关键词: 日向滩地震, GNSS, 同震形变, 库仑应力

Abstract:

In Southwest Japan, the Philippine Sea Plate subducts beneath the Eurasian Plate, which makes it one of the most seismically active areas in the world. This area nucleates primarily thrust-type earthquakes over the subduction interface. On August 8, 2024, a M_W 7.1 earthquake occurred in the Hyuganada region in southwestern Japan, offering a valuable opportunity to investigate the seismic slip of this event. Understanding the seismic slip plays an essential role in evaluating earthquake hazards, because it helps better understand the fault behaviors and stress interactions among active crustal faults and thus potential seismic impacts.
In this study, we first derived the coseismic displacements caused by the earthquake through Global Navigation Satellite System(GNSS)data. The maximum horizontal displacement is approximately 15cm, decreasing rapidly as the distance from the rupture zone increases. The vertical displacement shows distinct spatial characteristics. In the southeastern area of the Kyushu Island, near the rupture zone, the subsidence is predominant, with a maximum subsidence of about 6cm. Northwest of this area, the displacement gradually changes to uplift. This result indicates a complex deformation from the rupture area to the farther areas. Based on the elastic half-space model, we inverted the coseismic slip distribution using the derived GNSS displacements. In the model, we have considered a rupture fault with a length of 100km and a width of 60km. This fault is divided into 220 small rectangular elements, with 20 elements in the length direction and 11 in the depth direction. The length of each small rectangular tile is fixed at 5km, while the tile width varies with the depth. The minimum width is 5.2km, and the width increases with depth according to a proportionality coefficient of 1.01. We further use the grid search method to determine the fault dip angle and strike direction. The dip angle and strike direction of the fault are varied within ranges of -50° to 100° and 100° to 300°, respectively. Through these processes, the optimal fault dip angle and strike direction are determined to be 24° and 206°, respectively, consistent with the focal mechanism by the United States Geological Survey(USGS). The inverted coseismic slip distribution shows that the slip is primarily concentrated at 5~20km depths, featuring an elliptical pattern with a maximum slip of 1.47m. Based on the inverted fault slip model, we calculated the surface displacement using Okada’s half-space elastic dislocation theory. The results show that the model reproduces the first-order pattern of GNSS observations in both the horizontal and vertical directions. The total released moment is about 5.49×10¹⁹ Nm, corresponding to a moment magnitude of 7.09(assuming a shear modulus of 30GPa), which is close to the USGS of M_W7.1.
We calculated the Coulomb stress over the main fault using the inverted coseismic slip model. During this process, we assumed that the frictional coefficient of the fault, shear modulus, and Poisson’s ratio are to be 0.4, 30GPa, and 0.25, respectively. The fault plane’s strike, dip, and slip angles are set to be 206°, 24°, and 76°, respectively. The results reveal that outside the rupture zone, the Coulomb stress is positive, with a peak value of approximately 0.95MPa. In contrast, the overall stress is negative within the rupture zone, especially at depths between 7km and 17.5km, where it reaches a maximum value of -2.03MPa. Furthermore, the analyses on normal and shear stresses reveal a distinct pattern. In the shallow portion of the coseismic rupture zone, spanning from 0km to 7km, the normal stress is positive, with its maximum value reaching approximately 0.30MPa, whereas the shear stress is negative, peaking at around 0.73MPa. In other regions over the fault, the normal and shear stresses exhibit an increasing or decreasing trend.
In addition, we have also investigated the aftershocks that occurred within one month after the earthquake, using data from the USGS and JMA(Japan Meteorological Agency). The location of these aftershocks features a depth range of 10~60km. To further investigate the mechanisms of the aftershock occurrence, we analyzed the distribution of maximum shear stress at a depth of 40km. The results show that aftershocks mainly occur in areas where the shear stress is more than 100kPa. The majority of aftershocks concentrate in areas with the shear stress larger than 500kPa. This suggests at least hundreds of kPa are required to induce aftershocks of this event
To summarize, we have derived the coseismic slip and induced Coulomb stress of the 2024 Hyuganada earthquake. This reveals the seismic slip characteristics as well as impacts on the stress states and aftershock activities, which may contribute to further earthquake hazard assessment and mitigation strategies in the region.

Key words: Hyuganada earthquake, GNSS, coseismic deformation, Coulomb stress