Researchers Discover Two Epicenters in January 1 Noto Earthquake

The first seven months of 2024 have been so eventful that it’s easy to forget that the year began with a magnitude 7.5 earthquake centered beneath Japan’s Noto Peninsula on New Year’s Day. The quake killed more than 280 people and damaged more than 83,000 homes.

Geologists discovered that the earthquake started almost simultaneously at two different locations on the fault, allowing the seismic rupture to encircle and breach a resistant zone of the fault called a barrier. This rare “double initiation” mechanism applied intense pressure on both sides of the barrier, resulting in a powerful release of energy and significant shaking throughout the Noto Peninsula.

The Noto earthquake was preceded by intense seismic swarms, which are sequences of many small earthquakes that can sometimes lead to a larger catastrophic event. Using advanced seismic and geodetic technologies, the research team meticulously analyzed the movements inside the Earth during this swarm that led to the earthquake.

The study, published in the journal Scienceprovides insight into the role of fault barriers, also called asperities, in the genesis of earthquakes, and will help improve seismic hazard assessments and predictions of future earthquakes.

Earthquakes occur when fractures in the Earth’s crust, called faults, allow blocks of rock on either side of the fault to move past each other. This movement is localized and not continuous along the fault line because the fault line is not regular or smooth, which dissipates energy and eventually stops the movement.

A barrier is a rough area that blocks both sides of a fault. Barriers absorb the energy of the fault’s movement, slowing it down or stopping it completely. But the barrier can only absorb so much energy, and under the right conditions, the stored energy causes it to rupture violently, leading to powerful shaking. A series of small earthquakes may not be enough to break a barrier, but if a subsequent much stronger movement occurs on the fault, the rupture of the barrier will release all that stored energy.

Led by Lingsen Meng, associate professor of earth, planetary and space sciences at UCLA; Liuwei Xu, a UCLA graduate student; and Chen Ji, a professor of geophysics at UC Santa Barbara, an international team of researchers from the United States, France, China and Japan analyzed geospatial data and seismic wave recordings to understand the relationships between the swarm of small tremors and the larger earthquake that followed them. They identified a previously unknown barrier in the swarm region.

To their surprise, the New Year’s earthquake started almost simultaneously at two different locations on the fault. The energy from each location moved toward the barrier, causing a violent rupture and extremely strong shaking.

“The earthquake started in two places and spread in circles,” Meng explained. “The first one triggered waves that spread quickly and triggered a different epicenter. Then the two parts spread outward together and met in the middle, where the barrier was, and broke it.”

The mechanics are similar to bending a pencil at both ends until it breaks in the middle.

This finding is surprising because, although the dual initiation process, as it is known, has been observed in simulations, it is much more difficult to observe in nature. Dual initiation mechanisms require precisely tailored conditions, which can be defined in the laboratory but are less predictable in the real world.

“We were able to observe this because Japan has very good seismic monitoring stations and we also used GPS and satellite radar data. We collected all the data we could find! It was only because of all these data put together that we got a very good resolution on this fault and were able to get into these precise details,” Meng said.

The vast majority of earthquakes do not have this level of data collected, so it is possible that earthquakes with dual initiation mechanisms are more common than geologists think.

“It may be that with better imaging and better resolution, we will identify more like this in the future,” Meng said.

Earthquakes with two epicenters have a higher risk of stronger shaking because of the more intense motions. Meng’s group plans to study future scenarios to learn more about the conditions and probabilities of such earthquakes.

“Our results highlight the complex nature of earthquake triggering and the critical conditions that can lead to large-magnitude seismic events,” Meng said. “Understanding these processes is critical to improving our ability to predict and mitigate the impacts of future earthquakes.”