No 4 (2024)

The paper considers the distribution of ash during the paroxysmal eruption of the Shiveluch volcano located in the north of Kamchatka, which lasted from April 10‒13, 2023, and its impact on the water resources of the areas covered by the ashfall. The thickness of ash deposits in different localities and their granulometric composition are characterized. It is shown that the dynamics of the volcanic eruption plays the main role in the distribution of ash clouds, but the atmospheric circulation actually determines the spatial distribution of the thickness of deposits. The water-soluble salts contained in ashes and the dynamics of their leaching under natural conditions are determined. Calcium and magnesium sulfates and sodium chloride dominate in the composition of the water-soluble part of fresh ashes, with subordinate amounts of aluminum, potassium, and ammonium chlorides and fluorides. Well-soluble chlorides wash out of ashes first, followed by sulfates. Over time, the total content of soluble salts decreases and their qualitative composition changes: calcium, magnesium and sodium hydrocarbonates begin to dominate. Several months after the eruption, the impact of the ashfall on the waters of the settlements, including open springs on the day surface, leveled off.

The Tonga‒Kermadec subduction zone is located between the Pacific and Australian plates and is the site of the highest rates of Pacific plate subduction and dominant extension. In 2006 and 2009 in this region, two strong earthquakes occurred with magnitudes M_w = 8.0 and 8.1. There are about 170 islands in the Tonga region. They are volcanic centers that have erupted regularly over the past few decades. The paper presents the results of determining temporal variations in the slope of the earthquakes recurrence curve (b-value) in the Tonga subduction zone for 2005–2022 and variations in b depending on depth. Temporal variations in the b-value reflect the general tendency for the most powerful earthquakes to occur against the background of a decrease in b-value only in the surface layer at depths of 0–100 km. By comparing the variation of b-value with depth with a tectonic model of the Tonga subduction zone, it suggested that lower b-value might reflect greater stress at the top of the subducted slab due to its bending. Elevated b-value can apparently be associated with stretching mechanisms. For the Tonga subduction zone, as for other subduction zones, the increased b-value identified at a depth of 90‒100 km, which may be due to the presence at this depth of a magmatic front, which is associated with active volcanism.

Comprehensive studies (archaeological, archeo- and paleoseismological, georadar profiling) were carried out, which made it possible to establish the genesis of the destruction of the Yuzhno-Churubashskoe settlement (a large estate on the Nymphea Choir) in the Eastern Crimea. A strong seismic event with a possible focus along the southwestern limit of the Churubash estuary, one of the segments of the Parpach-Taman’ active fault, led to the formation of landslide bodies to the southwest of the fault. The submeridional plane of separation of one of them crossed the ancient settlement approximately in the middle, forming a visible flexure in the bedrock of the archaeological monument – a seismic-gravitational deformation. Strong seismic movements from the seismic source led to the collapse of all building structures and severe deformation of the preserved lower rows of masonry: tilts, horizontal shifts and rotations of parts of the walls – seismic-inertial deformations. Taking into account the proximity of the seismic focus and the severe destruction of all the buildings of the settlement, we assume that the estate fell into the epicenter zone of an ancient earthquake, where the intensity of seismic movements was at least Io ≥ IX points. Judging by the finds of amphoric stamps, chernolak ceramics, as well as a Bosporan coin, this large estate on the Nymphea Choir ceases to exist during a serious earthquake and a strong fire at the beginning of the fourth quarter of the IV century BC. It is possible that we observed traces of this earthquake earlier in Nymphaeum: the Nymphaeum structures built in the V–IV centuries BC were very seriously damaged – they were completely or partially destroyed. Further studies of active geological structures and archaeological sites will help to more accurately parameterize the identified seismic event, which will serve the purpose of a more accurate assessment of the seismic hazard of the Crimean Peninsula.