4.2.2 Effect of Depth Bins, constant β or Variableβ (Strategies 3 and 4)
To investigate the effects of allowing the ECS to vary with depth, and account for possible variation in attenuation with depth, we divide the dataset into four depth bins (1-4, 4-5, 5-8 and 8-15km) guided by the velocity structure (Figure S3) and earthquake distribution, ensuring sufficient earthquakes within each bin for a stable inversion. We repeat the analysis both assuming a constant shear wave (and rupture) velocity (Strategy 3) and allowing shear wave velocity to vary with depth (Strategy 4). The results are compared to those in the previous inversions in Figure 4 and Table 1.
At shallower depth, the depth-dependence of stress drop is significantly reduced by using variable β (slope reduces by 60% comparing Strategy 3 and 4), but only slightly reduced with only depth-dependent attenuation (slope only reduces by 22% comparing Strategy 1 and 3). However, at deeper depth, the depth-dependent attenuation correction has significant impact on the depth-dependence of stress drop (slope reduction of 85% between Strategy 1 and 3). Slopes of magnitude dependence are also reduced, albeit slightly.