Detailed spatial changes in the stress field of the 1984 western Nagano earthquake region

Abstract

[1] We estimate the stress field after the 1984 western Nagano earthquake from numerous focal mechanisms. To precisely determine the focal mechanisms, we analyze the earthquakes that occurred around the eastern part of the main shock fault, where station coverage is fairly good. Most of the earthquakes that occurred in this part, except the main shock fault, are reverse fault types. In contrast, earthquakes that occurred near the main shock fault have T axes distributed in a belt (strike slip, oblique slip, and reverse fault types occurred equally). Using the stress inversion method to quantitatively estimate the stress field of these earthquakes revealed that the s 2 axis near the main shock fault is close to the vertical direction, whereas it is close to the horizontal direction in other regions. At the western edge of the study area, we observe that the s 1 axis rotates toward the NS direction and that the stress ratio (s 1 − s 2 )/(s 1 − s 3 ) is low. We estimate that the magnitude of s hmin decreases near the main shock fault. Spatial variation of the stress field around the eastern part of the main shock fault is not generated by static stress changes caused by the main shock. Local stress anomalies might have occurred around the main shock fault before the main shock.

Which is heterogeneous, stress or strength? An estimation from high-density seismic observations

“…It is seen that nearly vertical σ 2 axes are concentrated around the mainshock fault. A similar result was reported by Yukutake et al (2010), and we confirmed their results in this study. Furthermore, stress ratios seemed to be larger around the mainshock fault than at grid points farther away from it.…”

“…13 (Upper) Relationship between focal distance and the difference in P-axis orientations (the angle between two P-axes) for each event pair in the cubic subregion for a grid point (x = 10, y = 9, z = − 2), and the frequency distribution of angles between two P-axes of each event pair. (Lower) Similar plots for Kagan angles between focal mechanisms of each event pair to aseismic slip in the downward extension of the mainshock fault, as pointed out by Yukutake et al (2010). The root mean squares (RMSs) of differences between the observed and calculated slip directions (misfit) are smaller than the errors for focal mechanisms at grid points away from the mainshock fault.…”

“…Furthermore, stress ratios seemed to be larger around the mainshock fault than at grid points farther away from it. These results can be explained by aseismic slip in the downward extension of the mainshock fault, as pointed out by Yukutake et al (2010). Figure 8b shows spatial changes in RMSs of differences between the observed and calculated slip directions (misfits) in the direction perpendicular to the fault strike.…”

“…Black circles are earthquakes used in this study, which occurred from June 1995 to June 2010. Red rectangle in the large-scale map indicates the study area 2009), source parameters of small earthquakes Matsuzawa et al 2004;Venkataraman et al 2006), precise hypocentral distributions (Rydelek et al 2002;Cheng et al 2007), 3D velocity structure at the kilometer scale (Doi et al 2013), and stress state (Yukutake et al 2010).…”

“…In the same study region, Yukutake et al (2010) conducted a stress inversion analysis using high-sampling data from 1999 to 2011. They estimated stress at points on a three-dimensional grid with 1 km spacing using the focal mechanisms for which hypocenters were located in a cubic subregion with dimensions of 2 km.…”