Darfield Earthquake Research Articles

Hydrological effects of the M W 7.1 Darfield (Canterbury) earthquake, 4 September 2010, New Zealand

The M W7.1 Darfield (Canterbury) earthquake on 4 September 2010 generated widespread hydrological effects ranging from near-instantaneous coseismic liquefaction and changes of groundwater levels in boreholes, to more sustained (days to weeks) post-seismic changes in spring flow, river discharge and groundwater piezometric levels, to longer term shifts in groundwater level one year after the earthquake. Groundwater piezometric responses include local groundwater level increases of >20 m around the Greendale Fault, particularly in deep aquifers (>80 m), whereas decreases occurred in coastal confined aquifers beneath Christchurch city. Increases of up to 5 m persisted within 20 km of the fault 12 h after the earthquake. Groundwater levels and springs were affected throughout New Zealand, from >350 km away in Southland to nearly 1000 km away in Northland, even where shaking intensities were less than Modified Mercalli Intensity (MM) 3–4 (weak to largely observed) and peak ground acceleration was much <0.01 g. Release of artesian groundwater pressure and groundwater flow are postulated to have played pivotal roles in Christchurch liquefaction.

Statistical analysis of the 2010 M W 7.1 Darfield Earthquake aftershock sequence

Statistical properties of the aftershock sequence of the M W 7.1 Darfield (Canterbury, New Zealand) earthquake are studied. The sequence exhibits rich scaling behaviour in magnitude and aftershock decay rates. In particular, we observe a marked variability in the frequency-magnitude statistics in space, between early and late times after the mainshock and over different magnitude ranges. The mainshock triggered two large earthquakes (22 February 2011 M W 6.2 Christchurch earthquake and 13 June 2011 M W 6.0 earthquake) that occurred later in the sequence and generated their own aftershock sequences. The frequency-magnitude statistics of the sequences are modelled using the Gutenberg–Richter scaling relation. We also study the difference between the magnitudes of the largest recorded aftershocks and the mainshock. This is analysed and discussed using the modified Båth law. In this context we consider the M W 6.2 Christchurch and 13 June 2011 M W 6.0 earthquakes as the largest aftershocks of the Darfield mainshock. It is also observed that the aftershock decay rates can be approximated by the modified Omori law. The obtained results indicate that the aftershock sequence exhibits self-similarity in both magnitude and time.

Performance of steel storage racks in the Darfield earthquake

The author inspected 18 racking installations at various food storage facilities in Christchurch for damage from the 4th September 2010 Darfield Earthquake. The type of racking installations and damage suffered are listed with some photographs of specific damage. The damage ranged from minor to complete collapse with large product loss. Although inspection access was limited, the collapse mechanism for racking installations is assessed and suggested. The damage inspected indicated some variation in the application of design approaches and potential areas where the behaviour of rack structures may not have been fully understood during design. General conclusions are drawn from the damage inspections, and suggestions offered for the refinement and improvement of racking design approaches.

Strong ground motion characteristics observed in the 4 September 2010 Darfield, New Zealand earthquake

This manuscript provides a physically-orientated and engineering-focused assessment of the near-source ground motions from the Darfield earthquake that were recorded by 29 strong motion stations in Christchurch city and the surrounding Canterbury Plains. In discussion of the observed ground motions particular attention is given to: (i) source features such as the complexity of the rupture on multiple fault planes and forward directivity effects; (ii) the effects of the Canterbury Plains sedimentary basin on waveguide effects through the region, and basin-edge effects observed around the Port Hills; and (iii) the importance of local site response as evidenced by observations of large high frequency amplification and liquefaction. Additional context is provided by comparison of ground motion amplitudes with empirical prediction models and reconciling ‘outliers’ based on inferred physical mechanisms. The severity of the horizontal and vertical components of ground motion are also emphasised via comparisons with response spectra prescribed for routine seismic design.

