Sismicidad inducida por la manipulación del subsuelo

Los terremotos inducidos por la inyección de gas realizadas en la plataforma continental del golfo de Valencia, proyecto El Castor, en septiembre-octubre 2013, han creado cierta alarma social en las poblaciones costeras próximas. Esto, que es novedoso en España, ha ocurrido con cierta frecuencia en otras áreas pobladas en distintos países y por causa de varios tipos de manipulación del subsuelo. Este artículo intenta ilustrar el problema de la sismicidad inducida por la actividad del hombre. Según se describe en el texto, la sismicidad inducida tiene unos límites, máximo terremoto inducido, que pueden ser investigados en cada caso. De esa manera, se pueden establecer protocolos de actuación que impidan que dichas actividades causen daños materiales en las localidades próximas. Earthquakes induced by the gas injection in the continental platform, of the golf of Valencia, Castor Project, last September-October 2013, has motivated some social alarm within the nearby coastal population. That, which is new for Spain, has happened with some frequency at some other populated areas, in different countries and due to various types of subsoil manipulations. This article tries to illustrate the induced seismicity problem due to human activities. As it is described in the text, the induced seismicity has some limits (maximum induced earthquake) that can be investigated for each particular case. By this way, protocols of operation can be established to avoid that theses activities might cause material damage to local population.

Modelización de las fallas activas para la estimación de la Peligrosidad Sísmica. Aplicación Metodológica en Haití

The development of this Master's Thesis is aimed at modeling active for estimating seismic hazard in Haití failures. It has been used zoned probabilistic method, both classical and hybrid, considering the incorporation of active faults as independent units in the calculation of seismic hazard. In this case, the rate of seismic moment is divided between the failures and the area seismogenetic same region. Failures included in this study are the Septentrional, Matheux and Enriquillo fault. We compared the results obtained by both methods to determine the importance of considering the faults in the calculation. In the first instance, updating the seismic catalog, homogenization, completeness analysis and purification was necessary to obtain a catalog ready to proceed to the estimation of the hazard. With the seismogenic zoning defined in previous studies and the updated seismic catalog, they are obtained relations Gutenberg-Richter recurrence of seismicity, superficial and deep in each area. Selected attenuation models were those used in (Benito et al., 2011), as the tectonic area of study is very similar to that of Central America. Its implementation has been through the development of a logical in which each branch is multiplied by an index based on the relevance of each combination of models. Results are presented as seismic hazard maps for return periods of 475, 975 and 2475 years, and spectral acceleration (SA) in structural periods: 0.1 - 0.2 - 0.5 - 1.0 and 2.0 seconds, and the difference accelerations between maps obtained by the classical method and the hybrid method. Maps realize the importance of including faults as separate items in the calculation of the hazard. The morphology of the zoned maps presented higher values in the area where the superficial and deep zone overlap. In the results it can determine that the minimum values in the zoned approach they outweigh the hybrid method, especially in areas where there are no faults. Higher values correspond to those obtained in fault zones by the hybrid method understanding that the contribution of the faults in this method is very important with high values. The maximum value of PGA obtained is close to Septentrional in 963gal, near to 460 gal in Matheux, and the Enriquillo fault line value reaches 760gal PGA in the Eastern segment and Western 730gal in the segment. This compares with that obtained in the zoned approach in this area where the value of PGA obtained was 240gal. These values are compared with those obtained by Frankel et al., (2011) with those have much similarity in values and morphology, in contrast to those presented by Benito et al., (2012) and the Standard Seismic Dominican Republic

Desarrollos metodológicos y aplicaciones hacia el cálculo de la peligrosidad sísmica en el ecuador continental y estudio de riesgo sísmico en la ciudad de Quito

