A systematic decay of the aftershock rate over time is one of the most fundamental empirical laws in Earth science. However, the equally fundamental effect of a mainshock on the size distribution of ...subsequent earthquakes has still not been quantified today and is therefore not used in earthquake hazard assessment. We apply a stacking approach to well‐recorded earthquake sequences to extract this effect. Immediately after a mainshock, the mean size distribution of events, or b value, increases by 20–30%, considerably decreasing the chance of subsequent larger events. This increase is strongest in the immediate vicinity of the mainshock, decreasing rapidly with distance but only gradually over time. We present a model that explains these observations as a consequence of the stress changes in the surrounding area caused by the mainshocks slip. Our results have substantial implications for how seismic risk during earthquake sequences is assessed.
Plain Language Summary
The effect of a mainshock on the size distribution of subsequent earthquakes has not been quantified and is therefore not used in earthquake hazard assessment. To quantify this effect, we develop a stacking approach centered on the mainshock time and apply it to for 31 well‐recorded aftershock sequences from around the world. We found that after a mainshock the earthquake size distribution shifts toward relative more smaller events, increasing the so‐called b value by 20–30%. One of the consequences of our finding is that the rates of large aftershocks are overestimated by the currently used models. Our result is fully consistent with both laboratory measurements and modeling, and we present a conceptual model that explains our findings.
Key Points
We develop a stacking approach to b value time series centered on the mainshock time in order to extract the generic behavior
Applying this approach to well‐recorded aftershock sequences, we demonstrate that the b value increases by 20–30% after a mainshock
We develop a Coulomb stress‐based model explaining the postmainshock b value increase and propose an empirical relationship to be used to forecast aftershock hazard
We present a first synoptic view of the seismotectonics and kinematics of the eastern sector of the European Alps using geodetic and seismological data. The study area marks the boundary between the ...Adriatic and the Eurasian plates, through a wide zone of deformation including a variety of tectonic styles within a complex network of crustal and lithospheric faults. A new dense GPS velocity field, new focal mechanisms and seismic catalogues, with uniformly re-calibrated magnitudes (from 1005), are used to estimate geodetic and seismic deformation rates and to develop interseismic kinematic and fault locking models. Kinematic indicators from seismological and geodetic data are remarkably consistent at different spatial scales. In addition to large-scale surface motion, GPS velocities highlight more localized deformation features revealing a complex configuration of interacting tectonic blocks, for which new constraints are provided in this work accounting for elastic strain build up at faults bonding rotating blocks. The geodetic and seismological data highlight two belts of higher deformation rates running WSW-ENE along the Eastern Southern Alps (ESA) in Italy and E-W in Slovenia, where deformation is more distributed. The highest geodetic strain-rates are observed in the Montello-Cansiglio segment of the ESA thrust front, for which the higher density of the GPS network provides indications of limited interseismic locking. Most of the dextral shear between the Eastern Southern Alps and the Eastern Alps blocks is accommodated along the Fella-Sava fault rather than the Periadriatic fault. In northern Croatia and Slovenia geodetic and seismological data allow constraining the kinematics of the active structures bounding the triangular-shaped region encompassing the Sava folds, which plays a major role in accommodating the transition from Adria- to Pannonian-like motion trends. The analysis of the seismic and geodetic moment rates provides new insights into the seismic potential along the ESA front.
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•GPS and seismological data show consistent kinematics in the eastern Alps•Kinematics and tectonics result from the interplay of several tectonic blocks•GPS show new evidence for active deformation across faults•Limited interseismic coupling found for the Montello-Cansiglio thrust faults•New insights into seismic potential from seismic and geodetic moment rates
We derive a unifying formulation, reliable at all scales, linking Anderson's faulting theory with the earthquake size-distribution, whose exponent is known as the b-value. Anderson's theory, ...introduced in 1905, related fault orientation to stress conditions. Independently, laboratory measurements on acoustic emissions have established that the applied differential stress controls their b-value. Our global survey revealed that observed spatial variations of b are controlled by different stress regimes, generally being lower in compressional (subduction trenches and continental collisional systems) and higher in extensional regimes (oceanic ridges). This confirmed previous observations that the b-value depends on the rake angle of focal mechanisms. Using a new plunge/dip-angles-based b-value analysis, we also identified further systematic influences of faulting geometry: steep normal faults (also typical of the oldest subduction zones) experience the highest proportion of smaller events, while low-angle thrust faults (typical of youngest subduction zones) undergo proportionally larger, more hazardous, events, differently from what would be expected by only allowing for rake-angle dependency. To date, however, no physical model has ever been proposed to explain how earthquakes size-distribution, differential stress and faulting styles relate to each other. Here, we propose and analytically derive a unifying formulation for describing how fault orientation and differential stresses determine b-value. Our formulation confirms that b-values decay linearly with increasing differential stress, but it also predicts a different dip-dependent modulation according to the tectonic environment, opening up new ways of assessing a region's seismic hazard.
•Unifying framework for b-value, faulting styles and differential stress.•b-Value patterns match the main global seismotectonic structures.•b-Value plunge-based ternary analysis firstly denotes a 2nd order variation scheme.•Dip-slip b-values trends fit faulting theory's differential stress curves.
