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  • 1
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    Teleseismic depth estimation of the 2015 Gorkha-Nepal aftershocks (2016)
    Oxford University Press
    Details
    Publication Date: 2016-10-26
    Description: The depth of 61 aftershocks of the 2015 April 25 Gorkha, Nepal earthquake, that occurred within the first 20 d following the main shock, is constrained using time delays between teleseismic P phases and depth phases ( pP and sP ). The detection and identification of these phases are automatically processed using the cepstral method developed by Letort et al ., and are validated with computed radiation patterns from the most probable focal mechanisms. The events are found to be relatively shallow (13.1 ± 3.9 km). Because depth estimations could potentially be biased by the method, velocity model or selected data, we also evaluate the depth resolution of the events from local catalogues by extracting 138 events with assumed well-constrained depth estimations. Comparison between the teleseismic depths and the depths from local and regional catalogues helps decrease epistemic uncertainties, and shows that the seismicity is clustered in a narrow band between 10 and 15 km depth. Given the geometry and depth of the major tectonic structures, most aftershocks are probably located in the immediate vicinity of the Main Himalayan Thrust (MHT) shear zone. The mid-crustal ramp of the flat/ramp MHT system is not resolved indicating that its height is moderate (less than 5–10 km) in the trace of the sections that ruptured on April 25. However, the seismicity depth range widens and deepens through an adjacent section to the east, a region that failed on 2015 May 12 during an M w 7.3 earthquake. This deeper seismicity could reflect a step-down of the basal detachment of the MHT, a lateral structural variation which probably acted as a barrier to the dynamic rupture propagation.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    The 2012 May 20 and 29, Emilia earthquakes (Northern Italy) and the main aftershocks: S-wave attenuation, acceleration source functions and site effects (2013)
    Oxford University Press
    Details
    Publication Date: 2013-09-10
    Description: We used strong-motion records from the 2012 May 20 and 29 Emilia-Romagna earthquakes ( M w 6.1 and 5.9, respectively) and four aftershocks with magnitudes ranging between 4.9 and 5.5 to analyse the S -wave spectral amplitude decay with distance and estimate acceleration source functions and site effects. The data set consists of six earthquakes, 44 stations and 248 records with hypocentral distances in the range 10 〈 r  〈 100 km. We rotated the accelerograms to calculate transverse and radial components of the acceleration spectrum. We found non-parametric attenuation functions that describe the spectral amplitude decay of SH and SV waves with distance at 60 different frequencies between 0.1 and 40 Hz. These attenuation functions provide an estimate of the quality factor Q at each frequency analysed. Assuming that geometrical spreading is 1/ r for r  ≤ r x and 1/( r x r ) 0.5 for r  〉 r x with r x  = 60 km and normalizing at 15 km (the recording distance where the attenuation functions start to decay), we find that the average Q for SH waves can be approximated by Q SH  = 82 ± 1 f  1.2±0.02 and by Q SV  = 79 ± 1 f  1.24±0.03 for SV waves in the frequency range 0.10 ≤ f  ≤ 10.7 Hz. At higher frequencies, 11.8 ≤ f  ≤ 40 Hz, the frequency dependence of Q weakens and is approximated by Q SH  = 301 ± 1 f   0.36±0.04 and Q SV  = 384 ± 1 f  0.28±0.04 . These results indicate that the S -wave attenuation is radially isotropic at local distances in the epicentral area. Nevertheless, we used these attenuation parameters separately to correct the radial (with Q SV ) and transverse (with Q SH ) components of the acceleration spectra and to separate source and site effects using a non-parametric spectral inversion scheme. We found that the source function of the main event and the bigger aftershocks show enhanced low frequency radiation between 0.4 and 3.0 Hz. We converted the source functions into far-field source acceleration spectra and interpreted the resulting source spectra in terms of Brune's model. The stress drops obtained range between approximately 0.9 and 2.9 MPa. Although all the recording stations used are located in the Po Plain, the site functions obtained from the spectral inversion show important amplification variability between the sites. We compared these site functions with the average horizontal to vertical spectral ratios calculated for each station, and we found consistent results for most stations.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Moderate Earthquake Teleseismic Depth Estimations: New Methods and Use of the Comprehensive Nuclear-Test-Ban Treaty Organization Network Data (2014)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2014-04-04
