Modelització analògica de l’efecte topogràfic: limitacions d’un estudi pilot i resultats

Màster Oficial en Recursos Minerals i Riscos Geològics, Universitat de Barcelona - Universitat Autònoma de Barcelona, Facultat de Geologia, Curs: 2015-2016, Tutora: María Ortuño CandelaThe topographic effect causes seismic wave amplification on the top of the mountains. This amplification reportedly produces more damage on the infrastructures of these parts of the reliefs and for this reason this effect has been widely studied using mainly numerical and experimental approximations. However, it has never been studied using analogue modelling. This study aims to start pointing out the main issues and results of carrying out this type of modelling. Two gelatin analogue models have been made in order to achieve a correct scaling of crust’s elastic parameters and acoustic waves have been used in order to reproduce seismic Pwave propagation. Two experiments have been performed in the models allowing the observation of attenuation by geometrical spreading and topographic amplification in most of the acoustic signals. The experimental setup also affects the signals for some configurations producing anomalies in the attenuation and amplification tendencies. Measurements of the acceleration in the model’s surface have also been performed, but these don’t show any enhanced motions at the top of the relief. The amplification depends on the frequency of the acoustic waves, thus the waves with wavelengths more similar to the mountain’s length are the most amplified (12% to 28% of maximum amplification), which is in agreement with most of the studies. Nevertheless, the limitations of this study must be taken into account when it comes to relating the processes in the model with the natural ones. The absence of shear waves in the experimental runs and scaling problems, especially for the acoustic wave frequencies, constitute the main limitations of this study. However, the fact that some processes identified in the model actually occur in the nature, ensure the validity of the method at least from a qualitative perspective. Several improvements must be applied in this method in order to obtain better results and get more useful clues for the natural hazard assessment in the future

Fault System-Based Probabilistic Seismic Hazard Assessment of a Moderate Seismicity Region: The Eastern Betics Shear Zone (SE Spain)

Including faults as seismogenic sources in probabilistic seismic hazard assessments (PSHA) has turned into a common practice as knowledge of active faults is improving. Moreover, the occurrence of earthquakes in multi-fault ruptures has evidenced the need to understand faults as interacting systems rather than independent sources. We present a PSHA for the Southeastern Spain obtained by including the faults of a moderate seismicity region, the Eastern Betics Shear Zone (EBSZ) in SE Spain, as the main seismogenic sources in two separate source models, one considering background seismicity. In contrast with previous studies in Spain, earthquake occurrence of the EBSZ system is modeled considering different hypotheses of multi-fault ruptures at the whole fault system scale and weighted in a logic tree. We compare the hazard levels with those from an area source PSHA and a previous fault-based approach. The results show a clear control of the EBSZ faults in the seismic hazard for all return periods, increasing drastically the hazard levels in the regions close to the fault traces and influencing up to 20 km farther with respect to the area source PSHA. The seismic hazard is dependent on the fault slip rates as peak ground accelerations and territorial extension of the fault influence appear higher around the Alhama de Murcia and Carboneras faults, while lower slip rate faults (Palomares Fault) show minor contribution to the hazard. For the return period of 475 years and near-fault locations, our models are more consistent with the ground motion values reached in the 2011 Mw 5.2 Lorca event than the building code or national seismic hazard map, which suggest that our fault system-based model performs more accurate estimations for this return period. Fault data, mainly slip rates, and its uncertainties have a clear impact on the seismic hazard and, for some faults, the lack of detailed paleoseismic studies can compromise the reliability of the hazard estimations. This, together with epistemic uncertainties concerning the background seismicity, are key discussion points in the present study, having an impact on further research and aiming to serve as a case example for other low-to-moderate seismicity regions worldwide

Modelling earthquake rupture rates in fault systems for seismic hazard assessment: the Eastern Betics Shear Zone.

Earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously. Consequently, modern fault-based probabilistic seismic hazard assessments (PSHA) need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties. We use SHERIFS, a recent approach of modelling annual rates of complex multi-fault ruptures, to obtain system-level magnitude-frequency distributions (MFDs) for the Eastern Betics Shear Zone (EBSZ, Spain) considering four fault rupture hypotheses. We then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies. The definition of the different rupture hypotheses was discussed within the frame of Fault2SHA ESC working group and critical fault input data is extracted from previous published studies. The four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics, with lengths varying from minimal fault sections to a rupture of nearly the whole system. The results suggest that multi-fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record. The method does not allow to completely discard any hypothesis, but it allows to weight the different models in a logic tree for seismic hazard assessment. The approach is revealed as a practical tool for obtaining fault-system MFDs and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed. This study aims to constitute a step forward in the consideration of complex multi-fault ruptures for future seismic hazard assessments in the region

Modelling earthquake rupture rates in fault systems for seismic hazard assessment: The Eastern Betics Shear Zone

Earthquake surface fault ruptures can show very complex geometries and involve different faults simultaneously. Consequently, modern fault-based probabilistic seismic hazard assessments (PSHA) need to account for such complexities in order to achieve more realistic modellings that treat fault systems as a whole and consider the occurrence of earthquake ruptures as aleatory uncertainties. We use SHERIFS, a recent approach of modelling annual rates of complex multi-fault ruptures, to obtain system-level magnitude-frequency distributions (MFDs) for the Eastern Betics Shear Zone (EBSZ, Spain) considering four fault rupture hypotheses. We then analyze the consistency of each scenario based on data from the earthquake catalogue and paleoseismic studies. The definition of the different rupture hypotheses was discussed within the frame of Fault2SHA ESC working group and critical fault input data is extracted from previous published studies. The four rupture hypotheses are defined as incremental scenarios based on fault geometry and kinematics, with lengths varying from minimal fault sections to a rupture of nearly the whole system. The results suggest that multi-fault ruptures involving lengths up to single to several whole faults are consistent with the annual rates from both the instrumental catalogue and paleoseismic record. The method does not allow to completely discard any hypothesis, but it allows to weight the different models in a logic tree for seismic hazard assessment. The approach is revealed as a practical tool for obtaining fault-system MFDs and as a useful tool for highlighting limitations and uncertainties in geological and paleoseismic data to be assessed. This study aims to constitute a step forward in the consideration of complex multi-fault ruptures for future seismic hazard assessments in the region

Geological characterization of the Llano del Águila fault in Campo de Dalías (Almería): possible seismogenic source of the 1804 earthquake

