Scientists have discovered a fascinating phenomenon deep beneath the eastern Pacific Ocean, where a seafloor fault exhibits remarkable consistency in its earthquake patterns. For decades, this fault has been producing magnitude 6 earthquakes with astonishing regularity, occurring every five to six years and repeatedly rupturing the same sections of the fault. This level of consistency is highly unusual in earthquake science, leaving researchers perplexed as to how it could be maintained so reliably.
Now, a groundbreaking study published in the journal Science has shed light on this mystery. The research reveals that special regions within the fault act as natural braking systems, repeatedly stopping earthquakes from growing larger. These braking systems are not just passive features but are active and dynamic parts of the fault system.
The study, led by seismologist Jianhua Gong, focused on the Gofar transform fault, located along the East Pacific Rise off Ecuador's western coast. The Gofar fault is a deep underwater fracture where the Pacific and Nazca tectonic plates slide past each other at a rate of about 140 millimeters per year. What makes this fault particularly intriguing is the pattern of its larger earthquakes, which start and stop in nearly the same locations.
To investigate this phenomenon, researchers used data collected during two major seafloor experiments, one in 2008 and another between 2019 and 2022. They placed ocean bottom seismometers directly on the seafloor along two parts of the Gofar fault, capturing tens of thousands of tiny earthquakes before and after two major magnitude 6 events. This detailed data revealed a consistent pattern in the barrier zones, which are quieter stretches of the fault that absorb stress without producing large ruptures.
The researchers discovered that in the days and weeks before a major earthquake, these barrier zones experienced bursts of small seismic activity. Immediately after the larger quake, these regions became almost completely quiet. This behavior was observed in two separate fault segments studied 12 years apart, indicating that the same physical process was at play in both cases.
The study explains that the barriers are not inactive sections of rock but are highly complex areas where the fault breaks into multiple strands. Small sideways offsets between these strands create localized openings within the fault structure, similar to small gaps inside a crack. Additionally, researchers found evidence that seawater seeps deep into these fractured zones, leading to a process called "dilatancy strengthening."
During a large earthquake, the sudden movement along the fault causes pressure inside the fluid-filled rock to drop rapidly, causing the porous rock to temporarily lock up, slowing or stopping the rupture. This natural braking system, formed by the barrier zones, acts like built-in brakes inside the fault, preventing the earthquake from escalating in size.
The implications of this discovery are far-reaching. The Gofar fault is located far from heavily populated coastlines, but the findings suggest that similar barrier zones may be common across the ocean floor. If so, these natural braking systems could be widespread, preventing some ruptures from escalating into even larger events. This knowledge could significantly improve earthquake models used to estimate seismic hazards 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, highlighting the importance of international collaboration in advancing our understanding of earthquake science.
