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Intense earthquake swarm in Noto Peninsula linked to ancient or hidden magmatic activity, Japan

Intense earthquake swarm in Japan linked to ancient or new hidden magmatic activity

An intense earthquake swarm that began around the end of 2019 in the northeastern Noto Peninsula, Japan, has been tied to fluids released by ancient, or possibly unknown modern, magmatic activity. This finding, published in JGR Solid Earth, challenges previous assumptions that such seismic activity could not occur in areas devoid of volcanic activity for over 10 million years.

The scientific study, published recently in JGR Solid Earth, has traced the ongoing intense earthquake swarm in Japan’s northeastern Noto Peninsula to the release of fluids from ancient or unrecognized modern magmatic activity. Until the end of 2019, the region, bereft of volcanic activity since the middle Miocene (15.6 million years ago), had never experienced such a swarm.

Researchers employed precisely determined earthquake locations and seismic reflectors to investigate the cause of this swarm. A review of the relocation of 10 940 earthquakes with magnitudes above 1 revealed that they all had a crustal origin and migrated upwards, activating a complex network of faults at depths shallower than 20 km (12.4 miles). The swarm’s beginning occurred at a locally deep depth (z = 17 km / 10.5 miles), and the local hypocenter distribution demonstrated a characteristic circular pattern.

Intense earthquake swarm in Japan linked to ancient or new hidden magmatic activity bg
(a) Map showing the location of the study region. The rectangle indicates the study area, and the triangles indicate Quaternary volcanoes. The thick black line approximates the surface trace of the Suzu-Oki active fault segment (Inoue & Okamura, 2010). Gray dots show the hypocenters of shallow earthquakes (z < 40 km) with the JMA magnitude MJMA ≥ 2.0 from 1 January 2003 to 30 September 2022. Yellow stars indicate earthquakes of ≥M6.5 that have occurred since 1919. Red star indicates the ∼M7 earthquake in 1729 (Usami, 2003). Beach balls show the moment tensor solutions by F-net (Kubo et al., 2002). Green color denotes the maximum event (M5.4) in 2022. Blue crosses indicate the distribution of stations used in the analysis. (b) Map showing the area of (a) by rectangle. (c) Magnitude–time diagrams of events in the source region of the present swarm. Blue circles with gray vertical bars indicate the magnitudes in the JMA unified catalog for M ≥ 1 events. The black line denotes the number of earthquakes with M ≥ 2 in each month. Credit: JGR Solid Earth/ Yoshida et al.

According to the study authors, “The earthquakes moved from deep to shallow depths via many planes, much like earthquake swarms near volcanoes.” The strongest of these earthquakes, at a magnitude of 5.4, occurred near the migration front on the most significant planar structure. The team has cautioned that further earthquakes could occur in this fault’s shallow parts.

The study’s key finding was the detection of a distinct S-wave reflector, suggesting a fluid source, in the vicinity of the swarm’s initiation point. The local hypocenters show a circular pattern analogous to the ring fault above a magma reservoir. These observations indicate that the ongoing seismic activity is influenced by fluids related to ancient or previously unrecognized modern magmatic activity.

The study’s findings highlight that even in regions where no volcanic activity has been observed for more than 10 million years, hidden magma-induced structures and fluids can generate earthquakes.


1 Upward Earthquake Swarm Migration in the Northeastern Noto Peninsula, Japan, Initiated From a Deep Ring-Shaped Cluster: Possibility of Fluid Leakage From a Hidden Magma System – Keisuke Yoshida et al. – AGU / JGR Solid Earth – June 2023 – https://doi.org/10.1029/2022JB026047 – OPEN ACCESS

Featured image: JGR Solid Earth / Yoshida et al.


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