Chapter One - Inferring Ocean Storm Characteristics from Ambient Seismic Noise: A Historical Perspective

Abstract

Abstract Microseisms seen on seismograms worldwide were once viewed as “noise” contaminating records of earthquakes. However, these low-amplitude oscillations generated by storms over the oceans are now recognized as carriers of an important meteorological “signal”. Decades-long archives of analog seismograms may thus represent a high-resolution record of climate change significantly longer than those based on traditional meteorological observations. One of the first phenomena investigated by the then-new field of seismology, microseism research began with their identification around 1870. Improved characterization came from subsequent investigations in Europe, Japan, and North America, which sought out their sources and source regions. Two-generation mechanisms were identified in the mid-twentieth century. In both, microseisms originate with atmospheric energy in the form of storms over the oceans. It is coupled into the water column via the generation of ocean swell, transmitted to the seafloor, and then travels as elastic waves at the seafloor. Analysis of secondary microseisms, recorded in eastern North America during August 1992 Saffir/Simpson category 5 hurricane Andrew, shows the feasibility of using these signals to identify North Atlantic Ocean hurricanes. The shift in dominant microseism frequency with Andrew intensification demonstrates that these microseisms were generated over the deep waters of the North Atlantic Ocean at or near the hurricane and are thus a near real-time record of hurricane changes. Variations in secondary microseism frequency and amplitude allow detection of the hurricane while over the ocean and up to ∼2000 km from the recording station. Analog seismograms from seismic stations in North America may thus document unobserved North Atlantic hurricanes. However, uncertainties remain. The relative contributions of deep- and shallow-water sources remain uncertain, and the generation of microseisms with transverse wave components lacks a satisfactory explanation. Better understanding of the various controls on microseism properties is necessary before information in these waveforms can be used to infer storm characteristics, especially for less-energetic storms.