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. 2023 Nov 1;14(1):6984.
doi: 10.1038/s41467-023-42673-w.

Increasing ocean wave energy observed in Earth's seismic wavefield since the late 20th century

Richard C Aster  1 , Adam T Ringler  2 , Robert E Anthony  2 , Thomas A Lee  3
Affiliations

Increasing ocean wave energy observed in Earth's seismic wavefield since the late 20th century

Richard C Aster et al. Nat Commun. .

Abstract

Ocean waves excite continuous globally observable seismic signals. We use data from 52 globally distributed seismographs to analyze the vertical component primary microseism wavefield at 14-20 s period between the late 1980s and August 2022. This signal is principally composed of Rayleigh waves generated by ocean wave seafloor tractions at less than several hundred meters depth, and is thus a proxy for near-coastal swell activity. Here we show that increasing seismic amplitudes at 3σ significance occur at 41 (79%) and negative trends occur at 3σ significance at eight (15%) sites. The greatest absolute increase occurs for the Antarctic Peninsula with respective acceleration amplitude and energy trends ( ± 3σ) of 0.037 ± 0.008 nm s-2y-1 (0.36 ± 0.08% y-1) and 4.16 ± 1.07 nm2 s-2y-1 (0.58 ± 0.15% y-1), where percentage trends are relative to historical medians. The inferred global mean near-coastal ocean wave energy increase rate is 0.27 ± 0.03% y-1 for all data and is 0.35 ± 0.04% y-1 since 1 January 2000. Strongly correlated seismic amplitude station histories occur to beyond 50∘ of separation and show regional-to-global associations with El Niño and La Niña events.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1
Fig. 1. Primary microseism vertical component acceleration amplitude histories at 52 long-operational seismic stations with associated robust trend estimates.
Trends (equation (6)) are estimated for time series with stationary seasonal harmonic functions (equation (1)) subtracted (Fig. 2, Table 2). Time series are displayed after applying 3-year moving median data smoothing for plotting clarity while trend values are calculated from daily sampled two-month (61-day) moving median filtered data. Trends for all available data, and for post-2000 data, are shown in red and black, respectively. Title colors indicate latitude, longitude (ϕ, ℓ) defined regions as follows: Blue: European North Atlantic (ϕ > 0∘, −43∘ < ℓ < 43∘); Red: Mid-North America and North Atlantic (0∘ < ϕ < 55∘, −111∘ < ℓ < −60∘); Cyan: Southwest Hemisphere (ϕ < 0∘, ℓ < 0∘); Green: Southeast Hemisphere (ϕ < 0∘, ℓ > 0∘); Black: Northern Hemisphere Pacific and Asia outside of Blue and Red groups. Time series with negative trends have italicized titles. Time axis tick marks correspond to 1 January of the indicated years. Corresponding seismic energy histories are shown in Supplementary Fig. 3.
Fig. 2
Fig. 2. Station locations and global trends (red positive, cyan negative) for vertical component acceleration amplitude, vertical component acceleration amplitude normalized by historical median, and vertical component seismic energy normalized by historical median.
a absolute (RA), b percentage (PA; equation (2); Table 2) seismic acceleration amplitude trends (red: positive; cyan: negative). c Percentage velocity squared energy proxy trends (PE; equation (3); Table 3). Circle radius is proportional to the trend value and white rims indicate 3σ trend significance (Fig. 3).
Fig. 3
Fig. 3. Microseism trend results by station (red: positive; cyan: negative) sorted from most postive to most negative.
a, b Acceleration amplitude percentage trends (Fig. 2b; Table 2; equation (2)) for absolute and percentage trends, RA and PA, respectively. c, d Energy (velocity amplitude squared) absolute and percentage trends RE and PE (Fig. 2c; Table 3; equation (3)). Blue and black data points and accompanying 3σ error bars reflect estimates obtained using the complete time series A(t) and E(t) and those obtained with associated seasonal harmonic trends (equation (1)) subtracted as indicated in subfigure legends. x axis rank indicates the largest-to-smallest order of trends fitted to data with stationary seasonal components subtracted (black data points).
