LICHEN DATING OF EARTHQUAKE-GENERATED REGIONAL ROCKFALL EVENTS, SOUTHERN ALPS, NEW-ZEALAND

Synchronous regional rockfall events triggered by large earthquakes in the Southern Alps of New Zealand were used to evaluate and improve th e lichenometry method for surface-exposure dating, Digital calipers we re used to measure the maximum diameter of the largest lichen on many rockfall blocks, using a fixed-area largest-lichen (FALL) sampling str ategy, Regional significance of FALL peaks can he tested bg confirming the occurrence of a coeval peak at multiple sites, and by showing an increase in peak size toward the earthquake epicenter, Significance of FALL peaks at a local site can be described in terms of peak size rel ative to a uniform density of FALL sizes, Measurements of 34000 FALL s izes on fully exposed rockfall blocks and outcrop joint faces at 90 si tes allow precise dating of geomorphic events of the past 300 to 500 S r, Uncertainties at the 95% confidence interval can be reduced to a le vel better than +/-10 Sr for ages within the calibrated time range rep resented by the lichen growth equation, Recognition of prehistorical r egional rockfall events in 1833, 1836, and 1840 demonstrates the excel lent resolution of this dating method, Precise dates result from excep tionally low measurement errors of lichen sizes relative to their grow th rate. tightly clustered FALL sizes for earthquake-induced rockfall events, and substrate exposure times for calibration sites that are kn own to the year or day, FALL peaks for synchronous rockfall events are the same for 20 sites with diverse climate, altitude, and substrate l ithology. A regionally consistent lichen growth rate allows use of a s ingle growth-rate equation for most species of Rhizocarpon subgenus Rh izocarpon on the South Island of New Zealand A nonlinear growth equati on suggests that the first colonization, on average, occurs in the 5th rr after formation of new, rock surfaces (similar to 0.5 m(2) unit ar eas) and is followed by rapid, exponentially declining growth for abou t 20 Sr(great-growth phase) that is largely completed by the 24th yr. Then, linear growth persists at about 15 mm per century (uniform-growt h phase).