<p>&#160;</p><p><span><strong>&#160;</strong></span></p><p>&#160;</p><p><span>Repeating earthquakes, sequences of microseismic events with highly similar seismograms and magnitudes, suggest quasi-periodic rupturing of the same asperity. They are observed on creeping fault segments surrounded by aseismic slip area and also in earthquake swarms. However, so far, they have not been documented in the West Bohemia/Vogtland seismic swarm area. These local swarms consist of thousands of M</span><sub><span>L</span></sub><span> < 4 events occurring along a small area of fault zone with repeated activation of some patches during the swarms and weak background activity in the intermediate periods. Detecting and analyzing the repeating earthquakes would help revealing the continuing background activity and identifying fault areas that are active permanently. This could point to the possible sources of fluids or aseismic creep that are supposed to play significant role in swarm generation. Repeating earthquakes are identified by waveform cross-correlation analysis comparing waveforms of repeaters with continuous seismic data set. We developed efficient detection algorithm to identify repeating earthquakes using selected event templates to reveal continuing seismic activity along the main Nov&#253; Kostel fault zone, namely in the areas with only episodic activity. The results provide a robust basis for routine application to the long-term seismic dataset that will allow also for further applications including analysis of the source parameters of the repeaters and/or detecting possible seismic velocity variations in the focal zone. </span></p><p>&#160;</p>
Summary Recent studies of source-time functions (STFs) of small earthquakes have shown that some of the ML < 3 events may display complicated waveforms indicating multiple rupturing episodes. The STFs of such earthquakes consist of several pulses whose relative positions provide information on the mutual position of the subevents. I have used the waveform modelling method to analyse multiple events in order to disclose the geometry of the rupture. The P and S waveforms of multiple events (MEs) are modelled as the sum of waveforms of single subevents with different hypocentre coordinates and scalar moments. To construct the waveform of each single event composing the ME, the waveform of a co-located small event is used as an empirical Green’s function (EGF). Assuming similar focal mechanisms of the subevents and of the EGF, the method seeks the coordinates and origin times of the subevents and their relative seismic moments. The non-linear problem is solved using the genetic algorithms method. Synthetic tests have shown that the method is capable of locating reliably up to three subevents with an accuracy better than 40 m. The method was applied to the records of the 2000 earthquake swarm in NW-Bohemia/Vogtland in Central Europe. By the EGF deconvolution, 54 MEs were identified in the magnitude range from 1.2 to 3.3, and 18 of them were successfully modelled as double or triple events with separate rupture positions. The separation of subsources reached 100 ms in time and 320 m in space. The relative positions of the subevents with respect to the orientation of the fault indicate that most of them occurred on a common fault plane. The space-time separation of the subevents corresponds to a speed of 3.0 ± 0.9 km s−1, a value typical for rupture propagation of large earthquakes. The later subevents occur farther than the nominal rupture radius of the first subevent, and their mutual distance scales with magnitude. These observations suggest that the analysed MEs share a common fault surface and that their subevents represent individual rupture episodes. The angular distribution of the position vectors of later subevents indicates that many of them result from slip-parallel rupture growth, while some of the ruptures propagate upwards.
The 2021 Fagradalsfjall volcanic eruption in the Reykjanes Peninsula, Iceland, was followed by effusive lava outflow lasting six months. It was preceded by an intensive earthquake swarm lasting one month with the largest earthquake exceeding ML 5. We analyze seismic data recorded by the Reykjanet local seismic network to trace the processes leading to the eruption to understand the relation between seismic activity and magma accumulation. Precise relocations show two hypocenter clusters of the 2021 swarm in the depth range of 1-6 km; a NE-SW trending cluster that maps the dyke propagation, and a WSW-ENE trending cluster that follows the axis of the oblique plate boundary. Additionally, we relocated the preceding earthquake swarms of 2017, 2019 and 2020 and found that they form two branches along the oblique plate boundary, which coincide with the WSW-ENE trending cluster of the 2021 swarm. These branches form a stepover of ∼1 km offset, forming a pull-apart basin structure at the intersection with the dyke. This is the place where the eruption occurred, suggesting that magma erupted at the place of crustal weakening. The strong seismic activity started with a ML 5.3 earthquake of 24 February 2021, which triggered the aftershocks on the oblique plate boundary and in the area of magmatic dyke, both in an area of elevated Coulomb stress. The seismicity shows a complex propagation of the dyke, which started at its northern end, migrated southwestward and then jumped back to the central part where the effusive eruption took place. The observed N-S striking focal mechanisms are interpreted as right-lateral antithetic Riedel shears that accommodate the left lateral slip along the oblique plate boundary. The co-existence of seismic and magmatic activity suggests that the past seismic activity weakened the crust in the eruption site area, where magma accumulated. The following ML 5.3 earthquake of 24 February 2021 triggered the seismic swarm and likely perturbed the magma pocket which led to the six-months lasting eruption that started on 19 March.
