Relationship between normal faults spacing and mechanical layer thickness : a case of Miyako-jima Island, southern Ryukyu arc, Japan

Estimating the active fault size is important for evaluating long-term activity on fault systems and predicting future earthquakes. The potential magnitude of earthquakes generated by an active fault can be evaluated from various parameters, such as the length of the rupture zone, area of the fault and slip rate (e.g., Matsuda, 1975; Anderson et al., 1996). However, multiple factors need to be considered when estimating fault size, because the difficulty remains in estimating such parameters.

We focused on fault spacing because the active faults in the Japanese islands sometimes have a regular distribution of fault spacing. Several previous studies have discussed the mechanism of the regular distribution of fault spacing. The development of regular fault spacing, defined by a characteristic horizontal distance between neighboring faults, has been attributed to a zone of stress reduction around each fault, which is strongly controlled by the fault size (e.g., Soliva et al., 2006; Fossen and Rotevatn, 2016). Soliva et al. (2006) have shown that the spacing between saturated normal faults is linearly related to the thickness of the mechanical layer. Similar relationships have also been demonstrated in analogue and numerical simulations (Rodríguez et al., 2007; Asahina et al., 2014). Thus, the thickness of the mechanical layer may be estimated from regularly spaced and saturated faults.

One of the clearest cases of active faults with a regular distribution of fault spacing in the Japanese islands is located in Miyako-jima Island, southern Ryukyu Arc. We have measured the spacing between a group of saturated normal faults on Miyako-jima Island (southern Ryukyu Arc, Japan), and estimated the thickness of the faulted mechanical layer. The measured and calculated results are compared with the previous studies to identify the characteristics of the estimated mechanical layer.

The results indicate that the shallow (<3 km) normal faults on Miyako-jima Island do not have the potential to cause medium-scale earthquake as individual active faults. The origin of the shallow normal faults might have been associated with a larger-scale deeper fault. The results show that fault spacing provides important information on the magnitude of earthquakes associated with active faults.