[1] The characteristics of earthquake swarms can neither be described by simple laws nor are the underlying mechanisms presently understood. Swarm activity is often assumed to be caused by an intrusion of fluids into the seismogenic zone. We have studied the earthquake catalog of the large earthquake swarm that occurred in the year 2000 in Vogtland, SE-Germany and NW-Bohemia, an area well known for its episodic swarm generation. We observe a significant decrease of the Gutenberg-Richter b value during the swarm evolution as well as a fractal temporal clustering of the earthquakes. The spatial spreading of the swarm's activity, which is approximately confined to one plane, cannot simply be explained by a process of fluid diffusion. Instead, we observe a simple relationship between the spatial spreading and the seismic moment release, which is in good agreement with empirical relationships derived for tectonically driven earthquakes and theoretical crack growth models. This observation points to a progressively growing main fracture underlying the swarm activity. In addition, we find that the swarm earthquakes themselves trigger aftershocks near the border of their rupture area. The stickslip behavior of the rupture propagation can be explained by stress transfers and induced fluid flows due to earthquakes in a fluid-permeated critically loaded fault zone. However, during the first phase, the temporal behavior is found to be different, pointing to intrusion of fluids initiating the swarm activity.
International audienceA swarm of earthquakes of magnitudes up to = 3.8 stroke the region of West Bohemia/Vogtland (border area between Czechia and Germany) in October 2008. It occurred in the Nový Kostel focal zone, where also all recent earthquake swarms (1985/1986, 1997, and 2000) took place, and was striking by a fast sequence of macroseismically observed earthquakes. We present the basic characteristics of this swarm based on the observations of a local network WEBNET (West Bohemia seismic network), which has been operated in the epicentral area, on the Czech territory. The swarm was recorded by 13 to 23 permanent and mobile WEBNET stations surrounding the swarm epicenters. In addition, a part of the swarm was also recorded by strong-motion accelerometers, which represent the first true accelerograms of the swarm earthquakes in the region. The peak ground acceleration reached 0.65 m/s. A comparison with previous earthquake swarms indicates that the total seismic moments released during the 1985/1986 and 2008 swarms are similar, of about 4E16 Nm, and that they represent the two largest swarms that occurred in the West Bohemia/ Vogtland region since the = 5.0 swarm of 1908. Characteristic features of the 2008 swarm are its short duration (4 weeks) and rapidity and, consequently, the fastest seismic moment release compared to previous swarms. Up to 25,000 events in the magnitude range of 0.5 < < 3.8 were detected using an automatic picker. A total of nine swarm phases can be distinguished in the swarm, five of them exceeding the magnitude level of 2.5. The magnitude-frequency distribution of the complete 2008 swarm activity shows a value close to 1. The swarm hypocenters fall precisely on the same fault portion of the Nový Kostel focal zone that was activated by the 2000 swarm ( ≤ 3.2) in a depth interval from 6 to 11 km and also by the 1985/1986 swarm ( ≤ 4.6). The steeply dipping fault planes of the 2000 and 2008 swarms seem to be identical considering the location error of about 100 m. Furthermore, focal mechanisms of the 2008 swarm are identical with those of the 2000 swarm, both matching an average strike of 170° and dip of 80° of the activated fault segment. An overall upward migration of activity is observed with first events at the bottom and last events at the top of the of the activated fault patch. Similarities in the activated fault area and in the seismic moments released during the three largest recent swarms enable to estimate the seismic potential of the focal zone. If the whole segment of the fault plane was activated simultaneously, it would represent an earthquake of ~5. This is in good agreement with the estimates of the maximum magnitudes of earthquakes that occurred in the West Bohemia/Vogtland region in the past
SUMMARY
Two recent major swarms in Western Bohemia occurred in the years 2000 and 2008 within almost the same portion of a fault close to Novy Kostel. Previous analysis of the year 2000 earthquake swarm revealed that fluid intrusion seemed to initiate the activity whereas stress redistribution by the individual swarm earthquakes played a major role in the further swarm evolution. Here we analyse the new swarm, which occurred in the year 2008, with regard to its correlation to the previous swarm as well its spatiotemporal migration patterns. We find that (i) the main part of the year 2008 activity ruptured fault patches adjacent to the main activity of the swarm 2000, but that also (ii) a significant overlap exists where earthquakes occurred in patches in which stress had been already released by precursory events; (iii) the activity shows a clear migration which can be described by a 1‐D (in up‐dip direction) diffusion process; (iv) the migration pattern can be equally well explained by a hydrofracture growth, which additionally explains the faster migration in up‐dip compared to the down‐dip direction as well as the maximum up‐dip extension of the activity. We use these observations to estimate the underlying fluid pressure change in two different ways: First, we calculate the stress changes induced by precursory events at the location of each swarm earthquake assuming that observed stress deficits had to be compensated by pore pressure increases; and secondly, we estimate the fluid overpressure by fitting a hydrofracture model to the asymmetric seismicity patterns. Both independent methods indicate that the fluid pressure increase was initially up to 30 MPa.
[1] We apply rock mechanics concepts to the seismological observations in order to explain why during hydraulic injection some events display tensile and some shear deformation. The presence of non-shear components depends on the differential stress and the fracture orientation with respect to the s 1 direction. Provided the slip vector is parallel to the traction we define four types of earthquakes according to the ratio of the shear and tensile components. Assuming a Griffith failure envelope, hybrid events containing both shear and tensile components can occur for fractures striking within 22.5°of s 1 . We argue that pure tensile fractures striking parallel to s 1 are unlikely in the presence of natural fractures. The low shear traction of tensile events also implies their small stress drops. By applying the analysis to two different data sets, Soultzsous-Forets and Cotton Valley, we show that different orientations of natural fractures and differential stress in the targeted formations made each region favorable for different non-DC components in the injection-induced seismicity. Citation: Fischer, T., and A. Guest (2011), Shear and tensile earthquakes caused by fluid injection, Geophys. Res. Lett., 38, L05307,
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