Evidence for surface faulting events along the Paganica fault prior to the 6 April 2009 L'Aquila earthquake (central Italy)

Abstract

We performed paleoseismological investigations at four sites across the normal Paganica fault (PF) (source of the 2009 Mw 6.3 L'Aquila earthquake), with the goal of reconstructing the rupture history and of contributing to the evaluation of the maximum event expected along the PF. We recognized five distinct surface faulting earthquakes (including the 2009) in the trenches. The age of the penultimate event is consistent with the 1461 earthquake; the third event back occurred around 1000 AD. The two oldest events have larger uncertainties and occurred in the interval 760 BC–670 AD and 2900–760 BC, respectively. The along‐strike vertical displacement for each paleoevent has a limited variability consistently with the fairly homogeneous slip observed in 2009 along the northern part of the rupture. Conversely, the throws change between distinct events and range between 0.15 m in 2009 (maximum estimate) and close to 0.4 (lower bound estimate) in earlier events. These paleorecords and the high fault escarpments imply that earthquakes larger than 2009 occurred on the PF, with implications for the level of hazard. Recurrence intervals also reflect a change with time, the average interval before ∼1000 AD is longer compared to that after this date. Two events occurred in the 2000–4000 years preceding ∼1000 AD, while three events occurred since ∼1000 AD. The age uncertainties affecting the interpreted events prevent the evaluation of a unique value for interevent interval; the older events appear closely spaced in time or far apart depending on the upper or lower boundary of the age interval. We tentatively assign an average interevent time of ∼500 years for the three youngest events, whereas the time elapsed between the previous ones could be larger, in the order of 1000–2000 years. We calculate a late Pleistocene dip‐slip rate for the PF of 0.2–0.4 mm/yr, consistent with 0.25–0.5 mm/yr for the early Pleistocene. Using age and throw of individual events, we calculate a similar late Holocene average dip‐slip rate of ∼0.3–0.4 mm/yr. This suggests that the portion of the PF where the 2009 continuous surface faulting occurred has fairly a constant average slip release since late Pleistocene. Finally, we discuss different rupture scenarios and alternative models of occurrence compatible with our data and their variability.

Control of pore fluid pressure diffusion on fault failure mode: Insights from the 2009 L'Aquila seismic sequence

“…As a consequence, the rupture that occurred on April 6, 2009 may not represent the full seismogenic potential of the activated structure. In fact, evidence of larger surface offsets, up to 0.8 m, were observed in a trench for a paleoseismic study on the same fault by Cinti et al [2011], as opposed to the maximum surface offsets of 0.1 m observed along the surface expression of the Paganica fault rupture of April 6, 2009.…”

“…[59] The two most recent events on the Paganica fault (M w ≈ 6, surface offsets of 0.1-0.3 m) experienced a return period of 500 years [Cinti et al, 2011]. The three older events recognized on the same fault were probably larger (M w ≈ 6.2-6.8, surface offsets 0.4-0.8 m), and spaced in time by 1000-2000 years.…”

When probabilistic seismic hazard climbs volcanoes: the Mt. Etna case, Italy – Part 1: Model components for sources parameterization

“…If a fault has a list of events associated with, the mean values (magnitude, recurrence time) and their variability derive directly from historical or paleoseismological observations. However, there are very few cases of effective repetition of major earthquakes on the same fault segment in Italy, mostly along the Apennines in Central Italy (Galli et al, 2010;Cinti et al, 2011;Moro et al, 2013;Peruzza et al, 2011). At Mt.…”

The anatomy of the 2009 L'Aquila normal fault system (central Italy) imaged by high resolution foreshock and aftershock locations

“…Tertulliani et al [2009], basing on both analogies in the distribution of the intensities and in the areas of the strongest effects produced by the shock, propose that the 1461 event may have occurred on the same fault responsible for the 2009 main shock, the Paganica-San Demetrio segment, identified by Bagnaia et al [1992]. More recently, Cinti et al [2011], performed paleoseismological investigations at four sites across the Paganica fault recognizing 5 surface faulting earthquakes (including the L'Aquila event of 2009) in the four trenches. Based on these observations, the Authors relate the retrieved age of the penultimate event to the 1461 event.…”