Seismological constraints on the down-dip shape of normal faults

We present a seismological technique for determining the down-dip shape of seismogenic normal faults. Synthetic models of non-planar source geometries reveal the important signals in teleseismic P and SH waveforms that are diagnostic of down-dip curvature. In particular, along-strike SH waveforms are the most sensitive to variations in source geometry, and have significantly more complex and larger-amplitude waveforms for curved source geometries than planar ones. We present the results of our forward-modelling technique for 13 earthquakes. Most continental normal-faulting earthquakes that rupture through the full seismogenic layer are planar and have dips of 30°–60°. There is evidence for faults with a listric shape from some of the earthquakes occurring in two regions; Tibet and East Africa. These ruptures occurred on antithetic faults, or minor faults within the hanging walls of the rifts affected, which may suggest a reason for the down-dip curvature. For these earthquakes, the change in dip across the seismogenic part of the fault plane is ≤30°.This work forms part of the NERC- and ESRC-funded project ‘Earthquakes without Frontiers’ and was partially supported by the NERC large grant ‘Looking inside the Continents from Space’

Rapid response to the M_w 4.9 earthquake of November 11, 2019 in Le Teil, Lower Rhône Valley, France

On November 11, 2019, a Mw 4.9 earthquake hit the region close to Montelimar (lower Rhône Valley, France), on the eastern margin of the Massif Central close to the external part of the Alps. Occuring in a moderate seismicity area, this earthquake is remarkable for its very shallow focal depth (between 1 and 3 km), its magnitude, and the moderate to large damages it produced in several villages. InSAR interferograms indicated a shallow rupture about 4 km long reaching the surface and the reactivation of the ancient NE-SW La Rouviere normal fault in reverse faulting in agreement with the present-day E-W compressional tectonics. The peculiarity of this earthquake together with a poor coverage of the epicentral region by permanent seismological and geodetic stations triggered the mobilisation of the French post-seismic unit and the broad French scientific community from various institutions, with the deployment of geophysical instruments (seismological and geodesic stations), geological field surveys, and field evaluation of the intensity of the earthquake. Within 7 days after the mainshock, 47 seismological stations were deployed in the epicentral area to improve the Le Teil aftershocks locations relative to the French permanent seismological network (RESIF), monitor the temporal and spatial evolution of microearthquakes close to the fault plane and temporal evolution of the seismic response of 3 damaged historical buildings, and to study suspected site effects and their influence in the distribution of seismic damage. This seismological dataset, completed by data owned by different institutions, was integrated in a homogeneous archive and distributed through FDSN web services by the RESIF data center. This dataset, together with observations of surface rupture evidences, geologic, geodetic and satellite data, will help to unravel the causes and rupture mechanism of this earthquake, and contribute to account in seismic hazard assessment for earthquakes along the major regional Cévenne fault system in a context of present-day compressional tectonics

Improved modeling of segmented earthquake rupture informed by enhanced signal analysis of seismic and geodetic observations

Earthquake source modeling has emerged from the need to be able to describe and quantifythe mechanism and physical properties of earthquakes. Investigations of earthquake ruptureand fault geometry requires the testing of a large number of such potential sets of earthquakesources models. Earthquakes often rupture across more than one fault segment. If such rupturesegmentation occurs on a significant scale, a simple model may not represent the rupture processwell. This thesis focuses on the data-driven inclusion of earthquake rupture segmentation intoearthquake source modeling. The developed tools and the modeling are based on the jointuse of seismological waveform far-field and geodetic Interferometric Synthetic Aperture Radarnear-field surface displacement maps to characterise earthquake sources robustly with rigorousconsideration of data and modeling errors.A strategy based on information theory is developed to determine the appropriate modelcomplexity to represent the available observations in a data-driven way. This is done inconsideration of the uncertainties in the determined source mechanisms by investigating theinferences of the full Bayesian model ensemble. Application on the datasets of four earthquakesindicated that the inferred source parameters are systematically biased by the choice of modelcomplexity. This might have effects on follow-up analyses, e. g. regional stress field inversionsand seismic hazard assessments.Further, two methods were developed to provide data-driven model-independent constraints toinform a kinematic earthquake source optimization about earthquake source parameter priorestimates. The first method is a time-domain multi-array backprojection of teleseismic datawith empirical traveltime corrections to infer the spatio-temporal evolution of the rupture. Thisenables detection of potential rupture segmentation based on the occurrence of coherent high-frequency sources during the rupture process. The second developed method uses image analysismethods on satellite radar measured surface displacement maps to infer modeling constraints onrupture characteristics (e.g. strike and length) and the number of potential segments. These twomethods provide model-independent constraints on fault location, dimension, orientation andrupture timing. The inferred source parameter constraints are used to constrain an inversion forthe source mechanism of the 2016 Muji Mw 6.6 earthquake, a segmented and bilateral strike-slipearthquake.As a case study to further investigate a depth-segmented fault system and occurrence of co-seismic rupture segmentation in such a system the 2008-2009 Qaidam sequence with co-seismicand post-seismic displacements is investigated. The Qaidam 2008-2009 earthquake sequence innortheast Tibet involved two reverse-thrust earthquakes and a postseismic signal of the 2008earthquake. The 2008 Qaidam earthquake is modeled as a deep shallow dipping earthquakewith no indication of rupture segmentation. The 2009 Qaidam earthquake is modeled on threedistinct south-dipping high-angle thrusts, with a bilateral and segmented rupture process. Agood agreement between co-seismic surface displacement measurements and coherent seismicenergy emission in the backprojection results is determined.Finally, a combined framework is proposed which applies all the developed methods and tools inan informed parallel modeling of several earthquake source model complexities. This frameworkallows for improved routine determination of earthquake source modeling under considerationof rupture segmentation. This thesis provides overall an improvement for earthquake sourceanalyses and the development of modeling standards for robust determination of second-orderearthquake source parameters

