The importance of overbank deposits and paleosol analyses for comprehensive volcanic hazard evaluation: The case of Holocene volcanism at Miravalles Volcano, Costa Rica

On the flanks of the dormant Miravalles volcano, systematic fieldwork and radiocarbon dating of buried humus-rich soils (paleosols) and wood fragments, augmented by mineralogical and geochemical analysis, reveal extensive and previously undocumented Holocene activity. Phase 1 consisted of 8.3 ka (~6300 BCE) volcanic debris avalanche and thick lapilli blast and fallout deposit that appear coeval. Hiatus 1 marks 2600 years of inactivity ensued followed by Phase 2 lapilli interbedded with ~5.5 ka lahars below a 5.3 ka basaltic lava flow (~3400 BCE). Hiatus 2 lasted 1800 years from 5.3 ka to 3.5 ka (3300-1500 BCE), after which a very active Phase 3 ensued (3.5 to 0.5 ka; 1600 BCE to 1500 CE) with > four lapilli eruptions, > 4 lahars, > 6 layers of ash and pumice, and small andesitic lava flows. The most recent evidence for eruption is an 880-year-old (0.9 ka; 1070 CE) lapilli overlain by gravels that may represent distal lahar sediments. Evidence indicates the occurrence of at least two, if not three, destructive lahars on the southwest flank of Miravalles in the past 500 years. The overbank sedimentary record indicates much more activity of Miravalles volcano over the past 3500 years (since 1500 BCE) than previously known, with a minimum of 24 events in that span. Overbank floodplain deposits are likely to contain the most compete record of recent activity in active and dormant volcanoes, and in the absence of dateable vegetation fragments, radiocarbon dating of paleosol A-horizons is very useful, with a precision of ~ 10%, i.e. 800 + 80 ybp.

Multiple Holocene Earthquakes on the Gales Creek Fault, Northwest Oregon Fore-Arc

ABSTRACT Several potentially hazardous northwest-striking faults in and around the Portland basin, within the fore-arc of Cascadia, are classified as Quaternary active by the U.S. Geological Survey, but little is known about their Holocene activity. We present new earthquake-timing constraints on the Gales Creek fault (GCF), a 73 km long, northwest-trending fault with youthful geomorphic expression located about 35 km west of Portland. We excavated a paleoseismic trench across the GCF in the populated northern Willamette Valley and document three surface-rupturing earthquakes from stratigraphic and structural relationships. Radiocarbon samples from offset stratigraphy constrain these earthquakes to have occurred ∼1000, ∼4200, and ∼8800 calibrated years before the present. The penultimate earthquake back-tilted a buried soil into the hillslope creating accommodation space that was infilled by a colluvial deposit. The most recent earthquake faulted and formed a fissure within the penultimate colluvial deposit. Our results suggest that the GCF has a recurrence interval of ∼4000 yr, and if the full 73 km length were to rupture, it would result in an Mw 7.1–7.4 earthquake, providing a significant seismic hazard for the greater Portland metropolitan area.

Evidence for Holocene Activity on the Jiali Fault, an Active Block Boundary in the Southeastern Tibetan Plateau

Most active block boundary faults within the Tibetan plateau have been thought of as Holocene active, and are able to produce large earthquakes. However, it is unknown whether the Jiali fault (JLF) has been seismically active in the Holocene, which currently hampers efforts to undertake meaningful seismic hazard assessments near the southeastern part of the Tibetan plateau. In this study, it is shown that the JLF has indeed ruptured during the Holocene, as evidenced from geological, paleoseismic, and radiocarbon dating investigations. Specifically, at least two surface-rupturing paleoseismic events were revealed with ages between 2160±30 yr B.P. and 2680±30 yr B.P., and prior to 2730±30 yr B.P., respectively. Combined with previous studies on the JLF, we suggest that the fault (1) can be considered an active block boundary fault and (2) accommodates crustal deformation related to the uplift of the Tibetan plateau since the late Cenozoic.