New Regional Moment Tensors in South Africa

Abstract

Regional moment tensors (RMTs) provide important information for seismotectonic and hazard studies in regions with low to moderate seismicity, where infrequent earthquakes of Mw ≥ ∼4.0–4.5 occur that are too small for global momenttensor techniques. Moment-tensor analysis involves fitting theoretical waveforms with observed broadband waveforms and inverting for the moment-tensor elements ( e.g. , Aki and Richards 1980; Jost and Herrmann 1989). One powerful tool to calculate RMTs is the time domain surface wave waveform inversion code TDMT_INVC (Dreger and Helmberger 1993; Pasyanos et al. 1996; Dreger 2003). In recent years RMTs have been routinely calculated with this software in many parts of the world such as western Canada (Ristau et al. 2003, 2007), California (Dreger and Helmberger 1993; Romanowicz et al. 1993; Pasyanos et al. 1996), Alaska (Ratchkovski and Hansen 2002), Japan (Kubo et al. 2002), Taiwan (Kao et al. 1998), the European–Mediterranean region (Bernardi et al. 2004), and New Zealand (Ristau 2008). Only a few moment tensors/focal mechanisms are available for South Africa. This is due to moderate tectonic and deep mine-related seismicity, as well as, until recently, a sparse distribution of broadband seismometers in the South African National Seismograph Network (SANSN) (Saunders et al. 2008). A unique opportunity presented itself when the dense, very broadband Incorporated Research Institutions in Seismology (IRIS) PASSCAL Kaapvaal craton array was deployed in South Africa ( e.g. , Nguuri et al. 2001). From this array we identified three near regional Mw ∼4.0 earthquakes with suitable waveform data to calculate RMTs with the TDMT_INVC software. Our goal is to determine the moment magnitude, earthquake mechanism, and focal depth in order to 1) make progress in resolving the difference between local and moment magnitudes routinely determined with the SANSN; and 2) expand our understanding of the regional …