An analysis of 542 moment tensor focal mechanisms across the Iberian Peninsula was conducted to infer active tectonic deformation and stress regimes. This study employed a suite of complementary methodologies, including focal mechanism classification (FMC) of the rupture type, composed focal mechanisms based on the average seismic moment tensor, rotation angle between tensor estimates, right dihedra composed focal mechanisms, slip model analysis to determine the strain conditions, and classical stress inversion methodology. Based on the slip model results and considering the tectonic constraints of Cenozoic deformation in Iberia, the study region was subdivided into several tectonically coherent zones, where the different methods were applied independently to ensure robust regional interpretations. The results indicate that thrust faulting stress regimes are active in the Gorringe-Horseshoe (GH) area and the easternmost Tell Atlas. In the south, most of the zones are transpressive, as is the southwestern corner of Iberia, south of Lisbon. The exception is the Granada Basin (GB), which exhibits a nearly radial normal faulting stress regime. Normal faulting stresses are dominant in the Pyrenees and the Mediterranean rim, north of the Betic Mountains. In the central part of the Pyrenees, the maximum horizontal extension is oriented perpendicular to the mountain range, indicating that local stresses related to post-orogenic collapse or isostatic rebound dominate over regional ones. The maximum horizontal compression along the Eurasia-Africa plate limit is consistently oriented around N154 degrees E, except in some parts of the Betics that are probably influenced by a remnant effect of the Alboran Slab. In the Central Ranges and offshore Atlantic, the maximum horizontal compression is slightly rotated anticlockwise to N140 degrees E.

Abstract. From the analysis of 542 moment tensor focal mechanisms in Iberia, active tectonic deformations and stresses were inferred by implementing different and complementary methodologies: FMC classification of the rupture type; composed focal mechanism based on the average seismic moment tensor; rotation angle between tensors estimates; Right Dihedra composed focal mechanisms; Slip Model analysis to determine the strain conditions and classical stress inversion methodology. By using the Slip Model results and considering the tectonic constraints of the Cenozoic deformation in Iberia, the study region was subdivided into a series of zones where the different methods were individually applied. The results indicate that thrust faulting stress regimes are active in the Gorringe-Horseshoe area and the easternmost Tell Atlas. In the south, most of the zones are transpressive, as well as in the southwestern corner of Iberia, south of Lisbon. The exception is the Granada Basin, which displays an almost radial normal faulting stress regime. Normal faulting stresses are dominant in the Pyrenees and in the Mediterranean rim, north of the Betics. In the central part of the Pyrenees, we find a maximum horizontal extension perpendicular to the range, indicating that local stresses related to post-orogenic collapse or isostatic rebound dominate over regional ones. The maximum horizontal compression along the Eurasia-Africa plate limit is very homogeneously close to N154° E, except in some parts of the Betics that are probably influenced by a remanent effect of the Alboran Slab. In the Central Ranges and offshore Atlantic, the maximum horizontal compression is slightly rotated anticlockwise to N140° E.

Monchique is a prominent 902 m topographic high in SW Iberia, which stands out in the general flat landscape of southern Portugal. It lies to the north of the Africa-Eurasia plate boundary, which locally accommodates a slow oblique convergence ( 5 mm/yr). Monchique comprises alkaline magmatic rocks of Late Cretaceous age, intruded in a post-rift context. It hosts the most active seismic cluster in mainland Portugal and important hydrothermal activity. This work investigates the relationship between the alkaline intrusion, local seismicity and hydrothermalism. We present magnetic and gravity modeling based on new drone-borne magnetic data and ground gravity data. New magnetic mapping of Monchique shows a 15 km long dipolar anomaly with 7-8 km wavelength and 2000 nT amplitude. 3D magnetic inversion models the main Monchique intrusion as a high-susceptibility body, 15 km long and 6 km wide, located below the Monchique mountain and extending 5-7 km depth. 2D forward modeling and geological interpretation further support the existence of ENE-WSW oriented dike-like gabbroic bodies that may extend deeper, around which syenite units have later emplaced. We relocate the seismicity using NonLinLoc and a 3D regional tomographic model, and find that earthquakes align along four main lineations that radiate outwards from the intrusion. We also find that most earthquakes cluster between 8 and 18 km depth, below the magmatic intrusion. The b-value at the core of the seismic cluster is higher than at the surrounding region, possibly related to the local hydrothermalism. We present five new focal mechanisms that are compatible with the regional stress field, supporting a regional tectonic control. The emplacement of the Monchique alkaline intrusion left fractures in the lithosphere that currently act as preferred pathways for fluids. In the context of the present-day stress field, the enhanced fracturing and fluid circulation facilitate the localization of small-magnitude earthquakes.

