Intending visitors to Milford Sound need to be aware that this area is exposed to a range of natural hazards and risks which are outlined below.
Earthquake
The Alpine Fault runs approximately 800km up the spine of the South Island of New Zealand and is one of the world’s major geological features. Responsible for the prominent geological features that are evident in Milford Sound, the Alpine Fault is active with rupture events occurring on average every 300 years. The latest significant earthquake was in 1717AD. For more information, see the link here: AF8 website.
With analysis indicating that major ruptures are likely to exceed Magnitude 8, the next Alpine Fault event is likely to be very destructive and could also trigger landslides and potentially a localised tsunami. An Alpine Fault rupture of Magnitude 8 is anticipated to damage critical infrastructure in Milford Sound such as key road networks.
Tsunami
Milford Sound is susceptible to both local and distant sourced tsunami events. Scientific analysis indicates that in the event of any severe earthquake (Magnitude 8 or higher) a destructive landslide (including submarine) induced tsunami could be generated. Supporting analysis also indicates that the substantial volume of water displaced by a landslide may result in a tsunami reaching very significant wave and run up heights.
In areas such as Milford Sound where numerous visitors are received each day, a landslide induced tsunami of any significant height would pose a significant risk to people at Milford Sound. The only warning these people would receive of a potential tsunami is the onset of an earthquake. MSTL is currently seeking a risk assessment related to landslide induced tsunami. This Notice will be updated after the risk assessment has been received and considered.
Severe Weather Events
Flooding
Milford Sound’s mountainous terrain results in significant annual rainfall with the area receiving in excess of 7 metres of rain each year on average. Low soil absorption typically sees rain running directly into the sea, lakes or high capacity rivers throughout the area. It is not unusual to get rainfall accumulations of 200 mm in 24 hours with intensities of up to 30 mm per hour.
Whilst the roading infrastructure is well developed to cope with most heavy rain events, significant and sustained rain events have the potential to close roads and isolate Milford Sound while repairs are carried out.
Landslide
Landslides are a regular occurrence in Milford Sound due to regular severe weather and the areas soil conditions. Landslides are primarily vegetation based near the coast where topsoil is limited, however further inland, they have a higher content of rock and soil.
Snow / Avalanche
Avalanches are also a threat in the Milford Sound area in that they pose a threat to the road networks which service them. While avalanche risk is significantly reduced by the preventative actions of the Milford Road Alliance (i.e. avalanche control), avalanches continue to occur on a periodic basis.