## 2 Geological and Volcanological Setting of the Changbaishan Volcano  
Changbaishan volcano (2774 m; \(\sim\)2 Ma-present) is the largest active volcano of China and lies above the flat portion of the Pacific slab subducting below the Eurasian plate (Figure 1). The volcano is located on the China-North Korea boundary and represents an intraplate, alkaline edifice with a summit caldera filled by a \(\sim\)5 km wide, 300 m deep lake (Heaven Lake) [40; 41]. A valley located in the northern flank, where the caldera rim is breached, hosts the only water outlet of the Heaven Lake. Such outlet forms the Erdaoba-he River (Figure 1). Changbaishan volcano lies on the Gaima basaltic plateau (29 Ma), which has an average altitude of 900-1000 m and covers the Sino-Korean Craton (Figure 1) [36; 42], and it is the northernmost edifice of a group of volcanoes including the 5 to 2 Ma old Wangtian'e and Namphothe strato-volcanoes and smaller scale monogenetic centers [41; 43]. The Changbaishan truncated cone has a basal diameter of about 50 km with radial valleys partially modified by glacial processes. Pre-caldera and post-caldera vents are located within and outside the present-day caldera depression (Figure 2). The Changbaishan activity is divided in three main stages: a 2-1 Ma old basaltic shield formation stage, a traclytic to rhyolitic composite cone formation stage (1-0.04 Ma), and the latest, caldera-forming explosive stage (Figure 1) [36; 43; 44]. The Changbaishan caldera formed by a collapse event related to the VEI -7, Millennium eruption (946 AD) [33], which ejected tephra of rhyolitic and traclytic composition (DRE \(\sim\)25 km\({}^{3}\)) [34; 35; 37; 40]. Other explosive eruptions occurred in Holocene times. These include the 8.1 ka pre-Millennium Pinian Qixiangzhan eruption, and the post-Millennium eruptions recorded in historical documents and recognized in distal tephra (1403, 1668, and the VEI \(\sim\)5 1702 AD) [42; 45; 46; 47; 48; 49]. [PERSON] et al. [36] report that the last eruption at Changbaishan occurred in 1903 AD within the summit caldera lake, however, [PERSON] et al. [37] ascribe the intra-caldera deposits of this event and the proximal deposits of the 1668 and 1702 AD eruptions to the final part of the Millennium eruption. This problem causes a difficulty in evaluating the volcanic hazard correctly [36]. Ash fall deposits of the Millennium and Qixiangzhan eruptions and of the 1215, 1403, 1668 AD eruptions have been found in the lakes around Changbaishan. For the Pinian events, fall deposits have been found in the Japan Sea, Japan mainland and Greenland (Millennium eruption) [49; 50]. The PDC deposits of Changbaishan, which frequently cover the fallout layers of the major eruptions, mainly outcrop in river gorges being significantly removed by fluvial and glacial erosion, and rainfall. A map of the spatial  
Figure 2: Simplified geological map of the Changbaishan caldera: 1, tracbyte lava flows; 2, rhyolite lava flow; 3, Millennium eruption (946 AD) pyroclastic deposits; 4, 1668 AD pyroclastic deposits; 5, 1702 AD pyroclastic deposits; 6, 1903 AD pyroclastic deposits; 7, rock and debris flows; 8, water table and river. distribution of PDC deposits at Changbaishan is, at the present, not available. The few outcrops allow us to estimate that PDCs cover an area of about 2430 km\({}^{2}\)[51]. The PDC deposits of the Millennium eruption can be traced 50-80 km away from the Changbaishan caldera [46,51]. In the valleys around the volcano, most PDCs evolve to lahars [51]. These latter have been found at distances up to 83 km north of the Changbaishan caldera. As concerns the next, possible eruption, the emplacement of fall and PDCs deposits with volumes of erupted magma in the range of 0.1-0.5 km\({}^{3}\), i.e.,VEI- 3-4, is expected [36]. However, gravity and tomographic models [52,53] show that two low Vs, low density 50 km (width) \(\times\) 5 km (height) and 20 km \(\times\) 2.5 km sinks occur at about 18 and 10 km depth.