Seismo Lab Seminar

Extensive continuous digital seismic data archives enable the analysis of Earth's long-period seismic wavefield across nearly four decades. This seminar considers primary and secondary microseism intensity between 4 and 20 s period between 1988 and 2024. 73 stations from 82.5 deg. N to 89.9 deg. S latitude from the Global Seismographic, New China Digital, and GEOSCOPE networks are used, all with >20 years of data and >80% data completeness. Acceleration power spectral densities are estimated using 50%-overlapping, 1-hr moving windows and are integrated in 2-s wide period bands to produce band-passed amplitude time series. We remove nonphysical outliers, earthquake signals, and stationary seasonal variations (using a fundamental period of 365.2422 d). Secular period-dependent trends are then estimated using L1 residual norm-minimizing regression. Increasing microseism amplitude is observed across most of the Earth in both the primary and secondary microseism bands (averages of 0.16 and 0.09 %/yr, respectively). The two microseism secular change rates correlate across global seismic stations at R=0.65 and have a regression slope of 1.04. However, secondary trends are systematically lower by about 0.05 %/yr, which is consistent with variable excitation of the secondary source relative to the primary due to its dependence on interfering ocean waves. Multiyear large-scale seismic intensity variations reflect interannual climate index (e.g., El Niño–Southern Oscillation) influence on large-scale storm and ocean wave energy. Microseism intensity histories in 2-s period bands exhibit regional to global correlations that reflect ocean-basin-scale teleconnected ocean swell, long-range Rayleigh wave propagation and attenuation, and the large-scale reach of climate variation. Secular trends in the global long-period wavefield as a function of period indicate greater intensification at longer periods, consistent with more frequent large-scale storm systems that produce longer-period ocean swell.