Characterization of barriers on an earthquake fault

TLDR

Seismograms from the 1966 Parkfield earthquake were reinterpreted using a finite‑dislocation source model in a layered medium. The study aims to discuss and classify earthquake barriers, distinguishing geometrical features such as fault bends and inhomogeneous high‑velocity anomalies. Barrier parameters were estimated using Ida’s formulas and surface‑fault‑break data, yielding breaking slip, cohesive stress, and end‑zone length values for both the Parkfield and 1857 earthquakes. The analysis indicates a barrier fracture energy of ~10⁹ erg cm⁻², predicts a maximum acceleration of ~1.5 g near the fault, and shows that barriers can both halt and trigger ruptures, acting as stress concentrators that drive twin earthquakes and fault‑boundary migration.

Abstract

Recently, Bouchon (1979 a ) reinterpreted strong motion seismograms obtained during the Parkfield earthquake of 1966 using a new method applicable to a finite propagating dislocation source in a layered medium. His results and other pertinent data, interpreted in terms of the barrier model of Das and Aki (1977), suggest that the rupture may be stopped by a barrier with the specific fracture energy of about 10 9 erg cm −2 . Using the formulas of Ida (1973 a ), we estimated parameters of the barrier as follows: breaking slip of about 20 cm, cohesive stress of about 100 bars, and length of end zone (nonelastic zone) of a few hundred meters. The barrier parameters for the great 1857 earthquake were also obtained from the description of surface fault breaks by Wallace (1968). The result led to the estimation of maximum acceleration of about 1.5g near the fault, under the assumption that the end zone length is proportional to the diameter of individual crack of the barrier model. Barriers for other earthquakes are discussed, and they are classified into geometrical barriers such as fault bend and corner and inhomogeneous barriers such as the high‐velocity anomaly straddling the San Andreas fault near San Juan Bautista. The barriers act not only as a stopper of rupture but also as an initiator of rupture, as well as a stress concentrator, causing twin earthquakes and migration or progression of major earthquakes along the plate boundary.