En el presente trabajo de tesis se desarrolla, en primer lugar, un estudio de peligrosidad sísmica en Ecuador continental, siguiendo una metodología probabilista zonificada. El estudio se plantea a escala regional y presenta como principales aportaciones: 1) la elaboración de un Estado del Arte sobre Tectónica y Geología de Ecuador, concluyendo con la identificación de las principales fuentes sísmicas; 2) La confección de un Catálogo Sísmico de proyecto, recopilando información de distintas agencias, que ha sido homogeneizado a magnitud momento, Mw, depurado de réplicas y premonitores y corregido por la falta de completitud para la estimación de tasas en diferentes rangos de magnitud; 3) la propuesta de un nueva zonificación sísmica, definiendo las zonas sismogenéticas en tres regímenes tectónicos: cortical, subducción interfase y subducción in-slab; 4) la caracterización sísmica de cada zona estimando los parámetros de recurrencia y Magnitud Máxima (Mmax), considerando para este último parámetro una distribución de valores posibles en función de la sismicidad y tectónica, tras un exhaustivo análisis de los datos existentes; 5) la generación de mapas de peligrosidad sísmica de Ecuador continental en términos de aceleración pico (PGA) y espectral SA (T= 1s) , en ambos casos para periodos de retorno (PR) de 475, 975 y 2475 años; 6) La estimación de espectros de peligrosidad uniforme (UHS) y sismos de control mediante desagregación de la peligrosidad, para PR de 475 y 2475 años en 4 capitales de provincia: Quito, Esmeraldas, Guayaquil y Loja. Una segunda parte del trabajo se destina al cálculo del riesgo sísmico en el Barrio Mariscal Sucre de Quito, lo que supone incidir ya a una escala municipal. Como principales contribuciones de este trabajo se destacan: 1) definición del escenario sísmico que más contribuye a la peligrosidad en Quito, que actuará como input de cálculo del riesgo; 2) caracterización de la acción sísmica asociada a ese escenario, incluyendo resultados de microzonación y efecto local en la zona de estudio; 3) Elaboración de una Base de Datos partiendo de información catastral e identificación de las tipologías dominantes; 4) Asignación de clases de vulnerabilidad y obtención de porcentajes de daño esperado en cada clase ante la acción sísmica definida previamente, con la consiguiente representación de mapas de vulnerabilidad y daño; 5) mapas de indicadores globales del riesgo sísmico; 6) Base de datos georreferenciada con toda la información generada en el estudio. Cabe destacar que el trabajo, aunque no formula nuevos métodos, si plantea una metodología integral de cálculo del riesgo sísmico, incorporando avances en cada fase abordada, desde la estimación de la peligrosidad o la definición de escenarios sísmicos con carácter hibrido (probabilista-determinista), hasta la asignación de vulnerabilidades y estimación de escenarios de daño. Esta tesis trata de presentar contribuciones hacia el mejor conocimiento de la peligrosidad sísmica en Ecuador y el riesgo sísmico en Quito, siendo uno de los primeros estudios de tesis que se desarrolla sobre estos temas en el país. El trabajo puede servir de ejemplo y punto de partida para estudios futuros; además de ser replicable en otras ciudades y municipios de Ecuador. -------------------- ABSTRACT: ------------------ This thesis first develops a study of seismic hazard in mainland Ecuador, following a zoned, probabilistic methodology. The study considers a regional scale and presents as main contributions: 1) The development of a State of Art on the Tectonics and Geology of Ecuador, concluding with the identification of the main seismic sources; 2) The creation of a Seismic Catalog project, collecting information from different agencies, which has been homogenized to Moment magnitude, Mw, purged from aftershocks and premonitories and corrected for the lack of completeness to estimate rates in different maggnitude ranges; 3) The proposal of a new seismic zoning, defining the seismogenic zones in three tectonic regimes: cortical, subduction interface and subduction in-slab; 4) The seismic characterization of each zone, estimating the parameters of recurrence and Maximum Magnitude (Mmax), considering the latter as a distribution of possible values, depending on the seismicity and tectonics, and after a thorough analysis of the existing data; 5) Seismic hazard maps of continental Ecuador in terms of peak ground acceleration (PGA) and spectral SA(T=1), and return periods (PR) of 475, 975 and 2475 years; 6) Uniform hazard spectra (UHS) and control earthquakes obtained by hazard disaggregation, for PR 475 and 2475 years in four provincial capitals: Quito, Esmeraldas, Guayaquil and Loja. The second section focuses on the calculation of seismic risk in the Quito Mariscal Sucre parish, which is already supposed to be influencing at a municipal level. The main contributions here are the: 1) Definition of the seismic scenario that contributes most to the hazard in Quito, which acts as an input in the risk calculation; 2) Characterization of the seismic action associated with that scenario, including results of micro-zoning and local effect in the study area; 3) Development of a database, based on cadastral data and identification of key typologies; 4) Allocation of vulnerability classes and obtaining percentages of damage expected in each class faced with the seismic action previously defined, with the consequent representation of maps of vulnerability and damage; 5) Global maps of seismic risk indicators; 6) Geo-referenced database with all the information generated in the study. It should be noted that although new methods are not prescribed, this study does set a comprehensive methodology for the calculation of seismic risk, incorporating advances in each phase approached, from the hazard estimation, or definition of seismic scenarios applying a hybrid (deterministic-probabilistic) method, to the allocation of vulnerabilities and estimation of damage scenarios. This thesis aims to present contributions leading to a better understanding of seismic hazard in Ecuador and seismic risk in Quito, and is one of the first studies in the country to develop such themes. This study can serve as an example and starting point for future studies, which could replicate this methodology in other cities and municipalities.

