The 2019–2020 Khalili (Iran) Earthquake Sequence— Anthropogenic Seismicity in the Zagros Simply Folded Belt?

This work was supported by the International Training Course "Seismology and Seismic Hazard Assessment,'' which has been funded by the GeoForschungsZentrum Potsdam (GFZ) and the German Federal Foreign Office through the German Humanitarian Assistance program, grant S08‐60 321.50 ALL 03/19. Furthermore, M. Jamalreyhani acknowledges support by a grant from the Iran National Science Foundation (INSF) under a research project “97013349.” E. Nissen was supported by the Natural Sciences and Engineering Research Council of Canada through Discovery Grant 2017‐04029, the Canada Foundation for Innovation, the British Columbia Knowledge Development Fund, and a Tier 2 Canada Research Chair. J. A. López‐Comino has also received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska‐Curie grant agreement 754446 and UGR Research and Knowledge Transfer Found–Athenea3i and by project 407141557 of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation). Most of the maps were prepared using the Pyrocko toolbox and GMT 5 software. We are grateful to all data research centers and networks for providing the data used in this study. InSAR interferograms were made using freely available Copernicus Sentinel data (2017; https://scihub.copernicus.eu/ ). We are thankful to Gudrun Richter, Christian Heberland, Sebastian Hainzl, Torsten Dahm, and Mir Ali Hassanzadeh for constructive comments on this work. M. Jamalreyhani and P. Büyükakpınar are very grateful to Claus Milkereit and Dorina Kroll for supporting them, during their visit to the GFZ Potsdam. Furthermore, we are grateful to Editor Rachel Abercrombie, the associate editor, and four anonymous reviewers for their valuable comments and suggestions that helped us to improve the manuscript.The seismic catalog and waveforms of the Iran network were downloaded from the Iranian Seismological Center (IRSC) available at http://irsc.ut.ac.ir/ . The IRSC waveforms are freely available for the events with magnitude larger than 4 and the IRSC catalog is available for 2.5+. The array data used for teleseismic modeling were downloaded from the Incorporated Research Institutions for Seismology (IRIS) Data Management Center (funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope proposal of the National Science Foundation under Cooperative Agreement EAR‐1261681) and the Institute for Geosciences and Natural Resources, Germany (BGR). InSAR interferograms were made using Copernicus Sentinel data available at https://scihub.copernicus.eu/ . Information on the Shanul and Homa gas reservoirs obtained from the Iranian Central Oil Fields Company webpage (ICOFC, https://en.icofc.ir/ ), Southern Zagros Oil and Gas Production Company ( https://www.szogpc.com/ ), and National Iranian Oil Company (NIOC). The geological map of the region, which is published by the Geological Survey of Iran (GSI), is available at https://gsi.ir/en . M n M nWe investigate the origin of a long-lived earthquake cluster in the Fars arc of the Zagros Simply Folded Belt that is colocated with the major Shanul natural gas field. The cluster emerged in January 2019 and initially comprised small events of Mn ∼ 3–4. It culminated on 9 June 2020 with a pair of Mw 5.4 and 5.7 earthquakes, which was followed by >100 aftershocks. We assess the spatiotemporal evolution of the earthquake sequence using multiple event hypocenter relocations, waveform inversions, and Sentinel-1 Interferometric Synthetic Aperture Radar (InSAR) measurements and models. We find that the early part of the sequence is spatially distinct from the 9 June 2020 earthquakes and their aftershocks. Moment tensors, centroid depths, and source parameter uncertainties of 15 of the largest (Mn ≥ 4.0) events show that the sequence is dominated by reverse faulting at shallow depths (mostly ≤4 km) within the sedimentary cover. InSAR modeling shows that the Mw 5.7 mainshock occurred at depths of 2–8 km with a rupture length and maximum slip of ∼20 km and ∼0.5 m, respectively. Our results suggest that the 2019–2020 Khalili earthquake sequence was likely influenced by operation of the Shanul field, though elevated natural seismicity in the Zagros makes the association difficult to prove. Understanding how to distinguish man-made from natural seismicity is helpful for hazard and risk assessment, notably in the Zagros, which is both seismically active and rich in oil and gas reserves.Claus Milkereit and Dorina KrollGerman Federal Foreign OfficeGerman Humanitarian Assistance program S08‐60 321.50 ALL 03/19Institute for Geosciences and Natural ResourcesTier 2 Canada Research ChairNatural Sciences and Engineering Research Council of Canada 2017‐04029Canada Foundation for InnovationDeutsche ForschungsgemeinschaftIran National Science Foundation 97013349, EAR‐1261681Horizon 2020 407141557, 754446British Columbia Knowledge Development FundHelmholtz-Zentrum Potsdam - Deutsches GeoForschungsZentrum GF

