Damage distribution maps from strong earthquakes and recorded data from field experiments have repeatedly shown that the ground surface topography and subsurface stratigraphy play a decisive role in shaping the ground motion characteristics at a site. Published theoretical studies qualitatively agree with observations from past seismic events and experiments; quantitatively, however, they systematically underestimate the absolute level of topographic amplification up to an order of magnitude or more in some cases. We have hypothesized in previous work that this discrepancy stems from idealizations of the geometry, material properties, and incident motion characteristics that most theoretical studies make. In this study, we perform numerical simulations of seismic wave propagation in heterogeneous media with arbitrary ground surface geometry, and compare results with high quality field recordings from a site with strong surface topography. Our goal is to explore whether high-fidelity simulations and realistic numerical models can – contrary to theoretical models – capture quantitatively the frequency and amplitude characteristics of topographic effects. For validation, we use field data from a linear array of nine portable seismometers that we deployed on Mount Pleasant and Heathcote Valley, Christchurch, New Zealand, and we compute empirical standard spectral ratios (SSR) and single-station horizontal-to-vertical spectral ratios (HVSR). The instruments recorded ambient vibrations and remote earthquakes for a period of two months (March-April 2017). We next perform two-dimensional wave propagation simulations using the explicit finite difference code FLAC. We construct our numerical model using a high-resolution (8m) Digital Elevation Map (DEM) available for the site, an estimated subsurface stratigraphy consistent with the geomorphology of the site, and soil properties estimated from in-situ and non-destructive tests. We subject the model to in-plane and out-of-plane incident motions that span a broadband frequency range (0.1-20Hz). Numerical and empirical spectral ratios from our blind prediction are found in very good quantitative agreement for stations on the slope of Mount Pleasant and on the surface of Heathcote Valley, across a wide range of frequencies that reveal the role of topography, soil amplification and basin edge focusing on the distribution of ground surface motion.
Surface rupture and slip from the Mw 7.8 2016 Kaikōura Earthquake have been mapped in the region between the Leader and Charwell rivers using field mapping and LiDAR data. The eastern Humps, north Leader and Conway-Charwell faults ruptured the ground surface in the study area. The E-NE striking ‘The Humps’ Fault runs along the base of the Mt Stewart range front, appears to dip steeply NW and intersects the NNW-NNE Leader Fault which itself terminates northwards at the NE striking Conway-Charwell Fault. The eastern Humps Fault is up to the NW and accommodates oblique slip with reverse and right lateral displacement. Net slip on ‘The Humps’ Fault is ≤4 m and produced ≤4 m uplift of the Mt Stewart range during the earthquake. The Leader Fault strikes NNW-NNE with dips ranging from ~10° west to 80° east and accommodated ≤4 m net slip comprising left-lateral and up-to-the-west vertical displacement. Like the Humps west of the study area, surface-rupture of the Leader Fault occurred on multiple strands. The complexity of rupture on the Leader Fault is in part due to the occurrence of bedding-parallel slip within the Cretaceous-Cenozoic sequence. Although the Mt Stewart range front is bounded by ‘The Humps’ Fault, in the study area neither this fault nor the Leader Fault were known to have been active before the earthquake. Fieldwork and trenching investigations are ongoing to characterise the geometry, kinematics and paleoseismic history of the mapped active faults.
A review of the week's news including... Mt Albert voters head to the polls to select a new MP this weekend, we hear from three candidates contesting the by election, Maori political leaders respond explosively to the Labour leader's comments that the Maori party is "not kaupapa Maori", how did a senior council roading manager receive over 1.1 million dollars in payments from a council contractor without his bosses knowing? the High Court rules Kim Dotcom is eligible for extradition US, the Fire Service's principal rural fire officer says lives may have been lost had firefighters not been told to return to their station, Sky TV says a decision to deny its billion dollar proposal to buy Vodafone is bizarre and disappointing, Pharmac considers funding women's sanitary products, we have coverage from the 6th anniversary of the Christchurch earthquake, the Prime Minister puts the board of the NZ Super Fund on notice after it approved a 23 percent pay increase for its Chief Executive, an Australian woman helping New Zealand dairy farmers across the Tasman who have fallen onto hard times is horrified by a lack of Government help and helicopters to Hercules and F-16s to classic spitfire replicas are on display at Ohakea airforce base during this weekend's Air Tattoo.
SARAH DOWIE to the Minister of Finance: What reports has he received on New Zealand’s trade exports?
EUGENIE SAGE to the Minister for Land Information: Has he asked Land Information New Zealand to withdraw the 997-hectare Riversdale Flats from the proposed sale of Mt White Station pastoral lease; if not, why not?
PHIL TWYFORD to the Minister for Social Housing: Will she confirm that as of 30 June the Government has only delivered 323 of the additional 1,400 emergency beds they promised at the start of November last year?
STUART SMITH to the Minister of Transport: What update can he provide on the reinstatement of State Highway 1 following the Kaikōura earthquakes?
STEFFAN BROWNING to the Minister for the Environment: Does he have confidence in the Environmental Protection Authority’s review of glyphosate?
Dr DAVID CLARK to the Minister of Health: Does he support the establishment of a cross-agency working group with Canterbury District Health Board on their finances, funding, and facilities?
DAVID SEYMOUR to the Minister of Health: Does he stand by all his answers to Oral Question No. 6 on 6 June regarding intraoperative radiotherapy for breast cancer?
BARBARA KURIGER to the Minister of Immigration: What recent announcements has he made in relation to immigration settings?
Hon NANAIA MAHUTA to the Minister for Treaty of Waitangi Negotiations: Does he believe that the signing of the Pare Hauraki Collective Settlement with the inclusion of redress disputed by Tauranga Moana iwi is a breach of Te Tiriti o Waitangi?
CHRIS BISHOP to the Associate Minister of Education: What recent announcements have the Government made on school property in the Wellington region?
PITA PARAONE to the Minister for Māori Development: Does he stand by all his statements; if so, why?
JENNY SALESA to the Minister of Education: Is she satisfied that the Government is doing all that it can to ensure an adequate supply of teachers, particularly in Auckland?
