12.10.2011 New Zealand Earthquakes

Categories

F_E and G_L Regions NZ

The globe is broken down into regions based on geographic and politcal boundaries. Each region is assigned a unique number.
The boundaries of these regions are defined at one-degree intervals.
The Flinn-Engdahl (F_E) seismic and geographical regionalization scheme was proposed in 1965, defined in 1974 and revised in 1995.
After the 1995 revision there are 754 F-E regions, subsequently numbered from 1 to 757 with three gaps (172, 299 and 550) at dissolved regions.
The regions are grouped into 50 larger seismic regions.
Under the F_E system New Zealand is known as Region 11
subdivided into;
158 - off W coast of North.
159 - North Island
160 - off E coast of North Is.
161 - off W coast of South Is.
162 - South Island
163 - Cook Strait
164 - off E coast of South Is.
165 - north of Macquarie Is.
166 - Auckland Islands
167 - Macquarie Island
168 - south of New Zealand

Due to the comprehensive data available from Geonet/GNS and the development of new computer programs since this Blog started, it has become necessary to re-define the New Zealand Regions further.
The 38 G_L sub- regions are designated based on thousands of earthquake epi-centers located over the last 3 years, actual physical geology and locally known regional divisions, divided primarily into half degree latitude sections.
They are not intended to replace the F_E numbers but further define F_E Regions 160, 159, 163, and 162.
Region Map (click on for interactive image)

Numbering these sub regions allows more precise calculations of energy released (or locked in) and other data processing.

Map Key

Earthquake Markers

On the daily maps each coloured dot orb or marker represents an earthquake, the colours indicate the size range.
The epicentre of the earthquake event is at the base of the orb.
Map Navigation
On the more recent maps clicking on an individual icon will open a small popup tag inside the map frame giving the detail information for that quake.
Once you have finished reading that tag you can simply click in the background map to close that tag and select another if you wish.
You can also move the map around inside the frame by clicking and holding down the left button of your mouse on the map itself, rather than having to use the up/down/left and right arrows.

You can see the earthquake maps in various versions, ie map, satellite, terrain and even Google Earth.

At the bottom of each map is text saying "see larger map". This will open a new page with the same map, but with the icons and magnitudes also as a list. The list is in a ascending time order. You can click on an icon on the map or a magnitude number on the list and a detail balloon (tag) will pop up on the map from the location of the event.
Google Earth Viewing
From here you can also click on the "View in Google Earth" button which will open the kml file in the real Google Earth rather than inside the Google Maps i-frame. This can be handy when you want to see multiple years events together.
You may have to zoom in on the map if you are looking for details of an icon that is in a cluster.

From May - Dec31st 2010 events marked with a gold star icon are quakes that have been reported felt by the public to the Geonet Responses page on the Geonet web site.

Because of the large number of Aftershocks from the 2010 Darfield quake the star icon is replaced in that area with a normal round colour coded icon with a F mark from Sept 3rd UTC. This icon is also adopted for all of New Zealand for 2011 and for updates and revisions.

Prior to May 2010 the largest quake of the day is marked with a tag showing the size in ML.
Dots that are marked with a yellow tag or have a larger set of rings or a larger watermark around them are earthquakes that have been reported to Geonet as felt by the public.
Details of Felt Responses can be viewed in the Comment box at the end of each post

Maps and Data are updated and revised as time allows.

Data

Data used in creating these maps is sourced from GEONET

From 01-08-2010 to 31-12-2011 clicking on this icon under each map will open a text document list, in another tab (or window in IE), with the text version of the data downloaded from Geonet.

Earthquakes that have been reported as Felt by the public to Geonet through the Internet based GeoNet Felt Earthquake Reports are underlined or marked with an F on the daily list.

Prior to May 2010 further information about Felt quakes can be viewed by clicking the blue Comments link at the bottom of each post. Some 2010 posts have Felt Maps.
The intensity of the reports is scaled according to the
Modified Mercalli scale.