El 25 de agosto de 1804 un fuerte terremoto sacudió la región del Campo de Dalías (Almería) llegando a sentirse con una intensidad EMS de IX (Mw ~6,4). En este estudio se muestran evidencias que sugieren una relación entre este episodio sísmico y la falla de Llano del Águila. Se trata de una falla normal sub vertical de dirección NW-SE y paralela a la falla de Loma del Viento, situada a unos 3 km al sur. Para la caracterización de la falla de Llano del Águila se ha llevado a cabo una nueva interpretación geomorfológica de los depósitos aluviales cuaternarios que se ven atravesados por la traza de la falla a lo largo del área de estudio. Se han identificado cuatro generaciones de abanicos aluviales provenientes de la Sierra de Gádor, y dos secciones de falla a escala cartográfica (Cantera Este y Rambla de la Maleza). La interpretación geomorfológica se basa en el análisis de fotografías aéreas históricas. Debido a la intensa antropización de la zona, los modelos digitales del terreno actuales no son útiles. Para solventar esta limitación se procesó un modelo digital de elevaciones mediante fotogrametría usando las fotos aéreas del vuelo interministerial (1977). El análisis de escarpes de falla mediante perfiles topográficos medidos en el nuevo modelo de elevaciones proporciona un salto vertical de 6,3 ± 1,9 m para la sección de la Cantera Este y de 12,1 ± 1,9 m para la sección de la Rambla de la Maleza. Estas interpretaciones han sido verificadas en el campo donde además se adquirieron nuevos datos sobre la cinemática de la falla. Todo ello ha permitido estimar la tasa de deslizamiento neta de cada sección: 0,016 ± 0,002 y 0,10 ± 0,02 mm/año para la sección de la Cantera Este y 0,031 ± 0,002 – 0,19 ± 0,.02 mm/año para la Rambla de la Maleza, respectivamente para los últimos 126 y 781 ka (Pleistoceno Medio). A partir de la longitud total de la traza de la falla se puede estimar mediante relaciones empíricas una magnitud máxima potencial de 6,59 ± 0,19.On August 25, 1804, an earthquake with a Mw ~6.4 and a maximum intensity of IX, caused serious damage in several locations of Campo de Dalías region (Almeria) This study provides new evidence of the relationship between this episode and the Llano del Águila fault. A NW-SE subvertical fault, with a normal-dextral slip, that runs parallel to the Loma del Viento fault, located at about 3 km to the south. For the characterization of the Llano del Águila fault, a new geomorphologic interpretation of the Quaternary alluvial deposits and their relationship with the trace of the structure has been carried out. Four generations of alluvial fans draining the Sierra de Gádor and two fault sections have been identified at a cartographic scale (Cantera Est and Rambla de la Maleza). All the geomorphologic interpretation is made by the analysis of historical aerial photos. Due the high anthropization of the area, modern elevation models are not sufficiently useful. To overcome this limitation, a digital elevation model was obtained through photogrammetry with the aerial photos of the interministerial flight (1977). Fault scarp analysis from topographic profiles measured on the new elevation model provides a 6.3 ± 1.9 m vertical slip for the Cantera East section and a 12.1 ± 1.9 m vertical slip for the Rambla de la Maleza section. All these interpretations have been verified in the field and new data on the kinematics have been acquired to estimate the net slip rate of each section. A 0.016 ± 0.002 – 0.10 ± 0.02 mm/yr slip rate has been estimated for the Cantera Est section and a 0.031 ± 0.002 – 0.19 ± 0.02 mm/yr for the Rambla de la Maleza one, for the last 126 – 781 ky (middle Pleistocene). A maximum magnitude of 6.59 ± 0.19 is estimated through empirical relationships from the total length of the fault trace.Depto. de Geodinámica, Estratigrafía y PaleontologíaFac. de Ciencias GeológicasTRUEpu

First evidence of paleoearthquakes along the Carboneras Fault Zone (SE Iberian Peninsula): Los Trances site

Seismogenic faults that have not produced historical large earthquakes remain unnoticed and, thus, are dangerously left out from seismic hazard analyses. The seismogenic nature of the Carboneras Fault Zone, a left-lateral strikeslip fault in the Eastern Betic Shear Zone (southeastern Spain), has not been fully explored to date in spite of having a morphological expression equivalent to the Alhama de Murcia Fault, a seismogenic fault in the same tectonic system. This study provides the first paleoseismic evidence of the seismogenic nature of the Carboneras Fault Zone, based on the analysis of 3 trenches at Los Trances site, on the northwestern edge of the La Serrata Range. Cross cutting relationships and numerical dating, based on radiocarbon, thermoluminescence and U-series, reveal a minimum of 4 paleoearthquakes: Paleoearthquake1 (the oldest) and Paleoearthquake2 took place after 133ka, Paleoearthquake3 occurred between 83-73ka and Paleoearthquake4 happened after 42.5ka (probably after 30.8ka), resulting in a maximum possible average recurrence of 33ka. This value, based on a minimum amount of paleoearthquakes, is probably overestimated, as it does not scale well with published slip-rates derived from offset channels or GPS geodetical data. The characterization of this fault as seismogenic, implies that it should be considered in the seismic hazard analyses of the SE Iberian Peninsula

First evidence of paleoearthquakes along the Carboneras Fault Zone (SE Iberian Peninsula) : Los Trances site

