Converting Strong-motion Networks to Arrays via Common Triggering

Abstract

Small-aperture, strong-motion arrays are of great importance for earthquake engineering and engineering seismology. Their capabilities of recording broadband ground motion over a wide dynamic range provide insights into details of fracture processes at the earthquake source ( e.g. , Spudich and Cranswick 1984), the heterogeneity levels of the medium ( e.g. , Sato and Fehler 1998), site effects ( e.g. , Ansal 1999), and the spatial variability of earthquake ground motion ( e.g. , Abrahamson and Bolt 1987). In addition, their wide dynamic range provides the opportunity to investigate the transition of strong to weak motion, e.g. , earthquake source and ground motion scaling over a wide magnitude range ( e.g. , Halldorsson and Papageorgiou 2005). The sensitivity of the strong-motion instruments generally controls how far this range extends to smaller magnitudes. However, when the waveform correlation is sufficient over the aperture of a group of instruments operating in an array configuration, the sensitivity can be increased beyond that of the individual instruments via array processing techniques (Steinberg 1965). Therefore, it is essential that as many instruments as possible within the array capture any earthquake disturbances, even weak motion, because the application of array processing techniques can produce good data sets from otherwise less useful data ( e.g. , Almendros et al. 1999; Gibbons and Ringdal 2006). While it may seem straightforward to capture a complete set of earthquake recordings, in practice it is difficult due to the variability of ground motion and noise levels across even a small-aperture array, especially when the instruments use adaptive triggers (such as short-term vs. long-term average energy levels, STA:LTA), whose sensitivity is dependent on the current background noise levels. High-sensitivity instruments such as velocity meters generally trade off high-amplitude response for a low noise floor while strong-motion (accelerometer) sensors are capable of capturing the largest ground motion, even …