On 21 July 2025, a very large rock avalanche occurred in the mountains of Hualien County, Taiwan. Initial measurements suggest that this ran out over about 6 km.
On 21 July 2025, an extremely large rock avalanche occurred in the administrative area of Wanrong Township in Hualien Count in Taiwan. This event was detected on seismic data and it has been described on Facebook by Chen-Yu Chen. In the days before the landslide, southern Taiwan had been affected by heavy rainfall associated with the passage of Tropical Storm Wipha.
The crown of the landslide is at [23.72645, 121.29021]. A rough measurement suggests that it is in the order of 6 km long and 2 km wide. The location is steep and rugged – this is a Google Earth image of the site of the landslide:-
As the image above shows, the area affected by the rock avalanche is exceptionally steep (even by Taiwan standards) and deeply dissected, suggesting regular landslide activity. I will return to this theme in a future post.
This is a Planet Labs image of the site, draped onto the Google Earth DEM, captured on 25 July 2025. So far, this is the only image of the site that I have been able to access – this part of Taiwan is exceptionally cloudy at this time of the year. Whilst some of the landslide is covered in cloud, most is visible.
Of particular note is the large-scale of the event, the long runout and the large amount of dust on the adjacent slopes. Note also the lake that has started to develop – it is reported on Facebook that the hazard associated with this is being managed.
The crown of the landslide is at about 2,450 metres and the toe is at roughly 700 metres, so this has a vertical extent of about 1,750 metres.
Here is an initial slider of the before and after images of the landslide:-
This is probably the largest landslide in Taiwan by volume since the Tsaoling rock avalanche and the Chiufengershan rock avalanche, both triggered by the Ch-Chi earthquake in 1999. However, the runout of the Wanrong landslide is, I think, larger than both of these landslides. I do not have a volume estimate at this point.
In the autumn, it is likely that clear imagery will become available of this exceptional landslide. However, Taiwan is likely to be affected by further heavy rainfall in the coming weeks, so the landslide might evolve further.
Reference and acknowledgement
Many thanks to Brian Yanites of Indiana University Bloomington for highlighting this event, and for his work on the landslide.
Planet Team 2025. Planet Application Program Interface: In Space for Life on Earth. San Francisco, CA. https://www.planet.com/