Buried large block revealed by gravity anomalies in the Tonankai and Nankai earthquakes regions, southwestern Japan

Abstract

We confirmed existence of a buried large block beneath seafloor off Cape Shionomisaki, Kii Peninsula, southwestern Japan, by improving marine gravity data. We found that the 1944 Tonankai and 1946 Nankai earthquakes occurred close to this block. The block is characterized by high gravity anomalies of 80 mGal larger than surrounding area with a diameter of about 40 km. Center of the anomaly is located 30 km off Cape Shionomisaki, southern end of the Kii Peninsula. This high gravity anomaly can be explained by existence of a higher density body with approximate thickness of 8 km just beneath the seafloor. This block coincides with the reported high velocity zone from recent seismic refraction survey in this area. Judging from seismic velocity, density, and geologic evidences, the block presumably consists of intruded acidic rocks, not seamount.

Structural control on the nucleation of megathrust earthquakes in the Nankai subduction zone

“…The 1946 Nankai ( M w 8.3) and the 1944 Tonankai ( M w 8.1) megathrust earthquakes occurred in or around patch B. Wide‐angle seismic surveys [ Kodaira et al , ] imaged patch B as a high‐ V body in the megathrust zone. Marine gravity data also show obvious high‐density anomalies in this area [ Honda and Kono , ]. Considering the widely exposed felsic plutons on Kii Peninsula of SW Japan, we think that patch B may reflect buried large plutons intruded into the upper plate directly upon the Nankai megathrust zone [e.g., Honda and Kono , ; Kimura et al , ].…”

An integrated geodynamic model of the Nankai subduction zone and neighboring regions from geophysical inversion and modeling

“…2a) in the forearc area of the Nankai Trough. While the sources for these gravity anomalies remain arguable, the high anomaly patch A is found to be intimately associated with great interplate earthquakes and strong seismic coupling (Honda and Kono, 2005;Kodaira et al, 2006), while the low anomaly patch B is related to seismic clustering (Tahara et al, 2006) plus aseismic (inter-seismic) uplifting and weak seismic coupling (Nakada et al, 2002). Another striking feature on the Bouguer gravity map is a low anomaly associated with the Middle Tectonic Line (MTL, Figs.…”

“…A conspicuous high Bouguer gravity anomaly patch located off the southern end of the Kii Peninsula (Figs. 1 and 2) is found to be coincidental with a high velocity zone in the crust, which may control the segmentation and synchronization of the rupture zones (Honda and Kono, 2005;Kodaira et al, 2006) or could be a asperity prone to great subduction earthquakes. The well known 1944 Tonankai and 1946 Nankai events all occurred around this high Bouguer gravity anomaly patch (site C in Fig.…”

“…3b). Considering the spatial correlations between these magnetic and gravity anomaly patches as well as the intimate relationships between gravity anomalies and earthquake clustering (Wells et al, 2003;Song and Simons, 2003;Llenos and McGuire, 2007;Honda and Kono, 2005;Kodaira et al, 2006), I speculate that magnetic anomalies and their zonal boundaries may also have controls on earthquake behaviors in the Nankai subduction zone. The landward extension of the magnetic boundary between zones the Kinan Seamount Chain zone and the Kii Peninsula zone seems to coincide with a possible tear or kink in the subducting plate revealed by seismic tomography and microearthquake hypocenters (Nakanishi and Honda, 2000;.…”

Application of cluster analysis based on waveform cross-correlation coefficients to data recorded by ocean-bottom seismometers: results from off the Kii Peninsula

“…Kodaira et al (2006) discovered a high-density rock body embedded in the overriding plate at almost the same location as the interplate clusters, although its center is shifted slightly southeastward ( Figure 12). They estimated the excess normal stress to be more than 30 MPa, taking account of the results obtained from gravity observations (Honda and Kono 2005), which suggests that the embedded rock is a strong asperity at the plate interface. Moreover, the rate of interplate coupling drastically changes at the northeastern edge of the rock body: the coupling rate is higher seaward and lower landward (Miyazaki and Heki 2001;Saiga et al 2011).…”