Beschreibung: Highlights • Unprecedented record of 3 unusually wide earthquakes (of 〉30) in last 4000 years. • Of 5 historical earthquakes on this subduction-zone segment, none is comparable. • Tephra correlation of coseismic subsidence, tsunamis and buried beach scarps. • New methods quantify subsidence, tsunami size, erosional retreat for each event. • New insights into millennial-scale variability of subduction-zone behavior. Abstract The prograding strand plain of Avachinsky Bay, Kamchatka, Russia, along the highly active Kamchatka subduction zone, exhibits geological evidence--buried erosional scarps--for coseismic subsidence only three times in the last four millennia, the last event about 1200 years ago. This same coast has a historical record (since A.D. 1737) of five subduction-zone earthquakes with large tsunami runup (〉5 m), the last of which was the 1952 Mw 9 Kamchatka earthquake, and a geological record of more than 30 large tsunamis in the last 4000 years. This rarity of buried scarps relative to large earthquakes contrasts with the Cascadia strand plain in SW Washington State, where most or all large events are represented by buried scarps. A strong factor in the amplitude and sign of coseismic deformation is distance from the seaward edge of a subduction zone (the trench); the Avachinsky Bay coastline is 180–200 km from the trench, with ∼25° slab dip, requiring unusually wide ruptures to generate significant coseismic subsidence. This coastal zone is undergoing net subsidence approximately equivalent to the total of the three coseismic subsidence events, generating a sequence of beach ridges that increase in elevation seaward. Each of the three unusual (coseismic subsidence) events comprises a) an earthquake whose deformation field caused b) onshore coseismic subsidence, thus local sea-level rise and c) sufficient deformation offshore to produce a large tsunami; a,b,c followed by d) a period of coastal erosion and shoreline retreat, leaving e) an erosional beach scarp that was f) subsequently buried once progradation resumed. We identified, dated and correlated the scarps and tsunami deposits from these events with several field methods, including trenching, tephrostratigraphy and ground penetrating radar. The scarps were correlated over an alongshore distance of 50–70 km. The most recent event (event 1) occurred ∼800 cal AD (1100–1250 14С years BP), event 2–600 cal BC (2400–2450 14С years BP), and event 3–1700 cal BC (3300–3500 14С years BP). We developed methods for quantifying subsidence, coastal erosion and tsunami size for each of these events. All three retain evidence of ∼0.4–1.2 m of coseismic subsidence; coastal erosion in the case of event 1 averaged more than 100 m; all three “event” tsunamis were amongst the largest in the last 4000 years.

Beschreibung: Kamchatsky Bay is the northernmost bay at the Pacific Kamchatka coast. It is located at the junction between the Kamchatka segment of the Pacific subduction zone and the dextral transform fault of the western Aleutians. The combination of the subduction and collision processes in this region results in the unique set of tectonic controls influencing its geological and geomorphological evolution. The Kamchatka River estuarine area is located on the northern coast of Kamchatsky Bay. The modern Kamchatka River valley, its estuary, and an aggradation marine terrace some 30 km long and up to 5 km wide were formed in this area during the Holocene. A vast area in the rear part of the terrace and in the Stolbovskaya lowlands is now occupied by the peats deposited directly above lacustrine-lagoonal and fluvial facies. These aggradational landforms record traces of tsunamis and vertical coseismic deformations associated with great subduction earthquakes, as well as strike-slip and thrust faulting associated with the collision. The results indicate that the average recurrence interval for major tsunamis in the Kamchatsky Bay is 300 years. The recurrence interval on individual fault zones associated with the collision between the western Aleutian and Kamchatka arcs is a few thousand years for earthquakes of magnitude between 7 and 7.5. For the entire region, the recurrence interval for major crustal earthquakes associated with motions along faults may be equal to a few hundred years, which is comparable with that for subduction-zone earthquakes.

Beschreibung: Detailed data are discussed on the rate of Holocene horizontal and vertical movements along a fault in the southeastern Kamchatsky Peninsula, which is situated between the converging Aleutian and Kamchatka island arcs. The fault is the northern boundary of the block invading into the peninsula under pressure of the Komandorsky Block of the Aleutian arc. The rate of right-lateral slip along the fault was increasing in the Holocene and reached 18–19 mm/yr over the last 2000 years and 20 mm/yr by contemporary time. Comparison of these estimates with those that follow from offsets of older rocks also indicates acceleration of horizontal movements along the fault from the early Quaternary to the present. The results obtained from rates of GPS station migration show that about half the rate of the northwestern drift of the Komandorsky Block is consumed for movement of the block of the southern side of the fault. The remainder of movement of the Komandorsky Block is consumed for movements (probably, underthrusting) at the eastern continental slope of the Kamchatsky Peninsula.

Beschreibung: Geochemical study of volcaniclastic material and radiocarbon dating of charred plant debris from Holocene deposits of the Guram site, which is located in vicinity of Vetrovoi Isthmus on Iturup Island, demonstrate that an explosive eruption (VEI 4-5) occurred there about 2000 years ago. The geochemical and age similarity with the tephra of marker layer CKr that was distinguished on Iturup, Urup, Simushir, Rasshua, and Matua islands of the Kuril Island Arc led to the conclusion that this eruption is possibly a source of this tephra. The data presented are proposed as a motivation for revision of the volcanic hazard on Iturup Island.

Beschreibung: Kamchatsky Peninsula lies within a complex meeting place of tectonic plates, in particular, the orthogonal interaction of the west-moving Komandorsky Island block with mainland Kamchatka. Examining the Holocene history of vertical deformation of marine wave-built terraces along the peninsular coast, we differentiated tectonic blocks undergoing uplift and tilting separated by zones of stable or subsided shorelines. We analyzed ~200 excavations along 〉30 coastal profiles and quantified vertical deformation on single profiles as well as along the coast using paleoshorelines dated with marker tephras. For the past ~2000 yr, the average rates of vertical deformation range from about –1 to +7 mm/yr. Uplift patterns are similar to those detected from historical leveling and from mapping of the stage 5e Quaternary marine terrace (ca. 120 ka). Average vertical deformation in the Holocene is highest for the shortest studied time period, from ca. A.D. 250 to 600, and it is several times faster than rates for marine oxygen isotope stage (MIS) 5e terraces. Vertical displacements observed along the coast are most likely coseismic and probably have included subsidence as well as uplift events. Because subsidence is generally associated with erosion, almost surely more prehistoric large earthquakes occurred than are recorded as topographic steps in these terraces. We suggest that the distribution of coastal uplift and subsidence observed along the Kamchatsky Peninsula coastline is qualitatively explained by the squeezing of the Kamchatsky Peninsula block between the Bering and Okhotsk plates, and the Komandorsky Island block.