Split-Mode Evidence for No Ultraslow Component to the Source

We present a comprehensive study of the Earth’s gravest spheroidal modes excited by the Maule, Chile, earthquake of 27 February 2010, using uninterrupted time series extending over a minimum of 19 and a maximum of 93 days. For each of 101 station–mode combinations, we use the formalism of Stein and Geller (1977) to compute the relative excitation of the 2l 1 singlets in the relevant geometry and obtain an estimate of the seismic moment by best fitting the observed spectrum to that of the resulting synthetic computed for the same recording window. The average results for seven spheroidal and two radial modes deviate no more than 12% from the Global Centroid Moment Tensor moment of 1:86 × 10 dyn·cm, with no evident trend with frequency. In other words, we fail to document an ultralowfrequency component (expressed as an increase of moment with period) in the source of the 2010 event. This result indicates that such components are not universal features of megaquakes, even though they had been documented for the 1960 Chilean, 2004 Sumatran, and 1964 Alaskan events (the only three events with larger moments in the past 50 years). In this respect, the 2010 earthquake is most comparable to the slightly smaller 2005 Nias event, which incidentally also featured a bilateral rupture.