1,720,967 research outputs found
Seismicity rate changes in association with the evolution of the stress field in northern Aegean Sea, Greece
No full textSeismicity rate changes in northern Aegean area were studied by applying the Dieterich (1994) Rate/State formulation. The study area was divided into four smaller ones that exhibit seismotectonic homogeneity and accommodate strong events with similar to identical focal mechanisms. Seismicity rate evaluation concerns the ‘study period’ between 1981 December 19 and 2010 August 31, whereas the reference seismicity rate was estimated for the ‘learning period’ lasting from 1970 January 1 to 1981 December 19. The coseismic slip of the strongest events (Mw ≥ 5.8) that occurred during the study period was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just before and after each strong event and their influence was then examined in connection with the occurrence rate of the smaller magnitude events above the individually determined magnitude of completeness in each subarea and for the respective time intervals. After defining the probability density function of seismicity distribution, a Rate/State model was used to combine static Coulomb stress changes (ΔCFF) with seismicity rates and to compare the observed with the expected rates of earthquake production for each time period and subarea. Different parameter values combinations were tested to evaluate the model sensitivity. Qualitative and quantitative correlations were performed for each one of the selected study periods. The modelling approach resulted to satisfactory correlation between real and synthetic seismicity rates and is expected to constitute a useful mean for the time-dependent seismic hazard assessment
An evaluation of coulomb stress changes from earthquake productivity variations in the western gulf of Corinth, Greece
No full textSpatial and temporal evolution of the stress field in the seismically active and well-monitored area of the western Gulf of Corinth, Greece, is investigated. The highly accurate and vast regional catalogues were used for inverting seismicity rate changes into stress variation using a rate/state-dependent friction model. After explicitly determining the physical quantities incorporated in the model (characteristic relaxation time, fault constitutive parameters, and reference seismicity rates), we looked for stress changes across space and over time and their possible association with earthquake clustering and fault interactions. We focused our attention on the Efpalio doublet of January 2010 (M = 5.5 and M = 5.4), with a high aftershock productivity, and attempted to reproduce and interpret stress changes prior to and after the initiation of this seismicity burst. The spatial distribution of stress changes was evaluated after smoothing the seismological data by means of a probability density function (PDF). The inverted stress calculations were compared with the calculations derived from an independent approach (elastic dislocation model) and this comparison was quantified. The results of the two methods are in good agreement (up to 80 %) in the far field, with the inversion technique providing more robust results in the near field, where they are more sensitive to the uncertainties of coseismic slip distribution. It is worth mentioning that the stress inversion model proved to be a very sensitive stress meter, able to detect even small stress changes correlated with spatio–temporal earthquake clustering. Data analysis was attempted from 1975 onwards to simulate the stress changes associated with stronger earthquakes over a longer time span. This approach revealed that only M > 5.5 events induce considerable stress variations, although in some cases there was no evidence for such stress changes even after an M > 5.5 earthquake
Forecasting seismicity rates in western Turkey as inferred from earthquake catalog and stressing history
Get PDFThe spatio-temporal variation in seismicity in western Turkey since the late 1970s is investigated through a rate/state model, which considers the stressing history to forecast the reference seismicity rate evolution. The basic catalog was divided according to specific criteria into four subsets, which correspond to areas exhibiting almost identical seismotectonic features. Completeness magnitude and reference seismicity rates are individually calculated for each subset. The forecasting periods are selected to be the inter-seismic time intervals between successive strong (M ≥ 5.8) earthquakes. The Coulomb stress changes associated with their coseismic slip are considered, along with the constant stressing rate to alter the rates of earthquake production. These rates are expressed by a probability density function and smoothed over the study area with different degrees of smoothing. The influence of the rate/state parameters in the model efficiency is explored by evaluating the Pearson linear correlation coefficient between simulated and observed earthquake occurrence rates along with its 95 % confidence limits. Application of different parameter values is attempted for the sensitivity of the calculated seismicity rates and their fit to the real data to be tested. Despite the ambiguities and the difficulties involved in the experimental parameter value determination, the results demonstrate that the present formulation and the available datasets are sufficient enough to contribute to seismic hazard assessment starting from a point such far back in time
A deterministic seismic hazard analysis for shallow earthquake in Greece
No full textThe maximum expected ground motion in Greece is estimated for shallow earthquakes using a deterministic seismic hazard
analysis (DSHA). In order to accomplish this analysis the input data include an homogeneous catalogue of earthquakes for the
period 426 BC–2003, a seismogenic source model with representative focal mechanisms and a set of velocity models. Because of
the discrete character of the earthquake catalogue and of errors in location of single seismic events, a smoothing algorithm is
applied to the catalogue of the main shocks to get a spatially smoothed distribution of magnitude. Based on the selected input
parameters synthetic seismograms for an upper frequency content of 1 Hz are computed on a grid of 0.2°×0.2°. The resultant
horizontal components for displacement, velocity, acceleration and DGA (Design Ground Acceleration) are mapped. The maps
which depict these results cannot be compared with previously published maps based on probabilistic methodologies as the latter
were compiled for a mean return period of 476 years. Therefore, in order to validate our deterministic analysis, the final results are
compared with PGA estimated from the maximum observed macroseismic intensity in Greece during the period 426 BC–2003.
