Scientists have discovered a fascinating phenomenon deep beneath the eastern Pacific Ocean, where a seafloor fault exhibits remarkable consistency in producing magnitude 6 earthquakes every five to six years. This regularity has long puzzled researchers, but a recent study published in the journal Science offers a groundbreaking explanation: natural braking systems within the fault itself. These braking systems, akin to built-in brakes, prevent earthquakes from escalating in size, providing valuable insights into earthquake science and forecasting.
The Gofar transform fault, located along the East Pacific Rise, has been a subject of extensive study due to its unique behavior. Unlike other faults, the Gofar's larger earthquakes consistently start and stop in nearly the same locations, with quieter stretches of the fault absorbing stress without producing large ruptures. These quieter sections, referred to as 'barriers', have now been identified as complex areas where the fault breaks into multiple strands, creating localized openings and trapping seawater.
The study, led by seismologist Jianhua Gong, and conducted in collaboration with various institutions, employed ocean bottom seismometers to capture tens of thousands of tiny earthquakes before and after major magnitude 6 events. The researchers discovered that the barrier zones experienced bursts of small seismic activity in the days and weeks leading up to a major earthquake, followed by a period of quiet after the larger quake occurred. This pattern was observed in two separate fault segments, studied 12 years apart, indicating a consistent physical process.
The barriers, it was found, are not inactive rock sections but rather highly complex areas where the fault breaks into multiple strands with small sideways offsets. These offsets create localized openings and, combined with trapped seawater, lead to a process called 'dilatancy strengthening'. During a large earthquake, the sudden movement along the fault causes a drop in pressure inside the fluid-filled rock, causing the porous rock to lock up temporarily, slowing or stopping the rupture.
This discovery has significant implications for earthquake science and forecasting. The Gofar fault, located far from heavily populated coastlines, may not directly threaten people, but the findings suggest that similar barrier zones could be common across the ocean floor. If so, these natural braking systems could prevent some ruptures from escalating into larger events, improving earthquake models and hazard estimates along underwater faults worldwide, including regions closer to major coastal populations.
The research was funded by the U.S. National Science Foundation and the Natural Sciences and Engineering Research Council of Canada. This breakthrough not only enhances our understanding of earthquake limits on these faults but also highlights the importance of natural mechanisms in controlling earthquake size, offering a more comprehensive approach to earthquake forecasting and hazard assessment.
