To examine the role of geological structures, especially related to the Heunghae fault [38
] in the liquefaction feature distribution, detailed literature review and field studies have been carried out. Previous studies [38
] reported that the Pohang basin is bounded by several E-W trending faults, which offset the tertiary formations and merged to the Yangsan fault. These faults are named as Hyongsan fault and Heunghae fault, which divide the Pohang basin into several sub-basins. During the post-earthquake survey, we were able to find several evidences of NE-SW striking small to medium scale normal faults in the northern part of the Heunghae basin with a fault gouge of 1-5 cm thick (see Figure 3
for the location and Figure 11
for the evidences of faults in seismic profile). These faults probably are subsidiary faults of the E-W trending Heunghae fault, because the structural parameters of these faults are well matched with the previously mapped Heunghae fault. The main Heunghae fault might have been covered by the basin fill deposits and was not clearly traced.
To confirm the existence of the main Heunghae fault within the basin, a shallow subsurface seismic refraction survey has been carried out using OYO McSeis SX 1125 instrument (Tsukuba, Japan) with 24 channel (28 Hz) seismographs. A Sledge Hammer was used for generating seismic waves at the surface and after recording the data it was processed using SeisImager software. The 1 km long seismic survey was taken perpendicular to the Heunghae fault towards the western margin of the Heunghae basin (see Figure 3
for location of seismic survey). The same 1 km long profile has been divided into six divisions (SP-1 to SP-6). The geophone spacing was taken at 5 m for SP-1 and SP-6, and 7.5 m for SP-2 to SP-5. The data processing of the seismic survey is mainly based on seismic refraction tomography techniques, as well as inverse travel time modelling of the refracted seismic waves. Using the estimated velocity (1.6–3.6 km/S), we have detected unconsolidated basin fill deposits lies from surface to 10 m deep, and consolidated basin deposits or weathered rock has been detected from 10 m to 80 m deep (Figure 11
On the basis of seismic profile and velocity difference, we have detected two south dipping normal faults which could be the traces of the Heunghae fault, which is well matched with the field data collected. The seismic profile and field evidences of normal faulting along the Heunghae fault is shown in Figure 11
. We argue that this fault played a role in the passage of seismic waves and amplifications within the Heunghae basin and the distribution of liquefaction features. Moreover, the mountain basin effect may do play an important role in extensive liquefaction within the basin and the distribution of sand boils (Figure 3
and Figure 11
]. On the basis of these observations, we will discuss the possible mechanism involved in the liquefaction and its distribution within this study area in the following section.
Possible Mechanisms Involved in Liquefaction Clustering in the South Part of the Heunghae Basin
The cause and distribution characteristics of liquefaction and related damages are associated with the combined effects of several factors such as earthquake magnitude, duration of shaking, distance from the epicenter, type of soil content, relative density, drainage condition, degree of consolidation, thickness of liquefiable sand/silt layer, and depth of groundwater table [5
]. Sometimes anthropogenic structures such as clay lining in rice fields and reclaimed land also influence to the severity of the hazard [5
By analyzing the source of the earthquake and the geological setting of the Heunghae basin area underwent liquefaction, we argue that the major structural factors for the liquefaction clustering during the Pohang earthquake might be the combination of mountain basin effect and trapping of seismic waves within fault zones. During the 2012 Emilia earthquake (Mw
5.9) in Italy, clustering of liquefaction features was observed with in the Po-Plain. This indicates that even if the affected area appears to be homogeneous from a geological point of view there are other local geological factors that controls the liquefaction susceptibility of the area within a basin or an alluvial plain [68
]. Furthermore, previous studies [70
] observed the mechanism involved in severe liquefaction within basins and suggested that the wedge shaped basement-to-sediment basin interface, which acted as acoustic lens, caused localized seismic wave amplification and extensive damage within the basin [7
]. Though the basin effect is poorly understood and included in the routine seismic hazard assessment, it has been well evidenced that several large and small magnitude earthquakes (1985 Mexico City earthquake, Mexico, Mw
8.0; 1994 North Ridge earthquake, USA, Mw
6.7; 1999 Izmit earthquake, Turkey Mw
7.6; 2008 Wenchuan earthquake, China, Mw
7.9; 2009 Olancha earthquake, USA, Mw
5.2; 2011 Tohoku earthquake, Japan (Mw
-9.0); 2012 Emilia earthquake, Italy, Mw
5.9; and 2017 Tripura earthquake, India, Mw
Other factor which controls the severity and distribution of liquefaction and seismic ground deformation is the trapping of seismic waves by the major fault zones within the basin. It was suggested that large faults within the sedimentary basin with fault gouges, fractured rocks and fluids can trap the seismic waves within the block bounded by fault zones [5
], which amplifies the upper bound in soft sediments of the basin. This amplification could be stronger within the basin surrounded by fault zones covered by unconsolidated Holocene alluvial deposits (Figure 11
and Figure 12
]. A similar observation was reported during the 2008 Wenchuan earthquake, where most of the liquefaction features were confined to the recent alluvial deposits close to the range front blind fault, and damaged buildings were clustered near or top of the Qingchuan blind fault in Sichuan province in China [7
]. During the 1994 Northridge earthquake in the USA, (Mw
], the basin structure was an important factor for the enhancement of liquefaction hazard. In the 2001 Bhuj earthquake in India (Mw
7.7), most of the liquefaction features were distributed close to the fault [83
]. This clustering of sand boils indicates a fault barrier mechanism for passage of seismic waves within a basin.
