Deep long-period (DLP) earthquakes observed beneath active volcanoes are sometimes considered as precursors to eruptions. Their origin remains, however, unclear. Here, we present a possible DLP generating mechanism related to the rapid growth of gas bubbles in response to the slow decompression of over-saturated magma. For certain values of the gas and bubble content, the elastic deformation of surrounding rocks forced by the expanding bubbly magma can be fast enough to generate seismic waves. We show that amplitudes and frequencies of DLP earthquakes observed beneath the Klyuchevskoy volcano (Kamchatka, Russia) can be predicted by our model when considering pressure changes of~10 7 Pa in a volume of~10 3-10 4 m 3 and realistic magma compositions. Our results show importance of the deep degassing in the generation of volcanic seismicity and suggest that the DLP swarms beneath active volcanoes might be related to the pulses of volatile-rich basaltic magmas rising from the mantle.

We review the current knowledge about Southeast Asian volcanoes and their eruption histories, and focus on identifying tephrochronologic markers representing major explosive eruptions in order to further future palaeoclimate and volcanological studies. Forty-one volcanic edifices in Southeast Asia have been classified as large calderas by Whelley et al. (2015) and thus have, or are likely to have, produced large explosive eruptions with a Volcanic Explosivity Index (VEI) of 6-8. Unfortunately, only 20 such eruptions have known ages, spanning from 1.2 Ma to 1991 AD, and fewer have geochemical data that can be used for tephrostratigraphic correlations. Volcanic products from different geodynamic regions and different sources can generally be distinguished on major element plots (e.g. K 2 O versus CaO) of matrix glass composition. However, the distinction of multiple eruptions from the same source often requires additional data such as trace element compositions of matrix glass and/or mineral compositions. Biotite, but also magnetite compositions (MgO and TiO 2 content in particular) appear to be very discriminating. Up to nine tuffs in addition to the three to four Toba tuffs can be utilised as widespread tephrochronologic markers and span a range from 1.2 to 1.6 Ma to recent. As only a few Holocene major eruptions have been well characterised and dated, many large calderas are still unstudied, and many distal tephra layers are still lacking a source, more tephrochronologic markers can certainly be defined in the future.

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We describe a new caldera-volcano in the volcanic front of Kamchatka named Verkhneavachinskaya caldera (VC). According to geological mapping, the VC is interpreted as an eroded shield volcano with a summit caldera exposing 1 km-thick lava-like ignimbrites. It is one of the largest (10 × 12 km diameter) and oldest (c. 5.78-5.58 Ma, Ar-Ar dating) morphologically preserved paleovolcano in Kamchatka. The welded ignimbrites with fiammes are andesites-basaltic andesites in composition, they are more mafic than most ignimbrites worldwide and than other post-Pliocene calderas in Kamchatka. The deposits of VC are interbedded layers of welded ignimbrites and volcanoclastic material, which we interpret as result of long-lived volcanic center activity with hot pyroclastic flows and subsequent accumulations of volcanoclastic suites (e.g. lahars). The most trace element ratios in VC rocks (e.g. La/Yb, Nb/La, Ba/Th) show similarity with post-accretion magmatism at Kamchatka and especially with Late Pleistocene-Holocene Bakening volcano located in the same area. The VC provides new insights into the early stages of Kamchatka frontal zone development after the Kronotsky Arc Terrane accretion. This potentially explains the origin of voluminous basaltic-andesitic ignimbrites formed on thin crust during the initial stage of the arc formation.

