Introduction:
The Mw 7.5 Noto earthquake, which struck the Noto Peninsula, Japan, on January 1, 2024, was one of the most significant seismic events in the region’s history. This major earthquake was preceded by an intense, nearly three-year-long earthquake swarm, which began in August 2018 and became highly active by December 2020. Crustal fluids were indicated as the primary trigger for the precursory swarm activity. This academic research aimed to estimate a detailed three-dimensional (3D) S-wave velocity structure beneath the Noto Peninsula to identify the geological factors controlling the complex spatial evolution of the earthquake swarm and the rupture pattern of the subsequent large earthquake.
Challenge:
Achieving a high-resolution image of the complex crustal structure was technically challenging because the region’s permanent seismic network had a sparse station spacing of approximately 20 km. This network density was insufficient for the detailed study required to resolve the fine-scale structural heterogeneities that dictate fluid migration and earthquake genesis. The specific “exploration” objective was to map subsurface features to a depth of around 15 km to understand the physical mechanism—what the paper identifies as a heterogeneity in fault-zone permeability—that enabled the precursor swarm and ultimately led to the significant event.
Solution:
To overcome the low-resolution challenge, the research team deployed a dense seismic observation network utilising SmartSolo nodal technology. The deployment consisted of 12 SmartSolo IGU-BD3C-5 seismic nodes installed across the Noto Peninsula. The nodes were set at an average separation of 5 km, thereby drastically increasing the spatial density relative to the permanent monitoring system. Data collection was continuous, spanning 32 days across two periods in October and November 2023, just one month before the Mw 7.5 earthquake. The study used Ambient Noise Cross-Correlation Functions, leveraging microseisms generated by ocean waves, to image the subsurface structure via surface wave tomography. The SmartSolo nodes produced high-quality data on these microseisms, which were essential for generating clear cross-correlation functions and for constraining the S-wave velocity structure down to an adequate depth of ~20 km.
Results:
The high-resolution 3D S-wave velocity structure derived from the dense SmartSolo data provided critical insights into the seismic process:
- The study successfully identified a distinct, high-velocity body (HVB) located immediately west of the earthquake swarm area. This HVB was continuously imaged down to a depth of ~20 km.
- Based on velocity analysis, the HVB was interpreted to be solidified ancient mafic magma, with S-wave velocities ranging from approximately 3.8 to 4.0 km/s.
- The preceding, fluid-driven earthquake swarm occurred only in the adjacent, less rigid low-velocity regions (at depths of 9 to 12 km) and critically avoided migrating into the HVB.
- This structural configuration showed that the solidified magma acted as an impermeable barrier to the crustal fluid migrations that drive swarm activity.
- The HVB’s location spatially coincided with the area of greatest slip and substantial uplift during the main 2024 Mw 7.5 event, proving that the solidified magma acted as the major asperity that eventually ruptured.
This case study highlights the effectiveness of SmartSolo nodes in addressing this complex geophysical challenge. The ability of the IGU-BD3C-5 nodes to capture high-quality ambient noise data over a short deployment period and at a high spatial density (5 km average separation) was instrumental in achieving the necessary resolution to image the deep, solidified magma body down to ~20 km. The project underscores the importance of utilising advanced, flexible, and dense seismic technology in modern earth science and seismic hazard assessment, revealing that pre-existing heterogeneity—in this case, ancient volcanic activity—is a critical factor controlling the generation of large earthquakes.
Figure 1. A) distributions of crustal earthquakes in the Noto Peninsula, Japan. B) Geology, coseismic uplift, and S-wave velocity structure, C) Vertical cross sections of the estimated S-wave velocity model with interpretation.
Reference: Ryota Takagi et al., Rupture of solidified ancient magma that impeded preceding swarm migrations led to the 2024 Noto earthquake. Sci. Adv. 11, eadv5938 (2025). DOI: 10.1126/sciadv.adv5938
Full article: www.science.org/doi/10.1126/sciadv.adv5938