Title: Multi-Decadal Global Microseism Intensity and Ocean Wave Climate
Abstract: It has been recognized since the dawn of global seismology that the oceans produce a planet-wide continuous and seasonally varying microseism signal. Extensive standardized global digital seismographic network archives such as the Global Seismographic Network (GSN) and GEOSCOPE now facilitate the uniform analysis of microseism energy across four decades, including signatures of anthropogenic climate change. The primary microseism signal near 20–14 s period sensed by long-operational seismic stations is a particularly apt proxy for global near-coastal wave energy because it is produced by ocean wave seafloor tractions applied at depths of less than about 300 m. The primary microseism is also less sensitive to ocean wave and bathymetric complexities that influence energetic and (more widely studied) secondary microseism signal. Examining continuous vertical component seismic records beginning in the late 1980s that are principally sensitive to Rayleigh waves, robust secular estimates reveal highly significant and geographically correlated increasing primary microseism amplitudes and energies at most long-running GSN stations. Particularly high absolute rates of increase are observed for station PMSA on the Antarctic Peninsula with seismic (acceleration) amplitude and (velocity-squared) energy trends through August 2022 (±3σ confidence intervals) of 0.36±0.08 %/y relative to the long-term station median) and 4.16±1.07 (nm/s)2/y (0.58±0.15 %/y), respectively. Assuming linearity and stationary ocean wave-seismic wave coupling, the corresponding inferred rate near-coastal ocean wave energy increase sensed by global seismographs is 0.27±0.03 %/y for the full record and 0.35±0.04 %/y since the turn of the 21st century. These inferred rates of ocean wave intensification corroborate independent estimates from oceanographic and meteorological data. Time-smoothed (e.g., 2-month) primary microseism signal station histories regionally cluster out to thousands of kilometers of interstation separation and demonstrate the geographically large-scale sensitivity kernels for the primary proxy. Multi-year primary microseism signal variations correlate well with El Niño and La Niña conditions that affect the distribution of geographically large-scale storm energy. Parallel analysis of the (much more energetic) 5–12 s period secondary microseism signal shows primary-consistent but higher temporal and geographic variability reflecting the ocean depth and wave-wave interference conditions necessary for its excitation. These multidecadal near-continuous time series contain additional deep information on the history of ocean waves. For example, primary - secondary comparative histories display station-specific seasonal phase and amplitude relationships reflecting seasonal ocean wave variability. These types of data products can also be analyzed down to the hourly level for analysis of extreme wave events over time and in near-real-time.
