USING TOPOGRAPHIC EXPRESSION TO IDENTIFY LANDSLIDES ALONG CROWLEY'S RIDGE IN SOUTHEASTERN MISSOURI AND NORTHEASTERN ARKANSAS

Crowley’s Ridge is a semi-continuous elevated upland west of the Mississippi River that extends for over 380 km between Cape Girardeau, MO and Helena, AR. The ridge lies between the western lowlands and the Holocene channel of the Mississippi River. Crowley’s Ridge is approximately 32 km wide and up to 90+ m high. The ridge appears to be locally constrained by late-Quaternary faulting along its northeastern escarpment, which may be responsible for locally tilting strata and is likely controlling prominent step-overs of the escarpment. The western side of the ridge appears to be constrained by erosion associated with the proto-Mississippi channel. Crowley’s Ridge was subjected to over 2,000 felt earthquakes during the 1811-12 New Madrid earthquake sequence, including four events with M_s > 8.0. At that time Crowley’s Ridge was not settled, but numerous earthquake-induced landslides were reported along the Chickasaw Bluffs east of the Mississippi River. 220 slides were subsequently mapped in the Chickasaw Bluffs by Jibson and Keefer (1988), between Walls, MS and Barlow, KY.

Landslides have been occasionally noticed by workers in the Crowley’s Ridge area, but all appear to be pre-historic. In an effort to identify possible seismically-induced earth movement along Crowley’s Ridge, the authors have begun a pilot study aimed at identifying old landslides and lateral spread features using topographic algorithms. These algorithms use drainage and topographic keys to recognize anomalous site characteristics typical of various landslide forms, including lateral spreads, slump-earthflows, slump flows, translational block slides, and shallow retrogressive slump complexes. To date, the authors have succeeded in identifying a large number of lateral spreads, earth flows, and translational block slides using the topographic algorithms on 52 USGS 7.5-minute quadrangles. The authors feel that this reconnaissance method may prove valuable as a screening tool in evaluating earth movement hazards across large areas that have not been previously studied.