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We present a new probabilistic seismic hazard model for the Canterbury region, the model superseding the earlier model of Stirling et al. (1999, 2001). The updated model incorporates new onshore and offshore fault data, new seismicity data, new methods for the earthquake source parameterisation of both datasets, and new methods for estimation of the expected levels of Modified Mercalli Intensity (MMI) across the region. While the overall regional pattern of estimated hazard has not changed since the earlier seismic hazard model, there have been slight reductions in hazard in some areas (western Canterbury Plains and eastern Southern Alps), coupled with significant increases in hazard in one area (immediately northeast of Kaikoura). The changes to estimated acceleration for the new versus older model serve to show the extent that major changes to a multidisciplinary source model may impact the final estimates of hazard, while the new MMI estimates show the added impact of a new methodology for calculating MMI hazard.
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Recent studies of the effects and Modified Mercalli (MM) intensities of New Zealand earthquakes have established criteria that will improve the reliability of intensities assigned using a number of effects, particularly the incidence of chimney damage and a wide range of environmental phenomena. The proportions of brittle chimneys which were damaged at intensities MM5–MM10 have been counted from the very detailed database of the 1968 Mw 7.2 Inangahua earthquake, and are shown to relate well to the proportions of chimneys which fell in 10 other earthquakes. Criteria based on environmental effects at intensities MM5-MM10 have been extended based on detailed studies of 22 earthquakes. These criteria have been adopted in an international intensity scale for environmental effects.
It was also found that the stopping of clocks should be a criterion for MM3, not MM5, and similarly the disturbance of liquids should be used at the threshold intensity of MM3 rather than MM4, as in the present MM intensity scale.
With the probable saturation of intensity at MM10, the criteria for MM12 have been omitted.
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The Mw 8.0 Pisco earthquake struck at 6.40pm local time with an epicentre offshore about 150 km south of Lima. At least 519 people were killed, and over 1,300 injured. Over 38,000 homes were destroyed and more than 100,000 were made homeless. 14 hospitals were destroyed and many other facilities damaged. The city of Pisco was worst affected with serious damage to the majority of adobe buildings. Other cities and towns nearby suffered similar damage to a lesser extent, depending on the distance from the epicentre. The capital Lima was not seriously affected, although there was some minor damage to buildings.
Strong ground motions were felt for over two minutes. In this subduction earthquake a tsunami was generated and affected tens of kilometres of coast.
The New Zealand Society for Earthquake Engineering Society (NZSEE) sent a 6-person reconnaissance team to Peru. The team spent three days in Lima meeting with key authorities and four days in the field observing some of the earthquake-affected area. This report describes the team’s observations and comments on the implications for earthquake engineering practice.
Highlights of the event in the eyes of the team were:
The long duration of the event – over 2 minutes of strong shaking
The unique geotechnical context – no rainfall and sandy soils
Significant liquefaction damage to roads and buildings
Poor performance of adobe construction
Generally good performance of reinforced concrete brick infill – but there were major collapses.
Good performance of some unreinforced masonry buildings
Widespread use of shear walls in major buildings in Lima
Engineered structures generally performed well
Damage to parts of Pan American Highway due to liquefaction
Minimal damage to a major steel mill, designed to international standards
Collapse and/or overload of telecom systems for up to four hours following the event, isolating Pisco and Ica
Water and waste water systems and storage were seriously affected in Pisco, and significantly in Ica
Port St Martin, serving Pisco, was seriously damaged but functional
Coordination of overseas / international aid needs careful consideration as part of response planning.
Management of response resources is critical.
There were significant tsunami effects which were variable in height up to 10 metres.
Relatively minor damage to architectural finishes and building services can render hospitals non-functional.
Survival of industrial facilities was important in reducing social impact by saving jobs.
The best of Peruvian earthquake engineering is international standard.
The development of earthquake-resistant standards in schools over the last three decades has paid dividends with modern designs performing well.
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Gisborne city experienced recorded peak ground accelerations exceeding 0.25g for the third time since 1966 in the magnitude Mw 6.6 earthquake at 075516 UT (8:55 pm local time) on 20 December 2007. The earthquake was at a hypocentral distance of 64 km from Gisborne at a depth of 40 km, well within the mantle of the subducted slab of the Pacific plate as it dips beneath the North Island of New Zealand. At this location the plate interface is about 10-15 km deep. The main event was followed by sparse aftershocks consistent with a rupture of the subducted plate, with the largest aftershock of magnitude 4.6 occurring on December 22nd. The GeoNet website received 3,257 felt reports, with a strongest intensity of MM8 (heavily damaging) assigned to the main shock.
The 122 strong motion records of this event show a clear regional directional variation in the wave propagation, as well as a distinct 2 Hz peak widely observed throughout the country. At a local scale, three sites in the Gisborne region recorded accelerations around 0.2g. Recordings in Gisborne city also revealed a predominant displacement direction, parallel to the main street where most of the damage occurred.
Source studies from moment tensor solution, aftershock relocations, GPS and strong motion data showed that the earthquake occurred within the subducted plate on a 45 degree eastward dipping fault plane. The mainshock rupture area is about 10 km2 reaching a maximum slip of 2.6 m. The computed high stress drop value of 17 MPa is as expected for an intraslab event and consistent with observations of very energetic seismic waves as well as heavy structural damage.
GPS data recorded by continuous GPS instruments have also shown that slow slip occurred for about three weeks after the main shock. The slow slip was triggered on the subduction interface, rather than on the same fault plane as the aftershocks. This is the first clear-cut case worldwide of triggered slow slip, although three non-triggered slow-slip events have occurred in the same region since 2002.
