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Dahm, Torsten
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- PublicationOpen AccessEstimation of long period volcanic sources by a frequency domain inversion approach(2006-04)
; ; ; ; ;Cesca, S.; Inst. f. Geophysik, Uni Hamburg ;Dahm, T.; Inst. f. Geophysik, Uni Hamburg ;Battaglia, J.; Laboratoire Magmas et Volcans, Toulouse ;Braun, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; The high interest of volcanologists to understand the physical phenomena which governs long period (LP) events is related to the fact that they may be directly generated by fluid transfer and could be indicators of the level of activity in the volcano and in some cases could act as precursor to eruptions. The wide variety of waveforms and spectral contents existing for LP events, as well as the existence of alternative models to explain the observations make it interesting to develop new inversion schemes. We propose an inversion methodology to determine source mechanisms and study these events through an exhaustive source inversion by using synthetic and observed data. Our method for source inversion is based on a frequency domain approach, which has its main advantage in reducing computational requirements. The resulting source mechanism is represented by the sum of two time-dependent terms: a full moment tensor and a single force. The method has been applied to different sets of synthetic and observed data, including data from Kilauea volcano. Green’s functions have been calculated using different layered crustal models, which have been proposed for volcanic areas. Inversion tests are established to check the stability of the method and the possibility of retrieving all source components. The method has been finally applied to volcanic data and results are interpreted in terms of possible source models.134 74 - PublicationRestrictedA probabilistic approach for the classification of earthquakes as ‘triggered’ or ‘not triggered’(2013)
; ; ; ; ; ; ; ; ;The occurrence time of earthquakes can be anticipated or delayed by external phenomena that induce strain energy changes on the faults. ‘Anticipated’ earthquakes are generally called ‘triggered’; however, it can be controversial to label a specific earthquake as such, mostly because of the stochastic nature of earthquake occurrence and of the large uncertainties usually associated to stress modelling. Here we introduce a combined statistical and physical approach to quantify the probability that a given earthquake was triggered by a given stress-inducing phenomenon. As an example, we consider an earthquake that was likely triggered by a natural event: the M = 6.2 13 Jan 1976 Kópasker earthquake on the Grímsey lineament (Tjörnes Fracture Zone, Iceland), which occurred about 3 weeks after a large dike injection in the nearby Krafla fissure swarm. By using Coulomb stress calculations and the rate-and-state earthquake nucleation theory, we calculate the likelihood of the earthquake in a scenario that contains only the tectonic background and excludes the dike and in a scenario that includes the dike but excludes the background. Applying the Bayes’ theorem, we obtain that the probability that the earthquake was indeed triggered by the dike, rather than purely due to the accumulation of tectonic strain, is about 60 to 90 %. This methodology allows us to assign quantitative probabilities to different scenarios and can help in classifying earthquakes as triggered or not triggered by natural or human-induced changes of stress in the crust.214 1 - PublicationOpen AccessLithospheric Sill Intrusions and Present‐Day Ground Deformation at Rhenish Massif, Central EuropeThe Rhenish Massif in Central Europe, which includes the Eifel Volcanic Fields, has shown ongoing ground deformation and signs of possible unrest. A buoyant plume exerting uplift forces at the bottom of the lithosphere was proposed to explain such deformation; the hypothesis of (possibly concurrent) melt accumulation in the crust/lithospheric mantle has not been explored yet. Here, we test deformation models in an elastic half-space considering sources of varying aspect ratio, size and depth. We explore the effects of data coverage, noise and uncertainty on the inferred source parameters. We find that the observed deformation would require melt accumulation in sub-horizontal sill-like structures expanding at the rate of up to ∼0.045 km3/yr. We discuss feasibility, limitations and possible interpretations of our resulting models and elaborate on further observations which may help constrain the structure of the Rhenish Massif magmatic system.
