The 1997 Umbria-Marche (central Italy) Earthquake Sequence: Insights on the mainshock ruptures from near source strong motion records

Abstract

A small size foreshock and the two mainshocks of the Umbria Marche earthquake sequence which occurred on September 26, 1997 have been recorded by two digital 3C accelerometers located at near source distances. The close epicentral distance and azimuthal location relative to the fault orientation and geometry make these records relevant to look at the detail of the rupture kinematics. S‐wave polarizations, apparent source time duration and waveforms from strong motion records are used to constrain the location of the fracture origin point, the fault geometry, the final slip distribution, size and mechanism of the events. The final model shows that the seismic ruptures occurred along two adjacent, sub‐parallel, low angle dipping normal faults. The relative timing, location and geometry of the mainshock faults suggest the presence of a transfer zone (barrier) which has probably controlled the amplitude increase of local stress released by the first rupture at its NW edge which triggered about 9 hours later the second rupture.

Analysis of seismological and geological observations formoderate-size earthquakes: the Colfiorito Fault System(Central Apennines, Italy)

“…6. Depths of major events (numbers 2, 3 and 18) are compatible with those estimated by strong-motion accelerometer data (Zollo et al 1999) P90 refers to the model derived by Ponziani et al (1990). V P /V S =1.9 is assumed to be constant.…”

“…The good geometry of the network (gaps lower thañ 60°for M>5.5) and the condition that the final solution was close to the one inferred by strong-motion analysis (Zollo et al 1999) allowed errors of ±1 km. In order to better constrain depths of M>5.5 earthquakes, we evaluated the influence of different initial depths on final ones.…”

“…Source kinematic properties (Zollo et al 1998(Zollo et al , 1999 for the three M>5.5 events are listed in Table 2 and represented in Fig. 6.…”

“…On 1997 September 26, two earthquakes occurred near the source properties and kinematics are analysed by Zollo et al (1998Zollo et al ( , 1999, Pino et al (1999), Ekströ m et al (1998, and village of Colfiorito, the first at 00:33 GMT (M W =5.7) and the second at 09:40 GMT (M W =6.0), causing severe damage, Olivieri & Ekströ m (1999). The analysis of geological features is treated in Basili et al (1999), Cinti et al (1999, Cello et al a few casualties and many injuries.…”

Plate Deformation

“…These findings represent evidence for horizontal postseismic deformation triggered by shallow earthquakes, in the Mediterranean region, as first shown in Aoudia et al (2003). For normal faulting, these applications show the relevance of source depth with respect to thickness and the Figure 44 2002-1999 baseline length variations along the GPS section with respect to the A 0 site using GIPSY and Bernese solutions, gray and black vertical bars, respectively, versus viscoelastic model predictions for FM1 (Salvi et al, 2000), FM2 (Zollo et al, 1999), and FM3 (Basili and Meghraoui, 2001), using a viscosity of 10 18 and 10 17 Pa s for the TZ and lower crust, respectively. Fine rheological layering in the crust has a major role in the interplay of relaxation in the TZ and LC with respect to relaxation in the mantle when shallow sources are considered in normal-mode relaxation theory.…”