1,720,975 research outputs found
Real-time discrimination of earthquake foreshocks and aftershocks
No full textImmediately after a large earthquake, the main question asked by the public and decision-makers is whether it was the
mainshock or a foreshock to an even stronger event yet to come. So far, scientists can only offer empirical evidence from
statistical compilations of past sequences, arguing that normally the aftershock sequence will decay gradually whereas the
occurrence of a forthcoming larger event has a probability of a few per cent. Here we analyse the average size distribution
of aftershocks of the recent Amatrice–Norcia and Kumamoto earthquake sequences, and we suggest that in many cases it
may be possible to discriminate whether an ongoing sequence represents a decaying aftershock sequence or foreshocks
to an upcoming large event. We propose a simple traffic light classification to assess in real time the level of concern about
a subsequent larger event and test it against 58 sequences, achieving a classification accuracy of 95 per cent
Detection of quarry and mine blast contamination in European regional catalogues
No full textSeismic catalogues often include man-made contaminations, that range from the changes in seismic networks operating in the region to the different processing of the data and to the inclusion of quarry explosions and marine shots. For example, the installation or closure of seismic stations produce an increase or a decrease in the detection of small events, whilst quarry blasts or mine explosions artificially enrich catalogues. Despite network operators’ best efforts to identify quarry blasts, it is easy to find these events in a catalogue, whereas the separation between explosion events and tectonic ones is a difficult task.
Detection and removing of artificial events from a seismic catalogue should be the
preliminary step in any analysis of statistical seismology.
Wiemer and Baer (BSSA, 2000) proposed an algorithm, based on a statistical
criterion, to identify and remove quarry explosions from catalogues. It is based on
the observation that quarry blasts generally take place during daytime hours: the areas with a high ratio of daytime and night-time events are likely to be regions with quarry activity.
In the first part of this work (Gulia, NH, 2010) we have modified the method and then tested it, using both a synthetic and a regional catalogue; in the second part the
procedure has been applied to some of the European regional catalogues available on
line.
The comparison between the results obtained and the location of known quarries and mines for the analyzed catalogues confirms both the presence of non-natural events in seismic catalogue and the reliability of the methodology in identifying mining areas.PublishedMontpellier5.2. TTC - Banche dati di sismologia strumentaleope
Detection of quarry and mine blast contamination in European regional catalogues
No full textThe presence of quarry and mine blasts in seismic catalogues is detected using
the Wiemer and Baer (Bull Seism Soc Am 90(2):525–530, 2000) algorithm. The procedure
is based on the observation that quarry blasts generally take place during daytime hours:
the areas with a high ratio of daytime and night-time events are likely to be regions with
quarry activity. In the first part of this work we have tested the method, using both a
synthetic and a regional catalogue; in the second part the procedure has been applied to
some of the European regional catalogues available on line. The comparison between the
results obtained and the location of known quarries and mines for the analysed catalogues
confirms the reliability of the methodology in identifying mining areas
Comment on “High-Definition Mapping of the Gutenberg–Richter b-Value and Its Relevance: A Case Study in Italy” by M. Taroni, J. Zhuang, and W. Marzocchi
Full text linkTaroni et al. (2021) published a statistical framework to reliably estimate the b-value and its uncertainties, with the goal being the interpretation in a seismotectonic context and improving earthquake forecasting capabilities. In this comment, we show that the results presented for the Italian region and the conclusions drawn by the authors, are heavily biased due to quarry-blast events in the Italian earthquake catalog used in the analysis. Without removing this anthropogenic component in the data, a meaningful analysis of the earthquake- size distribution for natural seismicity is, in our opinion, not possible. This comment highlights the need for basic data quality analysis before sophisticated statistical tools are applied to a dataset
Tectonic regimes and earthquake size distribution: new evidence f or PSHA
No full textEarthquake catalogues, seismotectonic zonations and ground-motion prediction
equations (GMPE) are the basic ingredients for probabilistic seismic hazard assessment
(PSHA). Seismotectonic zones are commonly defined considering the style-of-faulting;
contemporary GMPE’s also differentiate by the style-of-faulting. Here we present a
case study for Italy to show that style-of-faulting should also be incorporated into the
recurrence rates estimation.
In the past 40 years many studies relating b-values of the Gutenberg and Richter law to
physical properties have been performed, from laboratory rock specimens to observations in different tectonic regimes. Various authors analyzed the correlation
between b-value and tectonic regimes and the results are generally consistent: as power
laws indicate scale invariance, the inverse dependence of the b-value on the differential
stress is universally valid and the parameter can therefore be interpreted as a
‘stressmeter’ in the Earth’s crust. A consequence of the inverse dependency of the
b-value on differential stress is that tectonics regimes with different dominant faulting
styles should exhibit significantly different b-values, in particular the highest values for normal events (bNR), followed by strike-slip ( bSS) and reverse (bTH): bTH < bSS < bNR.