Slip Partitioning in the 2016 Alboran Sea Earthquake Sequence (Western Mediterranean)

This study was supported by FEDER/MINECO projects CGL2015-67130-C2-2-R and PID2019-109608GB-I00, FEDER/Junta de Andalucia project A-RNM-421-UGR18, and is part of the research group RNM104 of the Junta de Andalucia. JA has also received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 754446 and UGR Research and Knowledge Transfer Found-Athenea3i; and by project 407141557 of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation).A MW = 5.1 earthquake on January 21st, 2016 marked the beginning of a significant seismic sequence in the southern Alboran Sea, culminating in a MW = 6.3 earthquake on January 25th, and continuing with further moderate magnitude earthquakes until March. We use data from 35 seismic broadband stations in Spain, Morocco and Portugal to relocate the seismicity, estimate seismic moment tensors, and isolate regional apparent source time functions for the main earthquake. Relocation and regional moment tensor inversion consistently yield very shallow depths for the majority of events. We obtain 50 moment tensors for the sequence, showing a mixture of strike-slip faulting for the foreshock and the main event and reverse faulting for the major aftershocks. The leading role of reverse focal mechanisms among the aftershocks may be explained by the geometry of the fault network. The mainshock nucleates at a bend along the left-lateral Al-Idrisi fault, introducing local transpression within the transtensional Alboran Basin. The shallow depths of the 2016 Alboran Sea earthquakes may favor slip-partitioning on the involved faults. Apparent source durations for the main event suggest a ∼21 km long, asymmetric rupture that propagates primarily toward NE into the restraining fault segment, with fast rupture speed of ∼3.0 km/s. Consistently, the inversion for laterally variable fault displacement situates the main slip in the restraining segment. The partitioning into strike-slip rupture and dip-slip aftershocks confirms a non-optimal orientation of this segment, and suggests that the 2016 event settled a slip deficit from previous ruptures that could not propagate into the stronger restraining segment.FEDER/MINECO CGL2015-67130-C2-2-R PID2019-109608GB-I00FEDER/Junta de Andalucia project A-RNM-421-UGR18Junta de Andalucía RNM104European Union (EU) 754446German Research Foundation (DFG) 40714155

High resolution earthquake relocation along Idrija fault system in Western Slovenia and its application in understanding the deformation along the active faults of NW External Dinarides

