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Report on July 15th New Zealand Earthquake
Launched:
2009.07.19
Updated:
2009.07.21



An earthquake occurred at 8:22pm (local time) on July 15, 2009, off the west coast of the South Island in New Zealand (45.750S, 166.577E.).

Due to the large magnitude and the shallow source location, NOAA Pacific Tsunami Warning Center announced a tsunami warning, but was cancelled after about one hour.

The Following is a report on this earthquake. (Outreach Office)


Souce Process Inversion


Click the figure for the details.

by Natalia Poiata (DC 2)



Focal Mechanism by W-phase seismograms


Click the figure for the details.

by Yusuke Yokota (MC 2)




Tsunami Records by Seafloor Pressure Meter
The pressure meter placed NW of the hypocenter (between Australia and New Zealand) recorded the tsunami from this earthquake.

DART Tasman Sea 1

DART, Deep-ocean Assessment and Reporting of Tsunami, system is managed by NOAA to ensure early detection of tsunamis.


Observed Tsunamis

Observed Tsunamis reported by ITIC, International Tsunami Information Center.

Place Lat Lon Arrival Time(UT) Amplitude Period
Spring Bay (Tasmania) 42.5 S 147.9 E 11:45 6 cm 10 min
Port Kembla (Australia) 34.5 S 150.9 E 12:00 14 cm 12 min
Jackson Bay (Newzea Land) 44.0 S 169.6 E 9:56 50 cm 22 min
DART 5501 (Water Pressure Meter) 46.9 S 160.6 E 9:50 5 cm 8 min


Tectonic Background
New Zealand straddles the boundary of the Australian and Pacific Plates. Around the South Island, the Australian Plate is moving 35-45mm/yr in the direction of NE relative to the Pacific Plate.

The relative motion of the two plates is complex in this region. At the Puysegur Trench, SW off the South Island, the Australian Plate subducts beneath the Pacific Plate, whereas the relative motion is accomodated via oblique strike-slip movement at the Alpine Fault located at the west coast of the South Island. (Refer to the image below.)

The earthquake occurred at the Fiordland region where the strike-slip motion of the Alpine Fault is undergoing transition to a subdction at the Puysegur Trench. The initial report of the source mechanism shows that the subduction thrust movement is dominant to the strike-slip movement.

The seismic activity around the Fiordland is high. A M7.0 earthquake in August 2003 caused a landslide and minor damage. Some other earthquakes hit the region recently; a M7.0 in August 1993, a M6.4 in May 1989, and a M6.7 in 1988.




Left: Earthquake activities in NZ (compiled from the PDE catalogue of 1976- July 12, 2009. The cross line of AB-CD is the location of the earthquake on the 15th.)
Right:Tectonics in the vicinity of the South Island, NZ (PDF file from USGS website
(click to enlarge)


Enlargement of the vicinity of the source. Click to enalrge.


Local Seismic Intensity
The standard of seismic intensity is not unified worldwide, but many countries adopt the Modified Mercalli Intensity, which runs from degrees 1-12, whereas the Japanese Seicmic Intensity scale (Japan Meteorologist Agency Seismic Intensity Scale) has 9 degrees.

GeoNet of NZ adopted the the MM seismology for the recent earthquake.

GeoNet shaking map for Jul 15 Earthquake

Maximum of MM7 was reported.Note that this is not the observed intensity.


Links

   

Translated by Rie Tada. (Outreach Office)
Copyright 2009 Earthquake Research Institute, the Univ. of Tokyo All rights reserved