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Effect of the Kaikōura Earthquake on Velocity Changes In and Around the Ruptured Region: A Noise Cross-Correlation Approach

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posted on 08.12.2021, 17:38 by Kortink, Megan

Seismic velocity changes before and after large magnitude earthquakes carry information about damage present within the faults in the surrounding region. In this thesis, temporal velocity changes are measured before and after the 2016 Kaikōura earthquake using ambient noise interferometry between 2012 - 2018. This period contains the Mw 7.8 2016 Kaikoura earthquake as well as the 2013 Cook Strait earthquake sequence and a few deep large magnitude earthquakes in 2015 - 2016. Three primary objectives are identified: (1) investigate seismic velocity changes in the Kaikōura region and their connection to the 2016 Kaikōura earthquake to try and determine if there was a change before/after the earthquake, (2) determine how this change varied across the region, and (3) consider if ambient noise can lead to improved detection and understanding of geological hazard.   The primary approach used to measure velocity changes in the Kaikōura region involved cross correlating noise recorded by seismic stations across the region. Velocity changes are sought by averaging the best result from multiple onshore station pairs. A secondary approach was also used, in which specific station pairs were averaged to determine if there were more localised velocity changes over more specific regions. This was to determine if the velocity changes observed following the 2016 Kaikōura earthquake occurred over the entire ruptured region.   Following the 2016 Kaikōura earthquake a velocity decrease of 0.24±0.02% was observed on the average of the vertical-vertical components for eight stations. The remaining eight cross-component pairs showed a smaller seismic decrease with an average value of 0.22±0.05%. After the decrease following the Kaikōura earthquake, there is a steady velocity increase of 0.13±0.02% over a one-and-a-half-year period. This indicates that prior to the earthquake, seismic velocity was at a steady state until it was perturbed by the Kaikōura earthquake, and seismic velocities rapidly decreased over all stations. Across the region, stations with a longer interstation distance and further away from ruptured faults had a smaller decrease in velocity than station pairs with a smaller interstation distance that were closer to ruptured faults. We interpret the velocity decrease following the Kaikōura earthquake as a result of cracks opening during the earthquake. The velocity increase following the earthquake is indicative of the cracks slowly healing.   The Cook Strait earthquake sequence that occurred in 2013 did not cause any velocity changes at the stations used in this thesis. This has been interpreted to be because the changes were too small compared to the background noise or the stations were not recording during the time of the earthquake sequence. Two other decreases were also observed in the region following two deep earthquakes in April 2015 (Mw 6.2, depth = 52km) and February 2016 (Mw 5.7, depth = 48km). Both of these events resulted in a small seismic decrease of 0.1±0.02%. Although these earthquakes were close to seismic stations when they occurred, they were much deeper and had a smaller magnitude than the Kaikōura earthquake so did not cause a large velocity decrease. By understanding what causes velocity changes it is possible to have an improved understanding of the geological hazard in the region.

History

Copyright Date

01/01/2020

Date of Award

01/01/2020

Publisher

Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Geophysics

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level

Masters

Degree Name

Master of Science

Victoria University of Wellington Unit

Institute of Geophysics

ANZSRC Type Of Activity code

2 STRATEGIC BASIC RESEARCH

Victoria University of Wellington Item Type

Awarded Research Masters Thesis

Language

en_NZ

Victoria University of Wellington School

School of Geography, Environment and Earth Sciences

Advisors

Savage, Martha