The observation and analysis of Earth's free oscillations help seismologists to image Earth internal structure. In particular, free oscillation analysis provides the unique opportunity to study variations of density, a parameter that remains somewhat elusive to other seismological applications. On the other hand, density constraints play a key role in understanding processes proposed by neighboring disciplines in the Earth. For example, the transition of Earth's solid outer core to the inner core is associated with an abrupt increase in density. This density jump is an important constraint in discussions of the maintenance of Earth's geodynamo. Another area of interest is the "hot abyssal layer" in the lowermost few 100km of the mantle that has been proposed a few years ago. It carries the seismic signature of "hot" material with low seismic velocities, but it is nevertheless not buoyant. This layer is thought to be anomalously dense and its thickness must vary significantly laterally because seismology has been unable to detect an associated global discontinuity. Such a layer is thought to be the ultimate origin of lavas found on ocean islands whose chemical composition is very different from that found along mid-ocean ridges. Unfortunately, Earth's free oscillations have not been known precisely enough to prove or disprove with great confidence whether the "hot abyssal layer" is really dense. Another area for which free oscillation analyses contribute significantly is the structure and dynamics of Earth's core. It has been proposed in the 1990s that the inner core spins independently of the rest of the planet and a super-rotation of 6 degrees per year was initially published as being consistent with body waves that graze Earth's inner core beneath South America. Such a super-rotation has profound implications for Earth's geodynamo and the gravitational coupling of the mantle and core. Subsequent analyses of Earth's free oscillations have disproved such high rotation rates but the fidelity of free oscillation observations have so far not allowed seismologists to reduce uncertainties below 0.15 degrees/year.
The great December 6, 2004 Sumatra-Andaman earthquake excited Earth's free oscillations to a level not seen since the 1964 Good Friday Earthquake in Alaska. In fact, it nearly rivals the great May 22, 1960 Chile earthquake for which free oscillations were observed for the first time. This time, numerous high-quality digital seismic stations recorded the earthquake, with an unprecedented fidelity. PI Laske and her team measure free oscillation parameters for Earth's average and laterally varying internal structure. The team developed an analysis technique in which details of the earthquake process do not have to be known. This allows the analysis of events with relatively complicated source mechanisms, such as the Sumatra-Andaman earthquake whose shaking lasted for nearly 10 min. Laske essentially measures globally varying mode frequencies and attenuation rates. Earth's deviation from a non-rotating, uniformly layered planet removes the degeneracy of normal modes much like electron energy levels are split when an atom encounters a magnetic field. The measurement of this splitting allows Laske to image lateral heterogeneity that is symmetric. Earth structure that is not symmetric causes coupling between modes, hence the analysis of coupling coefficients allows her to fully image Earth's 3D heterogeneity. Prior to the Sumatra-Andaman earthquake, Earth's attenuating structure has been particularly difficult to assess because the seismic signal it causes is relatively small. It usually takes very deep earthquakes, such as the great 1994 Bolivia earthquake to excited modes that are sensitive to inner core structure. Due to its very large rupture area, the Sumatra-Andaman earthquake also excited these modes to a level that was not observed since the Bolivia earthquake. Though more recent, smaller earthquakes have been used to constrain inner core rotation rates, the Sumatra-Andaman earthquake adds an important, high-precision data point a decade after the Bolivia earthquake. Laske can now test inner core rotation rates over a timespan covering almost 30 years. Among the broader impacts of this project are the analysis of Earth's free oscillations provides key constraints on Earth structure to neighboring disciplines of the Earth sciences. Especially constraints on density are extremely difficult to obtain using other seismic methods, if not impossible. Furthermore, the project would contribute to the training of a graduate and an undergraduate student.
Very large earthquakes of the kind of the 26 December 2004 Sumatra-Andaman earthquake or the 11 March 2011 Tohoku, Japan earthquake cause Earth to ring for a long time at very specific frequencies, and subsets thereof. Sometimes, the signal is so large that it can be picked up by seismometers around the globe several months after the earthquake. The frequencies with which a planet vibrates after a quake depends on its internal structure. So by taking precise measurements, we can infer details about Earth's internal makeup. Earth's density in particular is usually hard to come by with other seismic methods. At the same time, accurate information on density is crucial for modelers that study the convection behavior of Earth's interior. Free oscillations are also excellent tools to study Earth's inner core. For this project, we assembled a new dataset of free oscillation observables. In collaboration with colleagues at the University in Nice, France, this new dataset has supported a PhD project to study details of Earth's mantle. In a second project, these data help improve our understanding of the makeup of Earth's inner core. The inner core is a particularly peculiar place in that seismic waves take different times when traveling in different directions. This property is called anisotropy. The anisotropy is almost but not perfectly aligned with Earth's rotation axis. Seismic evidence for small changes in the location of this symmetry axis has been used to suggest that the inner core may be rotating faster than the rest of the planet. Our own studies have shown that this is not the case. Over the last 30 years, the net rotation of the inner core has essentially been zero. However, we have collected evidence that the inner core may be rocking back and forth on shorter time scales. This grant also provided support to work toward improving the worldwide recording of Earth's normal modes. In particular, we have studied records collected on unburied ocean bottom seismometers (OBSs). Such recordings had previously been assumed to be too noisy to be useful for free oscillation studies. From data collected for the Hawaiian PLUME project, an NSF funded project to image the magma plumbing system of the Hawaiian hotspot, we extracted free oscillation spectra that sometimes rival those recorded on land. The burial of seismometers is currently prohibitively costly but we showed that seismologists may not need to depend on such deployments to obtain observatory-quality records. Our work in this field culminated in a funded major research initiative proposal to install a pilot station to test the feasibility if such deployments could stand in for the long planned but then abandoned idea of an ocean seismic network to fill crucial holes in global data coverage. Our pilot installation will provide near-realtime data access through an innovative wave glider developed by our industry partner. The autonomously operating wave glider replaces a moored buoy or an ocean cable to provide communication with the instrument. A pilot deployment is planned for 2013.
