The Cotentin peninsula is a region located on the northeastern edge of the Armorican Massif (France) that hosts several nuclear and radioactive waste facilities. A recent publication raised some issues regarding the source parameters of the 1926, Jersey earthquake, one of the key reference earthquakes for assessing the seismic hazard at the nuclear sites. In this study, we performed in-depth analyses on both the instrumental and macroseismic data to reappraise the source parameters. While the instrumental epicentral location was fairly well constrained at about mid-distance between the Jersey Island and the Cotentin peninsula, the focal depth could not be retrieved due to the resolution limits associated with a sparse seismological network and to the uncertainties on the phase picking. Different MS formulations applicable for moderate magnitude events recorded at regional distances were implemented to better account for the epistemic uncertainties, resulting in an estimation of MS equal to 5.1 +/- 0.3. Regarding the moment magnitude MW, we performed a broad series of waveform modeling accounting for a broad set of source parameterizations and signal processing techniques. The magnitude MW of the Jersey earthquake is estimated to 5.4 +/- 0.3 although these evaluations are strongly influenced by the period range adopted to fit the observations. We also carried out the evaluation of both MS and MW magnitudes by modeling the macroseismic field with results (MS = 5.1 +/- 0.3; Mw = 5.3 +/- 0.3) that are consistent with the instrumental estimates. Regarding the focal depth, the macroseismic analyses shed light on this parameter, even though the results depend on the assumption made on the presumed epicentral intensity (-18 km for I0 = VI-VII and - 12 km for I0 = VII).

ABSTRACTThe activation of tectonics and anthropogenic swarms in time and space and size remains challenging for seismologists. One remarkably long swarm is the Lacq swarm. It has been ongoing since 1969 and is located in a compound oil–gas field with a complex fluid manipulation history. Based on the overlap between the volumes where poroelastic model predicts stresses buildup and those where earthquakes occur, gas reservoir depletion was proposed to control the Lacq seismic swarm. The 2016 Mw 3.9, the largest event on the site, is located within a few kilometers downward the deep injection well. It questions the possible interactions between the 1955–2016 wastewater injections and the Lacq seismicity. Revisiting 60 yr of fluid manipulation history and seismicity indicates that the impacts of the wastewater injections on the Lacq seismicity were previously underevaluated. The main lines of evidence toward a wastewater injection cause are (1) cumulative injected volume enough in 1969 to trigger Mw 3 events, onset of Lacq seismicity; (2) 1976 injection below the gas reservoir occurs only a few years before the sharp increase in seismicity. It matches the onset of deep seismicity (below the gas reservoir, at the injection depth); (3) the (2007–2010) 2–3 folds increase in injection rate precedes 2013, 2016 top largest events; and (4) 75% of the 2013–2016 events cluster within 4–8 km depths, that is, close to and downward the 4.5 km deep injection well. As quantified by changepoint analysis, our results suggest that timely overlaps between injection operations and seismicity patterns are as decisive as extraction operations to control the Lacq seismicity. The seismicity onset is contemporary to cumulative stress changes (induced by depletion and injection operations) in the 0.1–1 MPa range. The interrelation between injection and extraction is the most probable cause of the Lacq seismicity onset and is sustenance over time. The injected volume–largest magnitude pair for Lacq field is in the same range (90% confidence level) than wastewater volume–magnitude pairs reported worldwide, in a wide variety of tectonic settings.

English Channel Islands are located off Normandy coast of France within an intraplate area not associated with high seismicity rate and active tectonics. However, in July 1926 a damaging and well-documented earthquake occurred there, followed by a strongly felt event in February 1927. In this paper, we reprocess macroseismic observations, analog seismograms and bulletin data in order to re-appraise the location and magnitude of these two earthquakes. We find that the macroseismic epicentre of the 1926 Jersey earthquake is shifted to the East when compared with the location offshore South of Jersey given in both the French database SisFrance and the recent French catalog FCAT-17. Arrival-times from published data, together with our own onset-time readings, are processed in order to obtain probabilistic hypocentral locations. The maximum-likelihood instrumental epicentre of the 1926 event is located about 15 km East of Jersey Island (49.20 degrees N, 1.82 degrees W). This epicentre is well constrained within a 10 km radius area. The location of the 1927 epicentre is slightly south of Jersey, at a location similar to that of SisFrance, but this latter epicentre is less constrained as fewer observations are available (the Probability Density Function for the 1927 epicentre is relatively "diluted"). Focal depth remains very poorly determined for both the 1926 and 1927 events. Analysis of historical seismograms is also performed in order to determine both surfacewave and moment magnitudes, M-s and M-w We find M-s = 5.6 +/- 0.2 for the 1926 event and M-s = 5.0 for the 1927 event. Waveform fitting of some of the highest quality seismograms of the 1926 event gives us M-w in the range [5.0, 5.5], depending on the chosen focal depth and focal mechanism. The 1927 Jersey earthquake is expected with an intensity magnitude MI about 0.6 smaller.

