Ocean wave sources of seismic noise

Abstract

[1] Noise with periods 3 to 10 s, ubiquitous in seismic records, is expected to be mostly generated by pairs of ocean wave trains of opposing propagation directions with half the seismic frequency. Here we present the first comprehensive numerical model of microseismic generation by random ocean waves, including ocean wave reflections. Synthetic and observed seismic spectra are well correlated (r > 0.85). On the basis of the model results, noise generation events can be clustered in three broad classes: wind waves with a broad directional spectrum (class I), sea states with a significant contribution of coastal reflections (class II), and the interaction of two independent wave systems (class III). At seismic stations close to western coasts, noise generated by class II sources generally dominates, but it is intermittently outshined by the intense class III sources, limiting the reliability of seismic data as a proxy for storm climates. The modeled seismic noise critically depends on the damping of seismic waves. At some mid-ocean island stations, low seismic damping is necessary to reproduce the observed high level and smoothness of noise time series that result from a spatial integration of sources over thousands of kilometers. In contrast, some coastal stations are only sensitive to noise within a few hundreds of kilometers. This revelation of noise source patterns worldwide provides a wealth of information for seismic studies, wave climate applications, and new constraints on the possible directional distribution of wave energy.

Continental microseismic intensity delineates oceanic upwelling timing along the west coast of North America

“…Numerical simulations of wind-generated microseisms by Ardhuin et al [2011Ardhuin et al [ , 2012 show that the wave reflection mechanism generally dominates in eastern boundary regions and is particularly dominant off central California in winter. Microseisms recorded at seismological stations typically represent sources within a few hundred kilometers of the coast [Bromirski et al, 2013].…”

“…A subsequent analysis by Hasselmann [1963] showed that seafloor Rayleigh waves are generated when the vector sum, K 12 = k 1 + k 2 , of the nearly identical magnitude and nearly oppositely directed wave numbers k 1 and k 2 of the interacting wave trains matches the wave number K of one of the possible seismic modes for given frequency ω [see also Ardhuin and Herbers, 2013]. This type of microseismic activity is common in coastal regions where onshore propagating trains of gravity waves are reflected from the continental boundary [Bromirski and Duennebeir, 2002;Ardhuin et al, 2011].…”

“…In addition to enabling us to use standard statistical methods, the log-transform has a physical justification in that the energy of the wind waves generating the microseisms has a power law dependence on wind duration, speed, and fetch [Komen et al, 1994;Ardhuin et al, 2011]. To more closely emulate the positive and negative phases of the upwelling index (UI), we next generated 6 hourly residual time series log S m j…”

“…There are numerous meteorological buoys measuring waves in the coastal ocean (http://www.ndbc.noaa.gov), but these report only significant wave height and peak wave period, which may be strongly affected by distal storm events and may not therefore be fully representative of regional wind forcing. These limitations, together with studies showing that storm-generated waves are responsible for the relatively intense~0.2 Hz ground motions recorded at land-based stations [e.g., Bromirski and Duennebeir, 2002;Kedar and Webb, 2005;Aster et al, 2008;Bromirski, 2009;Ardhuin et al, 2011], suggested that we could use microseismic activity as a reliable measure of wind-generated surface wave intensity, and hence the timing of the seasonal upwelling cycle along continental margins.…”

Wave height quantification using land based seismic data with grammatical evolution

“…Satellite altimetry can be used to track large storm cells as they evolve and move along a storm path since they occur over long time periods, but cannot be used as a real time ocean wave height monitoring tool. There have been significant improvements in numerical wave models in recent years with the introduction of coastal reflection and associated effects [4]. As with any numerical model, there are associated errors due to incomplete understanding of the physics of the medium.…”