| Popis: |
Trenching of sand blows in two liquefaction fields on late- to mid-Holocene alluvium within the southern Mississippi Embayment was conducted to determine the origin of the sand blows and their possible relationship to New Madrid seismic zone events. Trench stratigraphy and crosscutting relationships show evidence of multiple events of seismically induced sand venting. These two fields of moderate to intense liquefaction are 175 km and 240 km southwest of the southern limit of similarly intense sand blows within the New Madrid seismic zone (NMSZ) liquefaction field. The southernmost field (Ashley County, Arkansas) is ∼25 km in diameter, and sand blows are densely spaced. Sand blows are more sparsely spaced in the larger (∼30 km by 45 km) northern field (Desha County, Arkansas). Various correlations of liquefaction events between these two fields and between each of these fields and the NMSZ can be accommodated by our radiocarbon and infrared stimulated luminescence (IRSL) ages of sediments predating and postdating vented sand deposits in our trenches. These ages show a major liquefaction event at both the Ashley and Desha County fields between 4600 and 5500 years ago and a major event at the Ashley County field approximately 700 years ago. The early venting episode is older than the documented NMSZ chronology, and so a NMSZ seismic source cannot be assessed. The age of the later venting episode falls between two postulated NMSZ events, so it may reflect seismicity from a different source zone. Our trench logs show at least three other moderate to minor sand-venting episodes at the Ashley County field and two at the Desha County field, and one may be the same event at both fields about 2200 years ago. If these sand blows formed during large NMSZ events (≥175 km northeast), the area of strong ground motion for this seismic source has been underestimated. Alternatively, if these liquefaction fields formed due to local seismicity, then additional significant seismic source zones are present in the Mississippi Embayment outside the NMSZ. Each of the two fields sits astride seismicity alignments associated with the margins of a previously identified active fault block between the Arkansas River fault zone that projects through the Desha County field and the Saline River fault zone that projects through the Ashley County field. |