Evidence of earthquake-induced liquefaction obtained from GeoEye-1 images

A most significant effect of the Mw 7·1 Darfield earthquake of 4 September 2010 was widespread liquefaction and lateral spreading in the eastern parts of the city of Christchurch and the surrounding region. Large volumes of ejecta were deposited on the ground surface. A few hours after the earthquake, a GeoEye-1 image of part of the area affected by the earthquake was captured and subsequently appeared on Google Earth. Saturday 4 September was a clear day in Canterbury, so this image provided good evidence of liquefaction in the form of piles of ejected sand on the ground surface in Christchurch and the surrounding region. This letter illustrates the effectiveness of public domain GeoEye-1 images at indicating places where liquefaction has occurred. Not surprisingly, the colour contrast between the material ejected and the surface onto which the material was deposited was an important factor in identification. In addition, although a considerable volume of water was observed to have been ejected during the liquefaction, in some places, the images gave the impression that the extent of the liquefaction was much greater than was found to be the case during subsequent on-ground reconnaissance. These observations were confirmed following the Mw 6·2 Christchurch earthquake (an aftershock of the Darfield event) on 22 February 2011.

Ground Movement and Strains from Airphoto Measurements in Avonside Caused by 2010 Darfield Earthquake

In this work ground failure following the 2010 Darfield earthquake in Canterbury, NZ was studied. Lateral spreading and horizontal ground strains were analyzed in an area near the Central Business District of Christchurch city. The Avon river runs through the middle of the study area and includes natural, filled and artificially cut stream bed stretches. Lateral displacement vectors from photogrammetric data were interpolated and used to produce ground strains. Large compressive and tensile strains were identified. Natural point bars were found to correlate closely with tensile strains. Natural stream bed (filled and existing) was found to closely correlate with compressive strains. Vector magnitude values of lateral displacements were found to correlate with water supply pipe repair rates (R2 = 0.42). However, ground strains did not correlate with lateral displacements, water supply pipe repair rates or settlements from the photogrammetric data. Therefore, it may be concluded that ground strains have limited use in the indication of potential repair rates and do not directly predict settlement. However, ground strains were observed as useful in identifying paleo-geomorphology of the area in general and changing soil density profiles in particular.

The Mw 6.2 Christchurch earthquake of February 2011: preliminary report

A moment magnitude (Mw) 6.2 earthquake struck beneath the outer suburbs of Christchurch, New Zealand's second largest city, on 22 February 2011 local time. The Christchurch earthquake was the deadliest in New Zealand since the 1931 Mw 7.8 Hawkes Bay earthquake and the most expensive in New Zealand's recorded history. The effects of the earthquake on the region's population and infrastructure were severe including 181 fatalities, widespread building damage, liquefaction and landslides. The Christchurch earthquake was an aftershock of the Mw 7.1 Darfield Earthquake of September 2010, occurring towards the eastern edge of the aftershock zone. This was a low recurrence earthquake for New Zealand and occurred on a fault unrecognised prior to the Darfield event. Geodetic and seismological source models show that oblique-reverse slip occurred along a northeast–southwest-striking fault dipping southeast at c. 69°, with maximum slip at 3–4 km depth. Ground motions during the earthquake were unusually large at near-source distances for an earthquake of its size, registering up to 2.2 g (vertical) and 1.7 g (horizontal) near the epicentre and up to 0.8 g (vertical) and 0.7 g (horizontal) in the city centre. Acceleration response spectra exceeded 2500 yr building design codes and estimates based on standard New Zealand models. The earthquake was associated with high apparent stress indicative of a strong fault. Furthermore, rupture in an updip direction towards Christchurch likely led to strong directivity effects in the city. Site effects including long period amplification and near-surface effects also contributed to the severity of ground motions.

