11:15 AM - 11:30 AM
[SGD02-08] Crustal deformation associated with an earthquake swarm in the Noto Peninsula based on combined analysis of multiple GNSS observation networks (Part 2)
Keywords:Crustal deformation, GNSS stations operated by a private company, Earthquake swarm
An extensive seismic swarm started at the end of November 2020 at the northern tip of the Noto Peninsula, central Japan, which is a non-volcanic/geothermal area far from the major plate boundaries. The swarm activity has continued for more than two years and is still active as of February 2023. Transient deformation associated with the swarm activity was detected by the public GNSS observation network operated by the Geospatial Information Authority of Japan (GSI). We (Nishimura et al., 2022GSJ and 2022SSJ) reported that the combined analysis of the multiple GNSS observation networks including the ultra-dense network operated by the SoftBank Corp. revealed a detailed spatiotemporal evolution of the transient deformation. Here, we present recent deformation clarified by the combined GNSS analysis and updated source models and interpretation of the observed transient deformation.
The total GNSS displacement after the correction of the pre-event velocity shows a radiative horizontal pattern from the swarm area and a broad uplift in and around the swarm area. The largest total uplift exceeds 70 mm at a SoftBank original reference station in the swarm area from November 2020 to January 2023. The deformation rate is generally rapid in the first three months and almost constant until the largest M5.4 earthquake occurred on July 16, 2022. After the largest earthquake, the transient deformation is calmed down at the stations south of the swarm area but is still significant at the station north of the swarm area. We divide the activity before the largest earthquakes into three stages, that is, November 2020 to March 2021, March 2021 to June 2021, and June 2021 to June 2022 by examining observed transient deformation and seismicity. We estimated the source model of the observed deformation in these three stages by using a non-linear inversion method by Matsu’ura and Hasegawa (1989).
We present preferred source models following a hypothetical scenario of the earthquake swarm because it is difficult to estimate the unique source model due to the limited coverage of GNSS stations on the peninsula and the rather deep (e.g., >10 km) source. In the first stage, the opening of the low-angle tensile crack is estimated with a volumetric increase of ~1.4 x 107 m3 at a depth of ~16 km. In the second and third stages, the observed deformation is well reproduced by shear-tensile sources, which suggests an aseismic reverse-type slip and expansion of a southeast-dipping fault zone at a depth of 14-16 km. The estimated shear-tensile sources locate at the downdip of the clusters of the earthquake swarm in each stage, respectively. We propose the following hypothetical scenario of the earthquake swarm. In the swarm area, a southeast-dipping listric reverse fault zone extends in the upper and mid crust and the crustal fluid is rich in the lower crust as characterized by low seismic velocity (Nakajima, 2022EPS) and low electrical resistivity (Yoshimura et al., 2022SGEPSS). A large amount of upwelling fluid from the lower crust was trapped in the permeable fault zone and sub-horizontally spread around a depth of 16 km in the fault zone in the first stage. The upwelling fluid caused burst-type deep anomalous earthquakes in the southern swarm area. The fluid migrated up-dip along the fault zone and caused a sub-meter aseismic slip below the seismogenic depth (i.e., ~14 km). Large stress transfer of the aseismic slip triggers the updip intensive earthquake activities, though fault weakening due to upwelling fluid migration along the fault zone also contributes to the activation of earthquakes as suggested by a diffusive expansion of earthquakes.
Acknowledgments: We are grateful to SoftBank Corp. and ALES Corp. for providing us with the original GNSS observation data. We thank the Geospatial Information Authority of Japan and the Japan Meteorological Agency for providing GNSS observation data and earthquake catalog data. We also thank the Suzu City Government and the Noto Town Government for permitting the installation of temporary GNSS stations. This work was supported by JSPS KAKENHI Grant Number JP22K19949.
The total GNSS displacement after the correction of the pre-event velocity shows a radiative horizontal pattern from the swarm area and a broad uplift in and around the swarm area. The largest total uplift exceeds 70 mm at a SoftBank original reference station in the swarm area from November 2020 to January 2023. The deformation rate is generally rapid in the first three months and almost constant until the largest M5.4 earthquake occurred on July 16, 2022. After the largest earthquake, the transient deformation is calmed down at the stations south of the swarm area but is still significant at the station north of the swarm area. We divide the activity before the largest earthquakes into three stages, that is, November 2020 to March 2021, March 2021 to June 2021, and June 2021 to June 2022 by examining observed transient deformation and seismicity. We estimated the source model of the observed deformation in these three stages by using a non-linear inversion method by Matsu’ura and Hasegawa (1989).
We present preferred source models following a hypothetical scenario of the earthquake swarm because it is difficult to estimate the unique source model due to the limited coverage of GNSS stations on the peninsula and the rather deep (e.g., >10 km) source. In the first stage, the opening of the low-angle tensile crack is estimated with a volumetric increase of ~1.4 x 107 m3 at a depth of ~16 km. In the second and third stages, the observed deformation is well reproduced by shear-tensile sources, which suggests an aseismic reverse-type slip and expansion of a southeast-dipping fault zone at a depth of 14-16 km. The estimated shear-tensile sources locate at the downdip of the clusters of the earthquake swarm in each stage, respectively. We propose the following hypothetical scenario of the earthquake swarm. In the swarm area, a southeast-dipping listric reverse fault zone extends in the upper and mid crust and the crustal fluid is rich in the lower crust as characterized by low seismic velocity (Nakajima, 2022EPS) and low electrical resistivity (Yoshimura et al., 2022SGEPSS). A large amount of upwelling fluid from the lower crust was trapped in the permeable fault zone and sub-horizontally spread around a depth of 16 km in the fault zone in the first stage. The upwelling fluid caused burst-type deep anomalous earthquakes in the southern swarm area. The fluid migrated up-dip along the fault zone and caused a sub-meter aseismic slip below the seismogenic depth (i.e., ~14 km). Large stress transfer of the aseismic slip triggers the updip intensive earthquake activities, though fault weakening due to upwelling fluid migration along the fault zone also contributes to the activation of earthquakes as suggested by a diffusive expansion of earthquakes.
Acknowledgments: We are grateful to SoftBank Corp. and ALES Corp. for providing us with the original GNSS observation data. We thank the Geospatial Information Authority of Japan and the Japan Meteorological Agency for providing GNSS observation data and earthquake catalog data. We also thank the Suzu City Government and the Noto Town Government for permitting the installation of temporary GNSS stations. This work was supported by JSPS KAKENHI Grant Number JP22K19949.