    Description: We investigate the possibility to estimate the depths of earthquakes of moderate magnitude (3.5〉M〈5.5) from teleseismic data, using the Comprehensive Nuclear-Test-Ban Treaty Organization monitoring system. To face new problems that arise from the weakness of the magnitudes we want to deal with, two methods were developed. First, a depth-phase recognition method is applied, based on a new improved cepstral analysis. In addition, we develop a focal mechanism genetic algorithm inversion that focuses on depth resolution through an envelope-fitting procedure and adapted reproduction and mutation functions. Six events of interest were analyzed to validate these new proposed methods. First, the teleseismic depth of the recent Corsica earthquake (France, 7 July 2011) was estimated at about 26 km, whereas regional inversions do not provide any constraints. This proposed depth has a direct impact for regional tectonic studies, as this implies that this earthquake occurred in the continental crust. In contrast, the shallow Sardinia event (Italy, 3 January 2011) took place in the oceanic crust. The depth of the Virginia event (United States, 23 August 2011) was also constrained using these methods, and we show that it is possible to constrain the focal depth even for weak magnitude events, such as the m b  3.6 Ardèche earthquake (France, 3 August 2011). In the case of the Po Plain earthquake (Italy, 27 January 2012), this provides another example of the great interest in teleseismic analysis, where the detection of the Moho reflected phase ( pmP ) shows without doubt an upper-mantle event. Finally, the Murcia event (Spain, 11 May 2011) shows the limits of these methods for very shallow events (〈4–5 km). We show that these new depth determinations improve our ability to understand the geologic processes, as they provide complementary information on the source and are not affected by lateral crustal heterogeneities.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 4
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    A new, improved and fully automatic method for teleseismic depth estimation of moderate earthquakes (4.5 〈 M 〈 5.5): application to the Guerrero subduction zone (Mexico) (2015)
    Oxford University Press
    Details
    Publication Date: 2015-04-17
    Description: The depth of an earthquake is difficult to estimate because of the trade-off between depth and origin time estimations, and because it can be biased by lateral Earth heterogeneities. To face this challenge, we have developed a new, blind and fully automatic teleseismic depth analysis. The results of this new method do not depend on epistemic uncertainties due to depth-phase picking and identification. The method consists of a modification of the cepstral analysis from Letort et al. and Bonner et al. , which aims to detect surface reflected (pP, sP) waves in a signal at teleseismic distances (30°–90°) through the study of the spectral holes in the shape of the signal spectrum. The ability of our automatic method to improve depth estimations is shown by relocation of the recent moderate seismicity of the Guerrero subduction area (Mexico). We have therefore estimated the depth of 152 events using teleseismic data from the IRIS stations and arrays. One advantage of this method is that it can be applied for single stations (from IRIS) as well as for classical arrays. In the Guerrero area, our new cepstral analysis efficiently clusters event locations and provides an improved view of the geometry of the subduction. Moreover, we have also validated our method through relocation of the same events using the new International Seismological Centre (ISC)-locator algorithm, as well as comparing our cepstral depths with the available Harvard–Centroid Moment Tensor (CMT) solutions and the three available ground thrust (GT5) events (where lateral localization is assumed to be well constrained with uncertainty 〈5 km) for this area. These comparisons indicate an overestimation of focal depths in the ISC catalogue for deeper parts of the subduction, and they show a systematic bias between the estimated cepstral depths and the ISC-locator depths. Using information from the CMT catalogue relating to the predominant focal mechanism for this area, this bias can be explained as a misidentification of sP phases by pP phases, which shows the greater interest for the use of this new automatic cepstral analysis, as it is less sensitive to phase identification.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 5
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    High-resolution shallow seismic tomography of a hydrothermal area: application to the Solfatara, Pozzuoli (2012)
    Wiley-Blackwell
    Details
    Publication Date: 2017-04-04