A Short Term Seismic Hazard Assessment in Christchurch, New Zealand, After the M 7.1, 4 September 2010 Darfield Earthquake: An Application of a Smoothing Kernel and Rate-and-State Friction Model

The M(subscript w) 6.3, 21 February 2011 Christchurch, New Zealand, earthquake is regarded as an aftershock of the M 7.1, 4 September 2010 Darfield earthquake. However, it caused severe damage in the downtown Christchurch. Such a circumstance points out the importance of an aftershock sequence in seismic hazard evaluation and suggests the re-evaluation of a seismic hazard immediately after a large earthquake occurrence. For this purpose, we propose a probabilistic seismic hazard assessment (PSHA), which takes the disturbance of a short-term seismicity rate into account and can be easily applied in comparison with the classical PSHA. In our approach, the treatment of the background seismicity rate is the same as in the zoneless approach, which considers a bandwidth function as a smoothing Kernel in neighboring region of earthquakes. The rate-and-state friction model imparted by the Coulomb stress change of large earthquakes is used to calculate the fault-interaction-based disturbance in seismicity rate for PSHA. We apply this approach to evaluate the seismic hazard in Christchurch after the occurrence of the M 7.1, 4 September 2010 Darfield earthquake. Results show an increase of seismic hazards due to the stress increase in the region around the rupture plane, which extended to Christchurch. This provides a suitable basis for the application of a time-dependent PSHA using updating earthquake information.

Seismic performance of reinforced concrete buildings in the 22 February Christchurch (Lyttelton) earthquake

Six months after the 4 September 2010 Mw 7.1 Darfield (Canterbury) earthquake, a Mw 6.2 Christchurch (Lyttelton) aftershock struck Christchurch on the 22 February 2011. This earthquake was centred approximately 10km south-east of the Christchurch CBD at a shallow depth of 5km, resulting in intense seismic shaking within the Christchurch central business district (CBD). Unlike the 4 Sept earthquake when limited-to-moderate damage was observed in engineered reinforced concrete (RC) buildings [35], in the 22 February event a high number of RC Buildings in the Christchurch CBD (16.2 % out of 833) were severely damaged. There were 182 fatalities, 135 of which were the unfortunate consequences of the complete collapse of two mid-rise RC buildings. This paper describes immediate observations of damage to RC buildings in the 22 February 2011 Christchurch earthquake. Some preliminary lessons are highlighted and discussed in light of the observed performance of the RC building stock. Damage statistics and typical damage patterns are presented for various configurations and lateral resisting systems. Data was collated predominantly from first-hand post-earthquake reconnaissance observations by the authors, complemented with detailed assessment of the structural drawings of critical buildings and the observed behaviour. Overall, the 22 February 2011 Mw 6.2 Christchurch earthquake was a particularly severe test for both modern seismically-designed and existing non-ductile RC buildings. The sequence of earthquakes since the 4 Sept 2010, particularly the 22 Feb event has confirmed old lessons and brought to life new critical ones, highlighting some urgent action required to remedy structural deficiencies in both existing and “modern” buildings. Given the major social and economic impact of the earthquakes to a country with strong seismic engineering tradition, no doubt some aspects of the seismic design will be improved based on the lessons from Christchurch. The bar needs to and can be raised, starting with a strong endorsement of new damage-resisting, whilst cost-efficient, technologies as well as the strict enforcement, including financial incentives, of active policies for the seismic retrofit of existing buildings at a national scale.

Geotechnical aspects of the 22 February 2011 Christchurch earthquake

The 22 February 2011, Mw6.2-6.3 Christchurch earthquake is the most costly earthquake to affect New Zealand, causing 181 fatalities and severely damaging thousands of residential and commercial buildings, and most of the city lifelines and infrastructure. This manuscript presents an overview of observed geotechnical aspects of this earthquake as well as some of the completed and on-going research investigations. A unique aspect, which is particularly emphasized, is the severity and spatial extent of liquefaction occurring in native soils. Overall, both the spatial extent and severity of liquefaction in the city was greater than in the preceding 4th September 2010 Darfield earthquake, including numerous areas that liquefied in both events. Liquefaction and lateral spreading, variable over both large and short spatial scales, affected commercial structures in the Central Business District (CBD) in a variety of ways including: total and differential settlements and tilting; punching settlements of structures with shallow foundations; differential movements of components of complex structures; and interaction of adjacent structures via common foundation soils. Liquefaction was most severe in residential areas located to the east of the CBD as a result of stronger ground shaking due to the proximity to the causative fault, a high water table approximately 1m from the surface, and soils with composition and states of high susceptibility and potential for liquefaction. Total and differential settlements, and lateral movements, due to liquefaction and lateral spreading is estimated to have severely compromised 15,000 residential structures, the majority of which otherwise sustained only minor to moderate damage directly due to inertial loading from ground shaking. Liquefaction also had a profound effect on lifelines and other infrastructure, particularly bridge structures, and underground services. Minor damage was also observed at flood stop banks to the north of the city, which were more severely impacted in the 4th September 2010 Darfield earthquake. Due to the large high-frequency ground motion in the Port hills numerous rock falls and landslides also occurred, resulting in several fatalities and rendering some residential areas uninhabitable.

