# The Survey of Lava Tube Distribution in Jeju Island by Multi-Source Data Fusion
## 5 Discussions  
By overlaying the potential LTDPs on the strength of the spatial analysis mask, some insight into undiscovered lava tubes was revealed and shown in Figure 8a in which, 1034 V-migrations and 245 H-migrations were detected. The small portions of H-migrations (<0.0 mm/year) and V-migrations (>0.0 mm/year) were first cut off from LTDP to simplify analysis. Then, we split strong and weak migration points (Figure 8c,d) from the histogram distribution of V/H migrations. Figure 8: _Cont._First, potential LTDPs around the Jeju City area might include lots of instability induced by individual building structures rather than the potential lava tubes. The constraints identifying LTDPs on the undiscovered lava tubes are: (1) LTDPs should be distributed as a linear form in the direction of the lava flow, which is mostly from the cinder cone as the source of lava flows to the coastline; (2) V/H-migrations must be distributed together; (3) the lava tube networks producing LTDPs might cause regional linear deformation. Overlaying the H/V-migrations and the geological units is shown in Figure 8b. We defined the locations of potential lava networks from group \(A\) to \(G\) as assigned ellipses. The major axis of the ellipse is the estimated direction of the involved lava flow. The most interesting and distinguished group is \(B1\), which originated from the Gama cinder cone to the coastal area. It has a well-distinguished V/H-migrations along a pahoehoe lava flow. Around the Gama cinder cone, there is a 2 km length underground tunnel built by the Japanese army during World War II (WWII) and that becomes unstable. Although the military tunnel might cause unstable points, the spatial extent of group \(B1\) is too large to be considered as the consequence of the WWII military tunnel. However, it is worth noting that more than 700 WWII military tunnels are widely distributed in Jeju Island, and a large number of them are in structural failure and/or not discovered. This is the reason we classified the points only within the spatial analysis mask defined in Section 4.1. Group \(B2\) has a similar context, directed from a cinder cone to the coastline. The other obvious potential LTDP group is A1, which certainly fits the direction of a basalt-Trachytte unit (see Figure 1b) and follows the terminus of a lava flow. Since only a few lava tubes  
Figure 8: Distribution of LTDPs on (**a**) spatial analysis mask and pahoehoe lava flow and (**b**) geological units together with estimated clusters implying potential lava tube networks. (**c**) Histogram of V-migration points and average deformation/standard deviation/2 nd polynomial regression of strong V-migration points and (**d**) Histogram of H-migration points and average deformation/standard deviation/2 nd order polynomial regression of strong H-migration points. are identified there, we proposed an extensive and undisclosed lava tube network over _A1_. Group _A2_ is proposed as a lava tube trail, as it fits very well with an independent Basalt-Tracbyte lava flow unit starting a cinder cone, as shown in Figure 1b. Although the trail of potential LTDPs is unclear, groups _D1_ and _D2_ should be noticed, as the area will soon be reconstructed for the international airport. Groups \(E\) and \(C\) coincided with the trails of the two biggest lava tubes, the Manjang (length > 7.4 km)-Yunchen (length > 2.4 km) cluster and Biltemot (length > 11.7 km) cave, respectively. The average deformation pattern of detected V-migration (\"strong\" part \(<-1.82\) mm/year) shown in Figure 8c is more similar to that of the cave-road-crossing points in Figure 7a than other training data. The average H-migration pattern (\"strong\" part > 1.1 mm/year) in Figure 8d is quite different from H-migration training vectors but has a steady increase as expected. The geophysical exploration employing ground penetration radar (GPR) and electrical resistivity surveys reveal plausible underground caves which were presumably collapsing or being inundated by seawater around coastlines in area E [68].