More detailed investigation of earthquake-related documents has now led me to write something further about earthquakes in New Zealand. (For earlier posts about the Christchurch earthquakes see https://keithwoodford.wordpress.com in the earthquake category.) I have found four GNS maps (www.gns.cri.nz) which, when brought together, seem fundamental to understanding the ‘big picture’. I have therefore reproduced and interpreted them in this post, using information gleaned from a range of sources.
The first map (above) shows the three major fault systems in New Zealand: the North Island System, the Marlborough System and the Alpine Fault. What is also highlighted in this picture is that the North Island System actually separates the main Indo-Australian Plate (in this diagram more simply called the Australian Plate) from a ‘rootless fragment’ of this same plate. This Indo-Australian fragment has detached and is now taking a ride on top of the Pacific Plate. The real surface boundary of the Indo-Australian Plate is the Hikurangi Trough (also called a trench) to the east, where the Pacific Plate is being pushed under. The main NZ tsunami risk comes from an undersea earthquake in this trough. The figures printed within this map of 38, 39, and 41 are the average annual movement in millimetres along these parts of the trough. Elsewhere, I have seen GNS data (reported in earlier posts) indicating that the Alpine Fault moves at a similar rate to this, but that the two segments of the Indo-Australian trench are moving relative to each other at only 5-10 mm per annum. This recognition that movement between the two Indo-Australian fragments might be less than previously thought has led to suggestions that the risk to Wellington might also be less than previously thought. Basically, the smaller annual movement leads to longer return intervals between big earthquakes. (However, the earthquakes that Wellington feels are not only those between the two parts of the Indo Australian Plate, as the Pacific Plate does reach under Wellington at a depth of 30-40km. More on Wellington later.)
The picture above shows where the major shallow earthquakes have been in the last 160+ plus years. It is shallow earthquakes that do the most damage. There are two big swathes of the country where no big damaging earthquakes have occurred. One of these is all of the North Island to the west of the North Island faults. This is not unexpected, as it is generally an area of only moderate risk for shallow earthquakes (but not for volcanoes). The second is the area adjacent to (i.e. both west and east) of the Alpine Fault. This is not so reassuring, as it indicates that pressure is building up. (The Magnitude 7.1 Arthur’s Pass earthquake of 1929 was on the Poulter Fault which is one of several faults, this one relatively minor but others considerably larger, branching off the northern extremity of the Alpine Fault.) The earthquakes in Fiordland are associated with the Puysegur Trench where the Indo-Australian plate is subducting (diving beneath) the Pacific Plate.
The third map (above) shows all of the shallow earthquakes of more than Magnitude 3 over a ten year period. Most likely, this is up to the end of 2009, or perhaps even earlier, as the Magnitude 7.1 Darfield (Canterbury) earthquake of 4 September 2010 is not shown. Essentially, this map confirms how we really are the Shakey Isles but with the shallow earthquakes largely along the faults, and overlapping on this map as a splodge of red.
This last map (above) provides key information of several things. First, it shows how in the North Island the Pacific Plate is descending beneath the main part of the Indo-Australian Plate. The increasing depth going from east to west across the North Island gives a strong indication of the slope at which the Pacific Plate is descending. The second point is how there have been no earthquakes at depth along the Alpine Fault. Once again, this is indicative of pressure building up. The two plates are in a stalemate, but the pressure is continually building, and with nowhere else to go the fault must eventually unzip. The eastern side including the peaks of the Main Divide will move south. The western side will move North.
So what does all of this mean? My own interpretations are multiple. The first is that the GNS data reinforces the notion that the Alpine Fault, which last unzipped in a major way in about 1717, is a major menace. The greatest risk, because of proximity to the Fault, is on the West Coast. (See earlier posts.)
My second interpretation is that the Hikurangi Trough provides our greatest risk for a tsunami. I would like to learn more about that. I know that the Gisborne earthquake of 6.8 in December 2007 was at 40km depth within this trough. However, although this earthquake did considerable damage to Gisborne, it was too deep and of insufficient magnitude to create a tsunami. Apparently, the risk of tsunami from the Hikurangi Trough is enhanced by the possibility of major sediment deposits, deposited by the sea currents, being shaken off the sides as a landslide. It seems that the risk of sediment landslides is strongest at the southern end of the Hikurangi Trough. This is of particular concern for those living along the Kaikoura Coast in the South Island. However, for anyone who lives in low-lying areas along the east coast of both islands, from East Cape in the north to Christchurch in the south, this Hikurangi Trough seems far too close for comfort.
I indicated earlier that I would say more about Wellington. Despite some indications that they have already had their big one (officially known as the 1855 Wairarapa earthquake of magnitude 8.2, the biggest recorded in New Zealand since European settlement), and that the return intervals are longer than previously thought, there is still great scope for massive earthquakes. There are also some low-lying parts of Wellington that are susceptible both to liquefaction and tsunami. Similarly, cities like Blenheim, Nelson, Hastings, Napier, Gisborne and Tauranga, together with many smaller towns, would all seem to have significant risk from tsunami. All we can do is plan and prepare, and then get on with living.