Distinguishing seismic from nonseismic soft-sediment structures; criteria from seismic-hazard analysis

Abstract

Most studies of the geologic records of prehistoric earthquakes are driven by the need of seismic-hazard analyses for estimates of the locations, ages, and magnitudes of individual large earthquakes. In North America, most such studies analyze surface ruptures and earthquake-induced sand blows, dikes, and sills. In contrast, small, softsediment structures in one or a few beds, possibly induced by seismic shaking, are little used in North American hazard analyses. The reason is that present methods for studying such structures often cannot demonstrate an earthquake origin, rule out alternatives, or estimate location, age, and magnitude as well as can methods developed for the study of surface ruptures and sand blows, dikes, and sills. This chapter proposes six tests to distinguish seismic from nonseismic origins of small, soft-sediment structures in one or a few beds. The tests utilize evidence for sudden formation, synchroneity and zoned map distribution over many exposures, size of the structures, and tectonic and depositional settings. The main barriers to more rigorous utilization of these soft-sediment structures in hazard analysis are (1) few observations of their formation during historical earthquakes, and (2) few or no measurements of the threshold earthquake magnitudes and threshold horizontal accelerations that are required to form the structures.

Sedimentary facies and soft-sediment deformation structures in the late miocene-pliocene Middle Siwalik subgroup, eastern Himalaya, Darjiling District, India

“…Soft sediment deformation structures of similar dimensions have been established as seismites from western part of the sub-Himalayas (Kumar et al 2005;Singh et al 2007). These structures pass the tests for sudden formation, synchroneity, size, tectonic and depositional setting as proposed by Wheeler (2002). Any zoned map distribution for such structures in the Siwaliks is unavailable and thus the related test is not performed.…”

“…These include the presence of soft-sediment deformation structures in discrete sediment horizons (Mazumder et al 2006 and references therein), lateral continuity of such beds (Obermeier, 1996), their vertical repetition within a succession (Mazumder et al 2006), 2 presence of bounding undeformed beds (Sims 1973(Sims , 1975, presence of various softsediment deformation structures together (Kleverlaan, 1987;Obermeier, 1996;Pope et al 1997;Rosetti and Góes, 2000), association with sedimentary breccias, conglomerate and massive sandstone (Mazumder et al 2006), and deviation of palaeocurrent indicators from the normal palaeoflow pattern (Mazumder et al 2006). According to Wheeler (2002), there will always be some degree of uncertainty about the seismic origin of soft sediment deformation structures because other causes for formation of similar structures cannot be easily ruled out. He proposed that the seismic origin of a soft sediment deformation structure or a group of structures can be tested with evidences of sudden generation of such structures, synchroneity, wide spatial distribution and favourable tectonic and depositional setting.…”

Facies, stratigraphy, and evolution of a middle Ediacaran carbonate ramp: Khufai Formation, Sultanate of Oman

“…These beds are associated with large-scale slumping, contorted beds, and fluid escape structures and resemble seismites produced in carbonates or mixed sedimentary systems (Pope et al, 1997;Onasch and Kahle, 2002). Although similar features can be produced by loading, storm or tidal reworking, or gravity-driven slumping, the lateral continuity, size, and stratigraphic context of these features suggests faulting (Greb and Denver, 2002;Wheeler, 2002). The presence of early cemented shallow-water carbonates in the intraclast breccias at the Buah dome may indicate a fault-induced depth gradient.…”

Sand volcanoes in the Avon–Heathcote Estuary produced by the 2010–2011 Christchurch Earthquakes: implications for geological preservation and expression

“…Such deformed rocks where the deformation is interpreted to be the result of seismic events are referred to as seismites (Seilacher 1969) and are reviewed in Montenat et al (2007). Distinguishing seismically induced deformation from other non-seismic causes is crucial and criteria for the recognition of true seismites include evidence of sudden formation, synchroneity, distribution and depositional and tectonic setting (Wheeler 2002).…”