Source mechanisms and rupture processes of the Jujuy seismic nest, Chile-Argentina border

The Altiplano-Puna plateau, in Central Andes, is the second-largest continental plateau on Earth, extending between 22◦ and 27◦S at an average altitude of 4400 m. The Puna plateau has been formed in consequence of the subduction of the oceanic Nazca Plate beneath the continental South American plate, which has an average crustal thickness of 50 km at this location. A large seismicity cluster, the Jujuy cluster, is observed at depth of 150–250 km beneath the central region of the Puna plateau. The cluster is seismically very active, with hundreds of earthquakes reported and a peak magnitude MW 6.6 on 25th August 2006. The cluster is situated in one of three band of intermediate-depth focus seismicity, which extend parallel to the trench roughly North to South. It has been hypothesized that the Jujuy cluster could be a seismic nest, a compact seismogenic region characterized by a high stationary activity relative to its surroundings. In this study, we collected more than 40 years of data from different catalogs and proof that the cluster meets the three conditions of a seismic nest. Compared to other known intermediate depth nests at Hindu Kush (Afganisthan) or Bucaramanga (Colombia), the Jujuy nest presents an outstanding seismicity rate, with more than 100 M4+ earthquakes per year. We additionally performed a detailed analysis of the rupture process of some of the largest earthquakes in the nest, by means of moment tensor inversion and directivity analysis. We focused on the time period 2017–2018, where the seismic monitoring was the most extended. Our results show that earthquakes in the nest take place within the eastward subducting oceanic plate, but rupture along sub-horizontal planes dipping westward. We suggest that seismicity at Jujuy nest is controlled by dehydration processes, which are also responsible for the generation of fluids ascending to the crust beneath the Puna volcanic region. We use the rupture plane and nest geometry to provide a constraint to maximal expected magnitude, which we estimate as MW ~6.7.National Commission for Scientific and Technological Research (ANID-Becas Chile, Chile)European Unions Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant 754446UGR Research and Knowledge Transfer FoundAthenea3iGerman Research Foundation (DFG) 40714155

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

Preservation of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

We are grateful to the staff involved in the TransCorBe project. The Geophysical Instrument Pool at GFZ-Potsdam provided most of the seismic equipment. We are grateful to Christian Haberland for his support. We want to thank two anonymous reviewers for the careful reading of the manuscript and the interesting and constructive criticism they provided. This work was funded by the Spanish State Research Agency (SRA) under the grant PID2019-109608GB I00/SRA/10.13039/501100011033, FEDER/MINECO project CGL2015-67130-C2-2-R, FEDER/Junta de Andalucia project A-RNM-421-UGR18 and research group RNM104 of the Junta de Andalucia. The Granada University/CBUA funding for open access charge.We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the 50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching 45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought.Geophysical Instrument Pool at GFZ-Potsdam - Spanish State Research Agency (SRA) PID2019-109608GB I00/SRA/10.13039/501100011033Spanish Government CGL2015-67130-C2-2-RFEDER/Junta de Andalucia project A-RNM-421-UGR18 RNM104Junta de Andaluci

Preservation of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the 50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching 45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought

Relationship of Weather Types on the Seasonal and Spatial Variability of Rainfall, Runoff, and Sediment Yield in the Western Mediterranean Basin

Rainfall is the key factor to understand soil erosion processes, mechanisms, and rates. Most research was conducted to determine rainfall characteristics and their relationship with soil erosion (erosivity) but there is little information about how atmospheric patterns control soil losses, and this is important to enable sustainable environmental planning and risk prevention. We investigated the temporal and spatial variability of the relationships of rainfall, runoff, and sediment yield with atmospheric patterns (weather types, WTs) in the western Mediterranean basin. For this purpose, we analyzed a large database of rainfall events collected between 1985 and 2015 in 46 experimental plots and catchments with the aim to: (i) evaluate seasonal differences in the contribution of rainfall, runoff, and sediment yield produced by the WTs; and (ii) to analyze the seasonal efficiency of the different WTs (relation frequency and magnitude) related to rainfall, runoff, and sediment yield. The results indicate two different temporal patterns: the first weather type exhibits (during the cold period: autumn and winter) westerly flows that produce the highest rainfall, runoff, and sediment yield values throughout the territory; the second weather type exhibits easterly flows that predominate during the warm period (spring and summer) and it is located on the Mediterranean coast of the Iberian Peninsula. However, the cyclonic situations present high frequency throughout the whole year with a large influence extended around the western Mediterranean basin. Contrary, the anticyclonic situations, despite of its high frequency, do not contribute significantly to the total rainfall, runoff, and sediment (showing the lowest efficiency) because of atmospheric stability that currently characterize this atmospheric pattern. Our approach helps to better understand the relationship of WTs on the seasonal and spatial variability of rainfall, runoff and sediment yield with a regional scale based on the large dataset and number of soil erosion experimental stations

