Ongoing crustal deformation and earthquake swarm in the Noto Peninsula, central Japan

Earthquake swarms are generally interpreted as phenomena related to external stress perturbation including slow slip events and magma intrusion or weakening of fault strength due to pore pressure increase. Extensive swarm activities accompanying geodetically detectable deformation are often observed along plate boundary faults and volcanic areas. However, an extensive seismic swarm started in December 2021 at the northern tip of the Noto Peninsula, central Japan, which is a non-volcanic/geothermal area far from the major plate boundaries. We present a preliminary report of observed seismicity, crustal deformation, and their interpretation. The swarm activity started with several episodic earthquake bursts in the first several months and turned to be a continuous activity. The number of M≥1 earthquakes has been roughly constant at ~120 per week since July 2021, as of January 2022. The largest M5.1 earthquake occurred on September 16, 2021. Focal mechanisms of large earthquakes including the largest one suggest reverse faulting with a compressional axis of NW-SE. The focal depth ranges between 10-18 km. Transient displacements are observed at three permanent GNSS stations operated by the Geospatial Information Authority of Japan within 30 km from the epicentral region of earthquake swarms. The annual observed displacement from December 2021 suggests inflation with up to 12 mm of horizontal displacement and 30 mm of uplift. We installed four new GNSS stations near the epicentral area in September 2021 and found rapid extensional deformation around the epicentral area. Assuming a spherical inflation (Mogi) source, we estimated an annual volumetric increase of ~2.5 x 10⁷ m³ at a depth of ~12 km. We speculate the volumetric increase is caused by upwelling water originally dehydrated from the subducted Pacific plate. Although the estimated source predicts to increase of the Coulomb stress in the epicentral area, the temporal evolution of crustal deformation and earthquake activity is not always synchronized. It may suggest fault weakening due to pore fluid migration into the fault zone.