A band of unusually hard rock, formed from magma about 15 million years ago, near the epicenter of the 2024 Noto Peninsula earthquake likely worsened the disaster as internal faults collapsed following the initial tremor, Japanese researchers have said in a newly published paper.
The findings help explain why a series of small earthquakes, known as a swarm, over a period of about three years in northeastern Noto suddenly culminated in the magnitude 7.6 Noto Peninsula quake on Jan. 1, 2024, which registered shindo 7, the highest level on Japan¡¯s seismic intensity scale.
The paper, written by Tohoku University researchers and , states that the huge body of hard rock ¡ª referred to by the team as ¡°ancient magma¡± ¡ª sits under the ground just west of the epicenter, roughly 5 to 15 kilometers deep and about 10 to 15 km wide.
When the Noto earthquake caused faults inside to rupture, the epicenter area, or focal zone, expanded to span an area of about 150 km in total, the researchers said.
From mid-October to late November in 2023, Ryota Takagi, an associate professor at Tohoku University, and his research team had temporarily installed 12 high-performance compact seismometers across the area where the swarm of earlier quakes had been recorded.
They explored the underground structures by observing pressure changes caused by ocean waves that propagate underground as seismic waves, and combining their measurements with existing seismic data.
The researchers found a zone where seismic waves traveled faster than in the surrounding rock, indicating a span of hard rock being formed. Cross-checks with maps from the National Institute of Advanced Industrial Science and Technology (AIST) identified the rock as cooled, solidified magma from volcanic activity when the Japanese archipelago formed, about 15 million years ago.
Previous analyses suggested quake swarms generally migrate from south to north and from deeper to shallower depths, likely driven by the movement of fluids ¡ª mainly water ¡ª separated from deep rocks.
¡°¡®Ancient magma¡¯ does not allow water to pass, and this rock acted as a barrier that impeded fluid movement for three years,¡± Takagi said.
¡°During the Noto Peninsula earthquake, however, it accelerated fault rupture and became the trigger that extended the rupture in the east¨Cwest direction.¡±
Previous AIST research has shown that the peninsula¡¯s northern coast features three large marine terraces, likely formed by uplift during past major earthquakes. This suggests the same kind of fault failure within the ancient magma body may have occurred in the past as well.
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