Time

The days and hours, minutes and seconds posted in this blog are UTC time. This standard is "coordinated universal time", abbreviated UTC. This was formerly known as Greenwich mean time (GMT) Greenwich mean time was based upon the time at the zero degree meridian that crossed through Greenwich, England. GMT became a world time and date standard because it was used by Britain's Royal Navy and merchant fleet during the nineteenth century. Today, UTC uses precise atomic clocks, shortwave time signals, and satellites to ensure that UTC remains a reliable, accurate standard for scientific and navigational purposes. The New Zealand Time (NZST) is 12 hours ahead of UTC or 13 hours ahead during New Zealands Daylight Saving Time (NZDST) in the Summer.

NZ Seismic Background

Seismicity in New Zealand is dominated by the plate boundary. This boundary ( the thin blue line on the maps) runs down from the Tonga-Kermadec trench to the northeast of New Zealand, continues down the east coast of the North Island as the Hikurangi Trough, passes under the branch faulting systems of the Cook Strait-Marlborough Sounds region, and continues down the west coast of the South Island as the Alpine Fault. To the southwest of New Zealand it takes the form of the Puysegur Trench. Seismic activity is at its greatest to the northeast of the North Island. The Hikurangi Trough forms part of a westward-dipping subduction zone, the seismicity following the boundaries of the subducting plate down to depths of 300-400 km.

The seismicity also spreads out in a shallow belt (0-80 km) below the northeast-trending mountain ranges of the North Island and the Taupo Volcanic Zone behind them. (this is shown on the maps as the yellow "continental rift boundry", the red "transform fault" and the white " continental convergent boundry") The subduction zone continues below Cook Strait, but appears to come to an end, or at least the deeper earthquakes peter out, somewhere south of the Marlborough Sounds.

The northern part of the South Island, including the West Coast, has been among the most seismically active regions of New Zealand during this century; by contrast, the Alpine Fault itself, the major (transcurrent) fault accommodating plate motion in the South Island, has been seismically quiet. Seismic activity picks up rapidly in the Fiordland region and to the southwest of the South Island, and marks the beginning of a further, but eastward-dipping, subduction zone, with earthquakes recorded to depths of <200 km.

About 14,000 earthquakes, most of them minor, are recorded each year. About 200 of these are strong enough to be felt.

PGV and PGA

What is the Peak Ground Velocity (PGV)?

The PGV is measured in cm/s. For its calculation one uses the horizontal component of the ground movement, just like in the calculation for PGA. For medium and strong earthquakes the resulting velocity pattern mimics the geometry of the fault; the highest velocities occur close to the fault and in the direction of the spreading. The nature of the subsurface has an influence on the velocities as well, but this factor is much less important here than it is for the acceleration. Large-scale damage and damage to elastic structures are usually correlated with high velocities. The largest PGV ever measured was 183 cm/s. For small earthquakes (magnitude below 3) humans will mostly feel the acceleration (Wu et al., 2003).

What is the Peak Ground Acceleration (PGA)?

Earthquake waves cause a two-way movement of the ground in horizontal (parallel and perpendicular to the direction of wave propagation) and vertical direction. How big and how fast (acceleration and velocity) this deviation is depends on various factors: The length of the fault (magnitude), the distance between the station and the fault and the geology of the subsurface. The latter can have a large influence on the ground acceleration and wave form, which makes it very important to know about even slight changes in the subsurface below the station. It follows that the values of ground acceleration show a high variation even within small areas. This is especially true for medium to large-size earthquakes and interpolating the values is thus difficult. Generally the acceleration will decrease the further away we are from the fault. For the calculation of the PGA one considers only horizontal ground movements. The acceleration is given as a fraction or multiple of the Earth’s gravitational acceleration, g=9.81 ms−2. For small earthquakes (magnitude below 3) humans will mostly feel the acceleration (Wu et al., 2003).

October 2011
M	T	W	T	F	S	S
« Sep		Nov »
	1	2
3	4	5	6	7	8	9
10	11	12	13	14	15	16
17	18	19	20	21	22	23
24	25	26	27	28	29	30
31