Seismogenic faults that have not produced historical large earthquakes remain unnoticed and, thus, are dangerously left out from seismic hazard analyses. The seismogenic nature of the Carboneras Fault Zone, a left-lateral strikeslip fault in the Eastern Betic Shear Zone (southeastern Spain), has not been fully explored to date in spite of having a morphological expression equivalent to the Alhama de Murcia Fault, a seismogenic fault in the same tectonic system. This study provides the first paleoseismic evidence of the seismogenic nature of the CarbonerasFault Zone, based on the analysis of 3 trenches at Los Trances site, on the northwestern edge of the La Serrata Range. Cross cutting relationships and numerical dating, based on radiocarbon, thermoluminescence and U-series, reveal a minimum of 4 paleoearthquakes: Paleoearthquake1 (the oldest) and Paleoearthquake2 took place after 133ka, Paleoearthquake3 occurred between 83-73ka and Paleoearthquake4 happened after 42.5ka (probably after 30.8ka), resulting in a maximum possible average recurrence of 33ka. This value, based on a minimum amount of paleoearthquakes, is probably overestimated, as it does not scale well with published slip-rates derived from offset channels or GPS geodetical data. The characterization of this fault as seismogenic, implies that it should be considered in the seismic hazard analyses of the SE Iberian Peninsula

Actualizando la base de datos de fallas activas en el Cuaternario de Iberia a la versión 4 (QAFI v.4): cambios en la forma y en el fondo

Reunión Ibérica sobre Fallas Activas y Paleosismología (3ª. 2018. Alicante). - Texto en español con resumen y palabras claves en inglés y españolSe presentan a la comunidad científica los principales cambios que conllevará la actualización a la versión 4 de la base de datos QAFI. El nombre de la base de datos se modifica pasando a ser “Quaternary-Active Faults…”. Se sustituye el término “segment” por “section”. Se omiten las estimaciones automáticas de magnitud máxima y recurrencia. Se amplía la extensión de la base de datos al margen norteafricano. Se creará una nueva base de datos con aquellas fallas cuya actividad en el cuaternario ha quedado descartada en base a evidencias geológicas publicadas. También se exponen diversas mejoras en relación con la visualización de las fallas en la aplicación web, y se solicita colaboración para mejorar la parte de asociaciones con la sismicidad. Finalmente, se prospectan futuras aplicaciones de la QAFI: la clasificación de falla activa del Eurocódigo-8 y una base de datos de fuentes sismogénicas tipo falla. = We present to the scientific community the main changes that would affect the QAFI database in the process of updating to version 4. The name of the database changes, being now: “Quaternary-Active Faults…”. The term fault “segment” is substituted by “section”. Version 4 will not include automatic estimations of maximum magnitude nor recurrence interval. The spatial coverage of the database is extended to the north of Africa. A new complementary database with faults with proof of no Quaternary activity will be created. We also describe foreseen improvements in the visualization of the fault traces in the web application, and we ask for collaboration to improve the fault relationships with seismicity. Finally, we prospect about future uses of the QAFI database: active fault definition in Eurocode-8 and a seismogenic fault-sources database.Instituto Geológico y Minero de España, EspañaDepartamento de Geología y Geoquímica, Universidad Autónoma de Madrid, EspañaDepartamento de Geodinámica, Universidad Complutense de Madrid, EspañaGrupo RISKNAT, Departament de Dinàmica de la Terra i de l’Oceà, Universitat de Barcelona, EspañaGeosciences Research Division, Scripps Institution of Oceanography, University of California, Estados UnidosInstituto de Ciencias del Mar, EspañaDepartamento de Ciencias de la Tierra, Universidad de Zaragoza, EspañaDepartamento de Biología y Geología, Física y Química Inorgánica, Universidad Rey Juan Carlos, EspañaDepartamento de Ciencias de la Tierra y el Medio Ambiente, Universidad de Alicante, EspañaDepartamento de Geodinámica, Universidad de Granada, EspañaGNS Science, Nueva ZelandaDepartamento de Geologia, Universidade de Lisboa, PortugalUnidade de Geologia, Hidrogeologia e Geologia Costeira, Laboratório Nacional de Energia e Geologia, Portuga

