Cristina Crespo-Martín

The Beni-Ilmane (BI) seismic sequence, situated in the north-central region of Algeria, began on May 14, 2010 with a mainshock of ML 5.4 followed by 2 other shocks on the 16^th and 23^rd May of ML 5.3 for both. Using the complete dataset from the 14^th of May to the 31^st of August and the Epidemic Type Aftershock sequences (ETAS) model to separate background from aftershocks contribution gave a first insight around the uncertainty that surrounds the causes and mechanisms of the seismicity. According to our findings, two phases have been determined, the first one, beginning from May 14^th, shows low b and p values with most of the seismicity being self-triggered. In contrast, the second phase, beginning from May 23^rd, demonstrates an increase of both the b and p values with a high number of background events. In the second phase, the background contribution involves 53% of all seismic activity after May 23^rd’s peak which is high compared to typical mainshock-aftershocks sequences. A possible explanation is that the mainshocks began or assisted aseismic processes in addition to causing aftershocks. A sealed source at depth may have been broken by the third mainshock, opening the way for the following incursion of highly pressurized fluids (hydrocarbures) confirmed by a high Vp/Vs ratios.

Characterisation of surface deformation at stratovolcanoes is essential for a better understanding of the processes that can compromise edifice structural stability and potential for flank collapse. Spreading produced by the presence of a hydrothermal system or intrusion of a viscous magma body can produce similar deformation signatures, and both processes have implications for flank instability. In this work, we perform analogue models and consider examples from real volcanoes (Damavand, Ubinas, Semeru and Casita) so as to characterise and recognise surface deformation patterns produced by spreading due to the presence of a hydrothermal system and in response to magma intrusion. The experiments show that there are differences in the resulting surface deformation associated with each process. Magma intrusion results in a sharp transition between areas of subsidence and uplift, and is associated with faults with oblique strikes in the upper part of the edifice. Instead, asymmetric flank spreading is associated with hydrothermal system and results in flank bulging close to the base of the edifice. Although laboratory analogue models show different deformation responses that could be diagnostic of the associated processes, application in the field is difficult as often these diagnostic features are not preserved during evolution. However, basal bulging represents a potential diagnostic for the identification of asymmetric volcano flank spreading associated with hydrothermal activity, and the potential for instability. Remote sensing techniques can allow identification of such surface deformation features, providing a useful tool for hazard assessment and design of monitoring strategies at potentially unstable volcanoes.

SUMMARY: The Sarria-Triacastela-Becerreá seismicity is located in an intraplate region considered seismically stable, but in 1995 started an unprecedented earthquake activity in the area. Since then an anomalous long-term continuous seismicity remains until now in the same location. Despite the long-term seismicity and the large magnitudes for the region standards (5.1 Mw), the origin and mechanisms of this seismicity remains unclear. The isolation of background from the main shock–aftershock contribution by means of the application of the Epidemic Type Aftershock Sequences (ETAS) model, the spatial depiction and the resemblance to near seismic regions allow us to identify several seismicity behaviours. From 1995 to 1998/99, the seismicity in this location mainly consists of intensive and deeper earthquake sequences linked to the 1995 and 1997 main shocks. Our results suggest that the main shocks triggered aftershocks and additionally initiated or facilitated aseismic processes. A likely scenario is that the main shocks broke a sealed source at depth allowing a subsequent intrusion of high-pressurized fluids from depth. The resemblance of this period with Zamora seismic characteristics proposes that Triacastela seismicity was also initiated by tectonic activity. From 1998/99 to 2018, a change in seismicity is observed, the background contribution took control and swarm-type activity is predominant. While the earthquake rate decreases, the relative background contribution goes up. Actually, after 2013 the clusters almost disappear and background contribution achieves 55 per cent of the total activity. The spatial migration to the southeast and the upward trend to shallower depth support fluid migration as possible driving mechanism responsible for the transient seismicity in this period. The swarm-type activity in Triacastela in later periods and the resemblance of b-values with Ponte Caldelas and Ventaniella seismicity suggest that the seismicity in Triacastela is related to fluid migration and the reactivation of fractured areas. We propose that the mechanism of this anomalous and long-term seismicity in Triacastela is the mix of different mechanisms, starting with the tectonic seismicity, generated during the 1995 and 1997 seismic sequences, which initiated a fluid upward migration through fractured crustal fault patches, observed after 1998–1999, and responsible for the seismicity during the following 20 yr.

La base de los estudios de peligrosidad sísmica es disponer de catálogos sísmicos lo más completos posible. En regiones intraplaca, caracterizadas por largos periodos de recurrencia entre terremotos, es esencial obtener un catálogo sísmico con un extenso intervalo temporal. El noroeste de la Península Ibérica (Galicia, Asturias, Cantabria, Castilla y Leon y norte de Portugal) se ha considerado tradicionalmente una zona intraplaca sísmicamente estable, ya que se encuentra alejada de los bordes de placa sísmicamente activos. Sin embargo, crisis sísmicas como las Sarria-Triacastela-Becerreá (Lugo) (1995 y 1997; con eventos de magnitud hasta 5.3) ponen de manisfiesto en esta región un potencial sismogénico de magnitud moderada. En este trabajo se realiza una revisión y ampliación del catálogo sísmico de esta región previo a 1755 (periodo peor documentado del catálogo sísmico), con el objetivo de caracterizar esta sismicidad intraplaca y mejorar la completitud con un nuevo catálogo para el NO peninsular y así poder reconocer si la sismicidad reciente y posterior a 1755 es anómala o si, por el contrario, es característica de esta región. Para ello, se tomó como base el catálogo sísmico oficial español del Instituto Geográfico Nacional (IGN), que es la agencia responsable de la red y alerta sísmica. Se han buscado las fuentes documentales primarias de los terremotos descritos en dicho catálogo, para el margen espacial y temporal considerado, y se han revisado parámetros como la localización, fecha y zonas geográficas afectadas por el terremoto. En este catálogo a los 13 terremotos incluidos por el IGN, se han añadido 10 del catálogo de Ces Fernández (2015) y 4 nuevos, ampliando así el catálogo hasta los 27 terremotos. Tras revisar las fuentes se han modificado: 6 parámetros de fechas, 8 localizaciones y 10 zonas geográficas donde se sintió el sismo. Posteriormente ha sido posible calcular la intensidad de 18 de estos terremotos con dos escalas macrosísmicas (EMS-98 y ESI-07). En este nuevo catálogo las intensidades calculadas se encuentran entre IV y X en EMS-98 y entre VII y XI en la ESI- 07, es decir incluyen eventos de mayor o igual intensidad que los ocurridos después de 1755, que no han sobrepasado la intensidad VI o VII como ocurrió en la crisis de Sarria-Triacastela-Becerreá (Lugo) de 1997. Además, la distribución geográfica de los eventos anteriores a 1755 es similar a los ocurridos posteriormente. Todo ello indicaría que la sismicidad actual y posterior a 1755 no sería una anomalía de la tendencia de la región. En el futuro, para seguir ampliando el catálogo sísmico de esta región, sería necesario integrar estudios multidisciplinares de sismicidad histórica e instrumental y paleosismología, prestando atención a fallas que puedan ser activas, bajo el régimen tectónico actual.