Empirical Instrument Response Recovery for Analysis of Tropical Cyclone Generated Microseism at Harvard, Massachusetts, 1936–1940

Abstract Oceanic microseisms are an ever-present and ubiquitous feature recorded in seismic data, and have been robustly recorded by seismometers for over a century. Because these signals act as a proxy for oceanic wave activity, they can be used to assess climate-sensitive phenomena such as increases in near-coastal wave state and changes in the behavior of oceanic storms over the last century. Such analyses cast valuable light on the oceanic climate of the pre-satellite era. Here, rescued legacy seismic data, originally on paper, for 1936 through 1940 from Harvard, Massachusetts, are digitized into usable time series, and instrument corrections derived from earthquake doublets are applied to allow for comparisons to modern data. For historical and modern data, spectrograms are calculated, instrument responses removed, and microseismic signals are extracted. The results are consistent with observed changes to tropical cyclone behavior in the western North Atlantic for the last century, with a period of relatively high activity in the mid-twentieth century, followed by a lull in the 1980s. Crucially, microseism trends from these results give a four times lower rate of change than if only modern seismic data were used, illustrating the value of the additional information that can be obtained from historical data. Extraction of such oceanic information from legacy seismic data provides valuable and irreplaceable quantitative data for future climate modeling and verification of hindcasts.