SUMMARYDeveloping a model for anthropogenic seismic hazard remains an open challenge whatever the geo-resource production. We analyse the (Mmax) largest reported magnitude on each site where (RTS) Reservoir Triggered Seismicity in documented (37 events, 1933–2008), for aftershocks of reservoir impoundment loading. We relate each reservoir impoundment to its magnitude-equivalent M*reservoir = M*(Lr). We use (Lr) the reservoir length as a proxy for a rupture length of the reservoir main shock-equivallent. This latter is derived from the empirical relationship that exists for tectonic earthquake among magnitude and rupture length. We resolve (i) Mmax for RTS are bounded by M*reservoir at a 95 per cent confidence level; (ii) in average Mmax are smaller than M*reservoir by 2.2 units (iii) 50 per cent of the Mmax occurrence is within 2 ± 1 yr from the reservoir impoundment. These triggering patterns support the signature of fluid driven seismicity during the slow reservoir impoundment emerges as a weaker efficiency (larger ΔM = M*reservoir – Mmax) to trigger Mmax events than from earthquake interactions.

We analyzed the impact of the 26 largest impounded reservoirs on reservoir-triggered seismicity (RTS) patterns in the low-seismicity region of continental France. We treat reservoir-triggered earthquakes as tectonic earthquakes and apply similar concepts in our analysis. Generally, the spatial extent of an aftershock zone is controlled by the mainshock rupture length. In a similar manner, we use reservoir length as an equivalent length to the rupture length to assess the spatial extent of reservoir-triggering earthquakes and one to three reservoir lengths as a proxy for the nearfield distance where the stress change induced by reservoir impoundment may trigger seismicity. Accordingly, we define the 1L(r) distance as the near-field reservoir effect on seismicity and the 10L(r) distance as the far field, null effect of reservoir stress change on background seismicity. We find that (1) about a quarter of the reservoirs trigger M-max = 2.5-4.7 within the 1L(r) distance in a 15 yr space time window, and (2) as tested against a randomized series, superposed epoch analysis demonstrates a robust increase in the average seismicity rate within 2 yrs for the 1-3L(r) distance from reservoirs. The reservoirs that trigger in the (1L(r)) near-field distance are significantly larger than the nontriggering ones. While considering the distance of triggering of earthquakes from the reservoir, it is more appropriate to consider the normalized distance (the distance normalized by the reservoir length) to identify earthquake triggering reservoirs at a 1L(r) distance. While considering reservoir dimensions, the reservoir length appears to be a more important parameter than the reservoir depth, as the length is proportional to the area of significant stress change. Our results suggest that the RTS mimics the aftershock sequence of a slow reservoir-impoundment loading, with a corresponding M*reservoir = M(L-r) mainshock magnitude. Further, when considering mainshock-aftershock interactions, our analysis and observations support that the M-max for RTS for a given reservoir remains, on average, smaller than the reservoir magnitude equivalent.

