Volcanic similar earthquakes driven by aseismic slip associated with volcanic activity at Ogasawara Ioto

Deep beneath the ocean surface, where the Earth's crust is constantly shifting, volcanoes do more than just erupt. They breathe, swell, and shudder with a constant, low-level seismic hum. To understand these restless giants, scientists listen to the tiny tremors that ripple through the rock, treating them like a heartbeat that reveals the health of the volcano. Among these tremors are a special kind of event called "similar earthquakes." These are not random jolts; they are nearly identical copies of one another, repeating over and over from the exact same spot. When a volcano produces these repeating shocks, it suggests that a small, specific patch of rock is slipping again and again, much like a door hinge that creaks in the same rhythm every time it opens. By studying these repetitive signals, researchers can map the hidden movements of magma and fluids deep underground, even when the ground above is not visibly shaking.

In the remote Izu-Ogasawara island arc, a small volcanic island known as Ioto (or Iwo-Jima) has been a focal point for such activity. This island is an active caldera volcano, a massive crater formed by ancient eruptions, and it is currently undergoing a dramatic transformation. For centuries, the entire island has been slowly rising, a process driven by magma pushing up from below. While this uplift is well-documented, the precise way the rock inside the island is responding to this pressure has remained a mystery. A recent study by researchers from the National Research Institute for Earth Science and Disaster Resilience in Japan has finally peeled back the curtain on this hidden activity. By analyzing thousands of seismic records collected over five years, they discovered that the island's constant rising is not just a smooth, silent lift. Instead, the ground is being pushed upward by a relentless series of tiny, repetitive slips along hidden faults, creating a distinct pattern of similar earthquakes that tells a clear story of stress and release.

The researchers began by sifting through a massive catalog of roughly 60,000 earthquakes recorded at Ioto between 2018 and 2023. Most of these events were high-frequency tremors, sharp and brief, typical of volcanic regions. Using a computer algorithm designed to find patterns in noise, they grouped the earthquakes based on how closely their waveforms matched one another. It is a bit like sorting a pile of thousands of handwritten letters to find the ones written by the same person; the handwriting style is so consistent that the origin is unmistakable. The team identified 19 distinct groups of these "similar earthquakes," where the waves looked almost identical, suggesting they were all coming from the same tiny source. Remarkably, these repeating events made up about one-quarter of all the volcanic earthquakes detected during the study period, proving that this repetitive slipping is a dominant feature of the island's seismic life, not a rare anomaly.

Once the groups were identified, the scientists worked to pinpoint exactly where these slips were happening. By combining the timing of the waves, the direction they traveled, and their strength, they mapped the locations of the 19 clusters. The results revealed a striking pattern: the similar earthquakes were not scattered randomly across the island. Instead, they clustered tightly around known active faults, particularly in the southwestern part of the island. Strikingly, no similar earthquakes were found directly beneath the central resurgent dome, a large, uplifted mound in the middle of the caldera. This absence is significant; it suggests that the rock directly under the dome is likely too hot or too fluid-filled to fracture in the brittle way required to create these repeating shocks. Instead, the stress is being released along the edges, where the rising dome rubs against the surrounding rock, causing small, repetitive slips on narrow fault lines.

The timing of these events offered another crucial clue. The researchers found a strong link between the frequency of these similar earthquakes and the rate at which the island was rising. When the ground lifted faster, the earthquakes happened more often. In fact, the time between these repeating slips was incredibly short, often just a few days. This is much faster than the repeating earthquakes seen along major tectonic plate boundaries, where such events might wait years or decades to recur. The speed of the recurrence at Ioto is a direct result of the island's rapid uplift, which is occurring at a rate of up to one meter per year. This intense pressure loads the small fault patches so quickly that they fail and slip again almost immediately, creating a rapid-fire sequence of tiny earthquakes.

To understand the physical nature of these slips, the team estimated the size of the fault patches and the amount of stress released. They calculated that the area of rock slipping in each event was incredibly small, with a radius of only about 21 meters. The amount of stress released, known as the stress drop, was also very low, measuring less than one megapascal for nearly all the clusters. To put this in perspective, this is a very gentle release of energy compared to the massive stress drops seen in large, destructive earthquakes. The low stress and small size suggest that the rock in these areas is weakened, likely by the presence of hot volcanic fluids and steam that lubricate the fault lines. Because the rock is so weak and the slipping area is so small, the earthquakes remain tiny, rarely exceeding a magnitude of 1.0, which explains why the island does not experience the large, damaging quakes seen at other volcanic sites.

The study also looked at the total amount of movement caused by these slips. By adding up the slip from all the similar earthquakes over the five-year period, the researchers found that the cumulative movement reached about three meters. This is a significant amount of motion, yet it accounts for only about 60 percent of the total uplift observed at the surface. This discrepancy reveals a vital piece of the puzzle: while the similar earthquakes represent a major release of stress, they are not the whole story. A substantial portion of the island's rise is happening silently, through aseismic slip, where the rock slides smoothly without generating any earthquakes. This means that the island is deforming through a combination of violent, tiny jerks and a steady, silent flow.

The findings paint a picture of a volcano that is alive and active, but in a way that is distinct from the explosive eruptions often associated with such places. The repetitive, small-scale earthquakes at Ioto are the sound of the crust adjusting to the immense pressure of rising magma. They are not signs of an imminent, catastrophic eruption, but rather evidence of a system that is constantly releasing stress through a network of small, lubricated faults. The research confirms that the island's rapid uplift is driving these repetitive slips, and that the presence of volcanic fluids plays a key role in keeping the fault strength low. While the exact geometry of the faults remains difficult to pinpoint due to the limited number of seismic stations on the small island, the overall picture is clear: Ioto is a dynamic system where the ground is constantly shifting, slipping, and rising, driven by the slow, powerful push of the Earth from below. This understanding helps scientists better monitor volcanic activity, distinguishing between the harmless, repetitive adjustments of a rising dome and the more dangerous signals that might precede a major eruption.