Scale dependence in earthquake phenomena and its relevance to earthquake prediction.

The recent discovery of a low-velocity, low-Q zone with a width of 50-200 m reaching to the top of the ductile part of the crust, by observations on seismic guided waves trapped in the fault zone of the Landers earthquake of 1992, and its identification with the shear zone inferred from the distribution of tension cracks observed on the surface support the existence of a characteristic scale length of the order of 100 m affecting various earthquake phenomena in southern California, as evidenced earlier by the kink in the magnitude-frequency relation at about M3, the constant corner frequency for earthquakes with M below about 3, and the sourcecontrolled fmax of 5-10 Hz for major earthquakes. The temporal correlation between coda Q-1 and the fractional rate of occurrence of earthquakes in the magnitude range 3-3.5, the geographical similarity of coda Q-1 and seismic velocity at a depth of 20 km, and the simultaneous change of coda Q-1 and conductivity at the lower crust support the hypotheses that coda Q-1 may represent the activity of creep fracture in the ductile part of the lithosphere occurring over cracks with a characteristic size of the order of 100 m. The existence of such a characteristic scale length cannot be consistent with the overall self-similarity of earthquakes unless we postulate a discrete hierarchy of such characteristic scale lengths. The discrete hierarchy of characteristic scale lengths is consistent with recently observed logarithmic periodicity in precursory seismicity.

Intermediate-term earthquake prediction.

An earthquake of magnitude M and linear source dimension L(M) is preceded within a few years by certain patterns of seismicity in the magnitude range down to about (M - 3) in an area of linear dimension about 5L-10L. Prediction algorithms based on such patterns may allow one to predict approximately 80% of strong earthquakes with alarms occupying altogether 20-30% of the time-space considered. An area of alarm can be narrowed down to 2L-3L when observations include lower magnitudes, down to about (M - 4). In spite of their limited accuracy, such predictions open a possibility to prevent considerable damage. The following findings may provide for further development of prediction methods: (i) long-range correlations in fault system dynamics and accordingly large size of the areas over which different observed fields could be averaged and analyzed jointly, (ii) specific symptoms of an approaching strong earthquake, (iii) the partial similarity of these symptoms worldwide, (iv) the fact that some of them are not Earth specific: we probably encountered in seismicity the symptoms of instability common for a wide class of nonlinear systems.

Implications of fault constitutive properties for earthquake prediction.