The rank-sum multiple change-point method is a robust statistical procedure designed to search for the optimal number and the location of change points in an arbitrary continue or discrete sequence of values. As such, this procedure can be used to analyse time-series data. Twelve years of robust data sets for the Sechilienne (French Alps) rockslide show a continuous increase in average displacement rate from 50 to 280 mm per month, in the 2004-2014 period, followed by a strong decrease back to 50 mm per month in the 2014-2015 period. When possible kinematic phases are tentatively suggested in previous studies, its solely rely on the basis of empirical threshold values. In this paper, we analyse how the use of a statistical algorithm for change-point detection helps to better understand time phases in landslide kinematics. First, we test the efficiency of the statistical algorithm on geophysical benchmark data, these data sets (stream flows and Northern Hemisphere temperatures) being already analysed by independent statistical tools. Second, we apply the method to 12-yr daily time-series of the Sechilienne landslide, for rainfall and displacement data, from 2003 December to 2015 December, in order to quantitatively extract changes in landslide kinematics. We find two strong significant discontinuities in the weekly cumulated rainfall values: an average rainfall rate increase is resolved in 2012 April and a decrease in 2014 August. Four robust changes are highlighted in the displacement time-series (2008 May, 2009 November-December-2010 January, 2012 September and 2014 March), the 2010 one being preceded by a significant but weak rainfall rate increase (in 2009 November). Accordingly, we are able to quantitatively define five kinematic stages for the Sechilienne rock avalanche during this period. The synchronization between the rainfall and displacement rate, only resolved at the end of 2009 and beginning of 2010, corresponds to a remarkable change (fourfold increase in mean displacement rate) in the landslide kinematic. This suggests that an increase of the rainfall is able to drive an increase of the landslide displacement rate, but that most of the kinematics of the landslide is not directly attributable to rainfall amount. The detailed exploration of the characteristics of the five kinematic stages suggests that the weekly averaged displacement rates are more tied to the frequency or rainy days than to the rainfall rate values. These results suggest the pattern of Sechilienne rock avalanche is consistent with the previous findings that landslide kinematics is dependent upon not only rainfall but also soil moisture conditions (as known as being more strongly related to precipitation frequency than to precipitation amount). Finally, our analysis of the displacement rate time-series pinpoints a susceptibility change of slope response to rainfall, as being slower before the end of 2009 than after, respectively. The kinematic history as depicted by statistical tools opens new routes to understand the apparent complexity of Sechilienne landslide kinematic.

This study investigates the hypothesis of Feuillet et al. (2011) that the hypocenter of the seismic event on November 10, 1935 near Montserrat, Lesser Antilles (M-S 6 1/4) (Gutenberg and Richter, 1954) was mislocated by other authors and is actually located in the Montserrat-Havers fault zone. While this proposal was based both on a Ground Motion Prediction Equation and on the assumption that earthquakes in this region are bound to prominent fault systems, our study relies on earthquake localization methods using arrival times of the International Seismological Summary (ISS). Results of our methodology suggest that the hypocenter was really located at 16.90 degrees N, 62.53 degrees W. This solution is about 25 km north-west of the location proposed by Feuillet et al. (2011) within the Redonda fault system, northward of the Montserrat Havers fault zone. As depth phases that contribute valuable insights to the focal depth are not included in the ISS data set and the reassociation of these phases is difficult, the error in depth is high. Taking into account tectonic constraints and the vertical extend of NonLinLoc's uncertainty area of the preferred solution we assume that the focus is most probably in the lower crust between 20 km and the Moho. Our approach shows that the information of the ISS can lead to a reliable solution even without an exhaustive search for seismograms and station bulletins. This is encouraging for a better assessment of seismic and tsunami hazard in the Caribbean, Mexico, South and Central America, where many moderate to large earthquakes occurred in the first half of the 20th century. The limitations during this early phase of seismology which complicate such relocations are described in detail in this study. (C) 2015 Elsevier Ltd. All rights reserved.

Online Material: Tables of earthquakes and clustering results, maps of questionnaire results, ananimation showing the evolution of the Barcelonnette event questionnaire clustering with time, and figures showing clustering comparisons (zipped archive). In the early history of scientific seismology, the systematic collection of macroseismic information was performed through standardized paper questionnaires. The Internet is changing the way earthquakes are monitored by citizens (Wald et al. , 1999; Bossu et al. , 2011, 2014). With the phenomenal growth of the Internet, many Internet‐based macroseismic survey systems have been implemented in the last decade in many parts of the world (see Wald et al. , 2011, for a survey of these systems). These kinds of systems include automatic procedures for the assignments of intensity values. The two main processing stages in assigning intensity consist of grouping data by place then determining the intensity degree that best fits the descriptions. Grouping macroseismic observations is an elaborate process: groups should be large enough to allow a general assignment of the intensity (here, “general” means that the intensity value is not assigned to an individual questionnaire) but small enough to fairly represent local geographical conditions. In well‐recognized organizations dealing with Internet macroseismic questionnaires, some ad hoc partitioning techniques are currently being used. Among these techniques, the most commonly used in macroseismology makes regular tessellation with grid squares, perhaps because it is the easiest to create and deal with. Results from grid partitioning are expected to be influenced by grid geometries. Grid geometries are probably not significantly biasing results when raw data are already artificially discretized. Artificial distributions may be expected when city center point coordinates or postal ZIP codes are used to …