    Description: The Solfatara is one of the major volcanoes of the Phlegrean Fields (Campi Flegrei) volcanic complex, and it is located in a densely populated area a few kilometres west of the city of Naples. It is an active resurgent caldera that has been characterized by a rich history of surface–ground deformation and soil diffuse degassing and fumarolic emissions, which are indications of the top of a hydrothermal plume. A seismic survey was completed in May 2009 for the characterization of the main subsurface features of the Solfatara. Using the complete data set, we have carried out surface wave inversion with high spatial resolution. A classical minimization of a least-squares objective function was first computed to retrieve the dispersion curves of the surface waves. Then, the fitting procedure between the data and a three-sedimentlayer forward model was carried out (to a depth of 7 m), using an improved version of the neighbourhood algorithm. The inversion results indicate a NE-SW fault, which is not visible at the surface. This was confirmed by a temperature survey conducted in 2010. A passive seismic experiment localized the ambient noise sources that correlate well with the areas of high CO2 flux and high soil temperatures. Finally, considering that the intrinsic attenuation is proportional to the frequency, a centroid analysis provides an overview of the attenuation of the seismic waves, which is closely linked to the petrophysical properties of the rock. These different approaches that merge complete active and passive seismic data with soil temperature and CO2 flux maps confirm the presence of the hydrothermal system plume. Some properties of the top of the plume are indicated and localized.
    Description: Published
    Description: 1725–1733
    Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
    Description: 2.4. TTC - Laboratori di geochimica dei fluidi
    Description: 4.5. Studi sul degassamento naturale e sui gas petroliferi
    Description: JCR Journal
    Description: restricted
    Keywords: Inverse theory ; Tomography ; Hydrothermal systems ; Europe ; 03. Hydrosphere::03.02. Hydrology::03.02.04. Measurements and monitoring ; 03. Hydrosphere::03.04. Chemical and biological::03.04.05. Gases ; 04. Solid Earth::04.04. Geology::04.04.11. Instruments and techniques ; 04. Solid Earth::04.04. Geology::04.04.12. Fluid Geochemistry
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 6
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    Adventive hydrothermal circulation on Stromboli volcano (Aeolian Islands, Italy) revealed by geophysical and geochemical approaches: Implications for general fluid flow models on volcanoes (2011)
    Elsevier
    Details
    Publication Date: 2022-06-13
    Description: On March 15th 2007 a paroxysmal explosion occurred at the Stromboli volcano. This event generated a large amount of products,mostly lithic blocks, someofwhich impacted the ground as far as down to 200 m a.s.l., about 1.5 kmfaraway fromthe active vents. Two days after the explosion, a newvapouremissionwas discovered on the north-eastern flank of the volcanic edifice, at 560 m a.s.l., just above the area called “Nel Cannestrà”. This new vapour emission was due to a block impact. In order to investigate the block impact area to understand the appearance of the vapour emission, we conducted on May 2008 a multidisciplinary study involving Electrical Resistivity Tomography (ERT), Ground Penetrating Radar (GPR), Self-Potential (SP), CO2 soil diffuse degassing and soil temperature surveys. This complementary data set revealed the presence of an anomalous conductive body, probably related to a shallow hydrothermal level, at about 10–15 m depth, more or less parallel to the topography. It is the first time that such a hydrothermal fluid flow,with a temperature close to thewater boiling point (76 °C) has been evidenced at Stromboli at this low elevation on the flank of the edifice. The ERT results suggest a possible link between (1) the main central hydrothermal system of Stromboli, located just above the plumbing system feeding the active vents, with a maximum of subsurface soil temperature close to 90 °C and limited by the NeoStromboli summit crater boundary and (2) the investigated area of Nel Cannestrà, at ~500 m a.s.l., a buried eruptive fissure active 9 ka ago. In parallel, SP and CO2 soil diffuse degassingmeasurements suggest in this sector at slightly lower elevation fromthe block impact crater a magmatic and hydrothermal fluid rising system along the N41° regional fault. A complementary ERT profile, on May 2009, carried out from the NeoStromboli crater boundary downto the block impact crater displayed a flank fluid flowapparently connected to a deeper system. The concept of shallow hydrothermal level have been compared to similar ERT results recently obtained onMount Etna and La Fossa cone of Vulcano. This information needs to be taken into account in general fluid flow models on volcanoes. In particular, peripheral thermal waters (as those bordering the northeastern coast of Stromboli) could be contaminated by hydrothermal and magmatic fluids coming from regional faults but also from the summit.
