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Seismic fault surfaces have complex geometries over a broad-scale range. The Hurst exponent (H) is an important parameter characterizing the complexity of fault geometries. This exponent of seismic faults has only been estimated at outcrops and from surface traces of large earthquakes. This is because geometry data of subsurface faults usually have large uncertainties and many gaps. This study examined the applicability of the lifting scheme, which is one of the wavelet transform methods, to estimate H of irregularly sampled geometry of subsurface faults. We analysed the surface geometry of the subducting oceanic plate at the Nankai trough, Japan, which is part of the fault plane of interplate earthquakes. The geometries of the subducting plate were estimated along six survey lines by integrating seismic refraction and reflection surveys. Two-way traveltimes of reflected waves from the plate surface, which were measured from the reflection survey data, were converted to depth using the velocity structure estimated by refraction survey. The intervals between sampled points were irregular because the reflected waves were obscured or invisible in some places. The Hurst exponents were estimated from the scale dependence of the wavelet coefficients that were derived by the lifting scheme without interpolation. Analyses of the synthetic data simulating the irregularly sampled plate geometries indicated that the lifting scheme yields stable but largely biased estimates of H , especially for small H (<0.5). We introduced the empirical bias correction to achieve an unbiased estimation of the exponent. The analysis of plate geometry at the Nankai trough was conducted at narrow-scale ranges with consideration for the accuracy of velocity structures. We may conclude that H of five survey lines across the trough axis is > 0.8, and that of a line along trough is > 0.7. These estimated exponents had large uncertainties due to analyses at narrow-scale ranges, but were close to the estimates in studies of the surface traces of large earthquakes. [ABSTRACT FROM AUTHOR]

The offshore part of the North Anatolian Fault (NAF) beneath the Marmara Sea is a well-known seismic gap for future M > 7 earthquakes in the sense that more than 250 yr have passed since the last major earthquake in the Central Marmara region. Although many studies discussed the seismic potential for the future large earthquake in this region on the basis of historical record, geodetic and geological observations, it is difficult to evaluate the actual situation on the seismic activity and structure along the NAF beneath the Marmara Sea due to the lack of ocean bottom seismic observations. Using ocean bottom seismometer observations, an assessment of the location of possible asperities that could host an expected large earthquake is undertaken based on heterogeneities in the microseismicity distribution and seismic velocity structure. Specifically, seismic tomography and precise hypocentre estimations are conducted using offshore seismic data whose recording period is 11 months. About five times more microearthquakes are detected with respect to events recorded in a land-based catalogue. A comparison with previously published results from offshore observation data suggests that the seismicity pattern had not changed from 2014 September to 2017 May. The location accuracy of microearthquakes is greatly improved from only the land-based earthquake catalogue, particularly for depth direction. There are several aseismic and inactive zones of microearthquake, and the largest one is detected using land-based seismic observation, whereas other zones are newly detected via offshore observations. The obtained velocity model shows a strong lateral contrast, with two changing points. The western changing point corresponds to a segmentation boundary, where the dip angle of the NAF segments changed. High-velocity zones from tomographic images are characterized by low seismicity eastward of the segment boundary. To the east of 28.50°E, the high-velocity zone becomes thicker in the depth direction and is characterized by low seismicity. Although the low seismic activity alone could be interpreted as both strong coupling and fully creeping, the high-velocity features at the same can be concluded that these zones are consist of brittle material and strong coupling. From comparison with other geodetic and seismic studies, we interpret these zones as locked zones that had been ruptured by the past large earthquakes and could be ruptured by future ones. These zones might accumulate strain since the main shock rupture associated with the 1766 May M s 7.3 earthquake, the latest major earthquake in this region. [ABSTRACT FROM AUTHOR]