Insights on the seismogenic layer thickness from the upper crust structure of the Umbria‐Marche Apennines (central Italy)

Abstract

[1] We reconstruct the subsurface geology in a region of the northern Apennines (central Italy) where a protracted extensional sequence occurred in 1997-1998 with maximum magnitude M = 6.0. Our study is mainly based on the interpretation of three reprocessed seismic reflection profiles crossing the epicentral area, which constrain the subsurface geometry to a depth of about 12 km where most of the shallow seismicity occurs. Comparing the subsurface setting with accurately determined earthquake locations, we find that the seismicity is located entirely within the sedimentary cover and does not penetrate the underlying basement. This is explained by considering that the sedimentary cover is rather thick and composed of relatively strong lithologies (platform carbonates and evaporites), while the upper part of the basement consists of weak phyllites and siliciclastic rocks. This weak horizon is also evidenced by the low-Vp values measured in deep wells of the region. Its effect is to decouple the sedimentary cover from the crystalline basement, where only microseismicity occurs. Our study indicates that local structure and stratigraphy can significantly influence the distribution of seismicity within the upper crust, particularly in complex geological environments such as thrust-and-fold belts. Citation: Mirabella, F., M. Barchi, A. Lupattelli, E. Stucchi, and M. G. Ciaccio (2008), Insights on the seismogenic layer thickness from the upper crust

On the mechanical behaviour of a low-angle normal fault: the Alto Tiberina fault (Northern Apennines, Italy) system case study

“…In addition, the best fit of the GPS velocity data is obtained considering some degree of creeping along a portion of the GuF. The mechanical behaviour of the west-dipping GuF is still debated in the literature (Boncio et al, 2000;Mirabella et al, 2004Mirabella et al, , 2008Barchi and Ciaccio, 2009 Westaway et al, 1989) that occurred in 1984 in the area (namely, the Gubbio earthquake) occurred a few kilometres apart (see location in Fig. 1a).…”

Fault zone architecture and deformation processes within evaporitic rocks in the upper crust

“…It has been previously demonstrated that the mainshocks of the 1997 Colfiorito Earthquakes, in the Northern Apennines of Italy, nucleate within such formation along normal faults at a depth of about 6 km [Barchi, 2002;Miller et al, 2004;Mirabella et al, 2007]. In particular, the spatial and time evolution of the seismic sequence has been modeled as directly controlled by fluid migration, initially trapped by the Triassic Evaporites [Miller et al, 2004].…”

“…It has been recently observed that the mainshocks of one of the best documented extensional seismic sequences in the world, the 1997 Colfiorito sequence in the UmbriaMarche Apennines, were located along normal faults and TECTONICS, VOL. 27, TC4017, doi:10.1029/2007TC002230, 2008 nucleated within evaporitic rocks [i.e., the Triassic Evaporites; Barchi, 2002;Miller et al, 2004;Mirabella et al, 2007].…”

“…In this area two deep boreholes within the Triassic Evaporites, San Donato and Pieve Santo Stefano, recorded CO 2 fluid overpressure of $100 and 70 MPa at depths of $5000 and 4000 m respectively, corresponding to a fluid overpressure of 80% of the lithostatic load. In the present-day extending area the Colfiorito earthquake M w = 6.0 (1997) nucleated at a depth of $6 km within the Triassic Evaporites, as shown by the integration of seismic reflection profiles with well-located earthquakes [Barchi, 2002;Miller et al, 2004;Ciaccio et al, 2005;Mirabella et al, 2007]. The evolution of the aftershock sequence can be explained by a pressure pulse from depth migrating from the evaporites through the overlain carbonates [Miller et al, 2004].…”

Belt-Parallel Shortening in the Northern Apennines and Seismotectonic Implications

“…The proposed interpretation, sketched in Figure 2, provides that such activity is mostly produced by the relative motion among the outer sector of the belt, that stressed by the Adria plate is undergoing lateral escape and uplift, and the inner less mobile zones. The escaping material only includes the sedimentary cover, decoupled from its crustal basement at seismogenic depths (of the order of 6 -10 km) by mechanically weak lithological horizons, as evidenced by seismic survey [11] [12] and borehole data [13]- [15]. In particular, a Late Triassic evaporitic layer (Burano formation [16]) forms the base of the Meso-Cenozoic sedimentary cover of most of the Romagna-Marche-Umbria and Toscana-Emilia wedges [17]- [20].…”