Episodic transient deformation in the Noto Peninsula, central Japan, revealed by the analysis of multiple GNSS networks

Since November 30, 2020, an intense seismic swarm and transient deformation have been continuously observed in the Noto Peninsula, central Japan, which is a non-volcanic/geothermal area far from major plate boundaries. We modeled transient deformation based on a combined analysis of multiple Global Navigation Satellite System (GNSS) observation networks, including one operated by a private sector company (SoftBank Corp.), relocated earthquake hypocenters, and tectonic settings. Our analysis showed a total displacement pattern over two years shows horizontal inflation and uplift of up to ~70 mm around the source of the earthquake swarm. In the first three months, the opening of the sub-horizontal tensile crack had an estimated volumetric increase of ~1.4 x 107 m3 at a depth of ~16 km. Over the next 15 months, the observed deformation was well reproduced by shear-tensile sources, which represent an aseismic reverse-type slip and the opening of a southeast-dipping fault zone at a depth of 14–16 km. We suggest that the upwelling fluid spread subhorizontally at a depth of ~16 km through an existing permeable fault zone and then diffused into the fault zone, triggering a long-lasting sub-meter aseismic slip below the seismogenic depth. The aseismic slip further triggered intense earthquake swarms at the updip.