Theory and Observations: Normal Mode and Surface Wave Observations

Abstract

In this chapter, we attempt to review the observation of Earth's free oscillations and surface waves. The analysis of free oscillations and long-period surface waves can look back to a half-century long history, and milestone discoveries are numerous. We touch some of these, but our review of the history of developments in the field is incomplete. Here, we rather try to summarize what mode and surface wave analyses are good for and what their strengths and weaknesses are, and we concentrate on observational aspects rather than the detailed interpretation thereof. The principal theoretical background is laid out only briefly. We describe some of the analysis tools and applications and give examples for cases that we have personally been involved in. We hope to convey to the reader some of the excitement a mode seismologist experiences still today when encountering the occasional beautiful set of time series that she or he may have to wait for a decade.