29 5 - PublicationRestrictedNew insights on volcanic and tectonic structures of the southern Tyrrhenian (Italy) from marine and land seismic data(2013)
; ; ; ;Monna, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Sgroi, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma2, Roma, Italia ;Dahm, T.; Helmholtz-Zentrum Potsdam, Deutsches Geo Forschungs Zentrum, Potsdam, Germany; ; We present results from the first crustal seismic tomography for the southern Tyrrhenian area, which includes ocean bottom seismometer (OBS) data and a bathymetry correction. This area comprises Mt. Etna, the Aeolian Islands, and many volcanic seamounts, including the Marsili Seamount. The seismicity distribution in the area depends on the complex interaction between tectonics and volcanism. The 3-D velocity model presented in this study is obtained by the inversion of P wave arrival times from crustal earthquakes. We integrate travel time data recorded by an OBS network (Tyrrhenian Deep Sea Experiment), the SN-1 seafloor observatory, and the land network. Our model shows a high correlation between the P wave anomaly distribution and seismic and volcanic structures. Two main low-velocity anomalies underlie the central Aeolian Islands and Mt. Etna. The two volumes, which are related to the well-known active volcanism, are separated and located at different depths. This finding, in agreement with structural, petrography, and GPS data from literature, confirms the independence of the two systems. The strongest negative anomaly is found below Mt. Etna at the base of the crust, and we associate it with the deep feeding system of the volcano. We infer that most of the seismicity is generated in brittle rock volumes that are affected by the action of hot fluids under high pressure due to the active volcanism in the area. Lateral changes of velocity are related to a transition from the western to the central Aeolian Islands and to the passage from continental crust to the Tyrrhenian oceanic uppermost mantle.648 87 - PublicationRestrictedScaling and spatial complementarity of tectonic earthquake swarms(2018-01)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Tectonic earthquake swarms (TES) often coincide with aseismic slip and sometimes precede damaging earthquakes. In spite of recent progress in understanding the significance and properties of TES at plate boundaries, their mechanics and scaling are still largely uncertain. Here we evaluate several TES that occurred during the past 20 years on a transform plate boundary in North Iceland. We show that the swarms complement each other spatially with later swarms discouraged from fault segments activated by earlier swarms, which suggests efficient strain release and aseismic slip. The fault area illuminated by earthquakes during swarms may be more representative of the total moment release than the cumulative moment of the swarm earthquakes. We use these findings and other published results from a variety oftectonic settings to discuss general scaling properties for TES. The results indicate that the importance of TES in releasing tectonic strain at plate boundaries may have been underestimated.294 1 - PublicationOpen AccessThe TOMO-ETNA experiment: an imaging active campaign at Mt. Etna volcano. Context, main objectives, working-plans and involved research projects(2016)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ;Ibáñez, J.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain ;Prudencio, J.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain ;Díaz-Moreno, A.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain ;Patanè, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Puglisi, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Lühr, B.; Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany ;Carrión, F.; Instituto Andaluz de Geofísica, Universidad de Granada, Granada, Spain ;Dañobeitia, J. J.; Centro Mediterráneo de Investigaciones Marinas y Ambientales, ICM, Consejo Superior de Investigaciones Científicas, Barcelona, Spain ;Coltelli, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Catania, Catania, Italia ;Bianco, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Del Pezzo, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione OV, Napoli, Italia ;Dahm, T.; Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ, Potsdam, Germany ;Willmott, V.; Alfred-Wegener-Institut, Helmholtz-Zentrum für Polar-und Meeresforschung, International Cooperation, Bremerhaven, Germany ;Mazauric, V.; Ifremer, Centre Bretagne, Unité Navires et Systèmes Embarqués (NSE), Plouzané, France; ; ; ; ; ; ; ; ; ; ; ; ; The TOMO-ETNA experiment was devised to image of the crust underlying the volcanic edifice and, possibly, its plumbing system by using passive and active refraction/reflection seismic methods. This experiment included activities both on-land and offshore with the main objective of obtaining a new high-resolution seismic tomography to improve the knowledge of the crustal structures