TY - JOUR A1 - Maghsoudi, Samira A1 - Hainzl, Sebastian A1 - Cesca, Simone A1 - Dahm, Torsten A1 - Kaiser, Diethelm T1 - Identification and characterization of growing large-scale en-echelon fractures in a salt mine JF - Geophysical journal international N2 - The spatiotemporal seismicity of acoustic emission (AE) events recorded in the Morsleben salt mine is investigated. Almost a year after backfilling of the cavities from 2003, microevents are distributed with distinctive stripe shapes above cavities at different depth levels. The physical forces driving the creation of these stripes are still unknown. This study aims to find the active stripes and track fracture developments over time by combining two different temporal and spatial clustering techniques into a single methodological approach. Anomalous seismicity parameters values like sharp b-value changes for two active stripes are good indicators to explain possible stress accumulation at the stripe tips. We identify the formation of two new seismicity stripes and show that the AE activities in active clusters are migrated mostly unidirectional to eastward and upward. This indicates that the growth of underlying macrofractures is controlled by the gradient of extensional stress. Studying size distribution characteristic in terms of frequency-magnitude distribution and b-value in active phase and phase with constant seismicity rate show that deviations from the Gutenberg-Richter power law can be explained by the inclusion of different activity phases: (1) the inactive period before the formation of macrofractures, which is characterized by a deficit of larger events (higher b-values) and (2) the period of fracture growth characterized by the occurrence of larger events (smaller b-values). KW - Earthquake source observations KW - Statistical seismology Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggt443 SN - 0956-540X SN - 1365-246X VL - 196 IS - 2 SP - 1092 EP - 1105 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Cesca, Simone A1 - Grigoli, Francesco A1 - Heimann, Sebastian A1 - Gonzalez, Alvaro A1 - Buforn, Elisa A1 - Maghsoudi, Samira A1 - Blanch, Estefania A1 - Dahm, Torsten T1 - The 2013 September-October seismic sequence offshore Spain: a case of seismicity triggered by gas injection? JF - Geophysical journal international N2 - A spatially localized seismic sequence originated few tens of kilometres offshore the Mediterranean coast of Spain, close to the Ebro river delta, starting on 2013 September 5, and lasting at least until 2013 October. The sequence culminated in a maximal moment magnitude M-w 4.3 earthquake, on 2013 October 1. The most relevant seismogenic feature in the area is the Fosa de Amposta fault system, which includes different strands mapped at different distances to the coast, with a general NE-SW orientation, roughly parallel to the coastline. However, no significant known historical seismicity has involved this fault system in the past. The epicentral region is also located near the offshore platform of the Castor project, where gas is conducted through a pipeline from mainland and where it was recently injected in a depleted oil reservoir, at about 2 km depth. We analyse the temporal evolution of the seismic sequence and use full waveform techniques to derive absolute and relative locations, estimate depths and focal mechanisms for the largest events in the sequence (with magnitude mbLg larger than 3), and compare them to a previous event (2012 April 8, mbLg 3.3) taking place in the same region prior to the gas injection. Moment tensor inversion results show that the overall seismicity in this sequence is characterized by oblique mechanisms with a normal fault component, with a 30A degrees low-dip angle plane oriented NNE-SSW and a subvertical plane oriented NW-SE. The combined analysis of hypocentral location and focal mechanisms could indicate that the seismic sequence corresponds to rupture processes along shallow low-dip surfaces, which could have been triggered by the gas injection in the reservoir, and excludes the activation of the Amposta fault, as its known orientation is inconsistent with focal mechanism results. An alternative scenario includes the iterated triggering of a system of steep faults oriented NW-SE, which were identified by prior marine seismics investigations. KW - Earthquake dynamics KW - Earthquake source observations Y1 - 2014 U6 - https://doi.org/10.1093/gji/ggu172 SN - 0956-540X SN - 1365-246X VL - 198 IS - 2 SP - 941 EP - 953 PB - Oxford Univ. Press CY - Oxford ER -