BULL AND BRANDON LICHEN DATING OF EARTHQUAKE-GENERATED REGIONAL ROCK- FALL EVENTS, SOUTHERN ALPS, NEW ZEALAND William B. Bull, Geosciences Department, University of Arizona, Tucson, AZ, 58721 Mark T. Brandon, Department of Geology and Geophysics, Yale University, New Haven, CT, 06520-8109 ABSTRACT with diverse climate, altitude, and (Worsely, 1981), which restricts comparison substrate lithology. A regionally con- of results by different workers. In part, the Synchronous regional rockfall sistent lichen growth rate allows use lack of a standard approach has resulted events triggered by large earth- of a single growth-rate equation for from diverse study areas, each of which had quakes in the Southern Alps of New most species of Rhizocarpon subgenus a different purpose, sampling strategy, and Zealand were used to evaluate and Rhizocarpon on the South Island of assumptions. Locke et al. (1979) attribute improve the lichenometry method New Zealand. A nonlinear growth the lack of a universal method to the founder for surface-exposure dating. Digital equation suggests that the first colo- of lichenometry, Beschel (1950, 1957, 1959, calipers were used to measure the nization, on average, occurs in the 5th 1961, 1963), who emphasized only general maximum diameter of the largest li- yr after formation of new rock sur- guidelines instead of a preferred method. chen on many rockfall blocks, using faces (~0.5 m2 unit areas) and is fol- Most workers continue to estimate ages for geomorphic events, such as floods, glacier a fixed-area largest-lichen (FALL) lowed by rapid, exponentially declin- advances, and landslides, based on the single sampling strategy. Regional signifi- ing growth for about 20 yr (great- largest lichen or mean of five largest lichens cance of FALL peaks can be tested growth phase) that is largely com- for the entire deposit. Examples include by confirming the occurrence of a pleted by the 24th yr. Then, linear worthy papers by Matthews (1974), Locke coeval peak at multiple sites, and by growth persists at about 15 mm per et al. (1979), Birkeland (1981), Porter showing an increase in peak size to- century (uniform-growth phase). (1981), Rapp (1981), and Innes (1984, ward the earthquake epicenter. Sig- 1985a). nificance of FALL peaks at a local INTRODUCTION Some recent studies have departed from the site can be described in terms of peak conventional approach by sampling popula- size relative to a uniform density of Lichens can be used to estimate the ages tions of largest lichens on many blocks de- FALL sizes. of surfaces of young rocky deposits or the posited by rockfalls, snow avalanches, glaciers, Measurements of 34000 FALL times of recent exposure of outcrops. Lichen and debris flows (Bull, 1991a, 1994; Matthews sizes on fully exposed rockfall blocks sizes record both the initial time of expo- and McCarroll, 1994; McCarroll, 1993, 1994; and outcrop joint faces at 90 sites al- sure of the rock substrate upon which they Luckman and Fiske, 1995). Bull concentrated low precise dating of geomorphic grow, and subsequent disturbances to on dating of earthquake-generated (coseismic) events of the past 300 to 500 yr. Un- surficial boulders or joint blocks that expose rockfall events in California and New Zealand certainties at the 95% confidence in- fresh substrates for colonization by younger (Bull et al., 1994; Bull, 1996a, 1996b). His terval can be reduced to a level bet- lichens. The ability to record postformation new field and analytical procedures for disturbances also applies to other surface- ter than ±10 yr for ages within the lichenometry were developed in a variety of exposure dating methods, such as weather- geomorphic settings, and have been used to calibrated time range represented by ing rinds and cosmogenic isotopes. the lichen growth equation. Recog- study slush avalanches in northern Sweden Lichenometry directly dates times of spe- (Bull et al., 1995), which has virtually no earth- nition of prehistorical regional rock- cific events, such as earthquakes, by deter- fall events in 1833, 1836, and 1840 quakes. This paper expands on the lessons mining the time of earthquake-generated learned from Bull’s diverse studies to describe demonstrates the excellent resolution landslides. In contrast, radiocarbon strati- a new approach to lichenometry. of this dating method. Precise dates graphic dating estimates the time that or- We seek to advance lichenometry by em- result from exceptionally low mea- ganic matter grew. Deposition of organic phasizing the benefits of large sample sizes. surement errors of lichen sizes rela- matter in fluvial, colluvial, or swamp depos- Digital calipers were used to measure the tive to their growth rate, tightly clus- its at paleoseismic sites is not directly tied maximum diameter of the largest lichen on tered FALL sizes for earthquake-in- to specific earthquake events, because dat- many rockfall blocks in the Southern Alps, duced rockfall events, and substrate able organic matter is created before or af- where outcrops and hillslopes are disrupted exposure times for calibration sites