Portugal, located in the southwest region of the Eurasian plate, has been affected by several destructive earthquakes throughout its history, the most well-known being the 1755 Great Lisbon earthquake. The seismicity of the territory, both in the mainland and in the Azores and Madeira islands, has prompted the continuous development of seismic monitoring, from the first known macroseismic inquiry, following the 1755 Great Lisbon earthquake, to the current state-of-the-art seismic network. Once scattered in separate efforts, at present, most seismic stations in Portugal relay its data to a common data center, at Instituto Português do Mar e da Atmosfera, where data are automatically processed for the downstream generation of both manually revised and automatically generated products and services. In this article, we summarize the evolution of the permanent seismic network, its current status, the products and services that are publicly available, a recent effort of rapid deployment of a dense network following a mainshock, and state-of-the-art ocean-bottom seismometer developments.

Iberia, located at the southwestern end of Europe, displays a complex pattern of seismic activity, with most known active faults slipping at low rates (< 1 mm/yr). However, the seismic activity is remarkable, with numerous earthquakes in the historical record proving destructive. The earthquake cluster in mainland Portugal that has a highest rate of seismic activity is very localized (small spatial extent), extends vertically from 5 to 20 km depth and lays on the Monchique late Cretaceous magmatic intrusion, in SW Portugal. This magmatic intrusion forms strong rheological contrast between the intruded magmatic rocks and surrounding Paleozoic rocks. Furthermore, it is the locus of abundant natural water springs. Several pertinent questions remain to be answered concerning earthquakes in Monchique: Are earthquakes in Monchique simply a response to tectonic stresses (given the proximity of Monchique to the EU-AF plate boundary), with the localization of brittle failure in the region facilitated by the rheological contrast between the Cretaceous intrusion and surrounding Paleozoic rocks? Do fluids play a role in facilitating slip in existing fractures? Or, conversely, is the circulation of fluids facilitated by the faulting that results from the rheological contrasts? Are there hazardous faults in Monchique? In this presentation, we re-analyze in detail the seismic data recorded by the regional permanent seismic network, in order to better understand the relationship between seismic activity and igneous intrusion. In particular, we re-locate earthquakes using NonLinLoc and PRISM3D, a 3D velocity model for the region. At a subsequent step, we re-locate earthquakes using HypoDD. We also perform a clustering analysis based on waveform similarity and compute focal mechanisms for the region. The results show that earthquakes align along two main directions, E-W and NNE-SSW, coinciding with surface features of the magmatic intrusion. Focal mechanisms indicate dominantly strike-slip faulting, with the possible fault planes coinciding with the favored directions of earthquake lineations. We investigate the spatio-temporal evolution of seismicity and address possible forcing mechanisms, including tidal forcing. The author would like to acknowledge the financial support FCT through project UIDB/50019/2020 – IDL and PTDC/GEO-FIQ/2590/2014 - SPIDER.

We compute coda-wave attenuation for the Western Iberia and, for the first time, for its SW offshore area. Western Iberia has been well covered by the growing number of permanent seismic stations during the last decade, providing a large dataset. Using this dataset along with offshore seismograms of local earthquakes recorded by temporary ocean-bottom seismometers, and adopting the single backscattering model, we estimate the coda Q at five frequency bands centred at 1, 2, 4, 8 and 16 Hz. Then, we calculate coda Q average values on a geographical grid, 0.5 degrees x0.5 degrees resolution, using regionalization and inversion techniques. We observe that the coda Q at 10 Hz varies from 1057 in the NW continental Iberia to 448 in the South-western offshore area, where there is no crust. Additionally, we find that the area with similar low coda Q value is located in the SE Iberia (Betic range). Therefore, our coda Q grid results exhibit higher values over the older and stable crust and lower values in the offshore margin or active tectonic regions. This suggests that the regional variation in coda Q is less sensitive to crustal thickness than to the tectonic activity in each area and crustal age. Furthermore, we discuss our coda Q results through a comparison with those from other regions in the world, where coda Q has been determined with the same parameters. Our work highlights the importance of coda Q studies as an effective tool to investigate detailed regional differences in seismic attenuation that may be relevant for earthquake seismic hazard and risk assessment studies.