Since the results are obtained for point sources, with the frequency content scaled with moment magnitude, some sensitivity
tests are performed to assess the influence of the finite extent of fault related to large events. Sensitivity tests are also performed to
investigate the changes in the peak ground motion quantities when varying the crustal velocity models in some seismogenic areas.
The ratios and the relative differences between the results obtained using different models are mapped and their mean value
computed. The results highlight the importance in the deterministic approach of using good and reliable velocity models.
© 2007 Elsevier B.V. All rights reserved
Time-dependent earthquake occurrence rates along the hellenic arc
No full textSeismicity rate changes in the Hellenic subduction zone (southern Aegean Sea) were studied by applying the Dieterich (1994) rate/state‐dependent friction model combined with static Coulomb stress changes (ΔCFF). The coseismic slip of the strongest earthquakes (with moment magnitude, Mw≥6.0) was considered to contribute to the stress field evolution along with the continuous tectonic loading. Stress changes were calculated just after each strong event, and their influence was examined in connection with the smaller magnitude earthquake occurrence rates. Qualitative and quantitative comparison between the observed seismicity rates (smoothed by the means of a probability density function) and the expected ones, as they were forecasted by the rate/state model, were investigated for the interseismic periods (study periods) between subsequent strong earthquakes. The calculations aim to identify areas of expected increased seismicity rates as candidates to accommodate enhanced seismic activity. Results strongly depend on the determination (smoothing) of the unperturbed (reference) seismicity rates and data adequacy. Seismicity rate results were filtered by certain criteria and constraints, in an attempt to overcome model uncertainties (epicentral errors, rupture models, parameter values) and to provide reliable results for specific areas of major interest, that is, in areas with increased positive Coulomb stress changes values. The modeling approach resulted in satisfactory correlation between observed and synthetic seismicity rates and, in particular, the two strong (Mw≥6) earthquakes that occurred in 2013 are located in areas of increased expected seismicity rates
Temporal static stress drop variations due to injection activity at The Geysers geothermal field, California
No full textWe use a high‐quality data set from the NW part of The Geysers geothermal field to determine statistical significance of temporal static stress drop variations and their relation to injection rate changes. We use a group of 322 seismic events which occurred in the proximity of Prati‐9 and Prati‐29 injection wells to examine the influence of parameters such as moment magnitude, focal mechanism, hypocentral depth, and normalized hypocentral distances from open‐hole sections of injection wells on static stress drop changes. Our results indicate that (1) static stress drop variations in time are statistically significant, (2) statistically significant static stress drop changes are inversely related to injection rate fluctuations. Therefore, it is highly expected that static stress drop of seismic events is influenced by pore pressure in underground fluid injection conditions and depends on the effective normal stress and strength of the medium
IS-EPOS: a platform for anthropogenic seismicity research
No full textResearch in the feld of anthropogenic seismicity (AS) requires not only seismicity data but also data regarding the progress of the technological/production activities which is the origin of the induced or triggered seismic events. Such data are typically restricted and proprietary, and therefore, usually not available for independent researchers who wish to develop, perform and verify scientifc research. The induced seismicity-European plate observing system (IS-EPOS) web portal ofers to its user’s access to data, applications and documents in order to facilitate AS research. IS-EPOS web portal has been designed to serve as one of the main pillars of the Thematic Core Service—-Anthropogenic Hazards belonging to pan-European multidisciplinary research infrastructure created within the EPOS program. IS-EPOS platform is open for research community and general public according to its rules of access. The platform is operating since January 2016 and is now integrated in the EPOS Integrated Core Services. IS-EPOS e-platform promotes new opportunities to study and comprehend the dynamic and complex solid earth system by integrating the use of multidisciplinary data, data products, analysis models and online applications. The integration of existing and new national and transnational Research Infrastructures increases the access and use of multidisciplinary data recorded by the solid earth observing systems, acquired in laboratory experiments and/ or produced by computational simulations. In this paper, we describe the structure and the main innovative characteristics implemented in IS-EPOS. The platform is open to accommodate data integrated within other research projects, and it is continuously being updated with improvements in existing features and implementations of new ones. An appendix at the end of the article provides a summary of acronyms and abbreviations in order to make the reader familiar with the terms used throughout the manuscript
Significance of static stress transfer caused by coseismic slip of mining-induced seismic events in seismicity generation process in mines
No full textWe investigated an effect of static stress transfer for the mining-induced seismicity from Rudna Mine. We consider events of energy over 10/5J, which occurred in Rudna Mine from 1993-2006. We examine the possible triggering checking correlation between event locations and the stress-increased zones. We find that more than 50% of the analyzed seismic events occurred in areas where stress was enhanced due to the occurrence of previous events. Statistical test proved that for stress changes from 0.02 bar static stress triggering in Rudna Mine exists and this effect is statistically significant at the 95% confidence level
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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