Abstract
Lichenometry is a surface-exposure-dating procedure that complements traditional trench-and-date stratigraphic studies of earthquakes. Lichens on the surficial blocks of a slump in the Seaward Kaikoura Range, South Island, New Zealand provide precise, accurate (± 2 years) dating of 20 post-landslide rockfall events. The coseismic character of these rockfall events is apparent when ages of lichen-size peaks are compared with dates of historical earthquakes. Most local prehistoric lichen-size peaks are synchronous with peaks at other lichenometry sites in a 20 000 km2 region. Lichenometry may be the best paleoseismic tool for describing the extent and intensity of seismic shaking caused by prehistoric earthquakes, and for dating earthquakes generated by concealed thrust faults and subduction fault zones.
Highlights
Stratigraphic studies made in trenches excavated across fault scarps continue to be important for paleoseismology.Advantages of the trench-and-date approach include: 1) Identification of the fault responsible for a specific surface-rupture event.2) Confirmation of seismic slip as revealed by liquefaction disturbance and offset strata.3) Constraint of times of prehistorical earthquakes, usually by radiocarbon dating.4) Estimates of offset of strata and buried stream channels that can be used to estimate slip rates.Trenching along the plate-bounding Alpine Fault of New Zealand has provided age estimatesTrench-and-date paleoseismology has several drawbacks
Many rockfall events in the Sierra Nevada dated to times of magnitude Mw 7.5 to 8 San Andreas Fault earthquakes, some with epicenters more than 400 km away
Lichenometric studies of earthquakes begin with consecutive phases of work
Summary
Stratigraphic studies made in trenches excavated across fault scarps continue to be important for paleoseismology. Advantages of the trench-and-date approach include: 1) Identification of the fault responsible for a specific surface-rupture event. Time consuming, and provide little information about the extent and intensity of seismic shaking. Lichen dating of regional coseismic landslide events (Bull, 1996a; Bull and Brandon, 1998) complements stratigraphic dating of earthquakes. Advantages of this new approach to paleoseismology include: 1) Estimation of the time of the earthquake itself. Deficiencies of earthquake-lichenometry studies include inability to measure amounts of coseismic slip, possible lack of suitable slow growing crustose lichens in a study area, and possible lack of lichens older than 300 years at some sites. Unstable and rapidly changing hillslopes of tectonically active mountains favor detection of regional rockfall events generated by distant earthquakes
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