Milford Sound, while celebrated for its breathtaking natural beauty, is also a landscape shaped by powerful geological forces that present significant hazards. The steep-sided fjord, a product of glacial erosion, is prone to large-scale landslips and the potential for tsunamis, particularly when triggered by seismic activity. Managing these risks in a highly popular international tourist destination poses complex challenges for authorities, requiring
ongoing assessment and consideration of both individual and societal safety. Understanding these natural threats is crucial for appreciating the dynamic environment of Milford Sound.
The Persistent Threat of Landslides
Since the glaciers that carved Milford Sound melted approximately 16,000 years ago, the region has experienced at least 16 major landslips, each exceeding one million cubic meters in volume. The inherent geological characteristics of the fjord, including its steep slopes and confined waters, contribute to a significant risk of future landslips. These events are particularly concerning due to the lack of realistic evacuation time for visitors and the area's immense popularity. A large landslip into the fjord, most likely triggered by an earthquake, could generate a substantial local tsunami. Such an event, if it occurred during peak tourist density in the middle of the day, could potentially result in around 750 fatalities. Risk assessments suggest that a 1-in-1000-year wave runup could reach approximately 17 meters (56 feet) and arrive on shore within two to seven minutes. Historically, the highest estimated displacement wave runup reached about 100 meters (330 feet) at the Cleddau delta, occurring roughly every 2000 years.
Seismic Activity and Tsunami Generation
The potential for landslides and tsunamis in Milford Sound is closely linked to seismic activity in the region. The Alpine Fault, a major geological feature, passes across the head of the sound. Ruptures of the far southern portion of this fault have occurred seven times in the last 2000 years, with 27 events recorded since 6000 BCE. There is a 75% probability of a major Alpine Fault earthquake within the next 50 years, and these events have a 44% probability of generating a landslide-induced tsunami in the fjord. Furthermore, the probability of an earthquake similar in magnitude to the 1717 great earthquake (estimated at 8.1 ± 0.1) occurring along the southern Alpine Fault within the next 50 years is estimated at 30%. Beyond the Alpine Fault, significant trigger earthquakes can also originate from the Fiordland subduction-zone's Puysegur Trench to the south. While not all large landslides are solely earthquake-related, with some potentially being spontaneous rainfall-induced events, seismic activity remains a primary concern for triggering these hazardous events.
Risk Assessment and Societal Implications
In 2019, the individual risk for a visit to Milford Sound was estimated at 6.8 × 10⁻⁷, which is less than 1 in a million. This figure is two orders of magnitude lower than the known risk of visiting Whakaari / White Island, an active volcano. However, the societal risk associated with Milford Sound is considered higher than the level defined in New Zealand's tourism culture following the 2019 Whakaari / White Island eruption. This is primarily because Milford Sound is a much more popular tourist destination, meaning a single catastrophic event could impact a larger number of people. The decision regarding whether to accept this recently defined societal risk and its potential consequences has been assigned to the New Zealand government. This highlights the complex balance between promoting tourism to a world-renowned natural wonder and ensuring the safety of its many visitors in an environment prone to significant natural hazards.