Northeastern Italy and Western Slovenia lies in the region of moderate earthquake activity. In the past, rare strong earthquakes struck the region, namely 1511 Idrija M 6.9, Friuli 1976 series of M 6+ and 1998 and 2004 M 5.6 and 5.2 Ravne fault earthquakes. Multiple geodetic, geomorphologic and geologic studies show, that IFS is an active fault system, with slip rates up to 3.8 mm/yr over all the faults. Since 2006 area of IFS is covered by relatively dense network of broadband stations recording in continuous mode. To better understand the IFS itself, a detailed seismologic study was performed in this study. To gain the knowledge of fault system geometry, physics of faulting and temporal behavior, the data of all the stations in the region was analyzed. First, time series were analyzed using classic STA/LTA detection algorithm for the detection of earthquakes. After the initial scan of the database, arrival times of phases of detected earthquakes were manually repicked to obtain as precise arrival times of phases as possible. The magnitude of completeness for the catalog obtained in such way was 0.9. To obtain lower magnitude of completeness, additional manual inspection of waveforms was performed. New micro earthquakes were added to the existing database. For the detection of even lower magnitude earthquakes, all the earthquakes of magnitudes above 0.8 were selected as a template earthquakes. Matched filter detection of earthquakes was performed on the selected stations with simultaneous picking of precise phases as obtained from cross correlation around the P or S arrival times between template earthquake and the detected earthquake. The newly obtained earthquakes were added to the database lowering the magnitude of completeness to -0.7. Manually relocated earthquakes and earthquakes obtained from matched filter detection clearly illuminated active faults of IFS, showing activity along all the faults of the system. From cross sections perpendicular to the IFS clear geometry was defined for the central part of the system. Cross section showed that earthquakes along Ravne fault are much more shallow than on other faults of the system and illuminate a SW dipping fault which probably connects to Idrija fault at depth. Idrija fault was only active in its northern extend in the analyzed period, with deeper earthquakes illuminating vertical fault plane. Predjama and Selce fault were mostly active in the central and southern part of the system. Earthquakes along these two faults show vertically dipping faults. Finally, earthquake activity along Ra\u161a fault is mostly present in its central and southern extend and show a fault plane dipping towards NE. High resolution detection of the earthquakes also gave a better understanding of temporal earthquake evolution along the IFS. Temporal clustering was observed in the years 2009-2010 and in the late 2017. In 2009-2010 majority of the system shows high activity, with swarms taking place in northern and central part of the system and multiple mainshock-aftershock series in southern part. Similar, in 2017 multiple swarms and mainshock series happened in the southern part of the system. Combining the detected temporal clustering with the datasets of extensometer mounted on Predjama fault we observed that in the same time period as the 2009-2010 elevated activity happened, also a surface deformation was present. We modeled the surface deformation and earthquake activity using Coulomb stress changes. Best fit was given to the changes of normal stresses along the Idrija fault, which led to the unclamping of the faults parallel to Idrija fault, increasing the earthquake rates along these faults and causing the surface deformations due to the extraction and migration of fluids at depth

Geometry and mechanics of the active fault system in western Slovenia

Western Slovenia is part of an actively deforming region accommodating anticlockwise rotation of Adria and its continuous collision with Eurasia. The geometry of the active faulting system in this plate boundary is not well defined. In this study, detailed analysis of earthquake activity was performed with relocation of earthquakes in the period between 2006 and 2017. With inspection of the waveform data, slight temporal clustering of activity was observed. To increase the detection rate of microearthquakes we used a matched filter detection algorithm method. Templates of earthquakes were created and a database of continuous waveform data within the period 2006\u20132017 was investigated. As a result, high temporal correlation allowed us to identify swarms and earthquake sequences that affected the active fault system in the study region. Relocated seismicity allowed us to constrain the geometry of 5 nearly parallel faults, namely: Ravne, Idrija, Predjama, Selce and Ra\u161a faults. All these faults do have an expression in the geomorphology and reach a seismogenic depth of up to 20 km. Vertical and along strike extents of these active faults can favour earthquakes of moment magnitude equal to 7 or larger. The most recent large earthquake that occurred in this region is the 1511 earthquake with a magnitude 6.8. The leading fault in the system being the Idrija right-lateral strike-slip fault, experiences earthquake activity from 5 to 20 km on its northern segment, while on its southern segment no earthquake activity is detected over the decade of observations. We show that the interseismic loading on the southern segment of Idrija fault is likely unclamping the locked adjacent faults promoting the observed bursts of seismicity. Moreover, in 2009 the Predjama fault accommodated a sudden increase of the surface deformation at the extensometer accompanied by a simultaneous swarm activity at its seismogenic depth. This behaviour might correspond to velocity strengthening and weakening processes taking place at both the surface and depth terminations of a locked vertical fault. These processes can be driven by a slow-slip event on the deeper part of Idrija fault that would generate a temporary acceleration of the interseismic loading rate along with a change within the fluid circulation

