Along the shores of Santa Barbara, California, the calming sounds of waves crashing bring a familiar comfort to beachgoers. However, beneath the soothing surf lies a complex world of acoustic phenomena invisible to the human ear. Scientists at the University of California, Santa Barbara (UCSB), have uncovered a rich spectrum of low-frequency sounds produced by breaking ocean waves—sounds that are far below the threshold of human hearing, yet carry vital information about coastal dynamics. This recent breakthrough involves the detection and characterization of infrasound and seismic waves generated by the surf, revealing an entirely new method to monitor sea conditions through acoustic and ground-motion signals.
The surf’s roar is more than just what we perceive with our ears. While the audible crashing of waves is familiar, much of the acoustic energy produced by breaking waves exists at frequencies below 20 hertz (Hz), known as infrasound. These low-frequency pressure waves, along with associated seismic vibrations passing through the ocean floor, provide a rich but hidden acoustic landscape. The research team at UCSB employed sophisticated arrays of infrasound sensors coupled with seismometers to probe these subtle signals. Their findings were recently published in Geophysical Journal International, detailing the unique acoustic and seismic footprints left by the surf and demonstrating the feasibility of pinpointing wave-breaking locations along the coastline through this approach.
The genesis of these inaudible waves lies in the physical mechanics of wave breaking. When waves collide with the rocky shore or seabed, air pockets get entrained, forming and collapsing bubbles that oscillate collectively due to pressure instabilities. Jeremy Francoeur, lead author and former UCSB graduate student, describes this phenomenon as a synchronized expansion and contraction of bubble clouds, generating persistent pressure oscillations. These oscillations translate into infrasound waves that travel upward through the atmosphere and downward as seismic waves through the Earth’s crust. Though these pressure variations lie beneath the human auditory range, their amplitude reaches levels comparable to everyday urban noise, making them significant natural acoustic sources.
Infrasound below 20 Hz includes ordinary acoustic waves, but their low pitch means that they go unnoticed by humans. Senior author and geophysicist Robin Matoza emphasizes that these “hidden sounds” originate from a broad spectrum of natural and anthropogenic activities worldwide. They include major geological and atmospheric events such as volcanic eruptions, earthquakes, landslides, hurricanes, and even atmospheric phenomena like auroras and wind flowing over mountainous terrain. Each source produces distinct low-frequency acoustic signatures, which scientists can harness to better understand and monitor Earth’s dynamic processes.
Motivated by UCSB’s coastal location, Matoza’s research group turned their attention to the acoustic mysteries of surf noise. Deploying an array of sensors at Coal Oil Point Reserve—a protected site within the UC Natural Reserve System—the researchers recorded infrasound and seismic data synchronized with high-definition video of wave activity. This multi-modal data set allowed them to correlate specific acoustic pulses with precise moments of wave breaking, greatly enhancing signal identification compared to previous single-sensor studies. By aligning sound signals with video “snapshots,” the team was able to discern a robust acoustic fingerprint unique to breaking waves.
The infrasound signals identified arrived in repetitive bursts between 1 and 5 Hz frequency, distinctly marking the crashing surf’s rhythmic energy. Though “loudness” is a subjective notion tied to human hearing perceptions, the acoustic wave amplitudes measured reached remarkable levels. Typical surf-generated infrasound ranged between 0.1 and 0.5 pascals, comparable to the sound pressure of busy traffic, while stronger swells produced waves as intense as 1 to 2 pascals—akin to the noise of a factory floor. This quantitative analysis reveals that the ocean’s low-frequency acoustic emissions rival common urban soundscapes in energy, a startling discovery since these sounds remain inaudible.
A key insight emerged from the team’s exploration of how these infrasound signals relate to actual sea conditions. The researchers found a correlation between infrasound amplitude and significant wave height, a critical parameter measuring the vertical scale of open ocean swells. However, they noted that the relationship between acoustic data and observed wave behavior was more complex than initially hypothesized. The interplay of factors such as tides, wind patterns, and bathymetry introduced nonlinearities that challenged simplistic models, underscoring the need for further investigation into environmental influences on surf-generated acoustics.
The array’s capability extended beyond mere detection; by measuring minuscule variations in arrival times of infrasound waves at multiple sensors, the team applied reverse-time migration techniques to triangulate the exact origins of breaking waves. Remarkably, the analysis localized the acoustic source consistently to the rock shelf at Coal Oil Point. This led to the hypothesis that specific underwater topography concentrates wave impact zones, triggering synchronized bubble oscillations that amplify the infrasound output. Understanding these spatial patterns opens exciting avenues for mapping coastal processes through sound.
Looking ahead, the researchers aim to explore whether individual beach segments universally serve as primary infrasound emitters or if such patterns vary with geography and environmental conditions. Questions linger about how wave infrasound signatures might differ between globally diverse shorelines like Santa Barbara and Tahiti, or how dynamic factors like changing tides and fluctuating wind fields modulate these acoustic emissions. Unraveling these complexities will extend the applicability of surf acoustic monitoring as a powerful natural observatory tool.
Matoza’s lab enjoys unique advantages owing to the proximity of Coal Oil Point Reserve, only 2.5 miles from UCSB’s main campus. This closeness enables rapid deployment and iterative refinement of sensor arrays, facilitating extensive field experimentation and hypothesis testing. Moreover, students actively engaged in this project gain hands-on experience across the entire scientific workflow, from data collection and instrument installation to advanced signal analysis and scholarly writing. This immersion cultivates the next generation of geophysicists skilled in cutting-edge Earth science methodologies.
The ultimate goal is the development of an autonomous system capable of characterizing nearshore surf conditions solely from infrasound and seismic data, independent of visual observations. Current video monitoring technologies suffer from limitations imposed by darkness, fog, and adverse weather, which reduce visibility and reliability. Acoustic and ground-motion sensing could thus become indispensable complements, offering continuous, all-weather surveillance possibilities vital for coastal management, hazard preparedness, and environmental research.
This pioneering research sets a precedent in marine geophysics, unveiling a novel sensory interface through which the Earth’s atmospheric and oceanic interactions reveal themselves. By “listening” below the human audible spectrum, scientists can access a previously hidden domain of natural signals rich with information on coastal wave dynamics. As this technology matures and integrates with existing oceanographic tools, it promises significant advances in our understanding of ocean processes, coastal environments, and their responses to climate and anthropogenic changes.
Subject of Research: Acoustic and seismic signatures of breaking ocean waves
Article Title: Researchers characterize infrasound and seismic signals from surf to monitor coastal wave dynamics
News Publication Date: Not specified
Web References: https://academic.oup.com/gji/advance-article/doi/10.1093/gji/ggaf317/8236357
References: Study published in Geophysical Journal International
Image Credits: Elena Zhukova
Keywords: Space sciences; Seismology; Oceanography; Coastal processes
