The (top) observed and (bottom) predicted global excitation patterns can serve as a template for future investigations of global microseism hot spot activity.
Based on the typical frequencies of these wave fields, they are categorized into seismic hum (1-20mHz), primary microseisms (0.
The following subsections briefly introduce 20th-century research on this topic (for detailed, comprehensive reviews, see for microseisms (10) and for seismic hum, (11) for example).
Observations of microseisms date back to the late 19th century.
For example, Gherzi (1945) (18) considered microseisms to be excited by "pumping" of the storm associated with low pressure.
The earlier hypothesis by Wiechert (1904) (21) was that microseisms are excited by the impact of ocean waves breaking against a steep coast.
Moreover, the first-order effects are insufficient to excite secondary microseisms because the horizontal wavelengths of secondary microseisms are much longer than those of ocean surface gravity waves.
46,51) For the previous 20 years, seismic hum had been accepted as merely observational background noise, although microseisms above 0.
A beamforming method (62, 65, 66) with a dense seismic array can feasibly be used to locate the sources of microseisms (67, 68) and seismic hum.
A backprojection method is a powerful tool for locating the source of microseisms when travel time can be inferred accurately with a seismic velocity model.
However, body wave microseisms propagate in the mantle, which is much more transparent than the crust.
85) Figure 3 shows their precise locations of P-wave microseisms at 0.