ALBERT

Description: At the Decatur carbon capture and storage site (IL, USA) CO2 has been injected from 2011–2014 and from 2017 to present near the base of the Lower Mt. Simon Sandstone saline reservoir, resulting in microseismicity. Microseismicity is mainly located in the basement and distributed in distinct spatial clusters. The lack of significant impedance contrasts within the basement makes the interpretation of active-source seismic reflection data challenging, however, recent reprocessing allowed to resolve faults above and at the top of the basement. These faults generally do not coincide with the location of microseismic events and their continuation to the general depth of the seismic events cannot be assumed. This paper shows how the interpretation of the microseismicity can complement structural interpretations of active-source seismic reflection data. In particular, we analyze clusters and bursts (abrupt increases) of microseismicity, identify unresolved, smaller-scale weaknesses and extract statistical parameters. These parameters allow comparisons with the interpreted faults, and with fracture sets intercepted by boreholes. During injection at the Decatur site, the injection pressure was kept far below fracture pressure, nevertheless, seismic events were induced and spread far beyond the expected extent of the CO2 plume. We argue that local stress transfers related to the CO2 injection reactivated pre-existing fractures within the critically stressed basement. Finally, we conducted a slip tendency analysis for faults interpreted from active seismic, selected cluster, bursts and nodal planes from focal mechanisms to determine if the interpreted structures are optimally oriented with respect to the stress regime. Our results suggest that the orientation of fractures close to the injection well, generally shows slight deviations from the optimal orientation for slip. This might indicate either slight local deviations of the maximum horizontal stress azimuth from the average direction used in the analysis, or the lack of optimally oriented fractures at this location.

Description: The occurrence of felt earthquakes due to gas production in Groningen has initiated numerous studies and model attempts to understand and quantify induced seismicity in this region. The whole bandwidth of available models spans the range from fully deterministic models to purely empirical and stochastic models. In this article, we summarise the most important model approaches, describing their main achievements and limitations. In addition, we discuss remaining open questions and potential future directions of development.

Description: In the Koyna–Warna region, western India, an enormous number of microearthquakes was detected automatically on borehole records. Most of these events could not be identified on the surface network by a routine approach based on visual inspection primarily due to signal attenuation and the presence of noise. In this work, we implemented an automatic detection workflow to analyze the time series of an earthquake sequence that has clear foreshock and aftershock activity associated with an Mw 4.0 earthquake that occurred on 3 June 2017. Further, we applied a nested grid‐search algorithm to constrain the absolute earthquake locations. For about one month of data, a total of ∼1500 earthquakes were detected based on the automatic detection process, out of which ∼1000 earthquakes were locatable. All event detections, P‐wave and S‐wave phase readings were manually inspected and refined to ensure their quality. Previously, only about 435 events were well located based on the visual inspection approach for the same time period. Also, we analyzed repeated earthquakes based on waveform similarity leading to an improvement in the relocations of earthquakes of the aforementioned earthquake sequence. The relocated seismicity aligns parallel to a deep‐reaching lineament derived from recent investigations using airborne light detection and ranging measurements.

Description: The Alpine mountains in central Europe are characterized by a heterogeneous crust accumulating different tectonic units and blocks in close proximity to sedimentary foreland basins. Centroid moment tensor inversion provides insight into the faulting mechanisms of earthquakes and related tectonic processes but is significantly aggravated in such an environment. Thanks to the dense AlpArray seismic network and our flexible bootstrap-based inversion tool Grond, we are able to test different setups with respect to the uncertainties of the obtained moment tensors and centroid locations. We evaluate the influence of frequency bands, azimuthal gaps, input data types, and distance ranges and study the occurrence and reliability of non-double-couple (DC) components. We infer that for most earthquakes (Mw≥3.3) a combination of time domain full waveforms and frequency domain amplitude spectra in a frequency band of 0.02–0.07 Hz is suitable. Relying on the results of our methodological tests, we perform deviatoric moment tensor (MT) inversions for events with Mw〉3.0. Here, we present 75 solutions for earthquakes between January 2016 and December 2019 and analyze our results in the seismotectonic context of historical earthquakes, seismic activity of the last 3 decades, and GNSS deformation data. We study regions of comparably high seismic activity during the last decades, namely the Western Alps, the region around Lake Garda, and the eastern Southern Alps, as well as clusters further from the study region, i.e., in the northern Dinarides and the Apennines. Seismicity is particularly low in the Eastern Alps and in parts of the Central Alps. We apply a clustering algorithm to focal mechanisms, considering additional mechanisms from existing catalogs. Related to the N–S compressional regime, E–W-to-ENE–WSW-striking thrust faulting is mainly observed in the Friuli area in the eastern Southern Alps. Strike-slip faulting with a similarly oriented pressure axis is observed along the northern margin of the Central Alps and in the northern Dinarides. NW–SE-striking normal faulting is observed in the NW Alps, showing a similar strike direction to normal faulting earthquakes in the Apennines. Both our centroid depths and hypocentral depths in existing catalogs indicate that Alpine seismicity is predominantly very shallow; about 80 % of the studied events have depths shallower than 10 km.

Description: Even if faults are not visible on geological outcrops, source mechanisms of earthquakes occurring on those faults may distinguish between different types of faulting. We implemented a novel, probabilistic full-waveform moment tensor inversion for the application to shallow micro-earthquakes close to gas fields in North Germany. Due to the probabilistic approach, parameter trade-offs, uncertainties and ambiguities are mapped. The implemented bootstrap method implicitly accounts for modelling errors that may affect every station and phase in a unique way. Furthermore, event locations are provided by the algorithm. Such hypocentres, which are estimated simultaneously with moment tensors, are often less sensitive to uncertainties in crustal structure. We analysed events from the November 20th, 2019, Kirchlinteln series as well as the October 1st, 2018 ML 3.6 Lastrup event (00:28 UTC). We carefully investigated station quality, the usable frequency range, input data types, inversion parameters as well as velocity models and supplemented the analysis by a H/V study. The Kirchlinteln events show slightly oblique normal faulting and potentially, a complex faulting process involving several faults, fitting known local fault traces. The 00:28 UTC Lastrup event reveals thrust faulting, most likely on a single fault not fitting the regional fault trend.