SUMMARY
In a recent work, we computed the relative frequencies with which strong shocks (4.0 ≤ Mw < 5.0), widely felt by the population were followed in the same area by potentially destructive main ...shocks (Mw ≥ 5.0) in Italy. Assuming the stationarity of the seismic release properties, such frequencies can be tentatively used to estimate the probabilities of potentially destructive shocks after the occurrence of future strong shocks. This allows us to set up an alarm-based forecasting hypothesis related to strong foreshocks occurrence. Such hypothesis is tested retrospectively on the data of a homogenized seismic catalogue of the Italian area against a purely random hypothesis that simply forecasts the target main shocks proportionally to the space–time fraction occupied by the alarms. We compute the latter fraction in two ways (i) as the ratio between the average time covered by the alarms in each area and the total duration of the forecasting experiment (60 yr) and (ii) as the same ratio but weighted by the past frequency of occurrence of earthquakes in each area. In both cases the overall retrospective performance of our forecasting algorithm is definitely better than the random case. Considering an alarm duration of three months, the algorithm retrospectively forecasts more than 70 per cent of all shocks with Mw ≥ 5.5 occurred in Italy from 1960 to 2019 with a total space–time fraction covered by the alarms of the order of 2 per cent. Considering the same space–time coverage, the algorithm is also able to retrospectively forecasts more than 40 per cent of the first main shocks with Mw ≥ 5.5 of the seismic sequences occurred in the same time interval. Given the good reliability of our results, the forecasting algorithm is set and ready to be tested also prospectively, in parallel to other ongoing procedures operating on the Italian territory.
Backscatter radio is proposed for sensor networks. In that way, the transmitter for each sensor is simplified to a transistor connected to an antenna and therefore, the cost for each sensor's ...communicator becomes negligible, while energy used for wireless communication per sensor is minimized. A software-defined transceiver is built to transmit a carrier, receive the reflections from various sensors and extract their transmitted messages. This work presents a thorough model of the backscatter radio link, the system architecture and a set of data extraction techniques for each sensor's information, testing in practice a sensor communicating through backscatter at a range of approximately 15 meters indoors, with 5 milliwatt transmission power at 10 bits per second. This work highlights the idiosyncrasies of the backscatter channel and provides a new communication perspective in the fertile area of scalable sensor networks, especially when low bit-rate, ultra-low cost sensors are required.
We analysed the intensity data collected immediately after three strong earthquakes (Mw ≥ 6) that occurred in Italy in the last decade to infer location, orientation and size of the relevant ...seismogenic faults using the Boxer method. We show that, starting from a few hours after the earthquake, such macroseismic method could have provided a characterization of the seismogenic source comparable to instrumental methods, some of which would have required much more time to provide stable results. Particularly for the 6 April 2009 earthquake, the analysis of macroseismic survey data could have been able to accurately constrain, starting already from the early afternoon of the day of the main shock, location, orientation and size of the area of maximum coseismic displacement, related to the activation of the Paganica-S. Demetrio fault system. For the 20 May 2012 Emilia and the 24 August 2016 Amatrice earthquakes, preliminary macroseismic surveys were carried out within a few days from the main shocks, but their data were made available publicly a month later. For the 20 May 2012 Emilia earthquake, the orientation of the fault could have been correctly determined even if the size would have been underestimated. For the 24 August 2016 Amatrice earthquake, both the orientation and the size could have been correctly assessed. We also compared such macroseismic determinations with those obtained for some historical earthquakes in the same areas. The analysis of the cumulate intensities observed after the shocks of 24 August, 26 and 30 October 2016 indicates an overall Mw 6.8 comparable to that (Mw 6.9) of the largest earthquake occurred in the same section of the Apennines (14 January 1703). Even if the inferred orientations of the 2016 and 1703 boxes are very similar, the epicentres differ of about 20 km indicating a probable distinct fault system for the two earthquakes.
•Macroseismic intensities provide reliable parameters of the seismogenic source.•Macroseismic location, orientation and size comparable to instrumental ones•Fast and reliable assessment of the source of 6 April 2009 earthquake (Abruzzo, Italy)•We suggest the collection and near real-time analysis of macroseismic intensities.
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.
Key Points
We model the dependence of the b value on depth and tectonic style in Southern California
Statistical tests reveal that faulting style affects the b value more than depth
Simultaneous dependencies result with high statistical significance in the best data description
► Since 1997 systematically update the European–Mediterranean Regional Centroid Moment Tensor (RCMT) catalog. ► We present 354 new moment tensor solutions for 2005-2008. ► The RCMT catalog spans 12 ...years and contains more than 1000 definitive solutions.
Since 1997 we maintain and systematically update the European–Mediterranean Regional Centroid Moment Tensor (RCMT) catalog, which contains seismic-moment tensors for earthquakes with moderate magnitude (4.5
<
M
<
5.5) in the Mediterranean and European regions. We present 354 new solutions for the period 2005–2008. The Catalog now spans 12
years and contains more than 1000 definitive RCMT solutions. In addition to definitive solutions, we maintain and update a dataset of recent ‘quick’ solutions that are computed soon after an earthquake occurs using the more limited set of data available in quasi-real time. We investigate the reliability of the moment tensor results in the RCMT catalog. Comparison with Global CMT results, which is possible for the larger events (
M
<
5.0) in the RCMT catalog, shows that for 75% of the events, the Kagan angle measure of the similarity of two moment tensors is smaller than 20°, reflecting good agreement of the results. We describe improvements and enhancements in the dissemination of the RCMT results through our web site.
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