generation of fault-based seismic hazard and risk methodology are the user-friendly open source codes provided with this publication, documenting, step-by-step, the link between the geological database and the relative contribution of each section to seismic hazard and risk at specific localities

Quantifying Uncertainties for earthquakes' Magnitude and Depth

International audienceMagnitude estimates of earthquakes from the intensity macroseismic field are marred by uncertainties. Two main types of uncertainties can be identified: one linked to the quality of the intensity data and the epistemic uncertainties of the intensity prediction equations (IPE) used to estimate magnitude from the macroseismic field. Quality of the intensity data depends on how detailed the testimonies of the earthquake are and on their reliability. In some macroseismic databases, a quality factor is associated to each intensity data point. IPE are calibrated on earthquakes for which macroseismic data and instrumental data, i.e. magnitude and depth, are available. The coefficients of IPEs depend then on the quality of the instrumental data, the quality of the intensity data and the calibration dataset. We present here the QUake-MD methodology, acronym for Quantifying Uncertainties for earthquakes' Magnitude and Depth. QUake-MD quantifies uncertainties in magnitude/depth estimates for earthquakes known only by their macroseismic fields by taking into account the quality of intensity data and the IPE epistemic uncertainties. Intensity data quality is used to weight the inversion process of intensity data in the application of the IPEs and to associate uncertainties to the inverted depth and magnitude. IPE epistemic uncertainties are taken into account by the use of several IPEs. Uncertainties associated to the inverted depth and magnitude combined to the use of different IPEs can be used to build a probability density function of the plausible depth, magnitude and epicentral intensity associated to the considered earthquake. To illustrate the strength of the methodology we use the intensity data collected by the BCSF (Bureau Central Sismologique Français) following recent earthquakes and compare macroseismic and instrumental estimates of Mw, depth and associated uncertainties

QUake-MD: open source code to Quantify Uncertainties in Magnitude -Depth estimates of earthquakes from macroseismic intensities

International audienceThis paper presents a tool to quantify uncertainties in magnitude/depth/epicentral intensity (M/H/Io) estimates for earthquakes associated with macroseismic intensity data. The tool is an open-source code written in python and named QUake-MD (Quantifying Uncertainties in earthquakes' Magnitude and Depth). In QUake-MD uncertainties are propagated from the individual intensity data point (IDP) to the final M/H/Io solution. It also accounts for epistemic uncertainties associated to the use of different intensity prediction equations (IPE). For each IPE, QUake-MD performs a sequential least square inversion process to estimate the central value of the M/H/Io triplet. QUake-MD then explores the uncertainties around the M/H solution by constructing a probability density function of possible M/H solutions constrained to be consistent with the range of plausible Io, a plausible depth range and IDP uncertainties. The resulting probability density functions of all IPEs provided to QUake-MD are then stacked to obtain a final probability density function of possible M/H/Io solutions representative of both data quality and IPE epistemic uncertainties. This tool thus provides end-users with a more complete understanding of the uncertainties associated with historical earthquake parameters, beyond the classical standard deviation values proposed today in parametric earthquake catalogues

Spectral Matching in Time Domain: A Seismological and Engineering Analysis

International audienc