Waveform modeling of historical seismograms of the 1930 Irpinia earthquake provides insight on ‘‘blind’’ faulting in Southern Apennines (Italy)

The Southern Apennines chain is related to the west-dipping subduction of the
Apulian lithosphere. The strongest seismic events mostly occurred in correspondence of
the chain axis along normal NW–SE striking faults parallel to the chain axis. These
structures are related to mantle wedge upwelling beneath the chain. In the foreland,
faulting develops along E–W strike-slip to oblique-slip faults related to the roll-back
of the foreland. Similarly to other historical events in Southern Apennines, the
I0 = XI (MCS intensity scale) 23 July 1930 earthquake occurred between the chain
axis and the thrust front without surface faulting. This event produced more than
1400 casualties and extensive damage elongated approximately E-W. The analysis of the
historical waveforms provides the chance to study the fault geometry of this ‘‘anomalous’’
event and allow us to clarify its geodynamic significance. Our results indicate that
the MS = 6.6 1930 event nucleated at 14.6 ± 3.06 km depth and ruptured a north dipping,
N100 E striking plane with an oblique motion. The fault propagated along the fault strike
32 km to the east at about 2 km/s. The eastern fault tip is located in proximity of the
Vulture volcano. The 1930 hypocenter, similarly to the 1990 (MW = 5.8) Southern
Apennines event, is within the Mesozoic carbonates of the Apulian foredeep and the
rupture developed along a ‘‘blind’’ fault. The 1930 fault kinematics significantly differs
from that typical of large Southern Apennines earthquakes, which occur in a distinct
seismotectonic domain on late Pleistocene to Holocene outcropping faults.
These results stress the role played by pre-existing, ‘‘blind’’ faults in the Apennines
subduction setting