Revised hypocentre and fault rupture geometry for the 1980 November 23 Campania-Basilicata earthquake in southern Italy

Abstract

SUMMARY The fault rupture nucleation point of the 1980 November 23 Campania-Basilicata earthquake is relocated, following the recent identification of ∼20 km of surface faulting, the Carpineta and Picentini fault scarps, in addition to the ∼15 km previously documented on the Marzano and San Gregorio faults, all of which have ∼NW strike and ∼60° NE dip. This relocation indicates a range of revised origin times and nucleation point positions at 10-12 km depth that are ∼5–9 km SE of previously documented coordinates, between 18:34:52.0 ± 0.3 s, with latitude 40.724°± 1.4 km and longitude 15.414°± 1.4 km (preferred), and 18:34:52.5 ± 0.3 s, with latitude 40.742°± 1.4 km and longitude 15.373°± 1.4 km. My preferred nucleation point coincides with a downdip projection of the SE end of the Carpineta fault and indicates that this fault ruptured first, rather than the Marzano fault as was previously thought. With the fault rupture nucleation point adjusted to this new preferred position, field and seismological estimates of seismic moment match well, both overall and for individual fault scarps, and suggest the following sequence of fault rupture subevents. The initial fault rupture nucleated at or near the SE end of the Carpineta fault and propagated NW, releasing ∼2.5 × 1018 N m seismic moment. Rupture continued apparently without interruption onto the adjoining Marzano fault farther NW, where ∼6.5 × 1018N m of seismic moment was released. Rupture then paused for ∼0.5s, before continuing NW along the Picentini fault, where ∼4.5 × 1018N m more seismic moment was released. Probably early in this sequence of NW-propagating ruptures, a SE-propagating rupture released ∼2 × 1018 N m seismic moment on the San Gregorio fault. Each of these fault scarps and the corresponding rupture subevent was associated with intense aftershock activity. The existence of another aftershock cluster NW of the Picentini scarp suggests a fifth mainshock rupture subevent, at Castelfranci, which released up to ∼2 × 1018 N m more seismic moment. Rupture at this locality appears necessary to explain the form of ground acceleration recorded nearby, also. Following these ruptures on steep NE-dipping normal faults, two additional ruptures occurred on faults with different orientations, ∼20 s and ∼40 s after the initial mainshock rupture. The 40 s subevent involved release of ∼3 × 1018 N m of seismic moment on a steep normal fault that dips SW at ∼70° and projects to the earth's surface ∼11 km NE of the Marzano fault. The 20 s subevent apparently involved release of ∼4 × 1018 N m of seismic moment on a surface dipping NE at ∼20°, at the base of the brittle upper crust beneath this steep antithetic fault. Points where normal fault ruptures nucleated on the steep NE-dipping normal faults coincide with en echelon steps of < ∼1 km and abrupt ∼15° changes in strike. The Marzano and Carpineta faults, which have strike ∼315°, took up a small component of left-lateral slip, as is revealed by the first-motion focal mechanism, teleseismic waveform modelling, striations measured in-the field, and consistent rightward stepping: their slip vector azimuth is ∼N37°E. Assuming the same slip vector azimuth, a component of right-lateral slip is expected on the San Gregorio and Picentini faults that have strike ∼300°.