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Two new buildings at Wanganui Hospital, a peri-operative facility and an acute services block, have just been completed in June 2008 using RoGliders to provide seismic isolation in this earthquake prone region. This is the first application of the RoGlider, an isolator suitable for light structures providing in one compact unit the functions of support, damping and the required restoring force, while providing for a maximum displacement of ± 400 mm. The RoGlider provides for economical seismic protection under conditions not suitable for lead rubber bearings i.e. a combination of light vertical loads and large horizontal displacements.
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When evaluating the seismic performance of pile foundations in liquefiable soils, it is critically important to estimate the effects of cyclic ground displacements on the pile response. Advanced analyses based on the effective stress principle account for these effects in great detail by simulating the process of pore pressure build-up and associated stress-strain behaviour of soils. For this reason, the effective stress method has been established as a principal tool for the analysis and assessment of seismic performance of important engineering structures.
In this paper, effective stress analysis is applied to a case study of a bridge pier founded on piles in liquefiable soil. The study examines the likely effects of liquefaction, cyclic ground displacements and soil-structure interaction on the bridge foundation during a strong earthquake. A fully coupled effective stress method of analysis is used to compute the dynamic response of the soil-pile-bridge system. In the analysis, an elastoplastic deformation law based on a state concept interpretation is used for modelling nonlinear behaviour of sand. The seismic performance of the pile foundation is discussed using computed time histories and maximum values of ground and pile displacements, excess pore water pressure and pile bending moments. The advantages of the effective stress analysis are discussed through comparisons with a more conventional pseudo-static analysis of piles.
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In this paper, pseudo-static analysis of piles in liquefying soils is applied to a case study of a bridge foundation. The response of piles is separately evaluated for the cyclic phase during the intense shaking and development of liquefaction, and for the subsequent lateral spreading phase. With regard to the considerable uncertainties involved in predicting liquefaction and lateral spreading phenomena the effects of key parameters influencing the pile response are examined through parametric analyses. Particular attention is given to the variation in stiffness and residual strength of the liquefied soil, the magnitude of lateral ground displacement and the magnitude of inertial loads from the superstructure. The results shed light on the relative importance of key parameters for different combination of loads and ground conditions, and allow comparative evaluation between loads on the pile exerted by the crust layer and the liquefied layer.
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The paper presents the authors’ observations on the performance of buildings during the 8th October 2005, Kashmir earthquake in parts of Pakistan-administered Kashmir, and the North Western Frontier Province of Pakistan. A majority of the buildings in the earthquake region were non-engineered, owner-built, loadbearing masonry or reinforced concrete framed structures. Most of the masonry buildings were built with random or semi-dressed stone-walls without any reinforcement. The reinforced concrete frame buildings were deficient in strength, lacked ductile detailing and were poorly constructed. A large number of such buildings collapsed, leading to widespread destruction and loss of life. The building damage was the main cause behind the human and property loss. The collapse of floor and roof structures, the brittle behaviour of concrete buildings, a lack of integrity in masonry structures, and a lack of incorporation of seismically resistant features in building structures are found to be main reasons for the catastrophe.
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Viti Levu, the main island of Fiji, is located in a seismically active area within the Fiji Platform, a remnant island arc that lies in a diffuse plate boundary zone between the Pacific and Australian tectonic plates in the SW Pacific. The upper crust of Viti Levu is dissected by numerous intersecting fault/lineament zones mapped from remote sensing imagery of the land surface (topography, radar and aerial photos) and basement (magnetic) and have been subject to rigorous statistical tests of reproducibility and verification with field mapped fault data. Lineaments on the various imagery correlate with faults mapped in the field, and show spatial continuity between and beyond mapped faults, thereby providing a fuller coverage of regional structural patterns than previously known. Some fault/lineaments zones extend beyond the coastline to the offshore area from the SE Viti Levu study area. Multibeam bathymetry and seismic reflection data show the fault zones occur along and exert control on the location of a number of submarine canyons on the SE slope of Viti Levu. Evidence for Late Quaternary fault activity is only rarely observed in onshore SE Viti Levu (e.g. by displaced shoreline features), and in seismic reflection profiles from offshore.
The principal fault sets in Viti Levu represent generations of regional tectonic faulting that pervade the Fiji Platform during and after the disruption of the proto Fijian arc in the Middle to Late Miocene (~15Ma). These fault sets combine to form a complex network of interlocking faults creating a fault mesh that divides the upper crust into a number of fault blocks ranging from ~2-30 km wide. It is inferred that the fault mesh evolved throughout the Neogene as a response to the anticlockwise rotation of the Fiji Platform through progressive development of different fault sets and intervening crustal block rotations. Regional tectonic deformation is presently accommodated in a distributed manner through the entire fault mesh. Low magnitude earthquakes (<M4) occur regularly and may represent ruptures along short linking segments of the fault mesh, while infrequent larger earthquakes (>M4) may result from complex rupture propagation through several linking fault segments of the mesh that lie close to optimum stress orientations. The interpreted model of distributed deformation through the fault mesh for the study area in SE Viti Levu is inferred to be characteristic of the style of active deformation that occurs throughout the entire Fiji Platform.
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The undrained shear strength of partially saturated sand is examined based on a series of undrained triaxial compression and extension tests on Toyoura sand. The effects of partial saturation, density, and triaxial compression/extension modes are discussed in detail. The conditions of “flow” and “no flow” are categorised in terms of contractive and dilative behaviours. The threshold values of the undrained shear strength ratio dividing “flow” and “no flow” are found to be independent of the B-value and triaxial compression/extension modes. The empirical relations of the B-value against the initial state ratio rc and the undrained shear strength ratio Sus/p’c are discussed in detail.
