Constraining the Overall Kinematics of the 2004 Sumatra and the 2005 Nias Earthquakes Using the Earth’s Gravest Free Oscillations

The giant Aceh–Sumatran 2004 earthquake is the largest earthquake recorded since the great Chilean 1960 and the Alaskan 1964 events. The Earth’s free oscillations were strongly excited and recorded by numerous stations with an extremely good signal-to-noise ratio, even for the gravest modes. These particular modes are interesting because phases of these well-separated split free oscillations carry information on the overall kinematics of the source of large earthquakes (Mw 8) and, in particular, on the length, duration, and mean rupture velocity. Using the singlet-stripping technique, we study some of the Earth’s gravest free oscillations (0S2, 0S3, 0S4, 1S2, 0S0, and 1S0) recorded at several broadband permanent stations (Geoscope, Incorporated Research Institutions for Seismology [IRIS], Global Geodynamics Project [GGP]) to recover individual singlet parameters: phases, frequencies, and quality factors. We use these parameters to constrain the spatiotemporal extent of the source of the Sumatra earthquake of 26 December 2004 and the Nias earthquake of 28 March 2005. We mainly use vertical-component data from seismometers and superconducting gravimeters, because they are less noisy than the horizontal seismic data, but we also show that, for the 2004 event, horizontal components can also be used (1S2). On the basis of the initial phase measurements presented here, we obtain, for the 2004 event, a fault length of about 1250 km and a duration of about 550 sec. For the 2005 event, our measurements favor a model in which the southern segment breaks 40 sec later, but the bilateral nature of the rupture and its spatial dimension prevent us from properly constraining its spatial extent.