The 2014 Juan Fernández microplate earthquake doublet: Evidence for large thrust faulting driven by microplate rotation

We are thankful to the editor, Dr. K. Wang, to Prof. Dr. J. Zahradnik and to an anonymous reviewer for careful reviews and useful suggestions. We used pyrocko (Heimann et al., 2017) for processing and for the generation of some figures. C.V.M. appreciates the scholarship granted to her by the National Commission for Scientific and Technological Research (CONICYT -Becas Chile), N. 72180072. J.A.L.C. has received funding from the European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie Grant agreement N. 754446 and UGR Research and Knowledge Transfer Found -Athenea3i; and by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) -Projektnummer (407141557). T. Davis is funded by the DFG-ICDP grant N. RI 2782/3-1.On October 9, 2014, a Mw 7.1–6.7 seismic doublet occurred at the Juan Fern´andez microplate, close to the triple junction with Pacific and Nazca plates. The Mw 7.1 earthquake is the largest earthquake ever to have been recorded in the region. Its thrust focal mechanism is also unusual for the region, although the northern part of the microplate is expected to undergo compression. The region is remote and seismological data is limited to a seismic station at ~600 km distance on Easter Island and teleseismic observations for the largest events. We use a combination of advanced seismological techniques to overcome the lack of local data and resolve earthquake source parameters for the doublet and its aftershock sequence, being able to reconstruct the chronology of the sequence and the geometry of affected fault segments. Our results depict a complex seismic sequence characterized by the interplay of thrust and strike-slip earthquakes along different structures, including a second, reversed strike slip-thrust seismic doublet in November 2014. Seismicity occurred within the microplate and only in the late part of the sequence migrated northward, towards the microplate boundary. The first largest doublet, whose rupture kinematic is well explained by stress changes imparted by the first subevent on the second one, may have activated unmapped E-W and NE-SW faults or an internal curved pseudofault, attributed to the longterm rotation of the microplate. Few large, thrust earthquakes are observed within the sequence, taking place in the vicinity of mapped compressional ridges. We suggest that compressional stresses in the northern part of the microplate and at its boundary are partially accommodated aseismically. However, the occasional occurrence of large, impulsive thrust earthquakes, with a considerable tsunamigenic potential, poses a relevant hazard for islands in the South Pacific region.National Commission for Scientific and Technological Research (CONICYT -Becas Chile) 72180072European Commission 754446UGR Research and Knowledge Transfer Found -Athenea3iGerman Research Foundation (DFG) 407141557 RI 2782/3-

On the Source Parameters and Genesis of the 2017, Mw 4 Montesano Earthquake in the Outer Border of the Val d’Agri Oilfield (Italy)

On October 27, 2017, an Mw 4 earthquake occurred close to the municipality of Montesano sulla Marcellana, less than 10 km external to the concession of the largest European onshore hydrocarbon reservoir—the Val d’Agri oilfield (Southern Italy). Being a weak event located outside the extended monitoring domain of the industrial concession, the relevance of this earthquake and the possible links with the hydrocarbon exploitation were not extensively discussed. Actually, the analysis of shallow seismic events close to subsurface exploitation domains plays a significant role in the definition of key parameters in order to discriminate between natural, triggered, and induced seismicity, especially in tectonically active regions. The study of weak-to-moderate earthquakes can improve the characterization of the potentially destructive seismic hazard of this particular area, already struck by M > 6.5 episodes in the past. In this work, we analyze the source parameters of this Mw 4 earthquake by applying advanced seismological techniques to estimate the uncertainties derived from the moment tensor inversion and identify plausible directivity effects. The moment tensor is dominated by a NW–SE oriented normal faulting with a centroid depth of 14 km. A single ML 2.1 aftershock was recorded and used as the empirical Green’s function to calculate the apparent source time function for the mainshock. Apparent durations (in the range 0.11–0.21 s, obtained from S-waves) define an azimuthal pattern, which reveals an asymmetric bilateral rupture with 70% of the rupture propagation in the N310°Wdirection, suggesting a rupture plane dipping to the SW. Our results tally with the activation of a deeper fault segment associated with the Eastern Agri Fault System close to the basement as the origin of the Montesano earthquake. Finally, the Coulomb stress rate induced by depletion of the oilfield is calculated to quantify the trigger potential estimated for the Montesano earthquake yielding relatively low probabilities below 10%. Our analyses point toward the conclusion that the Mw 4 event was more likely due to the local natural tectonic stress, rather than induced or triggered by the long-term hydrocarbon extraction in the Val d’Agri oilfield.European Commission 754446UGR Research and Knowledge Transfer Found-Athenea3iGerman Research Foundation (DFG) 407141557Progetto di Monitoraggio Val d'Agri (INGV