Seismological and field observations of the 1984 Lazio-Abruzzo earthquakes: implications for the active tectonics of Italy

Abstract

Seismological and field studies of the 1984 Lazio-Abruzzo earthquakes provide new constraints on the pattern of Quaternary extension in the Apennine mountains of central Italy. Waveform modelling of the mainshock ( M , = 5.8; M, = 0.59 X 10l8 N m) at 17:49 on May 7 indicates centroid strike 123", dip 48", rake -96" and depth 10 km. The largest aftershock (M, = 5.2; M, = 0.21 X 10l8 N m), at 10:41 on May 11, had strike 170", dip 46", rake -67" and centroid depth also 10 km. The mainshock fault plane was part of the Sangro fault, one of three major SW-dipping normal faults that take up much of the extension of this part of central Italy in Lazio and Abruzzo. A much larger earthquake occurred on 1915 January 13 on an in-line continuation of this fault -30 km further NW.Scarps -10 m high suggest -1 mm yr-' Holocene slip rate on the Sangro fault, and imply -2 mm yr-' average Holocene extension rate across central Italy in good agreement with rates calculated from the seismicity of the last few decades. Topography and present-day dips of active normal faults in Abruzzo suggest that extension has been occurring locally for no more than -2Myr. Slip vector azimuths for the Lazio-Abruzzo earthquakes were -45", approximately perpendicular to the overall strike of the actively-extending zone and major active normal faults inside it. They thus imply that extension in this zone is accompanied by negligible local rotation of fault-bounded blocks around vertical axes. On a larger scale, the instantaneous present-day extension occurring across Italy between northern Campania and Tuscany can be regarded as anticlockwise rotation of the Adriatic Sea region to the NE relative to the Tyrrhenian Sea region to the SW around a pole. We suggest this pole is at present near 44"30'N, 9'30'E. Our 2 mm yr-' extension rate in Abruzzo implies -0.3" Myr-' angular velocity around this pole, giving -4 mm yr-' extension rate in Campania.Moving further southeast, some major normal faults in Campania and Calabria show substantial left-lateral strike-slip, suggesting blocks there may be rotating around vertical axes. The southernmost block of Calabria has rotated 36" clockwise since -4Myr ago, approximately the time when extension began locally. The leading edge of the extending zone appears to have propagated northwestward during the course of extension at -200 mm yr-' from Calabria to its present-day position.

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).…”

“…The synthetic and antithetic structures characterized by higher dip angles compared to the ATF have generated small-moderate magnitude earthquakes; the largest one was M W 5.1 that occurred in 1984, currently called the Gubbio earthquake ( Fig. 1a; Westaway et al, 1989;Collettini e al., 2003). Several authors suggest that this seismic event, originally called the Perugia earthquake (Haessler et al, 1988), nucleated on the Gubbio fault plane (GuF; Haessler et al, 1988; while Collettini et al (2003), by interpreting a set of multidisciplinary data, argued against this hypothesis.…”

“…Grey points represent the epicentral location (from the catalogue of the INGV national network available at: http://iside.rm.ingv.it/iside/standard/index.jsp) of the earthquakes (M< 3.5) that occurred between 1995 and 2010. The beach ball is the mechanism solution of the M w 5.1 1984 Gubbio earthquake (Westaway et al, 1989;Collettini et al, 2003). The red lines represent the projection at surface of the Alto Tiberina fault system (after Mirabella et al, 2011).…”

The role of pre-existing thrust faults and topography on the styles of extension in the Gran Sasso range (central Italy)

“…Seismological observations on the largest earthquakes in central Italy (Anderson and Jackson, 1987;Westaway et al, 1989) have shown that active deformation is accommodated by steep planar normal faults in the upper 10-15 km of the crust as usually observed in extensional settings (Jackson and White, 1989). Below this depth, it is generally considered that the high-angle normal faults (rupturing during seismic events) assume shallower dips rooting into the ductile lower crust (Eydogan and Jackson, 1985;Rigo et al, 1996).…”

Hydrological and Geochemical Changes Related to Earthquakes - Examples: Three Great Earthquakes of the XX Century in the Southern Apennines (Italy)

“…The main shock fault plane was part of the Sangro fault, one of three major SW-dipping normal faults that take up much of the extension of this part of central Italy in Lazio and Abruzzo (figure 1a) (Westaway et al, 1989). In order to evaluate the hydrological effects produced by the seismic sequence, we analyzed the data registered in 15 hydrometrical stations and the variations of levels of two lakes (Barrea and Scanno), all localised in the epicentral area and monitored by the Italian Hydrographic Survey.…”