The rate- and state-dependent constitutive formulation for fault slip characterizes an exceptional variety of materials over a wide range of sliding conditions. This formulation provides a unified representation of diverse sliding phenomena including slip weakening over a characteristic sliding distance Dc, apparent fracture energy at a rupture front, time-dependent healing after rapid slip, and various other transient and slip rate effects. Laboratory observations and theoretical models both indicate that earthquake nucleation is accompanied by long intervals of accelerating slip. Strains from the nucleation process on buried faults generally could not be detected if laboratory values of Dc apply to faults in nature. However, scaling of Dc is presently an open question and the possibility exists that measurable premonitory creep may precede some earthquakes. Earthquake activity is modeled as a sequence of earthquake nucleation events. In this model, earthquake clustering arises from sensitivity of nucleation times to the stress changes induced by prior earthquakes. The model gives the characteristic Omori aftershock decay law and assigns physical interpretation to aftershock parameters. The seismicity formulation predicts large changes of earthquake probabilities result from stress changes. Two mechanisms for foreshocks are proposed that describe observed frequency of occurrence of foreshock-mainshock pairs by time and magnitude. With the first mechanism, foreshocks represent a manifestation of earthquake clustering in which the stress change at the time of the foreshock increases the probability of earthquakes at all magnitudes including the eventual mainshock. With the second model, accelerating fault slip on the mainshock nucleation zone triggers foreshocks.

Dynamic friction and the origin of the complexity of earthquake sources.

We study a simple antiplane fault of finite length embedded in a homogeneous isotropic elastic solid to understand the origin of seismic source heterogeneity in the presence of nonlinear rate- and state-dependent friction. All the mechanical properties of the medium and friction are assumed homogeneous. Friction includes a characteristic length that is longer than the grid size so that our models have a well-defined continuum limit. Starting from a heterogeneous initial stress distribution, we apply a slowly increasing uniform stress load far from the fault and we simulate the seismicity for a few 1000 events. The style of seismicity produced by this model is determined by a control parameter associated with the degree of rate dependence of friction. For classical friction models with rate-independent friction, no complexity appears and seismicity is perfectly periodic. For weakly rate-dependent friction, large ruptures are still periodic, but small seismicity becomes increasingly nonstationary. When friction is highly rate-dependent, seismicity becomes nonperiodic and ruptures of all sizes occur inside the fault. Highly rate-dependent friction destabilizes the healing process producing premature healing of slip and partial stress drop. Partial stress drop produces large variations in the state of stress that in turn produce earthquakes of different sizes. Similar results have been found by other authors using the Burridge and Knopoff model. We conjecture that all models in which static stress drop is only a fraction of the dynamic stress drop produce stress heterogeneity.

Geometric incompatibility in a fault system.

Interdependence between geometry of a fault system, its kinematics, and seismicity is investigated. Quantitative measure is introduced for inconsistency between a fixed configuration of faults and the slip rates on each fault. This measure, named geometric incompatibility (G), depicts summarily the instability near the fault junctions: their divergence or convergence ("unlocking" or "locking up") and accumulation of stress and deformations. Accordingly, the changes in G are connected with dynamics of seismicity. Apart from geometric incompatibility, we consider deviation K from well-known Saint Venant condition of kinematic compatibility. This deviation depicts summarily unaccounted stress and strain accumulation in the region and/or internal inconsistencies in a reconstruction of block- and fault system (its geometry and movements). The estimates of G and K provide a useful tool for bringing together the data on different types of movement in a fault system. An analog of Stokes formula is found that allows determination of the total values of G and K in a region from the data on its boundary. The phenomenon of geometric incompatibility implies that nucleation of strong earthquakes is to large extent controlled by processes near fault junctions. The junctions that have been locked up may act as transient asperities, and unlocked junctions may act as transient weakest links. Tentative estimates of K and G are made for each end of the Big Bend of the San Andreas fault system in Southern California. Recent strong earthquakes Landers (1992, M = 7.3) and Northridge (1994, M = 6.7) both reduced K but had opposite impact on G: Landers unlocked the area, whereas Northridge locked it up again.