Locations for earthquake recorded in the Lesser Antilles subduction zone are processed separately by regional observatories, NEIC and ISC. There is no earthquake location catalog available compiling all available phase arrival data. We propose a new best complete earthquake catalog by merging all available phase arrival data for better constrains on earthquake locations. ISC provides the phase arrival data of 29243 earthquakes (magnitude range from 1.4 to 6.4) recorded by PRSN (Porto Rico), SRC (British West Indies), and from FUNVISIS (Venezuela). We add phases data from IPGP observatories for 68718 earthquakes from magnitudes 0.1 to 7.5 (OVSG, Guadeloupe, recorded 53226 earthquakes since 1981, and OVSM, Martinique, recorded 29931 earthquakes since 1972). IPGP also provides the accelerometer waveform data of the GIS-RAP network. We achieved automatic picking on the GIS-RAP data using the Component Energy Correlation Method. The CECM provides high precision phase detection, a realistic estimation of picking error and realistic weights that can be used with manual pick weights. The CECM add an average of 3 P-waves and 2 S-waves arrivals to 3846 earthquakes recorded by the GIS-RAP network since 2002. The final catalog contains 84979 earthquakes between 1972 and 2013, 24528 of which we compiled additional data. We achieve earthquake location using NonLinLoc, regional P and S waves data and a set of one dimensional velocity models. We produce improved locations for 22974 earthquakes (better residuals, on equal or larger arrival dataset) and improved duration magnitudes for 6258 earthquakes (using duration data and improved locations). A subset of best constrained 15626 hypocenters (with more than 8 phases and an average RMS of 0.48_0.77s) could be used for structural analysis and earthquake local tomography. Relative locations are to be applied in order to image active faulting. We aim to understand coupling in the seismogenic zone as well as triggering mechanisms of intermediate depth seismicity like fluid migration beneath the volcanic arc.

Proposed in the 1950's, Båth's law states that the largest aftershock has a magnitude that is typically 1.2 less than that of the mainshock. Thirty years of the global earthquake catalog allow us to extend Båth's law in time, space and focal mechanism. On average, reverse faults have a smaller magnitude and distance from the mainshock to largest aftershock than strike‐slip faults. The distribution of the time intervals between mainshocks and their largest aftershocks obeys power law, but with a somewhat faster rate of decay than for aftershocks, in general. This implies that the largest aftershocks are more likely to occur earlier rather than later in a given sequence of aftershocks.

Threshold magnitude (M0) is an important factor in determining the magnitude of completeness in calculating seismic b values. Seismic b values can assist in determining the likelihood of earthquake events; therefore, seismic b values should be re-determined repeatedly with ever increasing precision. In this study, we use a median-based analysis of the segment slope (MBASS) to detect change points in Gutenberg-Richter frequency-magnitude distributions (FMDs) to determine M0. Results give the b value for M0-0.5 to be smaller than the b-value for M0+0.5, and the difference in b values between M0 and M0-0.5 is larger than the difference in b values between M0 and M0+0.5. Therefore, b values resulting from M0+0.5 should therefore be more accurate than b values from M0-0.5 when calculating b values using the threshold magnitude (M0). This is especially true when earthquake events are few and M0 is large such as for the time periods 1900-1935, and 1936-1972 as noted in of Taiwan's earthquake catalog.

P>The Gutenberg-Richter b-value is thought to reflect the stress conditions in the crust; therefore, spatial and/or temporal variations of the b-value can provide important information regarding crustal tectonics. We investigate the variation of b-value with depth in seven selected areas of Southern California. A previous study provided a detailed mapping of the variations of b with depth in California; our study is less systematic than this study. Our approach is more similar to the regional one used by Mori & Abercrombie. In comparison to these previous studies, our investigation indicates that the variability of b is often not statistically significant and that the decrease of b with depth should be interpreted with caution. The seismic catalogues used are subsets of a set of about 100 000 seismic events recorded by the Southern California Seismic Network (SCSN) and relocated by Richards-Dinger & Shearer. We study the performance of Utsu's test compared to bootstrap tests for comparison of b-values. The results of our investigation also raise the question of the relevancy of Utsu's test when comparing b-values. Both simulations and real cases show that the Utsu's test is biased towards rejection of the null hypothesis in favour of the hypothesis that b-values are significantly different.

Computer-based geomorphometry using a DEM (Digital Elevation Model) allows the analysis of the three-dimensional properties of landscape. This methodology is particularly useful in an intraplate region like western Europe where the simple visual inspection of the topography cannot resolve the evolutionary trends of landforms. In these domains, the morphologies of the topographic surface may be controlled mainly by climate under a low rate of tectonic deformation. Among the geomorphometric parameters, the stream length index (SL) has been used to characterize fluvial systems in relation to tectonics movements. This work develops an algorithm to derive and map the SL index using a DEM and GIS, to investigate its spatial variations in a broad area. The algorithm is applied to a zone of weak intraplate deformation: the coastal lowlands of Normandy (France). The obtained spatial distributions of SL point to anomalous zones with high SL values. These zones are adjacent to mapped fault scarps and characterized by changes in flow direction. A Kruskal–Wallis test shows that the bedrock lithology has no impact on the SL value. Therefore, the SL variations can be related mainly to a differential uplift due to Quaternary tectonic forcing. Quaternary sea level fluctuations may also be responsible for high SL values in a part of the coastal lowland.

