With all the earthquake-related activity in central New Zealand going on at the moment, we took the opportunity to put your questions about what’s going on under our feet to our resident Subject Expert – Earth Science, Hamish Campbell from GNS Science.
Q: Did the January-May slow-slip event influence the Seddon sequence in any way?
HJC: We simply cannot say at this stage. More research is needed in order to establish any direct influence but there may be.
Q: Is this activity a sign of more to come spread further throughout New Zealand?
HJC: Seismic activity in New Zealand is a more or less constant natural phenomenon and is punctuated by bursts of more intense activity on a regular basis. Clearly, the Christchurch earthquakes are an example of this, as is this ‘event’ in Southern Cook Strait. Could it be ‘a sign of more to come spread further throughout NZ?’ Not really. We doubt that there is a significant or fundamental change going on, but again, there may be. Who knows?! Our knowledge of earthquakes extends only back to the 1850s, which is a very short time in geological terms. However, we should be able spot any ‘change’ in the pattern of earthquakes. What we do know is the instrumentally recorded pattern of seismic activity in New Zealand over the past 100 years. The question is: how representative and ‘normal’ is this record?
Q: Which fault lines are involved are in these quakes and could they trigger a large Wellington fault line quake?
HJC: So would we all, and my colleagues at GNS Science are working on this right now. On the basis of all data to hand, we think that the earthquakes are associated with a number of faults within the Kekerengu-Needles-Vernon-London Hill fault system (not the Wellington Fault system) and that some of the action relates to an area with no previously recognised fault. We don’t have any reason to suggest that these quakes will or could trigger a large quake on the Wellington Fault.
Q: Does reclaimed land in Wellington pushed up by past earthquakes have more chance of dramatic change in a new earthquake than non-reclaimed land?
HJC: No, not for naturally uplifted land. A distinction needs to be made between ‘reclaimed land’ as in man-made reclamation, and naturally uplifted land. There is a greater chance of deformation on uplifted man-made reclamation.
Q: How deep are earthquakes recorded as earthquakes. Crust? Lower mantle? Further?
HJC: Earthquakes are pulses of energy that are released when solid material is broken or snapped. Earthquakes can only occur in solid material i.e. within solid rock materials of the crust and the brittle upper mantle, collectively referred to as the lithosphere. Below this is the ductile (fluid) asthenosphere. You cannot break or snap a fluid. Now, having said this, solid ‘slabs’ of lithosphere can descend deep into the asthenosphere, breaking up as they do so. This explains why earthquakes have been recorded in excess of 700 kilometres below the earth’s surface. However, most are restricted to the lithosphere, normally only about 100-150 kilometres deep.
Q: In our house on Tinakori hill stuff stayed on the shelves, at my partner’s work on the docks near the stadium all sorts of things fell down. Is the reclaimed land always shakier or does it depend on the kind of earthquake?
HJC: Yes, the variation in experience of an earthquake largely depends on the nature (geological setting) of the ground on which the structure (house, building) is sited. The nature of the earthquake (location of the epicentre w.r.t. your site, magnitude, depth) also plays a role, but less so.
Q: Can you discuss the relationship between the type of soil a structure is built on and the risk of damage to the structure. Including liquefaction and other effects, not just seismic shakes?
HJC: Seismic energy travels as a wave. The denser the material it travels through, the greater the velocity and the less the wave height (amplitude). As it passes into less dense material, the velocity drops and the wave height greatly increases; the amplification effect can be enormous. Herein lies the problem: the density contrast between dense solid rock and much less dense soil, sand, peat and air is so great that seismic energy literally explodes into huge oscillating wave pulses. A good analogy is what happens to the tip of a whip when it is ‘cracked’ or the fly end of a fly fishing line being flicked. Liquefaction is a consequence of intense seismic shaking and is generally caused when ground acceleration exceeds 0.25 g. Liquefaction only occurs during this intense shaking (not before or after) and causes materials that normally behave as solid to flow. Liquefaction can only occur in substrates that comprise both a solid and fluid (liquid and/or gas) component. Most susceptible are sediments (sands, silts, muds and peats) that are charged with water.
Q: How believable is it that there was ground movement and floor damage at Cable Bay, Northland, as reported in the Northern Advocate?
