Statistics of the Seismic Sequence and Rupture Directivity of the M5.5 Earthquake in Orkney, South Africa

Abstract. Despite our general knowledge of earthquake processes, it is still not fully understood how earthquake ruptures nucleate and propagate and why they stop. Also, the controlling factors of the frequency and of the size of earthquakes are subject of ongoing research. We aim to address these questions with a comprehensive study of seismicity in deep South African gold mines. We find here the unique situation that the seismicity consists of both induced earthquakes and aftershocks triggered by the M5.5 Orkney earthquake which occurred in August 2014. We separate the cataloged seismicity and group the events into three classes: the aftershock sequence, seismicity induced by fluids, and seismicity induced by mining activities. We examine statistical properties of earthquakes in each of the three classes. We conclude that the magnitude statistics of both aftershocks and induced earthquakes are influenced by the finite size and geometry of the rock volume of stress perturbation resulting in an absence of larger magnitude events. The magnitude frequency distributions obey the Lower Bound model of magnitude probability. The statistics of dynamic stress drop of aftershocks and induced earthquakes satisfy log-normal distributions but the value range is different, it means that aftershocks are generally characterized by higher stress drops. Another key aspect in our study is the imaging of the propagating rupture of the M5.5 earthquake. We apply the back projection imaging approach using seismological data from two different local networks and retrieve similar results. We conclude that the rupture of the M5.5 earthquake propagated predominantly unilaterally, nearly from North to South over a distance of about 6km. The hypocenters of the aftershock sequence are situated unilaterally in respect to the hypocenter of the main shock and are aligned to the South which confirms the obtained rupture propagation image and the directivity of the main shock.