Chapter 3 Paleoseismology in extensional tectonic environments

Abstract

Publisher Summary This chapter describes selected historic earthquake ruptures on normal faults that serve as modern analogs for paleoseismic features. The paleoseismic studies were performed mainly in the semi-arid environment of the western United States. Geomorphic evidence for paleoearthquakes is discussed in the chapter proceeding from large-scale to microscale features. The chapter also describes stratigraphic evidence for paleoearthquakes. It also discusses the way to date paleoearthquakes using geomorphic and stratigraphic evidence. Large earthquakes accompanied by historic normal surface faulting form the modern analogs for the paleoearthquake studies of normal faults. In the United States, the understanding of normal surface faulting is heavily influenced by several M > 7 earthquakes that occurred in the semi-arid Basin and Range Province from 1915 to the present. These earthquakes produced normal fault scarps up to 70 km long and 6 m high in alluvium at the base of mountain fronts, and therefore the chapter focuses on fault scarp morphology, degradation, and trenching techniques. Cross-sections through historic normal surface rupture document extensive shattering of surface materials. Earthquake surface ruptures typically vary in sense and amount of displacement along strike and also vary at a given point on the fault between successive paleoearthquakes. In a general sense, vertical displacement on historic normal fault surface ruptures has been greatest at the center and least near the ends of rupture.