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Abstract-An association of adakites and rocks with intraplate geochemical parameters (NEAB) is encoun tered in Kamchatka, even though the Pacific plate that is plunging under Kamchatka is an old (older than 93 Ma) and cold plate. Inspection and comparative analysis of NEAB-adakite associations in Kamchatka and else where at Pacific subduction zones revealed the geodynamic settings that favor heating of oceanic crust until the crust is melted and adakites are generated. Two geodynamic settings are favorable for the process: (1) an initial period of subduction with the tip of the slab melted and (2) generation of subduction windows. Both of these settings have existed in Kamchatka for a short period of time. In eastern Kamchatka, melting affected the tip of the slab in a new subduction zone that was formed during the late Miocene, as the subduction zone was blocked beneath the Sredinnyi Range as this zone jumped into its present location. The NEAB-adakite asso ciation in the northern part of the Sredinnyi Range was also formed in the tip of the slab that was part of the near Commander Islands plate. Geological sections were modeled to study conditions for the generation of NEAB-adakite associations in Kamchatka on the background of the ongoing geodynamic evolution.

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<p>The correlation of subducted plate parameters with generated volcanism was studied along the Kamchatka arc. Increased slab age controls dip angle (25-45<sup>o</sup>) and length of the seismic zone (200-700 km slab depth)  from the north (~53<sup>0</sup>N) to the south (~49<sup>0</sup>N) of the Kamchatka arc. All listed above parameters generate various aged volcanic belts with different parameters of volcanism. The natural boundary between various aged slabs is on ~53<sup>0</sup>N, on the extension Avachinsky transform fault. It divides the Kamchatka arc on Southern Kamchatka with slab age ~ 103-105 Ma and Eastern volcanic belt, Central Kamchatkan Depression with slab age ~ 87-92 Ma. Complicated evolution and various ages of the slab control magmatism along the Kamchatka arc. Basic-intermediate magma compositions dominantly characterized Quaternary-Pliocene volcanoes in Central Kamchatkan Depression. In contrast, Neogene-Quaternary volcanism on Southern Kamchatka represents by strong explosions of acidic magmas (Gordeev, Bergal-Kuvikas, 2022).</p><p>Monogenetic volcanism marked a Malko-Petropavlovsk zone of transverse dislocations (MPZ), which is located on the extension Avachinsky transform fault. Monogenetic cinder cones in MPZ are randomly distributed along to these long-lived rupture zones. Here I present new geochemical and isotopic results of monogenetic volcanism in MPZ. Based on whole rock and trace element geochemistry, Pb-Sr-Nd isotopic ratios of monogenetic cinder cones magmas were shown to tap the enriched mantle source (low <sup>143</sup>Nd/<sup>144</sup>Nd isotopic ratios (0.512959-0.512999), as variated <sup>87</sup>Sr/<sup>86</sup>Sr (0.703356-0.703451) and <sup>206</sup>Pb/<sup>204</sup>Pb (18.30-18.45), <sup>208</sup>Pb/<sup>207</sup>Pb (38.00-38.12) isotopic ratios).  High Nb/Yb and La/Yb ratios, without significant inputs of the slab`s components (the lowest Ba, Th contents), indicate decompression melting predominately (Bergal-Kuvikas et al., 202X). Therefore, a combination of geophysical and geochemical methods enable us to conclude that monogenetic volcanism in MPZ   mark a natural boundary between various aged slab on Avachinsky transform fault. Various aged slabs under Southern Kamchatka and the Eastern volcanic belt generate volcanism with different magma compositions and ages of volcanoes.</p><p>This research was supported by Russian Science Foundation (grant number 21-17-00049,https://rscf.ru/project/21-17-00049/).</p><p>References</p><p>Bergal-Kuvikas O.V., Bindeman I.N., Chugaev A.V., Larionova Yu. O., Perepelov A.V., Khubaeva O.R. Pleistocene-Holocene monogenetic volcanism at Malko-Petropavlovsk zone of transverse dislocations on Kamchatka: geochemical features and genesis // Pure and Applied Geophysics. Special Issue: Geophysical Studies of Geodynamics and Natural Hazards in the Northwestern Pacific Region (in review)</p><p>Gordeev, E.I., Bergal-Kuvikas O.V. (2022). Structure of subduction zone and volcanism on Kamchatka. Doklady of the Earth Sciences. 2. 502. P. 26-30. 10.31857/S2686739722020086</p><p> </p><p> </p><p> </p>