ALBERT

Description: The region of West Bohemia/Vogtland in the Czech–German border area is well known for the repeated occurrence of earthquake swarms, CO 2 emanations and mofette fields. We present a local earthquake tomography study undertaken to image the Vp and Vp/Vs structure in the broader area of earthquake swarm activity. In comparison with previous investigations, more details of the near-surface geology, potential fluid pathways and features around and below the swarm focal zone could be revealed. In the uppermost crust, for the first time the Cheb basin and the Bublák/Hartoušov mofette fields were imaged as distinct anomalies of Vp and Vp/Vs. The well-pronounced low-Vp anomaly of the Cheb basin is not continuing into the Eger rift indicating a particular role of the basin within the rift system. A steep channel of increased Vp/Vs is interpreted as the pathway for fluids ascending from the earthquake swarm focal zone up to the Bublák/Hartoušov mofette fields. As a new feature, a mid-crustal body of high Vp and increased Vp/Vs is revealed just below and north of the earthquake swarm focal zone. It may represent a solidified intrusive body which emplaced prior or during the formation of the rift system. We speculate that enhanced fluid flow into the focal zone and triggering of earthquakes could be driven by the presence of the intrusive body if cooling is not fully completed. We consider the assumed intrusive structure as a heterogeneity leading to higher stress particularly at the junction of the rift system with the basin and prominent fault structures. This may additionally contribute to the triggering of earthquakes.

Description: Ambient noise tomography is used to retrieve Rayleigh wave group and phase velocity variations in the period range of 8–40 s based on the vertical component of cross-correlation functions from permanent broad-band and mid-band seismometers across the Iranian Plateau. The iterative, non-linear inversion method of fast marching surface tomography (FMST) is employed to produce 2-D group and phase velocity maps. Shear wave velocities are also estimated using a linear least-square method. Unlike most previous largescale tomographic results, our group, phase and shear wave velocity estimations, emphasize low velocity crustal structure (up to 50 km depth) beneath Zagros Fold and Thrust Belt (ZFTB) and Sanandaj-Sirjan metaphormic Zone (SSZ). The suture zone resulting from the subduction of the Arabian plate under the Central Iran is inferred along the boundary of SSZ and Urmieh-Dokhtar Magmatic Arc (UDMA). The velocity patterns show the main sedimentary basins, and reveal lateral velocity changes indicating the crustal thickening beneath ZFTB, SSZ and Lut Desert (LD), and the crustal thinning beneath Kavir Desert (KD) and UDMA are well inferred. A prominent low velocity is persistent in the whole crust beneath the central part of Alborz mountain range with high topography, and we suggest that it is likely due to elevated crustal temperatures within thin lithosphere.

Description: 〈span〉〈div〉Summary〈/div〉Presently ongoing geodynamic processes within the intracontinental lithospheric mantle give rise to different natural phenomena in the NW Bohemia/Vogtland region (Czech Republic, Germany), amongst others: earthquake swarms, mineral springs and degassing zones of mantle-derived fluids as well as highly concentrated CO〈sub〉2〈/sub〉 (mofettes). Their interaction mechanisms and relations are not yet fully understood, but fluid pathways within the crust are assumed, that allow efficient fluid transport between the main hypocentral swarm quake region and the degassing areas at the surface. Here, we focus on the location of the presumed fluid channels as well as on the investigation of their near-surface spatio-temporal variability, targeting a depth of a few hundreds of meters. We applied a 3D matched field processing (MFP) approach in the frequency band of 10-20 Hz considering the fluid flow as seismic noise source. Within three campaigns in 2015/16, we recorded continuous seismic noise data on the Hartoušov Mofette Field within the Cheb Basin (NW Bohemia, CZ), which is a key site to study fluid flow as it is characterized by strong and continuous surface degassing of CO〈sub〉2〈/sub〉. We used temporary arrays varying in extent (70-600 m aperture) and in the amount of stations (25-95 units). Assuming a homogeneous velocity model and applying conventional MFP phase-matching over a 3D grid search, we located two channel-like structures beneath the test site, which could be traced down to a common source area up to 2000 m depth. We thereby evaluated the influence of amplitude normalization of the measured noise signal on the MFP location considering water-filled or dry mofette channels. Additionally, a spatio-temporal analysis using time windows with a length of 10 min during 5 hrs of noise record shows variability of fluid flow activity in space and time and hence, its migration beneath the test site on a short time scale.〈/span〉