existing beneath the Etna volcano and northeast Sicily up to Aeolian Islands. The TOMO-ETNA experiment was divided in two phases. The first phase started on June 15, 2014 and finalized on July 24, 2014, with the withdrawal of two removable seismic networks (a short period network and a broadband network composed by 80 and 20 stations respectively) deployed at Etna volcano and surrounding areas. During this first phase the oceanographic research vessel (R/V) “Sarmiento de Gamboa” and the hydro-oceanographic vessel (H/V) “Galatea” performed the offshore activities, which includes the deployment of ocean bottom seismometers (OBS), air-gun shooting for wide angle seismic refraction (WAS), multi-channel seismic (MCS) reflection surveys, magnetic surveys and ROV (remotely operated vehicle) dives. This phase finished with the recovery of the short period seismic network. In the second phase the broadband seismic network remained operative until October 28, 2014, and the R/V “Aegaeo” performed additional MCS surveys during November 19-27, 2014. Overall, the information deriving from TOMO-ETNA experiment could provide the answer to many uncertainties that have arisen while exploiting the large amount of data provided by the cutting-edge monitoring systems of Etna volcano and seismogenic area of eastern Sicily.466 139 - PublicationOpen AccessOn the seismicity recorded in the geothermal area of Mt. Amiata. (Oral Presentation - ESC2016-461)(2016-09-05)
; ; ; ; ; ; ; ;Braun, T.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Caciagli, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Bologna, Bologna, Italia ;Dahm, T.; GFZ, Potsdam, Germany ;Famiani, D.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Gattuso, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Krueger, F.; Inst. of Earth & Environmental Sciences, Potsdam University, Potsdam, Germany ;Ohrnberger, M.; Inst. of Earth & Environmental Sciences, Potsdam University, Potsdam, Germany; ; ; ; ; ; ; ;ESC; European Seismological CommissionMt. Amiata in Tuscany (Italy) is an extinct volcano whose last eruptive activity was dated about 200 ky ago. Being still characterized by a high geothermal gradient the area lends itself for geothermal exploitation. Beneath the Tuscan Geothermal Areas seismicity is exclusively observed in the upper crust and is confined in depth by the so called K-horizon (400°C isotherme). The structure above contains permeable layers of highly fractured, volcanic rocks saturated with hot water and steam. Geothermal exploitation from these layers started in the 1960’s. Shallow earthquakes have occurred close to the geothermal wells, and the question is raised whether these event are of natural origin or related to the exploitation of heat. To monitor the seismic activity inside the geothermal field, an 8 station seismic network and a 7 element small aperture seismic array were installed in 2015 in the vicinity of the geothermal power plants during a joint field experiment by the Istituto Nazionale di Geofisica e Vulcanologia, the University of Potsdam and the GFZ-German Research Center of Geoscience. Already during the first 24 hours of seismic recording the array and the neighboring network stations recorded a M0.5 seismic event in the vicinity of the geothermal field of Bagnore. Since then micro-earthquake activity was recorded regularly. One of the main challenges of the seismic array/network installation, deployed in direct proximity to the geothermal energy production, is to identify seismic events caused by human operations. As hypocenters are located close to the geothermal power plants, at a similar depth as the production level, it is very difficult - if not impossible - to discriminate between natural earthquakes and anthropogenic events. The main goal of the seismic array/network deployed in the framework of our project is to shed some additional light on this question. The monitoring capabilities of the recording system permit a lowering of the detection threshold for local seismic events, performing high-resolution hypocentral determination, especially in the vicinity of the industrial operations, and calculating focal mechanisms. Array techniques and relative location methods will be used for a precise hypocentral determination. Polarization and spectral analysis, will be applied to discriminate seismic recordings from Mt. Amiata that sometimes resemble rather volcano-seismic waveforms with long-period characteristics, than typical tectonic events.140 101 - PublicationOpen AccessOn the origin of micro-earthquakes in geothermal areas (OMEGA): first results from a seismic experiment at Mt. Amiata (Italy)(2021-06)
