ALBERT

Description: Low-seismicity regions such as the United Kingdom (UK) pose a challenge for seismic hazard analysis in view of the limited amount of locally recorded data available. In particular, ground-motion prediction is faced with the problem that most of the instrumental observations available have been recorded at large distances from small earthquakes. Direct extrapolation of the results of regression on these data to the range of magnitudes and distances relevant for the seismic hazard analysis of engineered structures generally leads to unsatisfactory predictions. The present study presents a new ground-motion prediction equation (GMPE) for the UK in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped pseudospectral acceleration (PSA), based on the results of numerical simulations using a stochastic point-source model calibrated with parameters derived from local weak-motion data. The predictions from this model are compared with those of previous GMPEs based on UK data, other GMPEs derived for stable continental regions (SCRs), as well as recent GMPEs developed for the wider European area.

Description: Historically, hypocentral depths of earthquakes in South Africa are not well documented from instrumentally determined parameters. We, therefore, investigated a concise number of earthquakes in South Africa over a broad geographical region. We reviewed the hypocentral depths of instrumentally recorded tectonic earthquakes in South Africa through synthetic waveform modeling and through a separate study using multiple phase arrivals. The resulting depths of earthquakes in South Africa occur at shallow crustal depths. It is further shown that uncertainty in focal depth is higher using the multiphase method in comparison with the regional depth-phase modeling method. Methodology for determining focal depth from macroseismic observations is introduced in this article; however, poor coverage of intensity observations within the epicentral area limited the reliability of this method. Therefore, the study of hypocentral depths inferred from macroseismic observations was inconclusive, resulting from limited intensity data. Shallow depths are, however, inferred from the spatial patterns that do not exhibit the characteristic intensity patterns of deep earthquakes. It was noted during this study that the majority of events display the distinguishable isoseismal pattern of shallow earthquakes, whereas only a selected number of earthquakes exhibit spatial patterns expected of deeper crustal events. The present study did not find conclusive evidence of earthquakes occurring in the lower crust of South Africa from the limited number of earthquakes considered, which therefore indicates a unimodal depth distribution of earthquakes in the upper crust between the depths of 3 and 20 km. This observation contradicts reports in internationally published earthquake catalogs that indicate deeper earthquakes for individual events, for example, the M w ~6.2 1969 Ceres earthquake.