6
Discussion
6.1 3D reflection traveltime
modeling
To justify some of the seismic interpretations, we scrutinized the data
further. For the most eastern reflection (F1), because it comes near the
surface and it is clearly observed in several shot and receiver gathers,
we were able to model the reflection traveltime response based on Ayarza
et al. (2000), assuming a similar strike as the Wangsukcheon fault, a
velocity above the fault of 5000 m/s, and the position where the
reflection intersects the surface. A strike of N20E and dip of 60E can
explain the reflection traveltime observed in the real data, hence
further supporting its origin as being from the Wangsukcheon fault.
6.2 Origin of strong
surface-waves back
scatterings
Given the strong surface waves back-scattering observed in the data
(i.e., Figure 4), it was important to further analyze their properties.
The Zerwer et al. (2005) method, as implemented for multifold data by
Colombero et al. (2018), was applied to estimate the location of sharp
lateral variations in the subsurface and the corresponding maximum
affected wavelength. This method shows interesting results for locating
lateral variations at different places along the profile. These
locations were compared with the bedrock reflection from the
landstreamer data and tomography results from the wireless recorders
(Figure 5), as well as shot and receiver gathers where surface-waves are
clearly back scattered. Two of these surface-wave back scattering
sources are particularly strong: (1) where the Chugaryeong fault is
mapped and (2) at CMP location 2840. Although no corresponding faults
are represented in the geology at CMP 2840, a sharp geological contact
between Paleoproterozoic gneiss and Jurassic granite suggests the
presence of some tectonic structure that could be the reason for the
surface-wave back-scattering energy. The two locations show,
respectively, a minimum frequency of 12 Hz and 17 Hz with a maximum
affected wavelength of 110 m and 100 m. This distinguishes the
scattering to be geological in nature and not due to human constructions
such as road and bridge foundations. Similar strong back scattered
surface waves are also visible at Pocheon fault surface location, but
showing a lower intensity.
6.3 Crossdip analysis and
out-of-plane
structures
The crooked nature of the profile implies that the trace midpoints are
distributed along a 3D zone, allowing evaluation of out-of-the-plane
structures and apparent dips for several reflections. This effect was
further exploited using a crossdip analysis approach (Bellefleur et al.,
1995; Nedimović and West, 2003; Malehmir et al., 2006, 2009;
Rodriguez-Tablante et al., 2007; Beckel and Juhlin, 2019). For the
reflection (F2) underlying the domed-shaped reflections (Figures 6c and
7), we were able to estimate a crossdip angle of 10 degrees to the north
and a true dip of approximately 30 degrees towards NW. The crossdip
analysis could only be achieved thanks to the midpoint coverage provided
by the crookedness of the profile.
6.4 Fault 3D geometries and
seismicity
The information gained from all the analyses were compiled and used to
construct 3D surfaces of potential major fault systems along the profile
(Figure 8). Based on these surfaces, it is possible to interpret the
location and geometry of two of the major fault systems in the area,
namely the Wangsukcheon (F1) and Chugaryeong (F3) faults with high
reliability. The Wangsukcheon fault dips opposite to what was first
expected, especially if considering that it would be a splay fault from
the Chugaryeong fault. This implies that the Wangsukcheon fault is
likely a separate and unrelated fault system with respect to the two
other ones or that it makes a sharp turn as it extends to the northern
part of the country (Figure 1). Recent excavation works (Han and Lee,
2019) and historical studies (Kim, 1973) further support our
interpretation of the dip direction of the Wangsukcheon fault and the
reliability of the 3D reflection traveltime modeling work. The
Chugaryeong fault is not imaged as a reflection, as expected for a
sub-vertical feature that has also been argued from focal mechanism
solutions (Hong et al., 2018, 2021). Nonetheless, there are related
features that support the sub-vertical nature of the Chugaryeong fault
such as (i) the absence of coherent reflection, (ii) the sharp and
important lateral variation visible at the bedrock level in both the
landstreamer data and tomography and (iii) the extremely high surface
wave back-scattered energy. All these features are observed at the fault
surface expression (Choi et al., 2012). The reflections (F2) underlying
the domed reflectivity package (Figures 6c and 7) have a true dip of
approximately 30 degrees towards the NW. This dip angle was to a certain
degree speculated upon by Malehmir et al. (2022) for Pocheon fault but
is different from previous suggestions (Hong et al., 2021). Surface
projection of these reflections corresponds to the location of strong
surface-wave back scattered energy at the mapped position of the Pocheon
fault. Interestingly, projecting the reflections towards depth results
in them intersecting with the recorded seismicity, suggesting an
intersection with the Chugaryeong fault. While this is highly
speculative, a possible scenario might be that the Pocheon fault is a
splay fault from the Chugaryeong fault system and that the recorded
seismicity occurs at their intersection. Malehmir et al. (2022) argued
for the same geometry, however they had a much shorter profile, hence
their arguments were more speculative. Another possible scenario will be
that the F2 reflection is generated by a different fault that becomes
steeper close to the surface. Pocheon fault instead will be subvertical,
as suggested from seismological focal mechanism inversion studies (Hong
et al, 2021), hence not imaged. There may be a third scenario explaining
the NW-dipping package of reflectivity underlying the domed-shape
reflectivity (F2), namely dykes as they are also interpreted to be
present in the domed-shape reflectivity and diffractivity.
Given the curved-shaped nature of the reflectivity overlying these
planar sets of reflections, we argue for a thrust (reverse mechanism)
fault system associated with these reflections. Thrust faults are known
to generate fault-bend folds and this implies the dome-shaped
reflectivity might be the result of a growth fault system that was
active sometimes (even until now). In this scenario, the reflective
package would initially have consisted of sill-type intrusions, which
were then folded, and likely also faulted, forming the dome-shaped
reflectivity observed in the central part of the study area (Figure 7).
Assuming this interpretation scenario is correct, a direct implication
is that both the Chugaryeong and F2 faults might be active. Given the
opposite dip direction of the Wangsukcheon, it is likely that any
seismicity along this fault should be separately looked at, although the
fault appears to be inactive in terms of seismicity recorded in the
area.