June 2018 Project Update
Kaikoura Earthquake Work
Mark, together with David Barrell (GNS Science), Jack Williams (Cardiff University), Kat Sauer (University of Otago), and Ella van den Berg (University of Otago) completed the excavation of a paleoseismic trench across the Hundalee Fault, one of the 20+ faults that ruptured in the M7.8 2016 earthquake. The trench has revealed a prehistoric earthquake event that produced >1m fault displacement, which was at least double the 2016 displacement at the site. Current efforts are focused on radiocarbon dating of organic samples taken from the faulted sediments to get some age constraints on the timing of the prehistoric earthquake on the fault. The support of the Natural Hazards Research Platform is acknowledged.
Seismic source modelling of low seismicity regions: Otago case study
Work is commencing on an EQC-funded project that will seek to tackle the issues of characterising damaging earthquake sources in low-seismicity regions. Our Otago case study (see image below for Otago location in the seismic hazardscape of New Zealand) combines a multidisciplinary team of seismologists (Mark Stirling, Matt Gerstenberger, Erin Todd and Jonathan Griffin), Geodesists (Paul Denys), Mathematicians (Ting Wang), and broad spectrum South Island geologists (David Barrell). Initial efforts are in the compilation of long records of prehistoric earthquakes and cumulative deformation of marker horizons (e.g. Otago peneplain surface), optimising seismicity catalogues to constrain active seismic sources from the distribution of micro-earthquakes, and exploring mathematical and statistical method for combining data sets and quantifying uncertainties. EQC is acknowledged for their support of the project.
Mark Stirling has continued his role as the lead scientist in a long-standing seismic hazard re-evaluation of the Clyde Dam site in Central Otago. The final report was completed in mid-2018, and stands as a milestone “first” in the use of precariously-balanced rocks to constrain seismic hazard estimates for long return periods. He is also undertaking collaborative work of this nature for the Diablo Canyon nuclear power plant in California, and providing advice on earthquake source characterisation for the Ikata nuclear power plant in Japan. Early in 2018 he also undertook studies of active faults in the vicinity of a proposed wind farm in Hawkes Bay (see image below). Contact Energy Ltd, Pacific Gas and Electric Company, Shikoku Electric Power Company, and Meridian Energy are acknowledged for their support.
Earthquakes, tremor, seamount subduction, and shallow slow slip at the Hikurangi Margin
Erin Todd has continued to investigate the occurrence of tectonic tremor and microseismicity associated with shallow slow slip offshore Gisborne along the Hikurangi Margin. Prior to this work, it had been established that Gisborne slow slip events are associated with tectonic tremor downdip of the slow slip patch (Kim et al., 2011, Todd and Schwartz, 2016) and increases in local microseismicity (Delahaye et al., 2009). However, the shallow subduction interface is often poorly resolved due to its distance from land where near source experiments can be conducted. In addition to land-based seismic datasets, Erin has used offshore seismic data from the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) Experiment, which deployed 24 absolute pressure gauges and 15 ocean bottom seismometers to the seafloor above the Gisborne slow slip patch to conduct a detailed investigation into the spatial and temporal extent of slow slip and associated tremor and earthquake activity. This work is done in collaboration with researchers from GNS Science, Victoria University of Wellington, and international researchers from the United States and Japan. The image below of continuous GPS data for 5 stations along the northern Hikurangi shows the frequency and duration of slow slip events (identified by the coloured boxes) along the margin. Most of these slow slip events are accompanied by tectonic tremor and increases in microseismicity. Figure from Todd and Schwartz, 2016.
The close spatial relationship between slow slip, tremor, microseismicity, and subducting seamounts indicates that subducting seamounts may be an important factor in determining the distribution and style of the various styles of interplate slip observed along the northern Hikurangi. Erin is currently collaborating with researchers from GNS Science and NIWA to further investigate the role of seamount subduction in interplate fault slip and impacts to the stress state of the plate interface. The image below shows seamount “knolls” on the incoming plate, which when eventually subducted may greatly impact the style and distribution of interplate slip along the northern Hikurangi Margin.
