ALBERT

Description: Source, site, and propagation parameters are inverted from a U.K. database of weak-motion events (2.0〉M (sub L) 〉4.7). This results in the complete spectral parameterization of over 3200 velocimetric records of 273 events from the year 1992 to 2006. The S wave is extracted from the vertical records and is processed using a multitaper Fourier transform. We initially use a nonlinear least-squares log-space optimization to obtain estimates of the attenuation parameter for each spectrum. The estimates of t (super *) are then used to geometrically constrain a depth-dependent Q model using a technique adapted from velocity tomography. We then invert for the remaining frequency-dependent parameters and a collective amplitude parameter from the velocity spectra while fixing the newly computed attenuation parameters based on raytracing through our Q model. The resultant amplitude parameters are then split into source moments, apparent geometrical spreading, and site correction factors. We find a frequency-independent depth-dependent Q structure. A linear relationship proportional to 0.7 M (sub L) between moment magnitude (M (sub w) ) and local magnitude (M (sub L) ) is found in the range of 2-4.7 M (sub L) . The majority of stress drops are found to range on the order of 0.1-10 MPa. A multiple segment apparent geometrical spreading model is found to best describe the amplitude decay with distance, accounting for factors such as geometrical spreading and scattering, along with multiple phase interference in the analysis window. Site response functions are found to broadly correlate with regional geology, mean amplification occurring in the Cenozoic sedimentary rock sites to the southeast of England relative to the harder Palaeozoic rock sites of Wales and Scotland. We use a bootstrap analysis technique to analyze the dependence of our results on the data in order to estimate the variance of the results and check the robustness of different inversions. Synthetic spectra are also computed in order to obtain minimum variance and bias of model parameters associated with the method. In applying a geometrical Q constraint, through the use of Q tomography, we find that the robustness of the results is significantly increased. A thorough analysis of the trade-offs involved in the inversion is performed using synthetic datasets. We find strong trade-offs between the parameters, but we are able to show that this covariance is reduced when adopting the Q-tomography approach.

Description: A moderate size earthquake (M (sub L) 4.3, M (sub w) 4.0) occurred in southeastern England on 28 April 2007. The earthquake was of some significance as it caused damage in the town of Folkestone and produced the largest peak horizontal ground acceleration (PGA, 0.1 g) measured in the United Kingdom to date. It was followed by 12 aftershocks between M (sub L) 0.8 and 1.7. The earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the United Kingdom. The hypocenter of the event was at a depth of about 5 km beneath Folkestone, with an error ellipse indicating horizontal errors in a location of about 5 km. The depth was well constrained using a number of techniques, of which local travel-time inversion and teleseismic depth phase modelling are most reliable. A stress drop of 28.6 bars and a source radius of 0.5 km were determined from the analysis of displacement source spectra. We derived a near-surface attenuation factor kappa =0.02 from the aftershock data that were used in the spectral analysis of the mainshock. Applying the horizontal to vertical (H/V) spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0.4 and 3.9 Hz. This amplification is likely to have contributed to the mainshock PGA of 0.1 g measured at the same site. Similar site conditions may have been responsible for the damage in parts of Folkestone. The moment tensor computed from regional broadband data showed a strike-slip mechanism with a normal component and either right-lateral movement on a west-southwest-east-northeast-striking or left-lateral movement on a north-northwest-south-southeast-striking nodal plane. The north-northwest-south-southeast-striking nodal plane matches the trend of the main faults affecting the Kent coalfield and also possibly the Variscan front. It is thus possible that the causative fault was associated with the Variscan front, a major structural boundary at the northern limit of late Carboniferous folding and thrusting.

Description: In this study, we have evaluated the probabilistic and deterministic seismic hazard for the city of Almaty, the largest city in Kazakhstan, which has a population of nearly two million people. Almaty is located in the Tien Shan belt, a low‐strain‐rate environment within the interior of the Eurasian plate that is characterized by large infrequent earthquakes. A robust assessment of seismic hazard for Almaty is challenging because current knowledge about the occurrence of large earthquakes is limited, due to the short duration of the earthquake catalog and only partial information about the geometry, rupture behavior, slip rate, and the maximum expected earthquake magnitude of the faults in the area. The impact that this incomplete knowledge has on assessing seismic hazard in this area can be overcome using both probabilistic and deterministic approaches and integrating the results.First, we simulate ground‐shaking scenarios for three destructive historical earthquakes that occurred in the northern Tien Shan in 1887, 1889, and 1911, using ground‐motion prediction equations (GMPEs) and realistic fault‐rupture models based on recent geomorphological studies. We show that the large variability in the GMPEs results in large uncertainty in the ground‐motion simulations. Then, we estimate the seismic hazard probabilistically using a Monte Carlo‐based probabilistic seismic hazard analysis and the earthquake catalog compiled from the databases of the International Seismological Centre and the British Geological Survey. The results show that earthquakes of Mw 7.0–7.5 at Joyner–Boore distances of less than 10 km from the city pose a significant hazard to Almaty due to their proximity. These potential future earthquakes are similar to the 1887 Verny earthquake in terms of their magnitude and distance from Almaty. Unfortunately, this is the least well understood of the destructive historical earthquakes that have occurred in the northern Tien Shan.