Introduction

The steep, crumbly and wet mountains of New Zealand have frequent mass movements and floods, and landscape instability related to glaciers. The Southern Alps sit astride the Australian–Pacific plate boundary (Figure 2.1) with its highly active dextral-oblique thrust faults, active volcanoes, and subduction zones. This chapter summarizes a hazardous geomorphological consequence of widespread seismic shaking in the South Island of New Zealand.

Rocks tumble downhill during earthquakes and the times of arrival on a scree slope can be dated by measuring the largest lichen of Rhizocarpon subgenus Rhizocarpon on many blocks. This discussion focuses on how to use lichenometry to better understand the intensity and history of hazardous seismic shaking so as to minimize risk. A typical study site is described, the precision of dating is noted, and maps revealing areal intensities of seismic shaking are examined to better understand sizes and sequences of earthquakes.

Lichenometry site characteristics

The Mt. John site summarizes the desirable features needed for seismic-hazard evaluations. The valley side here was trimmed by a late Pleistocene glacier, creating cliffy outcrops of crumbly greywacke sandstone that have shed blocks to be stored in talus accumulation areas. Hazards such as snow avalanches and water floods are absent. I would have chosen an active talus cone if I had wanted to study the frequency and sizes of avalanches. Rhizocarpon subgenus Rhizocarpon grows well at this altitude and climate, but locally is impinged upon by other lichens and by mosses.