JOURNAL OF QUATERNARY SCIENCE (2019) 1–13 ISSN 0267-8179. DOI: 10.1002/jqs.3145 Tephrochronological dating of paleoearthquakes in active volcanic arcs: A case of the Eastern Volcanic Front on the Kamchatka Peninsula (northwest Pacific) EGOR ZELENIN,1* ANDREY KOZHURIN,1,2 VERA PONOMAREVA2 and MAXIM PORTNYAGIN3,4 1Geological Institute, Moscow, Russia 2Institute of Volcanology and Seismology, Petropavlovsk‐Kamchatsky, Russia 3GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany 4V.I. Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, Russia Received 16 April 2019; Revised 27 July 2019; Accepted 1 August 2019 ABSTRACT: Investigation of active faults is crucial for the seismic hazard assessment and, in the case of volcanic belts, it provides a deeper understanding of the interactions between volcanism and tectonic faulting. In this study, we report the results of the first paleoseismological and tephrochronological investigation undertaken on Holocene faulting in Kamchatka’s volcanic belts. The studied trenches and additional excavations are located along the axial fault zone of the Eastern Volcanic Front, where the earlier dated tephra layers provide a robust age control of the faulting events. Electron microprobe analysis of glass from 22 tephra samples permitted correlations among the disparate tephra profiles for constructing a summary tephra sequence. The latter, together with published geochronological data, allowed the construction of a Bayesian age model. Detailed examination of the tephra layers deformed by faulting allowed us to reconstruct and date six faulting events with the offsets of 1 to 20 cm indicating paleoearthquakes with magnitudes of Mw < 5.4. Holocene crustal seismicity of the Eastern Volcanic Front manifests temporal clustering rather than a uniform flux of events. However, no correlation between dated seismic events and the largest Holocene eruptions of proximal volcanoes was observed. Copyright © 2019 John Wiley & Sons, Ltd. KEYWORDS: paleoseismology; trenching; tephra; geochemical fingerprinting; age modelling Introduction continuously updated all‐Kamchatka tephrochronological framework extensively used for dating and correlation of Identification and characterisation of active faults are crucial various deposits including those formed by volcanic and for the study of crustal stress. When applied to the crust above co‐seismic processes (e.g., Braitseva et al., 1978b; Hulse et al., subduction zones, this approach provides a proxy for 2011; Kozhurin et al., 2006; Pinegina et al., 2018). geodynamics studies of the convergent plate boundaries. The The Kamchatka Peninsula overlies the northwestern margin main reasons for faulting studies above the subduction zones of the subducting Pacific plate and is one of the most include estimates of tectonic deformation rates, seismic hazard volcanically and tectonically active regions in the world (e.g. evaluation and the study of the interaction between volcanic Gorbatov et al., 1997). Kamchatka hosts about 30 active and tectonic activity in the arc (Kozhurin et al., 2006). All volcanoes and hundreds of monogenetic vents grouped into these investigations require robust age control, which in areas two major volcanic belts running northeast–southwest along of active volcanism can be readily provided with the help of the peninsula: the eastern volcanic belt including the Eastern tephrochronology. The applications of tephra in paleoseismol- Volcanic Front (EVF) and the Central Kamchatka Depression ogy vary from the use of tephra layers as isochrons (e.g. De (CKD) volcanic zone, and the Sredinny Range (Fig. 1). Active Lange and Lowe, 1990; Galadini et al., 1997; Townsend, crustal faults also tend to strike northeast–southwest along the 1998), to accurate dating of paleoseismic events (e.g. Berry- peninsula (Fig. 1) and group into two major fault zones: the man et al., 1998; Galli et al., 2010) and correlating those to the East Kamchatka Fault Zone (EKFZ), which bounds the CKD volcanic history (e.g. Kozhurin et al., 2006, Wils et al., 2018). from the east, and a fault system along the axis of the EVF However, to date very few studies have utilised recent spatially close to its volcanic centres (Fig. 2; Kozhurin advances in geochemical fingerprinting of tephra and in age et al., 2006). modelling (Ponomareva et al., 2017; Loame et al., 2019). Most of the instrumentally recorded seismicity in Kamchatka The Kamchatka Peninsula, in the northwest Pacific, is an is related to the subduction of the Pacific plate under the ideal location for tephrochronological studies of Holocene peninsula (Gusev and Shumilina, 2004). Recorded earth- faulting as its volcanoes are highly explosive and produce quakes within the peninsula’s crust, above the subduction