Near-source strong ground motions observed in the 22 February 2011 Christchurch earthquake

This manuscript provides a critical examination of the ground motions recorded in the near-source region resulting from the 22 February 2011 Christchurch earthquake. Particular attention is given to reconciling the observed spatial distribution of ground motions in terms of physical phenomena related to source, path and site effects. The large number of near-source observed strong ground motions show clear evidence of: forward-directivity, basin generated surface waves, liquefaction and other significant nonlinear site response. The pseudo-acceleration response spectra (SA) amplitudes and significant duration of strong motions agree well with empirical prediction models, except at long vibration periods where the influence of basin-generated surface waves and nonlinear site response are significant and not adequately accounted for in empirical SA models. Pseudo-acceleration response spectra are also compared with those observed in the 4 September 2010 Darfield earthquake and routine design response spectra used in order to emphasise the amplitude of ground shaking and elucidate the importance of local geotechnical characteristics on surface ground motions. The characteristics of the observed vertical component accelerations are shown to be strongly dependent on source-to-site distance and are comparable with those from the 4 September 2010 Darfield earthquake, implying the large amplitudes observed are simply a result of many observations at close distances rather than a peculiar source effect.

Observations of the performance of earth buildings following the February 2011 Christchurch earthquake

A reconnaissance survey of earth walled buildings in the Christchurch area was carried out following the February 2011 Christchurch Earthquake. Twenty six earth buildings were inspected during the survey including historic earth buildings and recent reinforced earth buildings. Some of these buildings had previously been inspected following the September 2010 Darfield Earthquake.&#x0D; The February 2011 Earthquake caused comparable patterns of damage to earth buildings as the September 2010 Darfield earthquake except for unreinforced pressed brick buildings which performed particularly badly. Reinforced earth buildings constructed since the 1990’s performed well during the February 2011 earthquake provided the overall wall bracing was adequate and detailing of the reinforcement and connections were generally in accordance with the NZ Earth Building Standards. Some older unreinforced rammed earth buildings constructed between 1950 and 1980, all of which had reinforced concrete foundations and bond beams, performed relatively well with only minor cracking. Unreinforced cob and adobe buildings in the area of strong shaking suffered significant damage and will require reconstruction or repair of the walls and strengthening of the upper floor or ceiling diaphragms. The performance of six houses are discussed as case studies that cover the range of buildings observed.

Structural Steel Performance Following Severe Earthquake Loading

The second Christchurch earthquake on February 22, 2011, Magnitude 6.35, generated more intense shaking in the Central Business District than the September 4, 2010 Darfield earthquake, Magnitude 7.1. The second earthquake was closer to the CBD and at shallow depth, resulting in peak ground accelerations 3 times higher. There was significant failure of unreinforced masonry buildings and collapse of a few reinforced concrete buildings, leading to loss of life. Steel structures on the whole performed well during the earthquake and the plastic, inelastic deformation was less than expected given the strength of the recorded ground accelerations. For steel buildings designed to withstand earthquake loading, a design philosophy is to have some structural elements deform plastically, absorbing energy in the process. Typically elements of beams are designed to plastically deform while the columns remain elastic. In the earthquake some of these elements deformed plastically and the buildings were structurally undamaged. The question which then arises is; the building may be safe, but will it withstand a further severe earthquake? In other words how much further plastic work damage can be absorbed without failure of the structural element? Previous research at Auckland on modern structural steel, where the steel was prestrained various levels, to represent earthquake loading, the toughness was determined, as a function of prestrain for the naturally strain-aged steel. Further research, on the same steel, investigated life to failure for cyclic plastic straining in tension and compression loading at various plastic strain amplitudes. This work has shown that provided the plastic strain in the structural element is in the range 2 – 5% the steel will still meet the relevant NZ Standards. To determine the remaining life the plastic strain must be determ ined then the decision made; to use the building as is, replace the structural element or demolish.

