Geology topics

Frederik Simons

Publications and source records attributed to Frederik Simons.

2 recordsLinked to original sources

USGS2026

Global seasonal microseism intensity and hemispheric asymmetry

Earth’s microseism wavefield dominates seismic background levels at periods between approximately 4 and 20 s, and reflects periodic and secular variations in ocean swell energy. Ocean wave energy couples to the seismic wavefield via distinct primary microseism and secondary microseism (PM and SM) source mechanisms, which are excited by basal swell tractions and seafloor pressure variations due to crossing seas, respectively. This study examines annual amplitude variations for the globally dominant PM (14–20 s) and SM (4–10 s) period bands. Annual harmonic variations are represented by four‐term Fourier series fits to vertical‐component seasonally smoothed acceleration time series from 73 stations in the Global Seismographic and GEOSCOPE networks with over 20 yr of recording and at least 75% data completeness. These annual periodic functions fit between 14%–95% (PM) and 22%–97% (SM) of signal variance. Station annual peak‐to‐peak variations range between 1.2–14.3 dB (PM) and 1.5–20.6 dB (SM). An asymmetry in microseism features exists between the Northern (NH) and Southern (SH) Hemispheres. High‐latitude NH stations show highly correlated PM and SM annual amplitude variations. This character dominates the wider extratropical NH but diminishes at tropical latitudes, and widespread relative PM–SM decorrelation is observed in the SH. These hemispheric characteristics reflect systematic differences in both extratropical storm activity and ocean wave state. Greater annual variation and seasonal predictability in the NH reflect the influence of the large continental landmasses that enhance both storm intensity seasonality and SM‐generating coastal wave reflection. Notable clusters of low PM–SM correlation stations are also observed in continental Antarctica due to seasonal sea ice influences, and in East Asia, reflecting unusual PM and SM source responses to South Asian monsoonal and tropical cyclone ocean wave influences.

Seismological Research Letters↗

USGS2026

Global primary and secondary microseism multi-decade geographic variation, secular intensification, and period lengthening

Earth's long-period background seismic wavefield is dominated by two distinct processes that couple ocean wave energy to a global microseism wavefield. We assess global microseism intensity in the secondary (4–10 s) and primary (14–20 s) bands, and across eight 2 s-wide period bands between 4 and 20 s. Robustly estimated primary and secondary secular amplitude trends are estimated at 73 globally distributed seismic station sites with continuous recording spanning at least 20 years, from as early as the late 1980s through October 2025. These trends are positive at 3⁢𝜎 significance for 61 (84%) and 46 (63%) stations with global average rates for vertical-component acceleration of 0.17 ± 0.04 and 0.11 ± 0.05%/yr, for the primary and secondary bands, respectively, with corresponding rates of energy increase of 0.27 ± 0.08 and 0.15 ± 0.09%/yr. Secular intensification is also observed within all 2 s period bands between 4 and 20 s. Amplitude histories for the longest primary microseism periods (18–20 s) correlate to near-antipodal distances, reflecting long-range teleconnections attributed to large-fetch storm systems, long-range swell and Rayleigh wave propagation, and geographically correlated El Niño Southern Oscillation and other geographically extensive atmospheric influences on storms and waves. The lower average rates of intensification for the secondary microseism suggest that crossing wave systems in remote regions are either under-observed or are intensifying more slowly than the primary microseism, possibly due to increasing swell unidirectionality. Secular intensification is greatest at the longest primary microseism periods. This is consistent with a broadening of the global ocean wave spectrum by approximately 0.01%/yr which may reflect an increasing occurrence of large storm systems.

JGR Solid Earth↗