“…Although the FK method may appear to be more flexible, owing to the arbitrary array shape, in reality this method requires a considerable number of seismometers (e.g., Capon, 1969;Asten and Henstridge, 1984). The "circular array" used in the SPAC method, on the other hand, can be satisfactorily constructed by arranging a single seismometer at the center of a circle and three seismometers on the circumference, to form a triangular shape (Figure 1a; this array is referred to as a "four-point array" in this paper) (Okada, 2003;Asten, 2006;Cho et al, 2008a). Furthermore, the SPAC method can also be applied for wavelength ranges much wider than those that can be handled using the FK method (e.g., Okada, 2003).…”