3.3 Observations from the Canterbury Earthquakes I
The Canterbury Accelerograph Network (CanNet) and some Results from the September 2010, M7.1 Darfield Earthquake
John Berrill, Hamish Avery, Michael Dewe, Andrew Chanerley, N.N. Alexander, Colin Dyer, Caroline Holden & Bill Fry
ABSTRACT: The majority of the close-in accelerograms from the Mw7.1 September 4th, 2010 Darfield Earthquake were recorded by the local Canterbury Network (CanNet), which was being installed in the central South Island of New Zealand in anticipation of both a great earthquake on the Alpine Fault and moderate events on the Marlborough fault system. At the time of the earthquake, 36 of the planned 60 free field instruments were in place on the Canterbury Plains, in and surrounding the epicentral region, and across the nearby City of Christchurch. This paper presents a brief history of the network, and examines some results. The underlying policy of having a dense network of lower-resolution instruments rather than fewer high dynamic-range ones is clearly vindicated.
Preliminary Source Model of the Mw 7.1 Darfield Earthquake from Geological, Geodetic and Seismic Data
C. Holden, J. Beavan, B. Fry, M. Reyners, J. Ristau, R. Van Dissen, P. Villamor & M. Quigley
ABSTRACT: The Mw 7.1 Darfield earthquake has provided geologists, geodesists and seismologists with well constrained surface fault rupture extent and displacements, densely spaced GPS coseismic displacements, striking InSAR images, and a globally unprecedented set of near-source strong motion data. Collectively, these datasets indicate that the Darfield earthquake was a complex event, involving rupture of multiple fault planes with most of the earthquake’s moment release resulting from dextral strike-slip movement on the previously unknown, east-west striking, Greendale Fault. They also point to important secondary sources such as a southeast-dipping blind reverse fault near Charing Cross that initiated the rupture sequence, and a northwest-dipping reverse fault near Hororata that increased rupture duration and spatial extent. Although the models are consistent and support each other, this analysis is still preliminary and ongoing research is focused on further integrating these data sets to better understand the nature, extent, depth and timing of sub-events of the Darfield earthquake.
Surface Rupture Displacement on the Greendale Fault during the Mw 7.1 Darfield (Canterbury) Earthquake, New Zealand, and its Impact on Man-Made Structures
R. Van Dissen, D. Barrell, N. Litchfield, P. Villamo, M. Quigley, A.B. King, K. Furlong, J. Begg, D. Townsend, H. Mackenzie, T. Stahl, D. Noble, B. Duffy, E. Bilderback, J. Claridge, A. Klahn, R. Jongens, S. Cox, R.M. Langridge, W. Ries, R.P. Dhakal, A. Smith, S. Hornblow, R. Nicol, K. Pedley, H. Henham, R. Hunter, A. Zajac & T. Mote
ABSTRACT: Surface rupture of the previously unrecognised Greendale Fault extended west-east for ~30 km across alluvial plains west of Christchurch, New Zealand, during the M w 7.1 Darfield (Canterbury) earthquake of September 2010. Surface rupture displacement was predominantly dextral strike-slip, averaging ~2.5 m, with maxima of ~5 m. Vertical displacement was generally less than 0.75 m. The surface rupture deformation zone ranged in width from ~30 to 300 m, and comprised discrete shears, localised bulges and, primarily, horizontal dextral flexure. About a dozen buildings, mainly single-storey houses and farm sheds, were affected by surface rupture, but none collapsed, largely because most of the buildings were relatively flexible and resilient timber-framed structures and also because deformation was distributed over a relatively wide zone. There were, however, notable differences in the respective performances of the buildings. Houses with only lightly-reinforced concrete slab foundations suffered moderate to severe structural and non-structural damage. Three other buildings performed more favourably: one had a robust concrete slab foundation, another had a shallow-seated pile foundation that isolated ground deformation from the superstructure, and the third had a structural system that enabled the house to tilt and rotate as a rigid body. Roads, power lines, underground pipes, and fences were also deformed by surface fault rupture and suffered damage commensurate with the type of feature, its orientation to the fault, and the amount, sense and width of surface rupture deformation.
Geological Engineering Study of Liquefaction After the 2010 Darfield Earthquake in an Area of Complex Fluvial Geology
S.D. Ward, M.K.H. Brown, I.R. Brown & T.J. Larkin
ABSTRACT: The liquefaction potential of an area of southwest Christchurch was assessed following the September 4 2010 Darfield earthquake. Site investigations were carried out where sand boils were observed, and on adjacent areas where no indication of liquefaction was found. At the site with sand boils, the path of ejected material was followed downwards to the source along thin (<5 mm) dykes that cut the near surface confining layers. The ejected material had followed an irregular path to the surface. Despite the fluidised material passing through dry, high porosity sands there did not appear to be any migration of fluid into the adjacent sands. Trenches at locations without sand boils also showed sands of low density and high pore pressures. This suggested that liquefaction may have occurred but near surface confining conditions did not allow migration of the liquefied material to the surface. Particle size analysis indicates grain sorting processes may have occurred during liquefaction. Analysis of existing topographic (LIDAR) and subsurface geological data has enabled us to determine which parts of the study area are likely to be susceptible to liquefaction.
Preliminary Assessment of Liquefaction in Urban Areas following the 2010 Darfield Earthquake
M.L. Taylor & M. Cubrinovski
ABSTRACT: In the weeks immediately following the 4 September 2010 Darfield Earthquake, some 80 Swedish Sounding (SWS) tests were carried out in Christchurch and the nearby township of Kaiapoi that were affected by extensive liquefaction related ground damage. SWS is a simple manually operated penetration test under a dead-load of 100 kg in which the number of half-rotations required for a 25 cm penetration of a rod (screw point) is recorded. One of the advantages of the SWS test which was heavily utilised in this investigation is the ability to perform the test within a confined space in backyards of residential properties. Other advantages include the fact that SWS has been successfully utilised in liquefaction studies and that SWS penetration resistance NSW can be expressed in terms of conventional SPT blow count using established N-NSW empirical correlation. Even though manually operated, the test setup used in the reconnaissance could probe soils up to 9 m depth. This paper presents an initial appraisal of the results of this data in terms of liquefaction triggering evaluation using semi-empirical methods. The evaluation methods have been applied to and scrutinised in areas including very severe, moderate-to-low or no liquefaction manifestation during the Darfield earthquake.
Observation and Characterisation of Land Damage due to Liquefaction and Lateral Spreading
M.E. Jacka & K.M. Murahidy
ABSTRACT: The September 2010, 7.1-magnitude Darfield earthquake caused significant land damage in Christchurch and outlying towns. This paper summarises the approach taken to compile land damage observations and a characterisation system which was established to map the effects of liquefaction and lateral spreading in affected residential areas. This information was used to guide the recovery, land remediation and public information efforts of the Earthquake Commission (EQC) and New Zealand Government.
Mapping of the affected areas by the authors commenced within hours of the main shock, giving opportunity for very early observation. More detailed work continued on behalf of the EQC, with overview mapping teams and detailed geotechnical assessment teams operating over the course of several months after the earthquake. This gave an overall picture of the ground movement patterns, severity in terms of magnitude and extent, types of land damage and effects on buildings and infrastructure.