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The Port Hills rockfall problem
The Port Hills area on the edge of Christchurch was very seriously affected by the Christchurch earthquake sequence. The Canterbury Earthquake Recovery Authority will today start the process of demolishing the rockfall affected houses in the Port Hills area. Yesterday they released a statement describing the challenges of this work; this statement is well-covered in an article in The Press, which also includes a nice video taken by a drone of some of the sites. A couple of years ago I visited many of these sites, and I have an old post that presents some of the images that I collected. As a reminder, this is typical of the state of some of the buildings at the top of the slopes on the Port Hills:
Dr Chris Massey, a researcher with GNS Science in New Zealand is currently visiting the Institute of Hazard, Risk and Resilience. During the 2011 Christchurch earthquake large boulders were unhinged from extremely steep slopes and crashed into houses below (see Christchurch earthquake secondary hazards). In this highly informative and exciting video, Chris presents some of the work they’re doing in studying how large rocks travel down steep slopes.
One of the legacies of the Christchurch earthquake sequence in New Zealand has been both increased levels of rockfall hazard and increased awareness of the level of hazard that predated the seismic events. The most seriously affected area is the Port Hills area to the southeast of the city, where large numbers of houses were affected by rockfalls during the earthquakes, and may more were left at risk in the aftermath. To assess the level of hazard properly, Christchurch City Council commissioned GNS Science to undertake a quantitative life risk assessment for the affected areas. This has been a huge piece of work, undertaken in the public spotlight on a very short timescale. The aim has been to generate the best possible assessment of the risk to life; this assessment can then be used as the basis for decisions on the viability of individual properties. Read more
The GeoNet map below shows the location of this series of events, which have occurred to the east of Christchurch:
Although one of these events were large earthquakes (the largest is M=6.0), the shallow depth (6 km for the largest), proximity to the city and vulnerable materials will have combined to make this a significant series of events once again. Chris Rowan at High Allocthonous has en excellent post on the earthquake focal mechanisms, so I won’t repeat that here. However, it is worth looking at the above data alongside this map of the early events, produced by the ever-impressive people at GNS Science:
Note that the first event (the Darfield earthquake) and aftershocks (in green on the map) occurred to the west of the city. The Feb 2011 events (red) were further to the east, and the June 2011 to the east once again. This most recent sequence is further east again. Read more
This is the first of several posts featuring incredible images of rockfall and landslide damage associated with the Christchurch earthquakes. It is mirrored from Dave Petley’s Landslide Blog.
This post shows the impact of a large boulder on a house.
This is the (beautiful) house in question – the cliffs that were the source of the boulder can be seen in the background. Note the distance that the rock has travelled:
NASA posted a new map yesterday that shows the shaking intensity of the earthquakes that took place in and around Christchurch. According to their website: ‘The deeper the red color of the circle, the more intense the “peak ground acceleration,” or shaking of the earth. Note how intensity is highest right around the most densely developed areas of Christchurch.’ Read more
The recent earthquake that struck Christchurch earlier this week is known so far to have killed 113 people, with hundreds reported missing. The greatest concern is that the death toll will continue to rise in the days ahead. There appears to be a number of factors involved that led to devastation worse than the Canterbury earthquake last year. Director of IHRR, Dave Petley, has covered the event extensively on his blog — On the causes of the high levels of loss in the Christchurch earthquake – identifying the high accelerations of the earthquake (up to 220% g) that would exceed the design strength of many buildings leading to excessive damage. Also, the fact that the high magnitude shallow earthquakes within Christchurch can cause more damage than deep earthquakes. Read more