The present seismicity in Western Provence and Eastern Languedoc (Southern France) is weak. However, when the historical seismicity is considered, these regions are certainly among the most seismic areas of southern France. The tectonic setting of both regions is one of an active intraplate zone. In comparatively “stable” areas like these, the study of small instrumental earthquakes (M<5) is an indispensable source of information. Unfortunately, in these regions, the instrumental seismicity is, as a rule, rare and diffuse. Therefore, interpreting the spatial pattern of this seismicity through a visual inspection is a difficult and subjective process. This paper presents a quantifiable analysis of the seismicity of the study regions. Earthquakes are associated with fault zones by examining the number of epicenters per unit area. The analysis is performed through the blade method applied on collapsed epicenters. The analysed data are extracted from the Laboratoire de Détection et de Géophysique Catalog. Our analysis highlights several significant epicenter alignments associated with known tectonic features.

A method based on nonparametric statistics (hereafter called “median- based analysis of the segment slope,” mbass) is applied for the detection of change points in frequency magnitude distributions (fmds). The determination of the lowest magnitude for which the Gutenberg–Richter relation still applies is a key point for the computation of reliable b -values. The change-point detection method presented here is used to determine automatically this threshold magnitude (called m 0 in this ing when fmds are analyzed by eye. Online material: Source code for detection of change-points and nied sample data set.

Physical modelling has been developed in order to simulate the effects of periglacial erosion processes on the degradation of slopes and scarps. Data from 41 experimental freeze-thaw cycles are presented. They attest to the efficiency of periglacial processes that control both erosion and changes in scarp morphology: (i) cryoexpulsion leads to an increase of scarp surface roughness and modifies significantly the internal structure of the active layer; (ii) combined effects of frost creep and gelifluction lead to slow and gradual downslope displacements of the active layer (0-3 cm/cycle); (iii) debris flows are associated with the most significant changes in scarp morphology and are responsible for the highest rate of scarp erosion; (iv) quantification of the erosion rate gives values close to 1 cm(3) cm(-2) for 41 freeze-thaw cycles. These experimental results are consistent with field data acquired along the La Hague fault scarp (Normandy, France) where an erosion rate of 4.6 +/- 1 m(3) m(-2) per glacial stage has been computed from the volume of natural slope deposits stored during the Weichselian glacial stage. These results show that moist periglacial erosion processes could lead to an underestimation of Plio-Quaternary deformation in the mid-latitudes. Copyright (c) 2006 John Wiley & Sons, Ltd.

A spatial statistical analysis method has been applied successfully on diffuse seismicity of southern Illinois and southeastern Missouri. The study area is a region of diffuse deformation and seismicity close to the New Madrid Seismic Zone. The New Madrid Earthquake Catalog is examined through the Blade Method applied on collapsed epicenters. This is a new approach since the Blade Method is applied on improved locations through the Best Estimate Method. The Blade Method performs tests on binomial distributions for the detection of significant seismic alignments within a sparse epicenter distribution. From the epicenters of the New Madrid Earthquake Catalog, using the Blade Method on collapsed epicenters, it is statistically valid to state that seismicity is associated with the New Madrid Seismic Zone and the Black, Ellington, Salem and Ste Genevieve faults.

The structural evolution of the English Channel area is controlled by structure and particularly by the pre-existing Cadomian and Variscan crustal discontinuities, which have been reactivated repeatedly in post-Variscan times. They controlled the crustal subsidence that produced basin development in the Mesozoic, prior to the sea-floor spreading in the North Atlantic region. They were then reactivated during the Cenozoic compression and basin inversion. The English Channel development is ascribed to mid-Tertiary differential uplift (Oligocene to Miocene). During late Tertiary to Quaternary times the Channel displays characteristics of a tectonically controlled fluvial basin periodically invaded by the sea. At the lithospheric scale, the Channel can be considered as an active intraplate area influenced by the NW-SE 'Alpine push', the NW-SE 'Atlantic ridge push' and glacial rebound stresses. Copyright (C) 2003 John Wiley Sons, Ltd.