abstract

A complex rupture history for the 23 November 1980 magnitude 6.9 Italian earthquake resulted in an unusual pattern of two high-angle, subperpendicular normal faults forming the corner of a down-dropped crustal block. Evidence for this interpretation is from aftershocks, the pattern of strong ground motion, focal mechanism of the mainshock, and two critically placed leveling profiles. Quantitative modeling of leveling profiles using smoothly varying dislocation segments provides confirmation of the fault pattern and specific verification of the existence of a secondary fault which is suborthogonal to the primary fault. Rupture began on the NW-trending main fault with downward motion of the northeast block. About 40 sec after initial motion began, rupture initiated on an orthogonal NE-trending subsidiary fault, propagating away from the main fault. This fault pattern is consistent with tectonic extension in a northeasterly direction.

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