Because many industrial and/or metropolitan areas are built on alluvial fans or basins, a correct quantification of the local site effects is necessary for a systematic and robust assessment of seismic hazards due to the direct relation with significant seismic damage and consequent loss of life. The amplification of an earthquake signal at a site plays a significant role in increasing seismic damage. The seismic site response due to fault zones has important implications on earthquake hazard on a local scale causing amplified motions near faults due to guided waves. Valuable research works by Li et al. (1994) on fault zone guided waves due to the San Andreas Fault zone grow attention to this issue. Constructive interference between reflected seismic waves is responsible for generating highly amplified fault zone guided waves and later arrivals of dispersive energy after S-waves (Igel et al. 1997). Fault gouge, clay-rich fault gouge, fracturing, increased porosity, remineralization, crack dilatation, and pore-fluid concentration near fault zones are the possible causes for this low-velocity zone (Ben-Zion 1998; Ben-Zion and Sammis 2003; Hickman et al. 2005).
Many researchers such as Rigano et al. (2008), Di Giulio et al. (2009), Pischiutta et al. (2012, 2013, 2015, 2017), Panzera et al. (2017a, 2017b), Burjánek et al. (2012), Panzera et al. (2019), Tortorici et al. (2019), and Kakhki et al. (2020) used the horizontal-to-vertical spectral ratios (HVSR) of earthquake and ambient vibration data to reveal the site effects related to fault zones. They observed systematic directional amplification normal to the dominant fault strike. Panzera et al. (2020) studied the site effects considering tectono-stratigraphic setting of the Santa Caterina area in Italy and observed maximum directional HVSR amplification of 8 in the perpendicular azimuth of the dominant N-S fracture zone. In contradict to these studies, Villani et al. (2018) did not observe any significant directional effects through their geophysical investigation at the Victoria fault in Malta. They interpreted their results because of the present inactivity in the fault zone, which is expressed in terms of ~ 0.6 Myr ago. Pischiutta et al. (2017) found a predominant high horizontal amplification in the NE-SW to NNE-SSW direction (i.e. fault perpendicular direction) across the Vado di Corno fault in Italy, which is considered as active normal faults during the L'Aquila 2009 earthquake. Di Giulio et al. (2009) conducted dense microtremor measurements along and across the intensely fractured zones of the Pernicana fault, Mount Etna, Italy. They observed strong directional horizontal amplification at 1 Hz close to the highly fractured zone.
The existence of seismic stations across the fault provides a good opportunity in identifying these guided waves. According to studies by Igel et al. (2002), Jahnke et al. (2002), and Fohrmann et al. (2004), moderate variations of fault zone properties could produce guided waves. Seismogenic depths > 10 km could act as fault zone waveguide according to studies by Li et al. (1997, 2000, and 2004), Li and Vernon (2001), Korneev et al. (2003), Mizuno et al. (2004), and Mizuno and Nishigami (2006). Moreover, Ben-Zion (1998), Rovelli et al. (2002), Peng et al. (2003), and Lewis et al. (2005) proved significant trade-offs between propagation distance along the fault zone, fault zone width, impedance contrasts between massive and damaged rock, and source location. Fault zone-related site effects are extensively studied by many researchers in fault zone stations due to sources not necessarily in the fault zone. Li et al. (1990, 1997) used a small number of selected waveforms to study the fault zone guided waves due to 100 m wide of low-velocity fault zone extending to the seismogenic depth. In California, 1500 weak ground motions on fault zone arrays were studied by Li and Vernon (2001). Shallow fault zone could behave as a waveguide due to very deep ground motions and far from the fault zone (Jahnke et al. 2002; Igel et al. 2002; Fohrmann et al. 2004).
The role of active fault zones in affecting the earthquake hazard estimation is playing an important role in evaluating the seismic risk analyses. In Japan, a comprehensive database of active faults is provided by the National Institute of Advanced Industrial Science and Technology (AIST: https://gbank.gsj.jp/activefault/index_e_gmap.html, last accessed January 2021). These active faults are subdivided into several strands and characterized by their surface trace geometry, rupture history, slip per event, slip rate, calculated future rupture probability, and recurrence interval. These characteristics are evaluated based on paleoseismic and geologic studies. To improve the database system, all these characteristics are compiled based on previous studies, such as, Matsubara and Obara (2011), Nishida et al. (2008), Matsubara et al. (2008), Nakamura et al. (2008), Abdelwahed and Zhao (2007), Nakajima et al. (2001), Zhao et al. (1996), to cite few among many others. Therefore, the present research work is originated based on the AIST database of active faults in Japan.
Motivated by these valuable previous studies and the valuable AIST database of active faults, the present research work is established taking into account that strong variation in amplitude and frequency content of surface ground motions are strongly related to fault zone’s geometry and seismic properties at depth. The present study is reported observed evidence of active reverse fault zone-related site spectral characteristics at three different localities in Japan using 26432 earthquakes recorded at 126 K-NET and KiK-net seismic stations. The spectral characteristics of the ground motions on the footwall (i.e. downthrown wall), fault zone, and the hanging wall (i.e. upthrown wall) are investigated by the Fourier acceleration spectrum of the surface orthogonal components, the HVSR, and the surface-to-borehole spectral ratio. Comparisons among these techniques are also implemented as a procedure for identifying and analyzing site spectral amplifications. Considering that reverse faults at those three different localities in Japan have similar main fault strike, the resulted site spectral amplifications could be possibly inferred to the dominant fault dip, the dominant frequency caused by the fault zone width, and the different fracture patterns due to the existence of a dense network of minor and major active faults, and the seismic properties of the fault zone (P-wave and S-wave velocities, Poisson's ratio, and perturbation velocities).