Citizen seismology helps decipher the 2021 Haiti earthquake

5 pages, 4 figures, supplementary materials https://doi.org/10.1126/science.abn1045.-- Data and materials availability: All data and code used in this study are openly available. RADAR data can be obtained through ESA (Sentinel) or JAXA (Alos-2). Aftershock data can be obtained from https://ayiti.unice.fr/ayiti-seismes/ (7). The codes used to process or model the data are published and public (8). The catalog of high-precision earthquake relocated with the NLL-SSST-coherence procedure (SM4) is available as supplementary dataOn 14 August 2021, the moment magnitude (Mw) 7.2 Nippes earthquake in Haiti occurred within the same fault zone as its devastating 2010 Mw 7.0 predecessor, but struck the country when field access was limited by insecurity and conventional seismometers from the national network were inoperative. A network of citizen seismometers installed in 2019 provided near-field data critical to rapidly understand the mechanism of the mainshock and monitor its aftershock sequence. Their real-time data defined two aftershock clusters that coincide with two areas of coseismic slip derived from inversions of conventional seismological and geodetic data. Machine learning applied to data from the citizen seismometer closest to the mainshock allows us to forecast aftershocks as accurately as with the network-derived catalog. This shows the utility of citizen science contributing to our understanding of a major earthquakeThis work was supported by the Centre National de la Recherche Scientifique (CNRS) and the Institut de Recherche pour le Développement (IRD) through their “Natural Hazard” program (E.C., S.S., T.M., B.D., F.C., J.P.A., J.C., A.D., D.B., S.P.); the FEDER European Community program within the Interreg Caraïbes “PREST” project (E.C., S.S., D.B.); Institut Universitaire de France (E.C., R.J.); Université Côte d’Azur and the French Embassy in Haiti (S.P.); the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant no. 758210, Geo4D project to R.J. and grant no. 805256 to Z.D.); the French National Research Agency (project ANR-21-CE03-0010 “OSMOSE” to E.C. and ANR-15-IDEX-01 “UCAJEDI Investments in the Future” to Q.B.); the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (grant no. 949221 to Q.B.); and HPC resources of IDRIS (under allocations 2020-AD011012142, 2021-AP011012536, and 2021-A0101012314 to Q.B.With the institutional support of the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000928-S)Peer reviewe

The 2013–2018 matese and beneventano seismic sequences (Central–Southern apennines). New constraints on the hypocentral depth determination

The Matese and Beneventano areas coincide with the transition from the central to the southern Apennines and are characterized by both SW-and NE-dipping normal faulting seismogenic structures, responsible for the large historical earthquakes. We studied the Matese and Beneventano seismicity by means of high-precision locations of earthquakes spanning from 29 December 2013 to 4 September 2018. Events were located by using all of the available data from temporary and permanent stations in the area and a 1D computed velocity model, inverting the dataset with the Velest code. For events M > 2.8 we used P-and S-waves arrival times of the strong motion stations located in the study area. A constant value of 1.83 for Vp/Vs was computed with a modified Wadati method. The dataset consists of 2378 earthquakes, 18,715 P-and 12,295 S-wave arrival times. We computed 55 new fault plane solutions. The mechanisms show predominantly normal fault movements, with T-axis trends oriented NE–SW. Only relatively small E–W trending clusters in the eastern peripheral zones of the Apenninic belt show right-lateral strike-slip kinematics similar to that observed in the Potenza (1990–1991) and Molise (2002 and 2018) sequences. These belong to transfer zones associated with differential slab retreat of the Adriatic plate subduction beneath the Apennines. The Matese sequence (December 2013–February 2014; main shock Mw 5.0) is the most relevant part of our dataset. Hypocentral depths along the axis of the Apenninic belt are in agreement with previous seismological studies that place most of the earthquakes in the brittle upper crust. We confirm a general deepening of seismicity moving from west to the east along the Apennines. Seismicity depth is controlled by heat-flow, which is lower in the eastern side, thus causing a deeper brittle–ductile transition

Episodic transient deformation revealed by the analysis of multiple GNSS networks in the Noto Peninsula, central Japan

流体とスロースリップに駆動された能登半島群発地震 --ソフトバンク独自基準点データを用いた地殻変動解析結果--. 京都大学プレスリリース. 2023-06-13.Since November 30, 2020, an intense seismic swarm and transient deformation have been continuously observed in the Noto Peninsula, central Japan, which is a non-volcanic/geothermal area far from major plate boundaries. We modeled transient deformation based on a combined analysis of multiple Global Navigation Satellite System (GNSS) observation networks, including one operated by a private sector company (SoftBank Corp.), relocated earthquake hypocenters, and tectonic settings. Our analysis showed a total displacement pattern over 2 years shows horizontal inflation and uplift of up to ~ 70 mm around the source of the earthquake swarm. In the first 3 months, the opening of the shallow-dipping tensile crack had an estimated volumetric increase of ~ 1.4 × 10⁷ m³ at a depth of ~ 16 km. Over the next 15 months, the observed deformation was well reproduced by shear-tensile sources, which represent an aseismic reverse-type slip and the opening of a southeast-dipping fault zone at a depth of 14–16 km. We suggest that the upwelling fluid spread at a depth of ~ 16 km through an existing shallow-dipping permeable fault zone and then diffused into the fault zone, triggering a long-lasting sub-meter aseismic slip below the seismogenic depth. The aseismic slip further triggered intense earthquake swarms at the updip