# The Survey of Lava Tube Distribution in Jeju Island by Multi-Source Data Fusion
## 1 Introduction  
Lava tubes, a major consequence of lava flow formation, are among the most representative geomorphic features in volcanic landscapes. Pioneering works have been accomplished to confirm the origin of lava tubes in Hawaiian volcanoes [1, 2, 3], Mt. Etna, [4], and elsewhere. Along with geomorphic interpretation [5, 6], numerical modeling of strain [7] and thermal erosion [8, 9] have been used to establish the outline of lava tube development. Based on the studies listed above, two scenarios have been proposed to understand lava tube development. First, a solidified lava roof blocked the liquid lava flow, and a single structured lava tube was created. Second, the repetitive contraction of lava evolved into a multi-structured lava tube [10, 11]. Recent research of lava tubes has focused on comparative analysis, as the lava tubes of solid planets and satellites have been identified by the spotting of skylights [12, 13] and are being proposed as critical habitable environments. Jeju Island, located on the southern coast of the Korean Peninsula, provides a valuable testbed for the geological/geomorphic studies of lava tubes due to their high spatial density, the unique diversity of cave morphology [14], and easy accessibility [15]. The more than 200 lava tubes, which extend up to 4-11 km and are distributed over an area of 1850 km\({}^{2}\), show a large variety of cave structures formed at all sorts of developmental stages [16]. The key element of the Jeju lava tubes is that they occurred inshallow lava flows and there is a tendency toward cave deformation and collapse [17]. Lava tubes are distributed along the gentle slopes of Jeju Island in some instances close to dense human settlements. Furthermore, it is estimated that a large number of lava tubes that have never been identified have already collapsed. This can be confirmed in the investigations conducted over decades by [18]. Ref. [19] also observed the risks involving potential lava tubes in Jeju Island, especially the cases of road-cave crossings. The risk of lava tube collapse is not a concern for Jeju Island alone, as pit craters induced by lava tube collapse have been identified in Hawaiian and Icelandic lava tubes as well [20; 21]. However, lava tube collapse in Jeju Island is especially significant due to the proximity to high-density settlements and the shallow depth of the Jeju lava tube system, as reported in [16]. Despite requirements to regulate risk and geological interests, studies of Jeju lava tube collapse have been limited to conceptual interpretations [19], in-situ explorations [16; 18], and research using geophysical surveys of potential locations [22]. That is, quantitative analysis is still lacking. Based on the observations mentioned above, ground stability should be inspected before recent urban extension and infrastructure construction. For instance, during the ongoing construction of the second international airport in western Jeju Island, identifying potential lava tubes is critical to avoid the risk induced by undiscovered cavities. However, such tasks cannot be achieved with conventional ground exploration and geophysical data mining approaches that are limited in their coverages and that produce unacceptably ambiguous data. Therefore, we tackled the challenge of assessing potential deformation risks caused by undiscovered lava tube networks through synthesized data interpretation employing Interferometric Synthetic Aperture Radar (InSAR) techniques [23; 24; 25] and other spatial/contextual analyses. Various technical approaches were used to improve the interpretation of line-of-sight (LOS) measurements of InSAR time series analysis at specific critical points. After combining machine learning techniques, an integrated approach was proposed to classify the actual deformed signal caused by lava tube instability, referred to as lava tube induced deformation point (LTDP).