HJC: I would say that this is extremely unlikely. More likely is that it relates to some local event (maybe weather-related) or to long-term settling effects associated with local groundwater effects.
Q: Is it possible to predict how different materials react to an earthquake – wood vs bricks vs concrete? How do the different waves treat them? And what do we know about how Wellington was formed that makes a difference to how the city fares during an earthquake. For instance, would how the hills and valleys were formed make a difference to how an earthquake pans out?
HJC: Yes it is. All these materials can be tested for their strength and resilience under simulated laboratory conditions. This kind of analysis is investigated and taught in university engineering departments and is routine in various science and technology providers (such as BRANZ). Wave properties of brick as opposed to wood or concrete will vary according to density, tensile strength, elasticity as well as dimensions and presence and relationship to other kinds of associated material (e.g. metal components, mortar etc). The nature of Wellington’s topography and the near-surface geology are major determinants in the considerable variation of earthquake experience from one part of the city to another.
Q: If a shock did cause a tsunami, where could we expect it to come in, how much time might we have, and what should be on the lookout for?
HJC: As a general rule, tsunami are caused by fault displacement of the sea floor. The greater the displacement (length and height of fault offset of the sea floor) the bigger the tsunami. An earthquake-induced tsunami could come from almost anywhere and could impact on almost all parts of the NZ coast. I say this because tsunamis are waves and waves travel in an orderly manner away from the source of the earthquake but like any wave they can be reflected, refracted, focussed, amplified and attenuated. This means they get around, so to speak. The magnitude of tsunamis varies enormously, so a great deal of the ‘impact’ will relate to the nature of the tsunami (distance to source, dimensions of the displacement of the sea floor). However, great efforts have been made to identify the most likely sources of tsunami that might impact on New Zealand from both near and far. The time we would have depends on distance to source. The further away the more warning and hence more time we would have to react. The closer the source the less time available. If by the sea, the thing to look out for (and react to immediately) would be a sudden retreat of the sea. This would be obvious and could only mean that a tsunami is approaching, and fast. You would need to rush inland to high ground as fast as possible.
Q: Is Wellington at risk of a tsunami if earthquakes and aftershocks keep occurring?
HJC: Wellington is always at risk from tsunami. Fortunately, few earthquakes cause rupturing of the sea floor. Nevertheless, active faults have been mapped on the sea floor in Cook Strait. It is indeed therefore possible that tsunami could be generated locally. There is a chance that there might be a severe earthquake in the near future (days to months) in Cook Strait and it may well be strong enough to displace the sea floor and generate a tsunami. Such a tsunami would be triggered less than 100 kilometres from Wellington and given that tsunami can travel at speeds of up to 800 kilometres per hour, it would impact on the Wellington coast in less than eight minutes. There would not be much time to warn people! Lets hope that if such an event were to occur, its impact was slight.
Q: How many building in Wellington are earthquake proof like the Beehive and other Parliament buildings?
HJC: I am not sure of the exact number but I think it is relatively small. I know that Wellington Central Police Building has base isolators as does the VUW Library Building, and Parliament. There are others. I know that the number was fewer than 10 in 1998. I suspect there are a few more now.
Q: Do earthquakes mean changes are going on with our tectonic plates?
HJC: Yes; it means that the plates are ‘at work’. They are constantly changing and deforming causing all sorts of multi-dimensional rearrangement at both the surface and subsurface. Earthquakes are largely about rock strength in the presence of enormous tectonic forces. A natural limit is reached and the rock fails; it breaks. The breakage points (which are really surfaces and are referred to as ‘faults’ if they are large scale, or ‘fractures’ or ‘cracks’ if they are small scale) may be thought of as valves. Earthquakes in New Zealand are largely due to break-up effects in either the Australian Plate or the Pacific Plate. There are diverse processes occurring in both plates as they variously move with respect to each other (colliding, extending, sliding). A complicating factor is the role of fluids which can act as both lubricants and as hydraulics, causing changes in pressure at a variety of scales.
Thanks for your time Hamish. If you’re interested in learning more about what’s going on beneath our feet and around us, Awesome Forces, edited by Geoff Hicks and Hamish Campbell, provides expert insight into the subject.