Fig. 4
Fig. 4. Vertical component acceleration amplitude trends RA calculated with seasonal harmonics subtracted versus historical station median acceleration amplitude with 3σ confidence intervals.
The correlation coefficient is 0.213 and 13 stations exhibit positive trends at 3σ significance that are greater in absolute value than at the most negative station (HKT; Hockley, Texas). Colors reflect geographic groups defined in Fig. 1. Dotted lines indicate representative percentage amplitude changes PA relative to the historical station median (Figs. 1b, 2b, 4; Table 2).
Fig. 5
Fig. 5. Clustering results for all stations based on correlated vertical component seismic acceleration histories.
a Microseism acceleration station clustering derived from correlation (Fig. 6, Supplementary Fig. 6) of detrended 61-day moving median microseism acceleration amplitude time series with seasonal harmonics (equation (1)) and secular trends (Fig. 1; Table 2) removed. D denotes the Ward dissimilarity metric. Regions of ocean bathymetry with depths of <300 m and candidate primary microseism source zones (equation (5)) correspond to the color bar. Transparent geographic boundaries and their colors, and station names below the dendrogram, correspond to general regional associations noted in Fig. 1 and Supplementary Fig. 3. b Dendrogram classification of station groups corresponding to a: SWP Southwest Pacific, SAIP South Atlantic, Indian, Pacific, SEPSWA Southeast Pacific and Southwest Atlantic, WCP Western and Central Pacific, ENA Eastern North America, NWPEA Northwest Pacific and east Asia, EUR Europe and Southwest Asia. ENSO and SOI indicate dendrogram correlation-based associations for equivalently smoothed El Niño and Southern Oscillation index time series (Supplementary Fig. 7).
Fig. 6
Fig. 6. Overlain vertical component acceleration time series (61-day smoothing) for station clusters and all stations.
Microseism acceleration time series clusters are defined from the associations shown in Fig. 5 and are normalized by respective station medians (Supplementary Fig. 4). Figure 7 shows corresponding 3-year smoothed time series. Black time series show the median of all smoothed time series for each subfigure. ENSO and SOI indices (Supplementary Fig. 7) scaled by seismic data are plotted in red within associated SWP and SEPSWA clusters, respectively.
Fig. 7
Fig. 7. Vertical component acceleration and vertical component seismic energy time series (3-year smoothing), and time series (61-day smoothing) correlations with inter-station angular distance.
a Microseism acceleration histories clustered using the dendrogram of Fig. 5 with three-year moving median data smoothing as in Fig. 1. Figure 6 shows the corresponding 61-day smoothed time series. ENSO and SOI indices, scaled by seismic time series amplitudes, are plotted in red within their associated SWP and SEPSWA clusters, respectively. Black time series show the median of all smoothed data across each cluster. b Global seismic energy time series with seasonal harmonics (equation (1)) removed, normalized by respective station medians (Supplementary Fig. 4), and smoothed with a 3-year moving median window. The median across all-time series is shown in black. Dashed energy trends correspond to 0.27% y−1 (green) and 0.35% y−1 (blue) from this study for the two indicated data periods, and to 0.47% y−1 (red, for 1948–2008, annually compounded) from ref. . Trends are normalized to one at 1 January 2005, and ±15% vertical shifts are imposed on the blue and red trends, respectively, for plotting clarity. Global energy excursions labeled A–E corresponding to 3-year moving median ENSO and SOI (Supplementary Fig. 7) excursions as indicated (red) in the SWP and SEPSWA panels in a. Time axis tick marks correspond to 1 January of the indicated years. c Correlation versus inter-station great-circle distance for demeaned and detrended 61-day median smoothed time series (Fig. 6). Black curves show correlation mean and ±1 standard deviation with 7.5° smoothing.
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