    Description: Published
    Description: 111-119
    Description: 1.2. TTC - Sorveglianza geochimica delle aree vulcaniche attive
    Description: 1.5. TTC - Sorveglianza dell'attività eruttiva dei vulcani
    Description: JCR Journal
    Description: reserved
    Keywords: Stromboli ; hydrothermal system ; adventive hydrothermal flow ; electrical resistivity tomography ; self-potential ; soil diffuse degassing ; temperature ; 2007 Stromboli eruptive crisis ; 04. Solid Earth::04.08. Volcanology::04.08.04. Thermodynamics ; 04. Solid Earth::04.08. Volcanology::04.08.07. Instruments and techniques
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 7
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    Hydrogeology of Stromboli volcano, Aeolian Islands (Italy) from the interpretation of resistivity tomograms, self-potential, soil temperature and soil CO2 concentration measurements (2011)
    Wiley Blackwell
    Details
    Publication Date: 2022-06-13
    Description: To gain a better insight of the hydrogeology and the location of the main tectonic faults of Stromboli volcano in Italy, we collected electrical resistivity measurements, soil CO2 concentrations, temperature and self-potential measurements along two profiles. These two profiles started at the village of Ginostra in the southwest part of the island. The first profile (4.8 km in length) ended up at the village of Scari in the north east part of the volcano and the second one (3.5 km in length) at Forgia Vecchia beach, in the eastern part of the island. These data were used to provide insights regarding the position of shallow aquifers and the extension of the hydrothermal system. This large-scale study is complemented by two high-resolution studies, one at the Pizzo area (near the active vents) and one at Rina Grande where flank collapse areas can be observed. The Pizzo corresponds to one of the main degassing structure of the hydrothermal system. The main degassing area is localized along a higher permeability area corresponding to the head of the gliding plane of the Rina Grande sector collapse. We found that the self-potential data reveal the position of an aquifer above the villages of Scari and San Vincenzo. We provide an estimate of the depth of this aquifer from these data. The lateral extension of the hydrothermal system (resistivity ∼15–60 ohm m) is broader than anticipated extending in the direction of the villages of Scari and San Vincenzo (in agreement with temperature data recorded in shallow wells). The lateral extension of the hydrothermal system reaches the lower third of the Rina Grande sector collapse area in the eastern part of the island. The hydrothermal body in this area is blocked by an old collapse boundary. This position of the hydrothermal body is consistent with low values of the magnetization (〈2.5 A m−1) from previously published work. The presence of the hydrothermal body below Rina Grande raises questions about the mechanical stability of this flank of the edifice.
