Earth's Density and Inner Core Rotation after the great Sumatra-Andaman Earthquake

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

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.