The borehole log drilled across the Heunghae basin for the pilot project of the potential CO2
storage site [84
] suggests that the Pohang basin has a typical wedge-shaped structure bounded and dissected by several faults and covered by soft sediments. The previous study [79
] for a CO2
storage project suggests that the Heunghae basin is bounded by the east dipping Gokgang fault to the east and the south dipping Heunghae fault to the north. The seismic refraction profile and field evidences about the presence of the two NE-SW/E-W striking and S-SE dipping subsidiary normal faults within the Heunghae basin help us to suggest the presence of the E-W striking Heunghae fault.
The potential seismogenic fault for the Pohang earthquake is the west dipping thrust fault, which might be an antithetic fault of the Gokgang fault (Figure 12
). On the basis of the geometry and location of the Heunghae fault and the seismogenic fault, the seismic waves generated during the earthquake were trapped and caused more amplification in the southern part of the basin than the northern part. The geophysical and field results suggesting the presence of the E-W trending Heunghae fault and the field evidence of clustered sand boils along the NE-SW causative fault for the Pohang earthquake proved the trapping of the seismic waves by fault zones (Figure 3
]. The distribution of sand boils within the Heunghae basin (Figure 3
) shows clustered sand boils indicating two preferred orientations. One set of sand boils shows E-W trend, whereas another set shows NE-SW trend, which are similar to the trend of the Heunghae fault and the antithetic fault caused the Pohang earthquake, respectively.
Based on the field observations, geological structures presented in the study area—i.e., spatial distribution plot of sand boils around the epicenter—it can be inferred that the distribution of liquefaction features is mostly controlled by the geological structures within the Heunghae Basin (Figure 3
and Figure 12
). Thus, the presence of the Heunghae fault and the antithetic blind fault led the differential amplification due to trapping of seismic waves within the same basin and the differential distribution of liquefaction features. Using this, we have proposed a conceptual model (Figure 12
) to explain the local clustering of sand boils within the Heunghae basin, which is well matched with the previous observations in Sichuan province, China [7
] and San Fernando Valley, USA [11
Many of the liquefactions that have caused damages within sedimentary basins have been reported in recent earthquakes such as the Sichuan basin during the 2008 Wenchuan earthquake in China (Mw 7.9), the Kanto basin near Tokyo during the 2011 Tohoku earthquake in Japan (Mw 9.0), Po-Plain in Italy during the 2012 Emilia earthquake (Mw 5.8), and the Kathmandu valley during the 2015 Nepal earthquake (Mw 7.8). The observed damages during these earthquakes poses a real seismic threat to the areas with similar geological settings around the globe. However, the involved mechanisms of geological aspect have received little to no attention, especially during small to moderate earthquakes. Although several numerical studies suggested the trapping and amplification of seismic waves within the basins, geological evidences were very rare and difficult to prove it during a small or a moderate earthquake.
Currently, the liquefaction during small to moderate earthquakes without any surface ruptures is another issue (e.g., such as in the 2009 Olancha earthquake in the USA and the 2017 Pohang earthquake in South Korea), because it can cause serious damages compared to its magnitude. This is especially true of the effect of geological structures within the basin, which is an important concern in earthquake hazard assessment. The present study and proposed inferences will help in understanding the geological phenomenon involved in more localized seismic damages, especially where serious liquefaction and related damages compared with its magnitude are reported in a similar geological and depositional setting. However, it needs more geophysical or seismic data for a conclusive interpretation.