Lichen dating of earthquake-generated regional rockfall events, Southern Alps, New Zealand

Abstract

Synchronous regional rockfall events triggered by large earthquakes in the Southern Alps of New Zealand were used to evaluate and improve the lichenometry method for surface-exposure dating. Digital calipers were used to measure the maximum diameter of the largest lichen on many rockfall blocks, using a fixed-area largest-lichen (FALL) sampling strategy. Regional significance of FALL peaks can be tested by 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 relative to a uniform density of FALL sizes. Measurements of 34 000 FALL sizes on fully exposed rockfall blocks and outcrop joint faces at 90 sites allow precise dating of geomorphic events of the past 300 to 500 yr. Uncertainties at the 95% confidence interval can be reduced to a level better than ±10 yr for ages within the calibrated time range represented by the lichen growth equation. Recognition of prehistorical regional rockfall events in 1833, 1836, and 1840 demonstrates the excellent resolution of this dating method. Precise dates result from exceptionally low measurement errors of lichen sizes relative to their growth rate, tightly clustered FALL sizes for earthquake-induced rockfall events, and substrate exposure times for calibration sites that are known to the year or day. FALL peaks for synchronous rockfall events are the same for 20 sites with diverse climate, altitude, and substrate lithology. A regionally consistent lichen growth rate allows use of a single growth-rate equation for most species of Rhizocarpon subgenus Rhizocarpon on the South Island of New Zealand. A nonlinear growth equation suggests that the first colonization, on average, occurs in the 5th yr after formation of new rock surfaces (~0.5 m 2 unit areas) and is followed by rapid, exponentially declining growth for about 20 yr (greatgrowth phase) that is largely completed by the 24th yr. Then, linear growth persists at about 15 mm per century (uniform-growth phase).