ter seismic disruption of the stratigraphic by large earthquakes. About 34000 lichen- that are known to the year or day. section. size measurements were made at 90 sites in FALL peaks for synchronous rock- Lichenometry dates geomorphic events, the South Island of New Zealand (Fig. 1). fall events are the same for 20 sites and like radiocarbon dating of stratigraphic Measurements were made from 1989 to events, it is in its fifth decade of develop- 1997 and, where necessary, are normalized ment. However, lichenometry continues to *e-mail: [email protected] to 1992 as a reference year. We use the syn- be hindered by a paucity of widely accepted chronous nature of rockfalls throughout [email protected] measurement and analytical procedures much of our 40000 km2 study region to pro- GSA Bulletin; January, 1998; v. 110; no. 1; p. 60-84; 23 figures; 5 tables. 60 Geological Society of America60 Bulletin, January 1998 LICHEN DATING OF NEW ZEALAND ROCKFALL EVENTS cesses loosen blocks that tumble downhill 168° E 172° E during times of seismic shaking. Each rock- N 0 100 200 Km NE fall block may have a different history of detachment from an outcrop, episodic travel WW 1 lichenometry site downhill, local microclimates, and coloni- 5 to 10 lichenometry sites zation by primitive plants. MB ° MU 42 S AR IN NM Regional Rockfall Events TN 2 CG,SK,WA Earthquake-induced landslides are wide- CH spread and common in many alpine moun- MO 1 FH tains. Earthquakes with Mw magnitudes Alpine fault CB AC NB greater than 7 can trigger rockfalls at distances CL of up to 400 km from their epicenters (Keefer, Mt. Cook 1984, 1994; Wieczorek et al., 1992; Wieczorek 44° S and Jäger, 1996). Mw refers to the seismic- moment magnitude scale as defined by Hanks Milford Sound 3 OH and Kanamori (1979). A coseismic landslide is a mass wasting event triggered by, and there- fore occurring during or shortly after an earth- Figure 1. Map of the South Island of New Zealand showing the Alpine fault quake. Lichens have been used to date system, and locations of lichenometry sites and historical earthquakes (Table rockfalls (Porter and Orombelli, 1981) and 2). Faults are shown by black lines (Officers of the New Zealand Geological coseismic landslides (Smirnova and Nikonov, Survey, 1983; Van Dissen and Yeats, 1991). Approximate locations of earth- 1990). A coseismic rockfall event is distinc- quake epicenters are AR, Acheron River; CH, Cheviot; IN, Inangahua; MA, tive. It occurs at many sites throughout a re- Marlborough; MO, Motunau; MU, Murchison; NB, New Brighton; NE, gion (Fig. 3), and rockfall abundance increases Nelson; SK, Seaward Kaikoura; TN, Tennyson, and WA, Waiau. The 1855 toward the earthquake epicenter. Coseismic West Wairarapa earthquake, WW, occurred in the southern part of the North rockfall events can be dated by measuring sizes Island. Locations of lichenometry sites are underscored: AC, Acheron rock of lichens with systematic growth rates that avalanche; CB, Craigieburn rock avalanche; CG, Cattle Gully; CL, Clyde colonize the newly exposed rock surfaces (Bull rock avalanche; NM, No Man’s Creek; and OH, Ohau. Circled 1 indicates the et al., 1994; Bull, 1996a, 1996b). general locations of the Arthur’s Pass, Lake Coleridge, and Bealey earth- Moderate earthquakes (Mw = 5.5 to 7), quakes, Falling Mountain rock avalanche, and the Otira Valley, Rough Creek, such as the Mw 6.1 Tennyson earthquake of and Zig Zag sites. Circled 2 includes the North Canterbury and Hossack 1990 in our study area, are accompanied by earthquakes, Raupo Swamp, and the Hope River Bridge landslide. Circled 3 clouds of dust caused by abundant rockfalls includes the Mueller and Tasman glaciers, and the Celmisia and Idyllic sites. near their epicenters. Newly arrived blocks in rockfall deposits are distinctive because vide new insights about factors influencing COSEISMIC ROCKFALL their freshly exposed surfaces have no li- the precision, accuracy, and resolution of LICHENOMETRY MODEL lichenometry as a surface-exposure dating chens. Fresh rockfall blocks are common only within 15 km of the 1990 earthquake method. Synchronous rockfall events at sites New Zealand is well suited for develop- epicenter. with different altitudes, substrate lithologies, ment and testing of the coseismic rockfall Coseismic events dominate the rockfall and climatic settings allowed us to determine lichenometry model. Large earthquakes ca- process in much of the Southern Alps. Many whether or not calibration of lichen growth pable of causing regionally extensive sites where the largest lichen on each block rates based on lichen-size measurements at rockfalls have occurred throughout the is at least 8 mm (Fig. 4) are indicative of a few control sites is valid on a regional Southern Alps. Most earthquakes are widely rockfall deposits that have not received an scale. Our scope also includes the impor- spaced in time relative to the moderately increment of new blocks since the Inangahua tance of site selection, identification of sig- rapid growth of the lichens used in this study, Mw 7.1 earthquake of 1968 (Downs, 1995).