Sophie Lambotte

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 ( M w >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 ( S 2, S 3, S 4, 1 S 2, S , and 1 S ) 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 (1 S 2). 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.

Seismic evidence for a change in the large scale tomographic pattern across the D” layer

We present SEISGLOB1, a pure SV tomographic model of Earths mantle based on Rayleigh phase velocities and normal mode self- and cross-coupling data. SEISGLOB1 is the first model that incorporates the cross-coupling of normal modes since the pioneering work of Resovsky & Ritzwoller [1999]. The simultaneous inversion of new cross-coupling normal modes and self-coupling of high order normal modes measured by Deuss et al. [2013] and Stoneley modes measured by Koelemeijer et al. [2013] allows us to show that the velocity structure at the base of the mantle is more complex than that expected from a dominant spherical hamonic degree 2 and that the relative strength of odd degrees has previously been underestimated. Near the CMB, the LLSVPs are less homogeneous than in previous studies, and various local maxima, often potentially associated with hotspots sources, are observed.

Toward Seeing the Earth's Interior Through Unbiased Tomographic Lenses

Geophysical tomographic studies traditionally exploit linear, damped least squares inversion methods. We demonstrate that the resulting models can be locally biased toward lower or higher amplitudes in regions of poor data illumination, potentially causing physical misinterpretations. For example, we show that global model S40RTS is locally biased toward higher amplitudes below isolated receivers where raypaths are quasi-vertical, such as on Hawaii. This leads to questions on the apparent low-velocity structure interpreted as the Hawaii hot spot. We prove that a linear Backus-Gilbert inversion scheme can bring the Earth’s interior into focus through unbiased tomographic lenses, as its model estimates are constrained to be averages over the true model. It also efficiently computes the full generalized inverse required to infer both model resolution and its covariance, enabling quantitative interpretations of tomographic models.