Year 11 geography teachers' response to the Darfield earthquake

AbstractWhen events such as the Darfield earthquake take place, it is assumed that the response from the geography teaching community provides the opportunity to reinforce the relevance of geography. This article reports the findings of a questionnaire which seeks to explore the question, ‘What were the characteristics of the learning and teaching of Year 11 Geography teachers in the weeks following the Darfield earthquake, September 4, 2010?’ The data offer empirical support for the agency of geography teachers' impressing the relevance of the subject to school students in response to an extreme natural event under New Zealand's collective backyard.

Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis

Research Article| January 01, 2012 Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis M. Quigley; M. Quigley 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar R. Van Dissen; R. Van Dissen 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar N. Litchfield; N. Litchfield 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar P. Villamor; P. Villamor 2GNS Science, Lower Hutt 5040, New Zealand Search for other works by this author on: GSW Google Scholar B. Duffy; B. Duffy 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar D. Barrell; D. Barrell 3GNS Science, Dunedin 9016, New Zealand Search for other works by this author on: GSW Google Scholar K. Furlong; K. Furlong 4Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA Search for other works by this author on: GSW Google Scholar T. Stahl; T. Stahl 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar E. Bilderback; E. Bilderback 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar D. Noble D. Noble 1Department of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand Search for other works by this author on: GSW Google Scholar Geology (2012) 40 (1): 55–58. https://doi.org/10.1130/G32528.1 Article history received: 03 Jun 2011 rev-recd: 06 Aug 2011 accepted: 24 Aug 2011 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. Quigley, R. Van Dissen, N. Litchfield, P. Villamor, B. Duffy, D. Barrell, K. Furlong, T. Stahl, E. Bilderback, D. Noble; Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis. Geology 2012;; 40 (1): 55–58. doi: https://doi.org/10.1130/G32528.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The September 2010 Mw 7.1 Darfield (Canterbury) earthquake in New Zealand is one of the best-recorded earthquakes of this magnitude. The earthquake occurred on a previously unidentified fault system and generated a 29.5 ± 0.5-km-long surface rupture across a low-relief agricultural landscape. High-accuracy measurements of coseismic displacements were obtained at over 100 localities along the Greendale fault. Maximum net displacement (Dmax) (5.3 ± 0.5 m) and average net displacement (Davg) (2.5 ± 0.1 m) are anomalously large for an earthquake of this Mw. Dmax / surface rupture length (SRL) and Davg/SRL ratios are among the largest ever recorded for a continental strike-slip earthquake. “Geologically derived” estimates of moment magnitude (MwG) are less than the seismologically derived Mw, derived using widely employed SRL-Mw scaling regressions. MwG is greater than Mw using Dmax- and Davg-Mw regressions. The “geologically derived” static stress drop of 13.9 ± 3.7 MPa provides a context with which to compare this earthquake rupture to interplate and intraplate ruptures of similar Mw. This data set provides fundamental information on fault rupture processes relevant to seismic-hazard modeling in this region and analogous settings globally. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Relationship between observed liquefaction at Kaiapoi following the 2010 Darfield earthquake and former channels of the Waimakariri River