Source models of mid-oceanic earthquakes are often based only on far-field, teleseismic data. The uncertainties of all source parameters are rarely quantified, which restricts our understanding of how these events slip and how oceanic lithosphere is formed. Here, we perform moment tensor inversions for five M-w 4.6-5.9 earthquakes that occurred in the Azores archipelago near theMid-Atlantic Ridge in 2013-2016, taking advantage of the recently expanded seismic network in the region. We assess moment tensor uncertainties due to data and Earthmodel variability as well as the robustness of teleseismic versus local data inversions. We find that for the events studied: (i) existing 1-D Earth models of the region based on receiver function data lead to a slightly improved data fit of local data compared to a widely used regional model based on active seismic surveys; and (ii) using different 1-D Earth models in the local data inversions leads to a variability in the retrieved source parameters of 15 degrees-30 degrees in fault strike, 5 degrees-20 degrees in dip, and 20 degrees-60 degrees in rake, depending on the earthquake's magnitude and location. We study in detail the M-w 5.9 2013 April 30 Povoacao basin earthquake using 1-D and 3-D waveform modelling, for which reported values of strike, dip, and rake in earthquake catalogues differ by 60 degrees, 35 degrees, and 80 degrees. We find that our moment tensor solutions show a lower variability than in the catalogues and exhibit a persistent non-double-couple component of similar to 40-60 per cent, which is not due to a volumetric change. We suggest that it is potentially due to geometrically complex faulting in the Povoacao basin, notably curved faults. We find that the retrieved moment tensor solutions depend strongly on the earthquake's location. If an accurate location is used, joint inversions of local and teleseismic data can help to stabilize moment tensor solutions of oceanic earthquakes and reduce parameter trade-offs, compared to inversions of local data alone.

Strong ground motion prediction based on finite-fault simulation requires the identification of the fault (strike, dip, length and width), source kinematics parameters (stress drop, rupture velocity and slip distribution), regional crustal properties (geometrical spreading, anelastic structure, and upper crustal amplification and attenuation parameters) and the determination of amplification effects due to the local site geology. The general purpose of this study is to understand source and attenuation properties in the Azores, by the determination of stress drop, quality factor and kappa, through records obtained by the Portuguese digital seismic and accelerometer network. Source Spectra were obtained, for each record, after correcting observed spectra from geometrical spreading and anelastic attenuation effect: quality factor was estimated based on coda decay in the time domain and the kappa parameter was estimated by fitting the high-frequency decay of the acceleration spectrum with a straight line in a log-linear scale. Mean stress drop value was obtained considering that ω-squared model for the source spectra prevails. Parameters kappa, k, and quality factor, Q, have been estimated to be k = (0.075 ± 0.02)s and \(Q(f) = (76 \pm 11)f^{0.69 \pm 0.09}\), respectively. A mean value of stress drop was estimated to be around 90–130 bars for the earthquakes in analysis. The knowledge of source and path parameters, in association with other ground motion parameters, allows improving ground-motion estimates for the Azores and, consequently, will lead to more accurate seismic hazard assessment for the Azores and better characterization of seismic scenarios.

The matching between linear trends of hypocentres and fault planes indicated by focal mechanisms (FMs) is frequently used to infer the location and geometry of active faults. This practice works well in regions of fast lithospheric deformation, where earthquake patterns are clear and major structures accommodate the bulk of deformation, but typically fails in regions of slow and distributed deformation. We present a new joint FM and hypocentre cluster algorithm that is able to detect systematically the consistency between hypocentre lineations and FMs, even in regions of distributed deformation. We apply the method to the Azores–western Mediterranean region, with particular emphasis on western Iberia. The analysis relies on a compilation of hypocentres and FMs taken from regional and global earthquake catalogues, academic theses and technical reports, complemented by new FMs for western Iberia. The joint clustering algorithm images both well-known and new seismo-tectonic features. The Azores triple junction is characterised by FMs with vertical pressure (P) axes, in good agreement with the divergent setting, and the Iberian domain is characterised by NW–SE oriented P axes, indicating a response of the lithosphere to the ongoing oblique convergence between Nubia and Eurasia. Several earthquakes remain unclustered in the western Mediterranean domain, which may indicate a response to local stresses. The major regions of consistent faulting that we identify are the mid-Atlantic ridge, the Terceira rift, the Trans-Alboran shear zone and the north coast of Algeria. In addition, other smaller earthquake clusters present a good match between epicentre lineations and FM fault planes. These clusters may signal single active faults or wide zones of distributed but consistent faulting. Mainland Portugal is dominated by strike–slip earthquakes with fault planes coincident with the predominant NNE–SSW and WNW–ESE oriented earthquake lineations. Clusters offshore SW Iberia are predominantly strike–slip or reverse, confirming previous suggestions of slip partitioning.

