Seismographs record an extraordinarily diverse variety of signals that range from earthquakes and volcanic eruptions to anthropogenic noise. It has been noted since the late 19th century that all long-period seismographs on Earth also record a continuous background signal between periods of approximately 8 and 20 s. This signal was understood by the mid-20th century to primarily consist of a global seismic surface wave field that is incessantly maintained by storm-driven ocean gravity waves.
The global microseism is produced by two distinct source processes. The most energetic of these, the secondary microseism mechanism, induces pressure fluctuations on the sea floor as a result of interfering ocean wave trains propagating in opposing directions. The requisite interfering wave fields may arise from coastal reflections, multiple storm sources, or interference within a single large and/or rapidly moving storm system. The resulting generalized standing wave pressures on the ocean floor occur at half the period of the ocean wave, so the secondary signal spectrum is accordingly period-shifted relative to the ocean wavefield. Generation of the secondary microseism requires additional (interfering wave) conditions in addition to the presence of ocean waves per se. The second (primary) component of the global microseism, which my co-authors and I analyzed in this study, arises from direct subsurface traveling ocean wave tractions upon the seafloor and is thus a more straightforward proxy for ocean wave energy than the secondary mechanism. The primary signal spectrum occurs at the ocean wave period and can thus be distinctly observed relative to the shorter period and more energetic secondary signal, particularly at periods between 14–20 s corresponding to deep water wave wavelengths between 310–620 m. Dynamic pressures of ocean waves fall off exponentially with depth and inverse wavelength, and the primary microseism at 14–20 s is generated by wave-induced pressure fluctuations at depths less than about 300 m, which constitute about 11% of the global seafloor and lie mostly near continental and island coasts. Measuring the intensity of the primary signal thus provides a highly spatially and temporally integrative proxy for wave intensity across Earth's near-coastal seafloor. Small perturbations in this ocean wave energy induce proportional changes in seismic wave energy that can be quantified from continuous digital seismographic data on time scales from seconds to decades.
We performed an analysis using data from 52 stations of the US National Science Foundation and US Geological Survey Global Seismographic Network that were selected solely for their long and continuous operation. Results show that median near-coastal median ocean swell energy has increased at 3σ significance over the past 30 to 35 years at 79% (41) of these stations. seasonal and long-term signal variations are also correlated across 40 or more degrees of spatial distance from station-to-station, confirming the extensive spatial averaging of primary microseism metrics. The greatest absolute rates of increase are observed near the Antarctic Peninsula by the seismograph at Palmer Station (photo above from earthquake.usgs.gov/monitoring/operations/stations/IU/PMSA/), which shows a positive energy trend of 0.58±0.15%/yr (±3σ). However, in proportion to historical median values, several stations in North America with preferential sensitivities to wave climate in the northern Atlantic Ocean exceed the Palmer Station rate of secular energy increase, with annualized rates there exceeding 0.8%/yr. Global average rates of energy increase for all 52 stations are 0.27±0.03%/yr since the late 1980s and 0.35±0.04%/yr (±3σ) since January of 2000. In addition the long geographic reach of El Niño/La Niña influences on ocean wave energy are evident in these data.
Anthropogenic global warming-driven increased global wave energy, compounded with global sea level rise and larger storm surge scenarios, poses dire challenges to coastal cities, infrastructure, and ecosystems. Microseism-based measures of increasing ocean wave energy provide complementary period-dependent metrics for monitoring and studying Earth's ocean wave climate, and the corresponding storms that produce large waves. Recent studies have also shown that analogue records from earlier seismographs can be processed to extract wave metrics extending as far back as the early 1930s, and thus expand quantitative understanding of evolving ocean wave climate to the centennial scale.