The Darfield earthquake caused widespread damage in the Canterbury region of New Zealand, with the majority of damage resulting from liquefaction and lateral spreading. One of the worst hit locations was the small town of Kaiapoi north of Christchurch, an area that has experienced liquefaction during past events and has been identified as highly susceptible to liquefaction. The low lying town sits on the banks of the Kaiapoi River, once a branch of the Waimakariri, a large braided river transporting gravelly sediment. The Waimakariri has been extensively modified both by natural and human processes, consequently many areas in and around the town were once former river channels.Using historical accounts and maps of the region, areas of land reclamation and old channels that had been cut off from the river since the beginning of European settlement in the 1850s were identified. These areas correlated well with many of the areas having significant liquefaction damage following the Darfield event. Substantial lateral spreads and sand boils developed in areas of reclamation along the current river path, causing significant damage to stopbanks and structures along the river, with fissures up to 2m deep and 1m wide. Much of the residential housing was damaged by lateral spreading, with cumulative displacement offsets up to 3m parallel to old channel beds that had aggraded over time due to river shifts. In former channel areas that were free of lateral spreading, large volumes of ejecta were present over wide areas, with depths of up to 400mm in places. Houses in these regions were damaged as a result of settlement and tilting. In all these areas underground services and roadways were severely impacted as a result of ground deformation. The severity of this damage indicates the importance of knowing the location of old channels when defining liquefaction prone regions.

Fault Location and Slip Distribution of the 22 February 2011 Mw 6.2 Christchurch, New Zealand, Earthquake from Geodetic Data

The 22 February (local time) MW ∼6.2 Christchurch earthquake occurred within the aftershock region of the 4 September 2010 MW 7.1 Darfield (Canterbury) earthquake (Gledhill et al. 2011). Both the Darfield and Christchurch earthquakes occurred on previously unknown faults in a region of historically low seismicity, but within the zone of plate boundary deformation between the Pacific and Australian plates. The Darfield earthquake caused surface rupture up to 5 m (Quigley et al. 2010, forthcoming), but none has been observed associated with the Christchurch earthquake. Geodetic data indicate that strain has been slowly accumulating within the region (Wallace et al. 2007; Beavan et al. 2002), and the presence of active subsurface faults was known or suspected ( e.g. , Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 in this region had been allowed for in the national seismic hazard model (Stirling et al. 2002), but the observed high apparent stresses (Fry and Gerstenberger 2011, page 833 of this issue) and high ground accelerations (Fry et al. 2011, page 846 of this issue) had not been anticipated, particularly those experienced in the Christchurch event. These and other factors (Fry and Gerstenberger 2011, page 833 of this issue; Fry et al. 2011, page 846 of this issue; Holden 2011, page 783 of this issue), plus the close proximity of the February earthquake to Christchurch city center, were responsible for the major damage caused by the earthquake ( e.g. , Kaiser et al. 2011). A large amount of geodetic ground-displacement data is available to constrain the source of the earthquake, in part because we reoccupied nearly 200 GPS sites that had been observed following the Darfield earthquake, and in part because a number of space agencies collected synthetic aperture radar (SAR) data over …

Kinematic Source Model of the 22 February 2011 Mw 6.2 Christchurch Earthquake Using Strong Motion Data

The Canterbury earthquake sequence began in September 2010 with the Mw 7.1 (source: GeoNet catalog, http://geonet.org.nz/canterbury-quakes/) Darfield earthquake that ruptured the previously unknown 40-km-long Greendale fault 30 km west of Christchurch (Gledhill et al. 2011). Extreme ground accelerations as high as 1.8 g near the epicenter were recorded. The event caused intense liquefaction in the eastern suburbs of Christchurch as well as closer to downtown, near the course of the Avon River. The Darfield earthquake was followed by a major aftershock on 22 February local time (21 February UTC) of magnitude Mw 6.2 (source: GeoNet), but Me 6.7 (source: USGS, http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001igm/neic\_b0001igm\_e.php). This earthquake was centred only a few kilometers south of the Christchurch city center. Extremely high accelerations (as high as 2.2 g) were also recorded near the epicenter (Kaiser, Benites et al. 2011). In addition to the extreme liquefaction seen after the Darfield earthquake, this event also caused landslides, large rockfalls, widespread damage to earthquake-risk buildings in Christchurch, and, most tragically, about 180 casualties. Another large aftershock of Mw 6.0 (source: GeoNet), but with Me 6.7 (source: USGS), subsequently occurred on 13 June local time (12 June UTC) just a few kilometers south of the February event, causing further damage, landslides, rockfalls, and liquefaction. Following the Darfield earthquake, the GeoNet network (New Zealand National Hazard Monitoring Network) and its regional component the CanNet network (Berrill et al. 2011) was supplemented by the deployment of 13 additional strong motion instruments regionally (and another nine following the February earthquake). We used this dense network of strong motion instruments to constrain the source kinematics of the February event. We present the inversion scheme and discuss its limitations. These results are preliminary, since more thorough data processing is needed; however, they already provide a key model that will help in understanding the sequence …

