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 a striking regularity, occurring every five to six years and rupturing nearly identical sections of the fault with similar magnitudes. This level of consistency is highly unusual in earthquake science, leaving researchers perplexed as to the underlying cause.
In a groundbreaking study published in the journal Science, a team of seismologists led by Jianhua Gong has finally unraveled the mystery. The research reveals that special regions within the fault act as natural braking systems, repeatedly stopping earthquakes from escalating in size. These braking systems are not just passive features but are highly complex and dynamic.
The Gofar transform fault, located along the East Pacific Rise off Ecuador's western coast, has been a subject of extensive study due to its unique characteristics. The fault is where the Pacific and Nazca tectonic plates slide past each other at a rate of about 140 millimeters per year. What sets Gofar apart is the consistent pattern of its larger earthquakes, which start and stop in nearly the same locations, with quieter stretches of the fault absorbing stress without producing large ruptures.
To investigate this phenomenon, researchers deployed ocean bottom seismometers along two segments of the Gofar fault during two major seafloor experiments. These instruments captured tens of thousands of tiny earthquakes, providing an unprecedented level of detail about the fault's behavior before, during, and after major ruptures. The study revealed a consistent pattern in the barrier zones, which are the quieter stretches of the fault.
In the days and weeks leading up to 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 is at play in both cases.
The researchers discovered that the barriers are not inactive sections of rock but rather 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, evidence suggests 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 a rapid drop in pressure inside the fluid-filled rock. This drop in pressure causes the porous rock to temporarily lock up, slowing or stopping the rupture from spreading and becoming larger. These barrier zones act as built-in brakes within the fault, preventing earthquakes from escalating in size.
The implications of this discovery are far-reaching. The Gofar fault, while located far from heavily populated coastlines, may have broader implications for earthquake science worldwide. Transform faults similar to Gofar are found throughout the Earth's oceans, and the study suggests that barrier zones like those found at Gofar may be common across the ocean floor. If so, they could function as a widespread system of natural earthquake brakes, preventing some ruptures from escalating into even larger events.
This research has the potential to improve earthquake models used to estimate seismic hazards along underwater faults worldwide, including regions closer to major coastal populations. The study 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 phenomena.