; ; ; ; ; ; ; ; ; ; ; In a joint project called OMEGA, between GFZ-Potsdam and the Istituto Nazionale di Geofisica e Vulcanologia (INGV), an experimental seismic monitoring system was installed in 2015 near the power plants of the geothermal area of Mt. Amiata (central Italy). The main objectives of this three-year experiment are: i) to monitor the seismic activity connected to any type of seismicity inside the geothermal field, ii) to verify if the low local seismicity rate near Mt. Amiata reported by the INGV bulletin is natural, or due to the sparse distribution of the INGV network, and iii) to discriminate natural from possibly induced seismicity. The eight-station network was extended by a sevenelement seismic array for the first four weeks. The aim of this paper is to present the first automatic hypocentre locations of the joint network/array analysis.396 139 - PublicationOpen AccessSeismicity observed in the Mt. Amiata Geothermal Area(2019-07)
; ; ; ; ; ; ; ; ; The crustal volume beneath Mt. Amiata is characterized by a high geothermal gradient, which makes the area particularly suitable for geothermal exploitation. Seismicity in the Tuscan Geothermal Areas is generally observed within the upper crust and is confined in depth by the K-horizon, a strong seismic reflector located in between 4-8 km b.s.l., often interpreted as the 400°C isotherme. The overlaying structure presents permeable layers of highly fractured volcanic rocks, saturated with hot water and steam. Geothermal exploitation from these layers started in the 1960's. Since then, shallow earthquakes have been occasionally observed close to the geothermal wells, and the question is whether these event are of natural origin or related to the exploitation of heat. To monitor the seismic activity inside the geothermal field of Mt. Amiata, we installed in 2015 a dedicated 8-station seismic network in the vicinity of the productive geothermal power plants for a 3-years recording period. The main challenges of our experiment are to automatically detect and locate the local microseismicity, trying to discriminate from natural seismicity those events caused by human operations. We use a waveform based detector to quickly scan the large dataset and automatically detect weak events in the target volume, providing also preliminary event locations, which are then refined in a second step, using standard and waveform based techniques. For hypocenters located close to the geothermal power plants, at a similar depth as the production level (3500 m b.s.l.), it remains very challenging to discriminate between natural and anthropogenic events.75 17 - PublicationOpen AccessAseismic transient driving the swarm-like seismic sequence in the Pollino range, Southern Italy(2015)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;ectonic earthquake swarms challenge our understanding of earthquake processes since it is difficult to link observations to the underlying physical mechanisms and to assess the hazard they pose. Transient forcing is thought to initiate and drive the spatio-temporal release of energy during swarms. The nature of the transient forcing may vary across sequences and range from aseismic creeping or transient slip to diffusion of pore pressure pulses to fluid redistribution and migration within the seismogenic crust. Distinguishing between such forcing mechanisms may be critical to reduce epistemic uncertainties in the assessment of hazard due to seismic swarms, because it can provide information on the frequency–magnitude distribution of the earthquakes (often deviating from the assumed Gutenberg–Richter relation) and on the expected source parameters influencing the ground motion (for example the stress drop). Here we study the ongoing Pollino range (Southern Italy) seismic swarm, a long-lasting seismic sequence with more than five thousand events recorded and located since October 2010. The two largest shocks (magnitude M w = 4.2 and M w = 5.1) are among the largest earthquakes ever recorded in an area which represents a seismic gap in the Italian historical earthquake catalogue. We investigate the geometrical, mechanical and statistical characteristics of the largest earthquakes and of the entire swarm. We calculate the focal mechanisms of the M l > 3 events in the sequence and the transfer of Coulomb stress on nearby known faults and analyse the statistics of the earthquake catalogue. We find that only 25 per cent of the earthquakes in the sequence can be explained as aftershocks, and the remaining 75 per cent may be attributed to a transient forcing. The b-values change in time throughout the sequence, with low b-values correlated with the period of highest rate of activity and with the occurrence of the largest shock. In the light of recent studies on the palaeoseismic and historical activity in the Pollino area, we identify two scenarios consistent with the observations and our analysis: This and past seismic swarms may have been ‘passive’ features, with small fault patches failing on largely locked faults, or may have been accompanied by an ‘active’, largely aseismic, release of a large portion of the accumulated tectonic strain. Those scenarios have very different implications for the seismic hazard of the area.290 23
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