    Description: INSU-CNRS, Laboratoire GeoSciences Reunion-IPGP, INGV, DOE (Energy Efficiency and Renewable Energy, Geothermal Technologies Program, awardDE-FG36–08GO018195)
    Description: Published
    Description: 1078-1094
    Description: 1.8. Osservazioni di geofisica ambientale
    Description: JCR Journal
    Description: restricted
    Keywords: Tomography. ; Electrical properties ; Hydrogeophysics ; Volcano monitoring ; 04. Solid Earth::04.08. Volcanology::04.08.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 8
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    Seismic catalogues of the 2018-2019 volcano-seismic crisis offshore Mayotte, Comoro Islands (2019)
    GFZ Data Services
    Details
    Publication Date: 2020-02-12
    Description: In May 2018 a volcano-seismic sequence accompanied the upward migration of a magmatic intrusion from Moho depth to the seafloor led to the drainage of the deep magmatic reservoir and to the birth of a submarine volcano offshore the island of Mayotte, Comoro Islands. This process of magma transport was accompanied by an intense seismic swarm and peculiar long-duration very long period signals. Between 1 January 2018 and 1 May 2019 we detected 407 sources of very long period signals and 6990 volcano-tectonic earthquakes. This report collects detection, location and source parameters catalogs for these two sets of earthquake sources. This data publication provides the catalogues of very long period (VLP) signals and volcano-tectonic (VT) earthquakes, as discussed in Cesca et al. (2019). Here, methods and data used to create the different catalogues are only briefly discussed; a more accurate description is given in Cesca et al. (2019), which furthermore discusses the different processes of dike migration, undersea eruption, deep reservoir drainage and overburden sagging which are responsible for the seismic activity.
    Language: English
    Type: info:eu-repo/semantics/workingPaper
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  • 9
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    The seismic sound of deep volcanic processes (2020)
    In:  Abstracts
    Details
    Publication Date: 2020-07-08
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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  • 10
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    Teleseismic and regional data analysis for estimating depth, mechanism and rupture process of the 3 April 2017 Mw 6.5 Botswana earthquake and its aftershock (5 April 2017, Mw 4.5) (2017)
    In:  Abstracts
    Details
    Publication Date: 2020-02-12
    Description: We constrain the source, depth, and rupture process of the Botswana earthquake of April 3, 2017, as well as its largest aftershock (5 April 2017, Mw 4.5). This earthquake is the largest recorded event (Mw 6.5) in the East African rift system since 1970, making one important case study to better understand source processes in stable continental regions. For the two events an automatic cepstrum analysis (Letort et al., 2015) is first applied on respectively 215 and 219 teleseismic records, in order to detect depth phase arrivals (pP, sP) in the P-coda. Coherent detections of depth phases for different azimuths allow us to estimate the hypocentral depths respectively at 28 and 23 km, suggesting that the events are located in the lower crust. A same cepstrum analysis is conducted on five other earthquakes with mb〉4 in this area (from 2002 to 2017), and confirms a deep crustal seismicity cluster (around 20-30 km). The source mechanisms are then characterized using a joint inversion method by fitting both regional long-period surface-waves and teleseismic high-frequency body-waves. Combining regional and teleseismic data (as well as systematic comparisons between theoretical and observed regional surface-waves dispersion curves prior to the inversion) allows us to decrease epistemic uncertainties due to lack of regional data and poor knowledge about the local velocity structure. Focal mechanisms are both constrained as normal faulting with a northwest trending, and hypocentral depths are confirmed at 28 and 24 km. Finally, in order to study the mainshock rupture process, we originally apply a kymograph analysis method (an image processing method, commonly used in the field of cell biology for identifying motions of molecular motors, e.g. Mangeol et al., 2016). Here, the kymograph allows us to better identify high-frequency teleseismic P-arrivals inside the P-coda by tracking both reflected depth phase and direct P-wave arrivals radiated from secondary sources during the faulting process. Secondary P-arrivals are thus identified with a significant azimuthal variation of their arrival times (until 4s), allowing the localization of the source that generated these secondary waves. This analysis shows that the mainshock is probably a mix of at least two events, the second being 20-30 km further northwest along the fault.
    Language: English
    Type: info:eu-repo/semantics/conferenceObject
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