Crustal deformation associated with the 2016 Kumamoto Earthquake and its effect on the magma system of Aso volcano

Abstract

An M JMA 6.5 earthquake (foreshock) and M JMA 7.3 earthquake (mainshock) struck Kumamoto Prefecture on April 14, 2016, and April 16, 2016. To evaluate the effect of crustal deformation due to the earthquake on the Aso magma system, we detected crustal deformation using InSAR and GNSS. From InSAR analysis, we detected large crustal deformations along the Hinagu Fault, the Futagawa Fault, and the northeast extension of the latter fault. It extended to more than 50 km, and the maximum slant-range change exceeded 1 m. Although the obtained crustal deformation was approximately explained by the right-lateral strike-slip on the fault, its details could not be explained by such simple faulting. Additionally, we found complex surface deformation west of the Aso caldera rim, suggesting that shallow fault slips occurred in many known and unknown faults associated with the earthquake. Most of the crustal deformation could be reasonably explained by four rectangle faults located along the Futagawa Fault, in the northeast extension of the Futagawa Fault, alongside the Hinagu Fault, and in the eastern part of the Futagawa Fault. The first three of faults have high dip angles and right-lateral slip. The other was a fault with a low dip angle that branched from the shallow depth of the fault along the Futagawa Fault. The normal-dip right-lateral slip was estimated for this segment. Based on the estimated fault model, we calculated the displacement and stress field around the Aso volcano by the finite-element method (FEM) to evaluate the effects on the Aso magma system. In this calculation, we assumed a spherical soft medium located at a 6-km depth beneath the area south of the Kusasenri region as the magma system and considered only static effects. The result shows complex distributions of displacements and stresses, but we can notice the following significant points. (1) The spherical magma system deformed to an ellipsoid, and the total volume was slightly increased, less than 1%. (2) The differential stress around the upper portion of the magma system was as large as 3.5 MPa. This is strong enough to open pre-existing cracks and can cause the migration of magma.

Source process of the 2016 Kumamoto earthquake (Mj7.3) inferred from kinematic inversion of strong-motion records

“…9b). Ozawa et al (2016) also explained the crustal deformation obtained from InSAR and GNSS data with a southeast-dipping fault model similar to our southeast-dipping F1 segment model. Our present fault model (Fig.…”

“…Most of these studies based on seismic data assume northwest-dipping fault planes. However, several studies based on the crustal deformation data obtained by interferometric synthetic aperture radar (InSAR) and global navigation satellite system (GNSS) networks suggest a southeast-dipping fault mechanism in the Aso caldera region (e.g., Ozawa et al 2016). Determining the fault plane dip direction based on the aftershock distribution is difficult because of the seismicity gap in the western part of the Aso caldera region (Aso gap in Fig.…”

Source rupture processes of the 2016 Kumamoto, Japan, earthquakes estimated from strong-motion waveforms

“…These planes are grouped into three major parts (north, central, and south) with transitional parts that smoothly connect the major parts. The strike angle and top location of each part follow Ozawa et al (2016). They identified a discontinuity of slant-range change of InSAR data along the Futagawa fault zone and a steep gradient of slant-range change along both the Hinagu fault zone and the eastern extension of the Futagawa fault zone (Fig.…”

“…The broken line denotes an additional fault model for the M 7.3 event that has a 56 km × 24 km rectangular plane with a strike of 224° and a dip of 65°. b Curved fault model for the M 7.3 event with mosaicked SAR interferograms made by comparing two SAR images before and after the M 7.3 event (Ozawa et al 2016). c Cross sections for the regions A, B, and C shown in Fig.…”

“…Aso. Surface displacements were also recorded by GNSS Earth Observation Network System (GEONET) and ALOS-2/ PALSAR-2 interferometric SAR (InSAR) (e.g., Ozawa et al 2016;Yarai et al 2016). The geodetic data indicate that right-lateral fault motion occurred along the Hinagu and Futagawa fault zones during the M 7.3 event.…”