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Request for content: please contact us for extra relevant info to be added to this page (Contact: Caroline Holden)

Mw 6.6 AUGUST 16th “LAKE GRASSMERE” EARTHQUAKE

General scientific info on the Mw 6.6 August 16th “Lake Grassmere” earthquake:

See GeoNet info here

Two maps comparing accelerations measured for the Cook Strait (left) and Lake Grassmere (right) earthquakes (Kaiser, GNS) - note that this is a preliminary map that will be updated as more data comes in

Downloading strong motion data:

STATIONS: See info provided below for the July 21st quake

STRONG MOTION DATA:

General info on GeoNet Strong-Motion data can be found here

Info on GeoNet accelerogram data files can be found here

Info on GeoNet response spectra data files can be found here

To convert GeoNet data into CSV format (Jim Cousins, GNS Science): csv_conversion_programs.zip

List of peak ground acceleration values:

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/08_Aug/2013-08-16_023105/20130816_023105.CSV

ACCELERATION data and plots (unfiltered):

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/08_Aug/2013-08-16_023105/Vol1/

ACCELERATION, VELOCITY, DISPLACEMENT data and plots (filtered 0.25 – 24.5 Hz)  :

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/08_Aug/2013-08-16_023105/Vol2/

RESPONSE SPECTRA data and plots( filtered 0.25 – 24.5 Hz) :

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/08_Aug/2013-08-16_023105/Vol3/

- Drs. Quincy Ma  and Liam Wotherpoon have produced an analysis of the response spectra in relations to the soil condition at each GeoNet station. This analysis is available here.

JULY 21st EARTHQUAKE

General scientific info on the Cook Strait earthquakes:

http://info.geonet.org.nz/display/home/Cook+Strait+Earthquakes

Selected info of interest to earthquake engineers:

Two maps comparing accelerations measured for the Cook Strait (left) and Christchurch (right) earthquakes (Kaiser, GNS)

The "ShakeMap" showing the shaking intensity as determined by GeoNet instruments and modelled against ground conditions (Ristau and Salichon, GNS Science). (for more info follow science info link provided above). This map is preliminary and people are advised to use actual recorded data if possible. The map is based on the attenuation relationship of B. S-J. Chiou and R.R. Youngs (2008). An NGA model for the average horizontal component of peak ground motion and response spectra (Earthquake Spectra, 24, 173-215). The Vs30 grid is 100 m resolution based on an assigned average velocity for 8 soil classes which was put together by Nick Perrin and Dave Heron based on geological maps of NZ (QMAP data, GNS Science). ShakeMap takes observed ground motions, and uses the attenuation relationship and Vs30 grid to interpolate predicted ground motions (PGA, PGV, MMI) onto a uniform grid.

DOWNLOADING STRONG MOTION DATA:

The following links lead to strong motion data, derivatives and plots of the Cook Strait earthquake.

STATIONS: Link to interactive map of stations (See figure below to tick correct boxes – “double-click” on a green square OR a red circle to get site-specific info)

http://magma.geonet.org.nz/resources/network/netmap.html

Screen-shot of http://magma.geonet.org.nz/resources/network/netmap.html

STRONG MOTION DATA:

General info on GeoNet Strong-Motion data can be found here

Info on GeoNet accelerogram data files can be found here

Info on GeoNet response spectra data files can be found here

To convert GeoNet data into CSV format (Jim Cousins, GNS Science): csv_conversion_programs.zip

List of peak ground acceleration values:

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/07_Jul/2013-07-21_050930/20130721_050930.CSV

ACCELERATION data and plots (unfiltered):

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/07_Jul/2013-07-21_050930/Vol1

ACCELERATION, VELOCITY, DISPLACEMENT data and plots (filtered 0.25 – 24.5 Hz)  :

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/07_Jul/2013-07-21_050930/Vol2

RESPONSE SPECTRA data and plots( filtered 0.25 – 24.5 Hz) :

ftp://ftp.geonet.org.nz/strong/processed/Proc/2013/07_Jul/2013-07-21_050930/Vol3

 

Other useful links:

- Instrumented buildings with seismic sensors:
http://info.geonet.org.nz/display/appdata/Structural+Array+Data

- From Dr Brendon Bradley’s research group at the University of Canterbury, a summary of the observed ground motions from the 21 July 2013 Mw6.6 earthquake in Cook Strait, off the coast of Seddon, can be found here.

Note that his info has been compiled from a wide range of observations but may be incomplete – please contact us for extra relevant information. The information presented has not been peer reviewed and readers should conduct their own validation.

the M 6.6 (Ml 6.5)  Cook Strait earthquake

The epicentre was located offshore, ~20km from the nearest settlement and ~50 km from the capital city of Wellington, where peak ground accelerations (PGAs) up to 0.2g were recorded.

In central Wellington spectral accelerations at a number of locations were recorded up to 30% of the current building design level for any given site class and periods ranging between 0.4 and 1 seconds.

Recorded response spectra were particularly extreme at the Victoria University Building site (VUWS) close to the foreshore and areas of reclaimed land, being over 50% the design level for periods of 0.4-0.5 seconds, as well as at the Karori Normal School site, further up inland and into the hills, where recorded levels were up to 100% of the design level at 0.4 seconds. It is worth noting that this peak record level is only apparent over a very narrow period range and that the overall spectral response at short periods is on average closer to 30% of the design level.

the M 6.6 Lake Grassmere earthquake

The epicentre was located onshore, 10 km from the nearest settlement where accelerations up to 0.5g were recorded.  The maximum recorded peak ground acceleration was 0.7g 6km from the epicentre. In Wellington, 80 km from the epicentre, PGAs of 0.2g were recorded.

In central Wellington a number of stations recorded spectral accelerations up to 20-30% of the current building design level for any given site class and periods ranging between 0.4 and 1.2 seconds.

Recorded spectra were again high for VUWS where levels reached 50% of the design level at 0.5 seconds, and at the Karori Normal School site where recorded levels were again found to be approximately 100% design level at 0.5 seconds. It is worth noting that this peak record level is only apparent over a very narrow period range and that the overall spectral response at short periods is on average closer to 30-50% of the design level.

Impacts on buildings in Wellington

  • Land in parts of the reclaimed port area typically received up to twice the ground shaking of other locations
  • Only some buildings, typically 6 to 12 storey, were significantly affected. This may have been  due to the nature of the shaking (0.4-1 seconds periods of peak spectral acceleration).
  • Other buildings including unreinforced masonry construction and houses were generally unaffected structurally.
  • Most damage related to the inability of parts to to accommodate seismic displacements:
    • Rigid glazing in older steel framed windows
    • Seismic flashings between buildings
    • Plasterboard panels in stairwells
    • Stairs and ramps
  • Damage to suspended ceilings and sprinkler pipework was common.
  • Most building structures are no worse than before the events.

See an overview of the geology reconnaissance trip here on GeoNet

Acknowledgements: we acknowledge the New Zealand GeoNet project and its sponsors EQC, GNS Science and LINZ, for providing data/images used in this page.