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[SSS11-26] Slip Characteristics in Shallow Areas above Seismogenic Zone for Inland Crustal Earthquakes with Surface Break
Keywords:Fault displacement, Shallow large slip, Strong ground motion, Source scaling relationship, LMGA (Long-period Motion Generation Area)
The recipe for predicting broadband strong ground motions (HERP, 2020) focuses on the modeling of source faults in deep seismogenic zones at depths ranging from a few kilometers to 15 km. On the other hand, Irikura et al. (2020) pointed out that setting up LMGAs (Long-period Motion Generation Areas) with large slips of about 2-4 m in the shallow area above the seismogenic zone is important to explain the long-period ground motions with permanent displacements observed near the fault during the 2016 Kumamoto earthquake (Mj7.3). We estimate the slips and source time functions in the LMGA to explain long-period ground motions (velocity waveforms and permanent displacements) observed mainly at near-fault sites. However, the spatial extent (length and width) of the LMGA and the characteristics of the slips are still in the research stage due to the lack of observation cases. To update the recipe for predicting broadband strong ground motions, it is important to study the characteristics of large slips in shallow areas above the seismogenic zone caused by past earthquakes with surface breaks.
First, we examine the relationship of source parameters for inland crustal earthquakes (Mw≧ around 6.5) with surface breaks and recognize that the relationship between the rupture areas and seismic moment, as well as between the average slip and seismic moment, agrees with the 2nd and 3rd stage of the three-stage source scaling relationship. Next, we compiled fault displacement data and recognized that the average fault displacement is nearly equal to the average slip in the rupture area and the maximum fault displacement is about 2.5 times the average slip in the rupture area. Finally, we compiled data on the shallow slips in LMGAs estimated from long-period ground motions at near-fault sites, fault displacements, and crustal deformation data. We recognized that the shallow maximum slips in LMGAs above the seismogenic zone are about twice the average slip in the rupture area. This scale factor of about 2 is underestimated compared to the 2.5. This is probably because the near-fault observation points used to model the LMGAs were far from the point where the maximum fault displacement occurred.
It is important to note that this study confirmed the proportional relationship between topographic and geological data, such as the maximum and average fault displacement at the surface during the earthquake, and seismological data, such as the average slip in the subsurface rupture area estimated from source inversion analysis. This relationship will contribute to the advancement of the recipe for predicting broadband strong ground motions, such as setting the slips and areas of LMGAs estimated from the distribution of fault displacements on the ground surface using topographic and geologic survey methods and verifying earthquake magnitude scale estimated from the empirical source scaling relationships based on the active fault length or the seismic rupture area.
Acknowledgments: This study was based on the 2024 research project “The study on improving the reliability of ground motion evaluation methods using fault rupture models” by the Secretariat of Nuclear Regulation Authority, Japan.
First, we examine the relationship of source parameters for inland crustal earthquakes (Mw≧ around 6.5) with surface breaks and recognize that the relationship between the rupture areas and seismic moment, as well as between the average slip and seismic moment, agrees with the 2nd and 3rd stage of the three-stage source scaling relationship. Next, we compiled fault displacement data and recognized that the average fault displacement is nearly equal to the average slip in the rupture area and the maximum fault displacement is about 2.5 times the average slip in the rupture area. Finally, we compiled data on the shallow slips in LMGAs estimated from long-period ground motions at near-fault sites, fault displacements, and crustal deformation data. We recognized that the shallow maximum slips in LMGAs above the seismogenic zone are about twice the average slip in the rupture area. This scale factor of about 2 is underestimated compared to the 2.5. This is probably because the near-fault observation points used to model the LMGAs were far from the point where the maximum fault displacement occurred.
It is important to note that this study confirmed the proportional relationship between topographic and geological data, such as the maximum and average fault displacement at the surface during the earthquake, and seismological data, such as the average slip in the subsurface rupture area estimated from source inversion analysis. This relationship will contribute to the advancement of the recipe for predicting broadband strong ground motions, such as setting the slips and areas of LMGAs estimated from the distribution of fault displacements on the ground surface using topographic and geologic survey methods and verifying earthquake magnitude scale estimated from the empirical source scaling relationships based on the active fault length or the seismic rupture area.
Acknowledgments: This study was based on the 2024 research project “The study on improving the reliability of ground motion evaluation methods using fault rupture models” by the Secretariat of Nuclear Regulation Authority, Japan.