Delahaye, E. J., Townend, J., Reyners, M. E., & Rogers, G. (2009). Microseismicity but no tremor accompanying slow slip in the Hikurangi subduction zone, New Zealand. Earth and Planetary Science Letters, 277(1–2), 21–28. https://doi.org/10.1016/j.epsl.2008.09.038
Kim, M. J., Schwartz, S. Y., & Bannister, S. (2011). Non-volcanic tremor associated with the March 2010 Gisborne slow slip event at the Hikurangi subduction margin, New Zealand. Geophysical Research Letters, 38(14). http://dx.doi.org/10.1029/2011GL048400
Todd, E. K., & Schwartz, S. Y. (2016). Tectonic tremor along the northern Hikurangi Margin, New Zealand, between 2010 and 2015. Journal of Geophysical Research: Solid Earth, 2016JB013480. https://doi.org/10.1002/2016JB013480
Postgraduate Research — June 2018
PhD funded by QuakeCoRE to undertake 3d shear wave velocity modelling for ground motion simulations in Dunedin and Mosgiel. The images below show the location of the Akatore Fault relative to Dunedin, and a synthetic seismogram produced in downtown Dunedin from a hypothetical Akatore Fault earthquake. The Akatore Fault is presently the most active fault in Otago (multiple earthquakes in the Holocene i.e. last 10,000 years, with two in the last c.1300 years) and the source of greatest seismic hazard for Dunedin. QuakeCoRE is acknowledged for their support of Anna’s PhD through their PhD scholarship programme.
MSc funded by University of Otago and QuakeCoRE to undertake groundwork for development of a 3D shear wave velocity model for Dunedin from multiple datasets. Most significantly, Catherine has led the acquisition, processing and interpretation of three seismic reflection profiles. The three profiles are shown below, with locations shown in the map (Profile A is the pink profile, B is the yellow one, and C is the blue one). In addition to providing valuable input to the 3d model development, Profile A has also been used to eliminate the possibility that the Akatore Fault is present within 10km of Dunedin city centre. QuakeCoRE is acknowledged for their partial support of Catherine’s work.
Ella van den Berg
MSc partially funded by Contact Energy Ltd to map the northern end of the Cardrona Fault in central Otago using field-based methods and interpretation of LIDAR (see image below). A significant part of Ellas work is also to determine the paleoseismic record of the fault. The study is seen as very important in the context of the rapidly growing Wanaka township nearby, and the ongoing safety of the Hawea Dam. Contact Energy Ltd is acknowledged for their support of Ella’s fieldwork costs.
PhD funded by Geoscience Australia (GA) to develop new methods of characterising earthquake recurrence in low seismicity regions like Otago and Australia (see Seismic source modelling of low seismicity regions: Otago case study above). The faults of the two regions tend to show strong aperiodicity in earthquake recurrence, where faults switch on and off, such as in the Akatore Fault example (See Anna Kowal’s section, above). Such behaviour may be characteristic of low seismicity regions in general, and be suggestive of fundamental controls on active tectonics at the regional scale. Jonathan commenced his studies in July 2018, and comes from a significant position in GA as interim leader of the Australian national seismic hazard modelling programme. GA is acknowledged for their support of Jonathan’s PhD scholarship.
Onshore and offshore fieldwork:
A great team of earthquake scientists conducted a 2km long seismic survey of the Dunedin city railway corridor. The purpose of the survey was to provide data to contribute to development of a 3D shear wave velocity model for the city (fieldwork funded by the QuakeCoRE project “A shear wave velocity model for Dunedin” and the NHRP project “Hidden faults under cities”).
Offshore, Erin Todd took part in the most recent Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) scientific cruise where five ocean-bottom absolute pressure gauges that recorded vertical deformation from the 2016 Te Araroa and Kaikoura earthquakes were recovered. Nine new absolute pressure gauges were deployed with an expected recovery in mid-2018.
Natural Hazards Research Platform and EQC Proposals
A great team effort went into two funding proposals. The proposal “A multidisciplinary seismotectonic source model for southern New Zealand” went into the Natural Hazard Research Platform contestable funding round, with Ting Wang, Matt Gerstenberger, Erin Todd, Paul Denys, Chris Pearson, Steven Smith, David Barrell and Mark Stirling as the proposal team.
Also Andrew Gorman, David Prior, Hamish Bowman, Jennifer Eccles (University of Auckland) and Mark Stirling prepared the proposal “Shear wave velocity model and ground motion simulations for Dunedin city” The proposal has got through the first round of EQC review, so we are now preparing the full proposal.