Scaling in geology: landforms and earthquakes.

Landforms and earthquakes appear to be extremely complex; yet, there is order in the complexity. Both satisfy fractal statistics in a variety of ways. A basic question is whether the fractal behavior is due to scale invariance or is the signature of a broadly applicable class of physical processes. Both landscape evolution and regional seismicity appear to be examples of self-organized critical phenomena. A variety of statistical models have been proposed to model landforms, including diffusion-limited aggregation, self-avoiding percolation, and cellular automata. Many authors have studied the behavior of multiple slider-block models, both in terms of the rupture of a fault to generate an earthquake and in terms of the interactions between faults associated with regional seismicity. The slider-block models exhibit a remarkably rich spectrum of behavior; two slider blocks can exhibit low-order chaotic behavior. Large numbers of slider blocks clearly exhibit self-organized critical behavior.

Geological risk assessment of Amtoudi Agadir in southern Morocco: a key case for sustainable cultural heritage

Medieval fortified granaries known as “agadirs” are very common in southern Morocco, being catalogued as world cultural heritage by United Nations. These Berber buildings (made of stones and tree trunks) usually located on rocky promontories, constitute historical testimonials related to the origin of Morocco, and, as tourist attractions, have a positive impact on the local economy. The sustainability of these ancient monuments requires geological-risk evaluations of the massif stability under the agadir with the proposal of stabilization measures, and an architectonic analysis with appropriate maintenance of the structural elements. An interdisciplinary study including climate, seismicity, hydrology, geology, geomorphology, geotechnical surveys of the massif, and diagnosis of the degradation of structural elements have been performed on the Amtoudi Agadir, selected as a case study. The main findings from this study are that the prevalent rocks used for construction (coming from the underlying substratum) are good-quality arkosic sandstones; the SW cliffs under the agadir are unstable under water saturation; some masonry walls are too thin and lack interlocking stones and mortar; and failures in the beams (due to flexure, fracture, and exhaustion in the resistance due to insect attacks or plant roots) are common. The basic risk assessment of ancient buildings of cultural heritage and their geologic substratum are needed especially in undeveloped areas with limited capacity to implement durable conservation policies. Therefore, recommendations have been provided to ensure the stability and maintenance of this important archaeological site.

AMS 14C and calibrated ages of sediment core GeoB10426-1

Newly acquired bathymetric and seismic reflection data have revealed mass-transport deposits (MTDs) on the northeastern Cretan margin in the active Hellenic subduction zone. These include a stack of two submarine landslides within the Malia Basin with a total volume of approximately 4.6 km**3 covering an area of about 135 km**2. These two MTDs have different geometry, internal deformations and transport structures. The older and stratigraphic lower MTD is interpreted as a debrite that fills a large part of the Malia Basin, while the second, younger MTD, with an age of at least 12.6 cal. ka B.P., indicate a thick, lens-shaped, partially translational landslide. This MTD comprises multiple slide masses with internal structure varying from highly deformed to nearly undeformed. The reconstructed source area of the older MTD is located in the westernmost Malia Basin. The source area of the younger MTD is identified in multiple headwalls at the slope-basin-transition in 450 m water depth. Numerous faults with an orientation almost parallel to the southwest-northeast-trending basin axis occur along the northern and southern boundaries of the Malia Basin and have caused a partial steepening of the slope-basin-transition. The possible triggers for slope failure and mass-wasting include (i) seismicity and (ii) movement of the uplifting island of Crete from neotectonics of the Hellenic subduction zone, and (iii) slip of clay-mineral-rich or ash-bearing layers during fluid involvement.