Published results of coda Q show a large variation in values. These variations are often claimed to be related to different tectonics, whereas they might just be related to using different assumptions in the processing, leading to different input parameters for the analysis. In this study, the effect of using different processing parameters is investigated and significant differences, particularly at low frequencies, are observed. We find a new set of optimal parameters, which we recommend using in future studies. Using a short lapse time of 30 s and optimal parameters, data from both similar and very different tectonic regions are used to calculate coda Q using the same program and the same parameters. The regions considered are eastern Anatolia, the Azores, Jan Mayen, northwestern and central Argentina, the Shanxi rift system in China, and southwestern Norway. We obtain the following relations: eastern Anatolia (Q = 88 f(0.66)), Azores (Q = 86 f(0.70)), Jan Mayen (Q = 90 f(0.72)), northwestern and central Argentina (Q = 89 f(0.94)), Shanxi rift system (Q = 99 f(0.89)), and southwestern Norway (Q = 124 f(0.91)).The results show that coda Q is very similar for regions of similar tectonics and significantly different for regions with varying tectonics. Using alternative, more common parameters gives different Q, but the regional differences remain, so which parameters to use to get correct coda Q values is still uncertain. However, coda Q can clearly distinguish different tectonic areas provided identical processing parameters are used, even if they are not optimal.

Introduction Conclusions References

This paper is a contribution to a better understanding of the tsunamigenic potential of large submarine earthquakes. Here, we analyze the tsunamigenic potential of large earthquakes which have occurred worldwide with magnitudes around M-w = 7.0 and greater during a period of 1 year, from June 2013 to June 2014. The analysis involves earthquake model evaluation, tsunami numerical modeling, and sensors' records analysis in order to confirm the generation of a tsunami (or lack thereof) following the occurrence of an earthquake. We also investigate and discuss the sensitivity of tsunami generation to the earthquake parameters recognized to control tsunami occurrence, including the earthquake location, magnitude, focal mechanism and fault rupture depth. Through this analysis, we attempt to understand why some earthquakes trigger tsunamis and others do not, and how the earthquake source parameters are related to the potential of tsunami generation. We further discuss the performance of tsunami warning systems in detecting tsunamis and disseminating the alerts. A total of 23 events, with magnitudes ranging from M-w = 6.7 to M-w = 8.1, have been analyzed. This study shows that about 39% of the analyzed earthquakes caused tsunamis that were recorded by different sensors with wave amplitudes varying from a few centimeters to about 2 m. Tsunami numerical modeling shows good agreement between simulated waveforms and recorded waveforms, for some events. On the other hand, simulations of tsunami generation predict that some of the events, considered as non-tsunamigenic, caused small tsunamis. We find that most generated tsunamis were caused by shallow earthquakes (depth < 30 km) and thrust faults that took place on/near the subduction zones. The results of this study can help the development of modified and improved versions of tsunami decision matrixes for various oceanic domains.

We present an analysis and characterization of the regional seismicity recorded by a temporary broadband seismic network deployed in the Cape Verde archipelago between November 2007 and September 2008.The detection of earthquakes was based on spectrograms, allowing the discrimination from low-frequency volcanic signals, resulting in 358 events of which 265 were located, the magnitudes usually being smaller than 3. For the location, a new 1-D P-velocity model was derived for the region showing a crust consistent with an oceanic crustal structure. The seismicity is located mostly offshore the westernmost and geologically youngest areas of the archipelago, near the islands of Santo Antao and Sao Vicente in the NW and Brava and Fogo in the SW. The SW cluster has a lower occurrence rate and corresponds to seismicity concentrated mainly along an alignment between Brava and the Cadamosto seamount presenting normal faulting mechanisms. The existence of the NW cluster, located offshore SW of Santo Antao, was so far unknown and concentrates around a recently recognized submarine cone field; this cluster presents focal depths extending from the crust to the upper mantle and suggests volcanic unrest No evident temporal behaviour could be perceived, although the events tend to occur in bursts of activity lasting a few days. In this recording period, no significant activity was detected at Fogo volcano, the most active volcanic edifice in Cape Verde.The seismicity characteristics point mainly to a volcanic origin. The correlation of the recorded seismicity with active volcanic structures agrees with the tendency for a westward migration of volcanic activity in the archipelago as indicated by the geologic record. (C) 2014 Elsevier B.V. All rights reserved.