SCALING OF EN ECHELON STEPS ALONG BASIN AND RANGE NORMAL FAULTS

Discontinuities such as en echelon steps in fault systems have long been considered potential barriers to earthquake rupture propagation. Recent studies suggest that the size of these discontinuities may be key in determining whether a step-over may stop, inhibit or allow rupture propagation. This study shows that linked en echelon segments along Basin and Range normal faults tend to show a consistent geometry, which may prove in useful hazard analysis.

The distance over which segments overlap along-strike and the segment spacing measured spacing perpendicular to strike have been compiled for range-front normal faults. The dataset includes faults along the eastern Sierra Nevada Mountains, faults within the central Nevada seismic zone, faults of the circum- Snake River plain region, and faults of eastern Oregon.

For fault segments that are linked – as evidenced by continuity of surface rupture, continuity of total displacement pattern, and relay faults - the amount of en echelon overlap is generally about twice the segment spacing. There is no characteristic size for a step-over; however, the majority of observations for linked segments show overlaps of up to about 12 km and hence spacings of up to about 6 km are typical. If the spacing of segments is greater than the seismogenic thickness, the overlap distance tends to be less than the spacing. This latter pattern is interpreted to reflect a limitation to fault interaction, which is consistent with poorly developed linking fault structures observed in many of the largest step-overs.