The Cascade rock avalanche: implications of a very large Alpine Fault-triggered failure, New Zealand

Abstract

Catastrophic deep-seated rock slope failures (RSFs; e.g., rock avalanches) can be particularly useful proxies for fault rupture and strong ground motion, and currently represent an underappreciated hazard of earthquakes in New Zealand. This study presents observations of the previously undescribed Cascade rock avalanche (CRA), a c. 0.75 km3 single-event, long-runout, catastrophic failure interpreted to have been coseismically triggered by a large to great earthquake c. 660 AD on the Alpine Fault. Despite its size and remarkable preservation, the CRA deposit has been previously identified as a terminal moraine and fault-damaged outcrop, highlighting the common misinterpretation of similar rock avalanche deposits. Comparisons are drawn between the CRA and other Alpine Fault-attributed rock avalanches, such as the better-studied c. 860 AD Round Top rock avalanche, to re-assess coseismic rock avalanche hazard. Structural relationships indicate the rock mass comprising the CRA may have formerly been a portion of a larger (c. 3 km3) RSF, before its catastrophic collapse on a deep-seated gravitational collapse structure (sackung). Sackungen and RSFs are common throughout the Southern Alps and other mountainous regions worldwide; in many cases, they should be considered potential precursors to catastrophic failure events. Two masses of rock in the Cascade River Valley show precursory signs of potential catastrophic failures of up to c. 2 km3; a similar mass may threaten the town of Franz Josef.