Paleoseismic transect across the Alhama de Murcia Fault and implications of a fault-based seismic hazard assessment for the Eastern Betics

[eng] The Eastern Betics Shear Zone (EBSZ) is one of the most active fault systems in SE Spain. It has caused damaging earthquakes in historical times, including the 2011 Lorca earthquake, which evidenced that the regional hazard estimations needed revision. In this context, the available fault data is usually too local and, for some faults, reliable and representative parameters are still lacking. Such heterogeneity has usually hampered seismic hazard models based on geological data. This thesis aims to be a comprehensive study allowing to: i) complete the paleoseismic record and parameters of one of the most active faults in the EBSZ, the Alhama de Murcia Fault (AMF), and ii) include fault data in a probabilistic seismic hazard assessment (PSHA). The study is thus divided in two main parts. (A) a local study presenting a structural characterization and integral paleoseismic study in four sites across a complete transect of the AMF. (B) a regional fault-based PSHA of the EBSZ with the main faults as inputs to define complex fault rupture scenarios using the SHERIFS code. A) The structural characterization has been carried out in the two central segments of the AMF, Lorca-Totana and Totana-Alhama. In the former, five fault branches are identified with partitioning of the deformation. From N to S: N1-AMF (lateral), N2a and b-AMF (dip slip), S-AMF (lateral) and F-AMF (dip slip). The Totana-Alhama segment shows more diffuse deformation, where the most prominent structure is the Amarguillo Fault (AF), a N-S transtensional ramification. The paleoseismic survey has been carried out in eight paleoseismic trenches across four fault branches of the Lorca-Totana segment. We obtained one of the completest paleoearthquake records in Iberia with seventeen events for the last ~100 ka in S-AMF. The recurrence varies from 5.7±1.7 to 3.1±1.4 kyr for the last 73-18 ka in S-AMF and F-AMF. The time compatibility of the last five events in these branches suggests that they could rupture synchronously. The total net slip rate of the segment since 18 ka is 1.55 +0.14/-0.18 mm/yr, considerably higher than previous estimations. The slip rate evolution shows fluctuations over time (super-cycles), some of which coincide with patterns identified in nearby faults, inferring possible activity synchronicities. B) The fault-based PSHA suggests that multi-fault ruptures involving lengths of single to several whole faults of the EBSZ are feasible, contrary to the rupture of the whole system (~400 km). The hazard shows a clear control of the EBSZ faults, increasing the accelerations close to their traces with respect to area source PSHAs. The seismic hazard is dependent on the slip rates, as lower slip rate faults (Palomares fault; PF, or NE end of AMF) have negligible contribution. Our results for the 475-year return period are also more consistent with the accelerations reached in the 2011 Lorca event than the building code or national hazard map. For some faults, the lack of detailed paleoseismic studies can compromise the reliability of the hazard. This is a key discussion in the present study, marking the need for better constrained and reliable slip rates in the EBSZ. The integral paleoseismic study of the AMF has revealed to be a crucial step towards a more representative characterization of its paleoseismic parameters (slip rates, recurrence), and thus, of the EBSZ. Acquiring refined and reviewed paleoseismic data is key to improve seismic hazard evaluations. Hereby, further research should focus on poorly researched faults and on performing integrative studies in other EBSZ faults. The comprehensive approach followed, from paleoseismic data collection to PSHA, contributes to perform more critical interpretations of the seismic hazard, and aims to serve as a case example for other low-to-moderate seismicity regions.[cat] La Zona de Cisalla de les Bètiques Orientals (ZCBO) és un dels sistemes de falles més actius del SE d’Espanya. Malgrat això, en aquesta zona les estimacions de perillositat sísmica rarament utilitzen dades de falles degut a que aquestes son sovint massa locals i poc acotades. En aquest context, la present tesi pretén ser un estudi exhaustiu que A) completi el registre paleosísmic i la representativitat dels paràmetres sísmics de la falla d’Alhama de Murcia (FAM), una de les més actives de la ZCBO, i B) porti a terme una avaluació probabilista de la perillositat sísmica (PSHA) de la ZCBO basada en dades geològiques de falles. A) La caracterització de la FAM ha permès obtenir dades paleosísmiques en quatre de cinc branques de falla que conformen un transsecte gairebé complet del segment central (Lorca- Totana). S’ha identificat un dels registres paleosísmics més complets de la Península Ibèrica amb disset paleoterratrèmols pels últims 100 ka. La recurrència d’aquests varia de 5.7±1.7 a 3.1±1.4 ka pels últims 73-18 ka i la seva compatibilitat temporal entre branques suggereix que les ruptures conjuntes son factibles. La velocitat de desplaçament neta total és de 1.55 +0.14/-0.18 mm/any pels últims 18 ka, malgrat aquesta ha presentat variacions cícliques almenys des del Pleistocè Superior també observades en d’altres falles de la ZCBO. B) El PSHA suggereix que les ruptures multi-falla que impliquen longituds d’una o varies falles completes son factibles a la ZCBO. En base a això, l’anàlisi portat a terme indica que les falles de la ZCBO dominen la perillositat a les àrees properes, incrementant considerablement els valors d’acceleració respecte a estudis previs que no consideren falles. Tot i això, les acceleracions son altament dependents de la velocitat de desplaçament de les falles, i en alguns casos, la manca d’estudis paleosísmics detallats por comprometre la fiabilitat dels models de perillositat. Cal doncs, focalitzar la futura recerca en aquestes falles menys estudiades. L’enfocament integral que s’ha seguit aqui, des de la recopilació de dades paleosísmiques fins al PSHA, contribueix a realitzar interpretacions més crítiques de la perillositat sísmica i pretén servir d’exemple per d’altres regions de sismicitat baixa-moderada

