Noto Quake Mystery: Dual Epicenters Uncovered

The first seven months of 2024 have been remarkably eventful, and it's easy to overlook that the year began with a magnitude 7.5 earthquake hitting Japan's Noto Peninsula on New Year's Day. This earthquake tragically claimed over 280 lives and damaged more than 83,000 homes.

Recent findings reveal that this earthquake started almost simultaneously at two distinct points on the fault line. This rare "dual-initiation" phenomenon allowed the seismic rupture to encircle and breach a tough section of the fault, known as a barrier. The intense pressure from both sides of this barrier led to a powerful release of energy, resulting in severe shaking across the Noto Peninsula.

Before the earthquake, the region experienced a series of seismic swarms, which are clusters of smaller tremors that can sometimes lead to a larger, catastrophic quake. Researchers employed advanced seismic and geodetic technologies to analyze these pre-quake tremors in detail.

Published in the journal Science, the study provides new insights into the role of fault barriers, or asperities, in the development of earthquakes. This research is set to enhance our understanding of seismic risk and improve future earthquake predictions.

Earthquakes occur when blocks of rock on either side of a fault shift past each other. This movement isn’t uniform along the fault because the surface is uneven, which helps to dissipate energy and eventually halts the movement.

A fault barrier is a rough segment that holds the fault sides together, absorbing the energy and potentially preventing further movement. However, if the energy exceeds the barrier's capacity, it can break violently, causing strong shaking. While a series of minor earthquakes might not breach the barrier, a subsequent stronger movement can release the stored-up energy all at once.

Led by Lingsen Meng from UCLA, alongside graduate student Liuwei Xu and Chen Ji from UC Santa Barbara, an international team of scientists from the U.S., France, China, and Japan analyzed geospatial data and seismic recordings to understand the relationship between the swarm of smaller quakes and the larger earthquake that followed. They identified a previously unknown barrier in the area affected by the tremors.

The researchers were astonished to find that the New Year's Day earthquake began almost simultaneously in two separate locations on the fault. The energy from these two points converged on the barrier, causing a violent rupture and extreme shaking.

Meng explained, "The earthquake started in two places and the waves met at the barrier, causing it to break." This process is similar to bending a pencil from both ends until it snaps in the middle.

The discovery was unexpected because while dual-initiation has been simulated, it is rarely observed in nature. This mechanism needs very specific conditions, which can be replicated in a lab but are less predictable in the real world.

Meng highlighted that Japan’s advanced seismic monitoring, combined with GPS and satellite radar data, allowed the team to achieve a high level of detail on the fault and the dual-initiation process.

Most earthquakes don’t have such detailed data, so it’s possible that dual-initiation events might be more common than previously thought. Meng's team plans to further investigate these scenarios to better understand the conditions and probabilities of such earthquakes.

"Our results underscore the intricate nature of earthquake initiation and the critical conditions that can lead to major seismic events," Meng said. "Grasping these processes is essential for improving our ability to forecast and mitigate the impact of future earthquakes."