Description: To investigate temporal seismic velocity changes due to earthquake related processes and environmental forcing in Northern Chile, we analyse 8 yr of ambient seismic noise recorded by the Integrated Plate Boundary Observatory Chile (IPOC). By autocorrelating the ambient seismic noise field measured on the vertical components, approximations of the Green's functions are retrieved and velocity changes are measured with Coda Wave Interferometry. At station PATCX, we observe seasonal changes in seismic velocity caused by thermal stress as well as transient velocity reductions in the frequency range of 4–6 Hz. Sudden velocity drops occur at the time of mostly earthquake-induced ground shaking and recover over a variable period of time. We present an empirical model that describes the seismic velocity variations based on continuous observations of the local ground acceleration. The model assumes that not only the shaking of large earthquakes causes velocity drops, but any small vibrations continuously induce minor velocity variations that are immediately compensated by healing in the steady state. We show that the shaking effect is accumulated over time and best described by the integrated envelope of the ground acceleration over the discretization interval of the velocity measurements, which is one day. In our model, the amplitude of the velocity reduction as well as the recovery time are proportional to the size of the excitation. This model with two free scaling parameters fits the data of the shaking induced velocity variation in remarkable detail. Additionally, a linear trend is observed that might be related to a recovery process from one or more earthquakes before our measurement period. A clear relationship between ground shaking and induced velocity reductions is not visible at other stations. We attribute the outstanding sensitivity of PATCX to ground shaking and thermal stress to the special geological setting of the station, where the subsurface material consists of relatively loose conglomerate with high pore volume leading to a stronger nonlinearity compared to the other IPOC stations.

Description: Monte Carlo solutions to the radiative transfer equations are used to model translational and rotational motion seismogram envelopes in random elastic media with deterministic background structure assuming multiple anisotropic scattering. Observation and modelling of the three additional components of rotational motions can provide independent information about wave propagation in the Earth's structure. Rotational motions around the vertical axis observed in the P -wave coda are of particular interest as they can only be excited by horizontally polarized shear waves and therefore indicate the conversion from P to SH energy by multiple scattering at 3-D heterogeneities. To investigate crustal scattering and attenuation parameters in south-east Germany beneath the Gräfenberg array multicomponent seismogram envelopes of rotational and translational motions are synthesized and compared to seismic data from regional swarm-earthquakes and of deep teleseismic events. In the regional case a nonlinear genetic inversion is used to estimate scattering and attenuation parameters at high frequencies (4–8 Hz). Our preferred model of crustal heterogeneity consists of a medium with random velocity and density fluctuations described by an exponential autocorrelation function with a correlation length of a few hundred metres and fluctuations in the range of 3 per cent. The quality factor for elastic S -waves attenuation $Q_i^S$ is around 700. In a second, step simulations of teleseismic P -wave arrivals using this estimated set of scattering and attenuation parameters are compared to observed seismogram envelopes from deep events. Simulations of teleseismic events with the parameters found from the regional inversion show good agreement with the measured seismogram envelopes. This includes ringlaser observations of vertical rotations in the teleseismic P -wave coda that naturally result from the proposed model of wave scattering. The model also predicts, that the elastic energy recorded in the teleseismic P coda is not equipartitioned, unlike the coda of regional events, but contains an excess of shear energy. The results confirm that scattering generating the teleseismic P -wave coda mainly occurs in the crustal part of the lithosphere beneath the receiver. Our observations do not require scattering of high frequency waves in the mantle, but weak scattering in the lithospheric mantle cannot be ruled out.