Terry Binding WEMO Photograph 20
Images, UC QuakeStudies
A photograph of a damaged fence near the Greendale fault line in Canterbury.
A photograph of a damaged fence near the Greendale fault line in Canterbury.
A photograph of an excavator on the side of a road in Canterbury. This stretch of road is near the Greendale fault line which caused the 4 September 2010 earthquake.
A photograph of a volunteer from the Wellington Emergency Management Office standing in a crack in a field near the Greendale fault line.
A photograph of a volunteer from the Wellington Emergency Management Office standing in a crack in a field near the Greendale fault line.
We present ground motion simulations of the Porters Pass (PP) fault in the Canterbury region of New Zealand; a major active source near Christchurch city. The active segment of the PP fault has an inferred length of 82 km and a mostly strike-slip sense of movement. The PP fault slip makes up approximately 10% of the total 37 mm/yr margin-parallel plate motion and also comprises a significant proportion of the total strain budget in regional tectonics. Given that the closest segment of the fault is less than 45 km from Christchurch city, the PP fault is crucial for accurate earthquake hazard assessment for this major population centre. We have employed the hybrid simulation methodology of Graves and Pitarka (2010, 2015), which combines low (f<1 Hz) and high (f>1 Hz) frequencies into a broadband spectrum. We have used validations from three moderate magnitude events (ππ€4.6 Sept 04, 2010; ππ€4.6 Nov 06, 2010; ππ€4.9 Apr 29, 2011) to build confidence for the ππ€ > 7 PP simulations. Thus far, our simulations include multiple rupture scenarios which test the impacts of hypocentre location and the finite-fault stochastic rupture representation of the source itself. In particular, we have identified the need to use location-specific 1D ππ /ππ models for the high frequency part of the simulations to better match observations.
During the 2011 M7.8 KaikΕura earthquake, ground motions recorded near the epicentre showed a significant spatial variation. The Te Mara farm (WTMC) station, the nearest to the epicentre, recorded 1g and 2.7g of horizontal and vertical peak ground accelerations (PGA), respectively. The nearby Waiu Gorge (WIGC) station recorded a horizontal PGA of 0.8g. Interestingly, however, the Culverden Airlie Farm (CULC) station that was very closely located to WIGC recorded a horizontal PGA of only 0.25g. This poster demonstrates how the local geological condition could have contributed to the spatially variable ground motions observed in the North Canterbury, based on the results of recently conducted geophysical investigations. The surficial geology of this area is dominated by alluvial gravel deposits with traces of silt. A borehole log showed that the thickness of the sediments at WTMC is over 76 metres. Interestingly, the shear wave velocity (Vs) profiles obtained from the three strong motion sites suggest unusually high shear wave velocity of the gravelly sediments. The velocity of sediments and the lack of clear peaks in the horizontal-to-vertical (H/V) spectral ratio at WTMC suggest that the large ground motion observed at this station was likely caused by the proximity of the station to the causative fault itself; the site effect was likely insignificant. Comparisons of H/V spectral ratios and Vs profiles suggest that the sediment thickness is much smaller at WIGC compared with CULC; the high PGA at WIGC was likely influenced by the high-frequency amplification caused by the response of shallow sediments.
On 4 September 2010, a 7.1 magnitude earthquake struck near Darfield, 40 kilometres west of Christchurch, New Zealand. The quake caused significant damage to land and buildings nearby, with damage extending to Christchurch city. On 22 February 2011, a 6.3 magnitude earthquake struck Christchurch, causing extensive and significant damage across the city and with the loss of 185 lives. Years on from these events, occasional large aftershocks continue to shake the region. Two main entomological collections were situated within close proximity to the 2010/11 Canterbury earthquakes. The Lincoln University Entomology Research Collection, which is housed on the 5th floor of a 7 storey building, was 27.5 km from the 2010 Darfield earthquake epicentre. The Canterbury Museum Entomology Collection, which is housed in the basement of a multi-storeyed heritage building, was 10 km from the 2011 Christchurch earthquake epicentre. We discuss the impacts of the earthquakes on these collections, the causes of the damage to the specimens and facilities, and subsequent efforts that were made to prevent further damage in the event of future seismic events. We also discuss the wider need for preparedness against the risks posed by natural disasters and other catastrophic events.