Ⓔ Holocene Paleoearthquakes and Early–Late Pleistocene Slip Rate on the Sulmona Fault (Central Apeninnes, Italy) by Paolo Galli, * Biagio Giaccio, Edoardo Peronace, and Paolo Messina Abstract The 23 km long Sulmona normal fault flanks the southwestern slope of the Mount Morrone range in the central Apennines. To date, the recent activity of this struc- ture is uncertain, as the faulting evidence concerns only undated deposits and the fault cannot be associated with any of the strong historical earthquakes (M w > 6:5) of the region. Our observation from ∼1 Ma offset slope breccias, coupled with new tephrochro- nological data on faulted early Last Glacial lacustrine infilling of the Sulmona basin, implies a vertical slip rate of ∼0:5 mm=yr over both the long term and middle term. Moreover, at the apex of a Late Pleistocene alluvial fan, radiocarbon dating of offset stratigraphy uncovered in four paleoseismological trenches shows that repeated earth- quakes resulted in more than 4 m of vertical offset since ∼9 ka, providing again a mini- mum Holocene vertical slip rate of ∼0:5 mm=yr. In combination with the results of the trench exposure, we define four faulting events in the past ∼9 ky B:P:, and at least one before then. The most recent is constrained by robust radiocarbon dates as the middle of the second century A.D. The penultimate event was around the middle of the fifth mil- lennium B.P., whereas the other two events are dated in a time between 8.5 ky to 6.5 ky B.P. and around 9 ky cal B.P. The oldest occurred before 9.5 ky B.P. As well as revealing the unexpected paleoseismic history of the Sulmona silent fault, our data provide a re- currence time for M w ≥ 6:5 earthquakes of ∼2:4 ky versus an elapsed time of ∼1:85 ky since the last event. The latter matches with an earthquake previously hypothesized through archaeoseismic clues collected from several Roman settlements of the Sulmona Plain, all of which were dated to halfway through the second century A.D. Online Material: Table of major-element compositions, field photos, and Harker diagrams. Introduction At the end of the last century, Galadini and Galli (1999) claimed the existence of at least two parallel sets of seismo- genic normal-fault systems running along the backbone of the central Apennines. These systems accommodate the dominant northeast–southwest extensional processes that have affected the Apennine chain since the Middle Pleistocene, with a velocity currently rated at ∼3 mm=yr using Global Position- ing System data (D’Agostino, 2014). The western fault set was responsible for all of the M w 6.3–7.0 historical earth- quakes (and last, but not least, the 2009 M w 6.3 L ’Aquila event), whereas the eastern fault set has been defined as silent (no historical earthquakes), and with an elapsed time since the last fault rupture of longer than 1500 years (Galadini and Galli, 2000). As historical and paleoseismological studies demonstrated that this time span often exceeds the average recurrence interval for M w ≥ 6:5 Apennine earthquakes, it appears likely that future strong earthquakes might also oc- cur along faults of this eastern set (i.e., the Vettore, Laga, Campo Imperatore, and Sulmona faults [SFs] in Galli et al., 2008, and references therein; see v, l, c, and s, respectively, in Fig. 1b). The northwest–southeast-striking SF is one of these silent structures. It affects the western slope of the Mount Morrone ramp anticline (2061 meters above sea level [m.a.s.l.]), with two parallel splays that run at different ele- vations (Fig. 1a), and that join at a few kilometers in depth beneath the Quaternary Sulmona basin. Previously, Vittori et al. (1995) reported on the metric displacement of Pleisto- cene alluvial fan sediments along secondary, north–south- trending faults, whereas Galadini and Galli (2000) showed both the offset of Late Pleistocene slope deposits along the eastern splay and of Early Pleistocene breccias along the *Also at Consiglio Nazionale delle Ricerche-Istituto di Geologia Ambi- entale e Geoingegneria, Area di Ricerca Roma1, Via Salaria km 29.3, Mon- telibretti, Italy; e-mail: paolo.galli@protezionecivile.it. 1 Bulletin of the Seismological Society of America, Vol. 105, No. 1, pp. 1–13, February 2015, doi: 10.1785/0120140029