Reassessing seismicity and seismic hazard in offshore areas: The case of the Alboran Sea (western Mediterranean)

8th International Colloquium on Historical Earthquakes, Palaeo- Macroseismology and Seismotectonics, Past earthquakes and advances in seismology for informed risk decision-making, 17-20 September 2023, Lixouri, Kefalonia Island, Greece.-- 5 pages, 2 figuresThe potential occurrence of large earthquakes and their capacity to trigger devastating tsunamis poses a significant geohazard that raises crucial societal concerns. Such events possess the ability to disrupt submarine structures, impact coastal regions, and have far-reaching consequences on global economies, thereby posing a risk to local populations (Bilham, 2010). Recent history serves as a stark reminder of the catastrophic nature of these occurrences, exemplified by the colossal Sumatra earthquake and tsunami in 2004 within the Indian Ocean (Mw8.7) or the Tohoku-Oki earthquake and tsunami that struck the northwest region of Japan in 2011 (Mw9.0–9.1). Nonetheless, it is vital to recognize that earthquakes with moderate to large magnitudes (Mw>5.5) in regions characterized by low to moderate tectonic deformation, and featuring long intervals between events, can also exert a substantial impact. The Alboran Sea situated in the Western Mediterranean is one such area of concern. Consequently, in recent decades, there has been a notable expansion in the interest to identify the active faults and to enhance our understanding about their seismic activity and probable aftermaths. [...]This research was supported by the grant STRENGTH (PID2019-104668RB-I00) funded by MCIN/AEI/10.13039/501100011033. This project acknowledges the „Severo Ochoa Centre of Excellence‟ accreditation (CEX2019-000928-S) and the grant UNrIDDLE (2018-T1/AMB-11039) “Atracción de Talento Investigador” call 2018 funded by Comunidad de MadridPeer reviewe