17:15 〜 18:30
[SSS09-P05] Effects of fine-scale heterogeneity distribution in the Pacific slab on the generation of anomalously large intensities from deep-focus earthquakes
キーワード：Deep earthquake、Fine-scale heterogeneity、Oceanic lithosphere
From the analysis of OBS records in northwest Pacific and 3-D FDM simulations of high-frequency oceanic Pn/Sn (Po/So) wave propagations, lateral variations in fine-scale heterogeneity in the oceanic lithosphere have been revealed in detail (Furumura & Kennett, SSJ2020 Fall meeting). The Po/So propagation variability can be explained with a von Karman distribution function for elastic properties in the lithosphere with a longer correlation distance of 20 km in the NE direction, and much shorter correlation distances in the perpendicular (0.5 and 2 km) direction and in depth (0.5 km), with standard deviation from the background wavespeed of 2%. Since the longer axis of the heterogeneity distribution aligns with the observed Pn/Sn-wavespeed anisotropy in the northwest Pacific, it is highly likely that it was also developed during the formation and growth of the oceanic plate. Such a structure is very effective to guide high-frequency Po/So with long codas by multiple forward scattering in the direction parallel to the elongated heterogeneity.
The complex heterogeneity in the Pacific plate is expected to be carried by subduction deep beneath Japan, and can then produce unusually large signals in certain directions from deep earthquakes within the subducted slab. For example, on 2007 July 16, a 378 km deep Mw 6.8 earthquake beneath the Sea of Japan, produced anomalously large shaking over central to northern Japan. The shaking intensity near the epicenter was less than 1 (JMA), but the maximum intensity of 4 occurred in Obihiro, Hokkaido (Fig.1a).
Usually, such a peculiar intensity pattern occurs from deep earthquakes, as high-frequency (>1-2 Hz) signals traverse the cold, high-Q slab. In addition, the waveguide effect is reinforced by multiple forward scattering of seismic waves in the heterogeneous slab that captures high-frequency signals (e.g., Furumura & Kennett, 2005, JGR; Kennett & Furumura, 2015, G3). For this earthquake the significantly extended PGA contours into Hokkaido, more than 1000 km away from the hypocenter, is rather anomalous compared to other deep-focus earthquakes, indicating that an additional directional waveguide effect is needed.
We have undertaken 3-D FDM simulations of high-frequency wave propagation (up to 4.6 Hz) using the heterogenous slab structure (Fig.1b) that was identified from the Po/So propagation variability in the northwest Pacific. The area of simulation was 1536 km by 768 km in horizontal direction and 665 km in depth (Fig.1c), discretized by a uniform grid of 0.875 km. The crust and mantle structure was based on Crust 1.0 (Laske et al., 2013) and deeper structure was ak135 (Kennett et al., 1995) with earth flattening. The Pacific plate model was based on Yokota et al. (2017).
The simulation results show that the presence of an elongated fine-scale heterogeneity in N45oE direction in the Pacific slab (Model A in Fig.2a) can reproduce the main pattern of larger PGA extending to Hokkaido, similar to the observations, though the level of simulated ground motion was about a half of observation due to high-frequency limitations in the FDM simulation. The importance of the orientation of the heterogeneity distribution for enhanced waveguide effects is demonstrated by comparison with the results using another model (Model B) in which the longer correlation length is orthogonal (N135oE; Fig. 2b). The simulation for Model B displays large PGA contours from Tokyo to the offshore of Tohoku, but these do not extend to Hokkaido.
A further deep event beneath Sakhalin (2013 July 23; depth=487 km, Mw6.8) did not display large PGA over Japan. FDM simulation shows that this effect arises from the propagation of seismic waves to Japan perpendicular to the elongated heterogeneity in the Pacific slab.
The complex heterogeneity in the Pacific plate is expected to be carried by subduction deep beneath Japan, and can then produce unusually large signals in certain directions from deep earthquakes within the subducted slab. For example, on 2007 July 16, a 378 km deep Mw 6.8 earthquake beneath the Sea of Japan, produced anomalously large shaking over central to northern Japan. The shaking intensity near the epicenter was less than 1 (JMA), but the maximum intensity of 4 occurred in Obihiro, Hokkaido (Fig.1a).
Usually, such a peculiar intensity pattern occurs from deep earthquakes, as high-frequency (>1-2 Hz) signals traverse the cold, high-Q slab. In addition, the waveguide effect is reinforced by multiple forward scattering of seismic waves in the heterogeneous slab that captures high-frequency signals (e.g., Furumura & Kennett, 2005, JGR; Kennett & Furumura, 2015, G3). For this earthquake the significantly extended PGA contours into Hokkaido, more than 1000 km away from the hypocenter, is rather anomalous compared to other deep-focus earthquakes, indicating that an additional directional waveguide effect is needed.
We have undertaken 3-D FDM simulations of high-frequency wave propagation (up to 4.6 Hz) using the heterogenous slab structure (Fig.1b) that was identified from the Po/So propagation variability in the northwest Pacific. The area of simulation was 1536 km by 768 km in horizontal direction and 665 km in depth (Fig.1c), discretized by a uniform grid of 0.875 km. The crust and mantle structure was based on Crust 1.0 (Laske et al., 2013) and deeper structure was ak135 (Kennett et al., 1995) with earth flattening. The Pacific plate model was based on Yokota et al. (2017).
The simulation results show that the presence of an elongated fine-scale heterogeneity in N45oE direction in the Pacific slab (Model A in Fig.2a) can reproduce the main pattern of larger PGA extending to Hokkaido, similar to the observations, though the level of simulated ground motion was about a half of observation due to high-frequency limitations in the FDM simulation. The importance of the orientation of the heterogeneity distribution for enhanced waveguide effects is demonstrated by comparison with the results using another model (Model B) in which the longer correlation length is orthogonal (N135oE; Fig. 2b). The simulation for Model B displays large PGA contours from Tokyo to the offshore of Tohoku, but these do not extend to Hokkaido.
A further deep event beneath Sakhalin (2013 July 23; depth=487 km, Mw6.8) did not display large PGA over Japan. FDM simulation shows that this effect arises from the propagation of seismic waves to Japan perpendicular to the elongated heterogeneity in the Pacific slab.