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Report on July 15th New Zealand Earthquake
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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.
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