numerous tephras. The Holocene tephra sequence has been zone, are rare and of moderate magnitudes (Gordeev et al., thoroughly studied over the last four decades (e.g., Braitseva 2006). Only one of those, the 1996 Karymsky earthquake et al., 1978a, 1995, 1997, 1998; Kyle et al., 2011; Pevzner within the EVF (surface wave magnitude MS = 6.6) resulted in et al., 2006; Plunkett et al., 2015; Ponomareva, 1990; surface ruptures (Leonov, 2009), but the actual process of the Ponomareva et al., 2015, 2017). These studies resulted in the surface deformations is still unclear due to ambiguous field data. The period of documented seismicity for Kamchatka is *Correspondence: E Zelenin, as above. extremely short, <100 a (Ruppert et al., 2007), which makes E‐mail: [email protected]; [email protected] paleoseismology especially important for geodynamic and Copyright © 2019 John Wiley & Sons, Ltd. 2 JOURNAL OF QUATERNARY SCIENCE (2017), faulting, spatially linked to EVF volcanism, appears to result from the superposition of regional ocean‐directed lateral extension on a magmatic thinning of a brittle crust along the volcanic belt, similar to the Taupo zone. Previous work on EVF faulting (Legler and Parfenov, 1979; Florensky and Trifonov, 1985; Leonov, 1989; Kozhurin and Zelenin, 2017) considered spatial distribution, fault type, and average deformation rate, whereas paleoseismological parameters, such as earthquake recurrence and magnitude, remained unexplored. Here, for the first time, we identify and date Holocene faulting events within the EVF and estimate the magnitude of the related earthquakes based on trenching and tephrochro- nological data, supported by aerial imagery interpretation and rupture scaling relations. We study the northern EVF segment, between Krasheninnikov and Zhupanovsky volcanoes, where a 150 km‐long axial fault zone forms a prominent, 60–65 m‐deep graben (Fig. 2; Kozhurin and Zelenin, 2017). An equally important result of our studies is the detailed Holocene tephra record for the central part of the EVF, supplied with the electron microprobe (EMP) data on glass from most of the tephras. The record includes 18 marker tephra layers from distal volcanoes, which link the northern and southern Kamchatka tephrostratigraphies. Figure 1. Main tectonic features, volcanic belts and active fault zones of Kamchatka. Central Kamchatka Depression (CKD) and smaller half‐ grabens (as in Kozhurin and Zelenin, 2017) are filled with yellow; Methods volcanic belts of the Sredinny Range and the Eastern Volcanic Front (EVF) are filled with red, Holocene volcanic centres (Ponomareva Mapping et al., 2007) are indicated. Faults are black lines with hatches for normal faults, triangles for reverse and thrust faults, and one‐sided Detailed mapping of the fault scarps was implemented using arrows for strike‐slip faults. Kamchatsky Peninsula block with stereoscopic pairs of detailed (1:6000) aerial imagery, which predominant compressional faulting is labelled “KP”. Sources of the permitted us to trace the scarps of individual ruptures. identified tephras off the extent of Fig. 2 are labelled (SH, Shiveluch volcano). [Color figure can be viewed at wileyonlinelibrary.com] Paleoseismological trenching earthquake hazard studies. Paleoseismological trenching can Trenching is an approach used to study deposits deformed by provide data on ages of faulting events and their recurrence faulting events in an excavation across the fault scarp. It was rate along with direct observations of fault kinematics and performed in two key areas of the EVF fault zone (Fig. 2): tundra‐ displacements. Paleoseismological trenching of crustal faults covered Shirokoe Plateau bounding the Uzon caldera from the in Kamchatka was initiated in the EKFZ, where it enabled the south (Fig. 3), and a forested southern slope of the Bolshoi refining of fault kinematics as well as identification and dating Semiachik massif (Fig. 4). Both areas lie on the late Pleistocene of magnitude (M) ~6.5 faulting events recorded in a Holocene ignimbrite plateaus heavily dissected by a system of normal faults. – soil tephra sequence (Kozhurin et al., 2006, 2008). Later, The plateaus are mantled with the Holocene soil–tephra sequence, paleoseismological trenching accompanied by the tephrochro- which is 2.5–3 m thick on flat surface and gradually thins up the nological studies was implemented northeast of the EKFZ, in fault scarps. The trenches were positioned across the scarps of the Kamchatsky Peninsula (Fig. 1), a locus of collision between typical height and dug into the soil–tephra sequence down to the the Aleutian Island arc and Kamchatka (Pinegina et al., 2013; underlying ignimbrite. Studied soil–tephra sequences record Kozhurin et al., 2014). Here it helped to