Ground Motions versus Geotechnical and Structural Damage in the February 2011 Christchurch Earthquake

The Mw = 6.3 earthquake of February 22 was the strongest seismic event in a series of damaging aftershocks in and around Christchurch after the Darfield earthquake on 4 September 2010. The source of the Darfield earthquake was in a sparsely populated area and thus it caused no loss of life. Serious damage was mainly due to extensive liquefaction. By contrast, the Christchurch earthquake was generated on a fault in close proximity to the city, resulting in a death toll of 181 people. The Canterbury Plains are covered with river gravels that hide any evidence of past fault activity in this region. The newly revealed Greendale fault was therefore completely unknown. Only a portion of it was revealed on the ground surface during the Darfield earthquake. The second fault (the one that ruptured in February 2011) appears to be a continuation of the first, although no fault structure directly connecting the faults has been recognized. There is a debate among seismologists at this point whether this is a different fault from Greendale one or not (NHRP 2011a; NHRP 2011b; Geonet 2011). Due to its magnitude, shallow depth and close proximity to the city, the February earthquake proved particularly destructive for the central business district (CBD) of Christchurch, where buildings suffered extensive damage. Thanks to a dense network of strong ground motion stations, a large number of records have been obtained, which provide valuable information on the event and offer the possibility of relating the extent of damage to actual measurements of ground shaking. Apart from the southern part of the city on the hills and the Lyttelton port area, Christchurch is built on deep estuarine soil, which has been shaped in the last thousands of years by the ever-changing riverbed. Fine sands—the dominant soil type—and the …

Fine-scale Relocation of Aftershocks of the 22 February Mw 6.2 Christchurch Earthquake using Double-difference Tomography

On 22 February 2011 New Zealand time (21 February UTC), the MW 6.2 Christchurch earthquake occurred just 7 km southeast of the center of Christchurch city, New Zealand (Fry et al. 2011, Holden 2011, page 783 of this issue). There were 181 confirmed fatalities, and the damage to Christchurch city is estimated to be NZ$15 billion–$NZ20 billion (US$12 billion–US$16 billion). The event was well-recorded by the broadband and strong-motion national-scale GeoNet network (Petersen et al. 2011) as well as by the Canterbury regional strong-motion network (Avery et al. 2004). Since the 22 February earthquake, more than 2,700 further aftershocks have been recorded up to 1 May 2011, including 21 events with local magnitude ( ML ) greater than 5. Here we describe the initial relocation analysis for these aftershocks. The Mw 6.2 Christchurch earthquake is part of the larger aftershock sequence of the Mw 7.1 Darfield earthquake, which occurred at 16:35 3 September UTC, 2010. Seismological, GPS, and InSAR data all suggest that the earthquake rupture process for the Mw 7.1 Darfield earthquake involved failure of multiple fault segments (Beavan et al. 2010; Gledhill et al. 2011). A surface rupture for that earthquake, now termed the Greendale fault, extending ∼29.5 km and located ∼4 km south of the epicenter, is consistent with strike-slip faulting with an average horizontal surface displacement of ∼2.5 m (Quigley et al. 2010). The vast majority of the 7,400+ aftershocks following the Darfield earthquake are shallow, at less than 15 km depth. Figure 1 shows that, although many of the aftershocks occurred near the surface trace of the Greendale fault, intense clusters of aftershock activity have also occurred at the western and eastern ends of the Darfield fault trace, as well as north-northwest from the Darfield epicenter. The distribution of …