Oceanic transform faults are strike-slip boundaries—faults that move horizontally rather than up and down and connect offset mid-ocean ridge segments. They have long been regarded as simple "conservative" plate boundaries that slide past each other without creating or destroying Earth's crust. However, mounting evidence suggests that these faults are influenced by magmatism and hydrothermal circulation, exhibiting complex three-dimensional structures.

In a study published in Science on June 25, a team led by Prof. ZHANG Haijiang from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences demonstrated that these faults are dynamic systems governed by fluids, tides, and magmatism.

In this study, the researchers used data from a dense ocean-bottom seismometer array deployed on the Gofar transform fault of the East Pacific Rise—an area that arrests large earthquake ruptures but hosts abundant microseismicity. Analyzing continuous waveforms from 2019 to 2022, they discovered for the first time persistent harmonic tremors originating within the upper 4.5 km beneath the seafloor.

Tremor is a sustained, non-impulsive seismic signal. It is highly sensitive to weak stress perturbations such as tides and can serve as a proxy for fluid activity within fault zones. However, direct evidence of tremor within oceanic transform faults had previously not been obtained.

The researchers discovered that these signals were modulated by semidiurnal tides and revealed a cyclic "sealing-pressurization-rupture-drainage" mechanism. For example, they observed that tremor amplitude showed a strong correlation with semidiurnal tides before an M4 earthquake on September 8, 2020, indicating that the fault was in a near-critical state. Immediately after the rupture, however, this correlation collapsed, microseismicity surged, and compressional to shear wave velocity ratios (Vp/Vs) increased, indicating that the rupture had opened sealed fracture networks, expelling accumulated gases and allowing liquid reinfiltration. Over subsequent weeks, as mineral precipitation gradually resealed the fractures, the tide-tremor correlation slowly recovered, initiating a new cycle. Similar behavior was observed around multiple M4 earthquakes.

Based on these observations, the researchers proposed a "valve-like" cycle model. In the sealing stage, mineral precipitation closes fractures, deep volatiles accumulate, pore pressure rises, and the system becomes highly sensitive to tidal stresses, producing harmonic tremor. In the rupture stage, earthquakes break seals, expel gases, weaken tide-tremor coupling, and trigger microseismicity. In the recovery stage, hydrothermal sealing resumes, and the system returns to a pressurized state, restarting the cycle.

This study provides the first evidence of tidally modulated tremor on an oceanic transform fault, demonstrating the dynamic nature of oceanic transform faults. In addition, it offers a new framework for understanding transform fault mechanics and seismic hazards, with implications for understanding deep-sea hydrothermal systems and guiding mineral exploration.