Evaluation of Potential Post-Glacial Faults Using Bare-Earth LiDAR Topographic Data Near Ridley Island, British Columbia

I present results of my evaluation to identify topographic lineaments that are potentially related to post-glacial faulting using bare-earth LiDAR topographic data near Ridley Island, British Columbia. The purpose of this evaluation has been to review bare-earth LiDAR data for evidence of post-glacial faulting in the area surrounding Ridley Island and provide a map of the potential faults to review and possibly field check. My work consisted of an extensive literature review to understand the tectonic, geologic, glacial and sea level history of the area and analysis of bare-earth LiDAR data for Ridley Island and the surrounding region. Ridley Island and the surrounding north coast of British Columbia have a long and complex tectonic and geologic history. The north coast of British Columbia consists of a series of accreted terranes and some post-accretionary deposits. The accreted terranes were attached to the North American continent during subduction of the Pacific Plate between approximately 200 Ma and 10 Ma. The terrane and post-accretionary deposits are metamorphosed sedimentary, volcanic and intrusive rocks. The rocks have experienced significant deformation and been intruded by plutonic bodies. Approximately 10 Ma subduction of the Pacific Plate beneath the North America Plate ceased along the central and north coast of British Columbia and the Queen Charlotte Fault Zone was formed. The Queen Charlotte Fault Zone is a transform-type fault that separates the Pacific Plate from the North America Plate. Within the past 1 million years, the area has experienced multiple glacial/interglacial cycles. The most recent glacial cycle occurred approximately 23,000 to 13,500 years ago. Few Quaternary deposits have been mapped in the area. The vast majority of seismicity around the northwest coast of British Columbia occurs along the Queen Charlotte Fault Zone. Numerous faults have been mapped in the area, but there is currently no evidence to suggest these faults are active (i.e. have evidence for post-glacial surface displacement or deformation). No earthquakes have been recorded within 50 km of Ridley Island. Several small earthquakes (less than magnitude 6) have been recorded within 100 km of the island. These earthquakes have not been correlated to active faults. GPS data suggests there is ongoing strain in the vicinity of Ridley Island. The strain has the potential to be released along faults, but the calculated strain may be a result of erroneous data or accommodated aseismically. Currently, the greatest known seismic hazard to Ridley Island is the Queen Charlotte Fault Zone. LiDAR data for Ridley Island, Digby Island, Lelu Island and portions of Kaien Island, Smith Island and the British Columbia mainland were reviewed and analyzed for evidence of postglacial faulting. The data showed a strong fabric across the landscape with a northwest-southeast trend that appears to mirror the observed foliation in the area. A total of 80 potential post-glacial faults were identified. Three lineaments are categorized as high, forty-one lineaments are categorized as medium and thirty-six lineaments are categorized as low. The identified features should be examined in the field to further assess potential activity. My analysis did not include areas outside of the LiDAR coverage; however faulting may be present there. LiDAR data analysis is only useful for detecting faults with surficial expressions. Faulting without obvious surficial expressions may be present in the study area.

Statistical tests of load-unload response ratio signals by lattice solid model: Implication to tidal triggering and earthquake prediction

Statistical tests of Load-Unload Response Ratio (LURR) signals are carried in order to verify statistical robustness of the previous studies using the Lattice Solid Model (MORA et al., 2002b). In each case 24 groups of samples with the same macroscopic parameters (tidal perturbation amplitude A, period T and tectonic loading rate k) but different particle arrangements are employed. Results of uni-axial compression experiments show that before the normalized time of catastrophic failure, the ensemble average LURR value rises significantly, in agreement with the observations of high LURR prior to the large earthquakes. In shearing tests, two parameters are found to control the correlation between earthquake occurrence and tidal stress. One is, A/(kT) controlling the phase shift between the peak seismicity rate and the peak amplitude of the perturbation stress. With an increase of this parameter, the phase shift is found to decrease. Another parameter, AT/k, controls the height of the probability density function (Pdf) of modeled seismicity. As this parameter increases, the Pdf becomes sharper and narrower, indicating a strong triggering. Statistical studies of LURR signals in shearing tests also suggest that except in strong triggering cases, where LURR cannot be calculated due to poor data in unloading cycles, the larger events are more likely to occur in higher LURR periods than the smaller ones, supporting the LURR hypothesis.