In this study, we evaluate this hypothesis for the first time, using data from the Italian Peninsula, whose complex geology is reflected in a strongly variable stress field and distinctly different faulting regimes. Extensional, compressional and strike-slip regimes are simultaneously present. T he study region fulfils two other critical requirements: 1)the regional seismic monitoring of the microseismicity of the past two decades was good enough to allow detailed mapping of the b-value and 2) a rich catalogue of focal mechanism exists that allows a detailed seismotectonic zonation. Because the b–value is
a critical parameter in PSHA, linking it firmly to regional faulting style has significant implications for future regional PSHA studies. At present the b-values are not used for zonation purposes, but they are either assigned regionally or computed for each zone, where zones are in general defined based on expert judgment. We suggest that future seismotectonic zonation models should take into account the knowledge on faulting style dependence of b-values. T here are a variety of way how this can be achieved, for example using high resolution mapping of b as an input for zonation, or by using the b-values of the large scale tectonic zones as a prior, deviating only if local b-values are found to be significantly different from the regional ones.PublishedMontpellier3.1. Fisica dei terremotiope
Tectonic regimes and earthquake size distribution: new evidence f or PSHA
No full textEarthquake catalogues, seismotectonic zonations and ground-motion prediction
equations (GMPE) are the basic ingredients for probabilistic seismic hazard assessment
(PSHA). Seismotectonic zones are commonly defined considering the style-of-faulting;
contemporary GMPE’s also differentiate by the style-of-faulting. Here we present a
case study for Italy to show that style-of-faulting should also be incorporated into the
recurrence rates estimation.
In the past 40 years many studies relating b-values of the Gutenberg and Richter law to
physical properties have been performed, from laboratory rock specimens to observations in different tectonic regimes. Various authors analyzed the correlation
between b-value and tectonic regimes and the results are generally consistent: as power
laws indicate scale invariance, the inverse dependence of the b-value on the differential
stress is universally valid and the parameter can therefore be interpreted as a
‘stressmeter’ in the Earth’s crust. A consequence of the inverse dependency of the
b-value on differential stress is that tectonics regimes with different dominant faulting
styles should exhibit significantly different b-values, in particular the highest values for normal events (bNR), followed by strike-slip ( bSS) and reverse (bTH): bTH < bSS < bNR.
In this study, we evaluate this hypothesis for the first time, using data from the Italian Peninsula, whose complex geology is reflected in a strongly variable stress field and distinctly different faulting regimes. Extensional, compressional and strike-slip regimes are simultaneously present. T he study region fulfils two other critical requirements: 1)the regional seismic monitoring of the microseismicity of the past two decades was good enough to allow detailed mapping of the b-value and 2) a rich catalogue of focal mechanism exists that allows a detailed seismotectonic zonation. Because the b–value is
a critical parameter in PSHA, linking it firmly to regional faulting style has significant implications for future regional PSHA studies. At present the b-values are not used for zonation purposes, but they are either assigned regionally or computed for each zone, where zones are in general defined based on expert judgment. We suggest that future seismotectonic zonation models should take into account the knowledge on faulting style dependence of b-values. T here are a variety of way how this can be achieved, for example using high resolution mapping of b as an input for zonation, or by using the b-values of the large scale tectonic zones as a prior, deviating only if local b-values are found to be significantly different from the regional ones.PublishedMontpellier3.1. Fisica dei terremotiope
HALM: A Hybrid Asperity Likelihood Model for Italy
No full textThe Asperity Likelihood Model (ALM), first developed and currently tested for California, hypothesizes that
small-scale spatial variations in the b-value of the Gutenberg and Richter relationship play a central role in forecasting
future seismicity (Wiemer and Schorlemmer, SRL, 2007). The physical basis of the model is the concept
that the local b-value is inversely dependent on applied shear stress. Thus low b-values (b < 0.7) characterize the
locked paches of faults –asperities- from which future mainshocks are more likely to be generated, whereas the
high b-values (b > 1.1) found for example in creeping section of faults suggest a lower seismic hazard.
To test this model in a reproducible and prospective way suitable for the requirements of the CSEP initiative
(www.cseptesting.org), the b-value variability is mapped on a grid. First, using the entire dataset above the
overall magnitude of completeness, the regional b-value is estimated. This value is then compared to the one
locally estimated at each grid-node for a number of radii, we use the local value if its likelihood score, corrected
for the degrees of freedom using the Akaike Information Criterion, suggest to do so.
We are currently calibrating the ALM model for implementation in the Italian testing region, the first region
within the CSEP EU testing Center (eu.cseptesting.org) for which fully prospective tests of earthquake
likelihood models will commence in Europe. We are also developing a modified approach, ‘hybrid’ between a
grid-based and a zoning one: the HALM (Hybrid Asperity Likelihood Model). According to HALM, the Italian
territory is divided in three distinct regions depending on the main tectonic elements, combined with knowledge
derived from GPS networks, seismic profile interpretation, borehole breakouts and the focal mechanisms of the
event. The local b-value variability was thus mapped using three independent overall b-values. We evaluate the
performance of the two models in retrospective tests using the standard CSEP likelihood test.UnpublishedVienna3.1. Fisica dei terremotiope
Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment
Full text linkISSN:0895-0695ISSN:1938-2057ISSN:1938-205
Short-term probabilistic earthquake risk assessment considering time-dependent b values
No full textLaboratory experiments highlight a systematic b value decrease during the stress increase period
before failure, and some large natural events are known to show a precursory decrease in the b value. However,
short-termforecastmodels currently consider only the generic probability that an event can trigger subsequent
seismicity in the near field. While the probability increase over a stationary Poissonian background is substantial,
selected case studies have shown through cost-benefit analysis that the absolute main shock probability
remains too low to warrant significant mitigation actions. We analyze the probabilities considering both
changes in the seismicity rates and temporal changes in the b value. The precursory b value decrease in the
2009 L’Aquila case results in an additional fiftyfold probability increase for a M6.3 event. Translated into
time-varying hazard and risk, these changes surpass the cost-benefit threshold for short-term evacuation
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