New Zealand is comprised of two large principal islands, and is located southeasterly of Australia and southerly of Fiji, Samoa and Tonga in the Southern Pacific Ocean. The islands of New Zealand are situated in an area of complex plate tectonics. Despite New Zealand being geographically islands, it shares many similarities to California with regard to structural geology and earthquakes.
The islands of New Zealand are located on an active margin of the
Australian tectonic plate and predominantly continental crustal materials (ie. felsic – granitic type rocks) comprise their basement, along with much of the submarine “rise” areas surrounding the islands. These basement complexes are overlain by volcanic and a variety of sedimentary rocks, both marine (ie. limestone) and continental (ie. sandstones, coal). Interwoven with the in-situ rocks are areas of faulted and deformed ocean crustal plate remnants and suspect terrain consisting of islands and continental fragments rafted to and literally plastered onto the islands from plate tectonic activity.
The geologic history, present, and future of the area of New Zealand is strongly controlled by its being located on an active leading plate edge where the Australian plate interacts with the Pacific crustal plate. Unlike the geologic materials associated with the continental Australian plate system, the Pacific plate is comprised of basaltic – mafic type rocks that are much denser than their continental counterparts. Because of this, convergent interactions between the two usually results in the oceanic plate being thrust – or subducted – under the continental plate. This is the case with regard to the North Island, where the Pacific plate is subducting beneath the northeast and east portions of the island. This subduction zone continues northerly as the Kermadec Trench which extends as far as southern Samoa and Tonga. The subduction zone north of the island involves principally ocean crusts on both sides, Pacific plate to the east converging with seafloor / oceanic crust of the Fiji Basin and the Kermadec Ridge. This subduction is active, and is (along with predecessor subduction zones in the geologic past) a primary source of much of the volcanic rock and volcanic and geothermal activity on the North Island. This subduction is also causing a “lift” effect on the North Island, most prominently in the east.
Due to the fact all this tectonic activity takes place on the surface of a sphere, combined with plate geometry and the structural geology of the South Island and adjacent submarine rise, the Kemadec subduction zone northeast off of the North Island is replaced with another intermediary thrust, mapped as the Hikurangi Trough system off the east coast of the North Island. In the northern-northeast area of the South Island, these thrusts transition in the area of the Median Tectonic Line into a major transform fault system dominated by the Alpine Fault Zone. This transform fault system is similar mechanically to the San Andreas Fault Zone of California. Both fault systems are predominately strike-slip, meaning the principal motions are horizontal and roughly parallel to the axis of the fault. Both fault systems are associated with the boundary between the Pacific Plate and a continental margin. Both fault systems are right-lateral, meaning they have offsets where a person on one side of the fault line facing the other side would perceive objects on the opposite fault block to move to the right. These two systems are also a part of the overall Pacific Rim of Fire – referring to the strong concentration of heavy earthquake and volcanic activity associated with the perimeter of the Pacific Ocean.
Unlike the subduction zones to the north, the transform faults on the South Island introduce areas of pronounced uplift in the west, forming the high Southern Alps, and areas along the east (ie. Canterbury and Christchurch) that are depressed and actually are sinking.
The above described tectonic setting is highly active, and involves approximately 45-mm per year of driving force. As would be expected, this area is a focus of earthquake activity. As can be seen on the plots of earthquake activity, literally thousands of earthquakes have occurred in New Zealand historically, including several large to very large events. The exceptional historic earthquakes include the estimated M8.2 event of Wairarapa in 1855, located in the southeast portion of the North Island; the volcanic eruptions of Tarawera (situated in the northeast portion of the North Island) in 1886; a M7.8 event in Hawkes Bay 1931, also situated in the northeast portion of the North Island. The South Island has been relatively quiet historically with respect to large earthquakes, however this prior relative calm has been disrupted by the occurrence of the two relatively recent large earthquakes. These earthquakes include the 2010 M7.1 Earthquake, the mainshock of which occurred in the area of Canterbury on September 3, 2010; and the most recent February 21, 2011 M6.3 event whose mainshock was situated in the area of Christchurch. Both Canterbury and Christchurch are located on the east side of the South Island. The South Island also experienced M6.8 and M5.9 earthquakes in June 1994. These 1994 events occurred about 40 km to the northwest of the Canterbury area in the mountains. An M7.1 earthquake also occurred in the area of Authurs Pass in 1921, about 50 km northwest of the Canterbury area.
Both of these events included hundreds of small to mid sized aftershocks, and many M4.5 and larger. These historic earthquakes of the South Island described herein were all generally shallow in nature (ie. approximately 5 km depth), and were situated near population centers. Based on current understanding, these earthquakes did not occur on the main transforms associated with the Alpine and Hope Fault Zones, but rather on some of the numerous sympathetic faults driven by these main systems. These earthquakes, with the exception of the most recent 2011 Christchurch event, cause widespread property damage but cause little severe personal injury. The 2011 event, however, even though it was on the low end of the major historic earthquakes in terms of absolute magnitude, was responsible for over 160 known deaths and caused severe damage to structures and infrastructure.
One reason for this is the interaction of the epicenter of the Christchurch earthquakes with the structural geology and geometry, causing a local “focusing” of seismic waveforms, especially where these waveforms refract / reflect and combine, forming local ground motions well in excess of 1g. This combined with the location of the epicenter being significantly closer to the highly developed areas of Christchurch – literally right underneath the south east suburbs – than previous events was a main reason for the exceptional damage. Seismic instrumentation indicated peak ground motions of over 1.68g at the Heathcote Valley Primary School, and 0.98g at the Lyttleton Port Company stations. Most of the ground motions were of about 0.5g or less repeatable. An acceleration of 0.5g is considered to be a very strong ground motion capable of considerable damage to failure of ordinary structures and causing damage even in well built structures. The 2011 earthquake also included numerous aftershocks in the M4 to M5 range, which unlike the aftershocks of the previous earthquakes, were in close proximity to developed areas and cause significant additional damage.
In addition to the primary effects of earthquake shaking, severe damage was caused by seismically induced liquefaction in the lowlying / valley and coastal areas underlain by poorly consolidated sandy sediments with shallow groundwater, seismically induced settlement, and landsliding and slope lurching. Since the earthquakes in this event are shallow and all situated roughly directly under the city, and much of the area is directly underlain by soft, alluvial sediments, the waveforms act with little attenuation, and are locally actually amplified and focused like light through a lens by portions of these sediments and confining geology. This combination produced disastrous consequences by literally focusing the earthquake forces into the developed areas of the city. These occurrences related to focusing and amplification of seismic waveforms by soft sedimentary geology were also associated with large earthquakes in California, including the January 1994 Northridge event, and the Easter 2010 Calexico – Laguna Salada event. These California earthquakes caused widespread property damage due to both primary shaking and secondary liquefaction-type actions.
