Our work over the last five years to find and analyze waveform doublets, pertinent to the study of time-dependent features of the inner core, has greatly strengthened the observational evidence. We are extending this work, which for several reasons can be expected to result in the discovery of relevant waveform doublets at an increasing rate, and to take it in new directions that will greatly increase the spatial sampling of time-dependent travel times of seismic waves passing through the Earth’s inner core. We are pursuing this work with the scientific goal of understanding the cause or causes of the time-dependence, at the same time as we are improving the sampling.
For the last eight years, the scientific evidence for change in travel times of seismic waves traversing the inner core has largely been associated with efforts to remove contaminating effects, whether derived from inhomogeneities of Earth structure or from event mislocations. But our new approach, based on waveform doublets, simply avoids such contaminants and allows the research to focus on the interpretation of far more precise measurements of the travel-time change itself.
The main reasons it is important for researchers to measure and analyze such travel-time changes, are (1) that it promotes a mindset which recognizes the dynamic nature of the Earth's deep interior; and (2) that it has implications for the big picture of how our planet differentiated, and the degree of vigor with which convection in the fluid core is maintained. Accepting that the evidence from waveform doublets is sufficiently persuasive to require changes within the inner core on a time scale that can lead to perceptible effects over a decade or less, there are consequences in the study of overall heat flow and energy balance; geomagnetism; and gravity, as well as seismology (that is, the four geophysical disciplines associated with observable features measurable at the Earth's surface).
Our work has attracted significant public interest, because (a) it is a new twist on research that led to hundreds of news stories in the 1990s; (b) it is quite easily understandable by people, young, old and in-between, who lack scientific training but who are curious about the planet on which we live; and most fundamentally because (c) the inner core, as a Moon-sized object apparently moving inside the Earth, is a fascinating object, and the background to good stories of how science is done.
It can also be noted that our methods for studying travel-time change are a contribution within the general subject of precision seismology, in which even teleseismic signals are measured with attention to detail at the level of ~ 10 milliseconds. We note finally that the discovery of new waveform doublets in subduction zones is of great interest in the context of earthquake physics, whether the events occur close together in time or separated by several years
Cross Cutting Themes:
Hazards and Risk
National Science Foundation