7.5 Earthquake in NZ

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Let me tell you, a 7.5 magnitude earthquake is a BIG one. The scale is logarithmic - a 7.0 quake has 10x as much energy as a 6.0 one and (correction) [-]100X[/-] 1000x more than a 5.0 one.

For the technically interested from the USGS:

Tectonic SummaryThe November 13, 2016 M 7.8 earthquake in North Canterbury, New Zealand, occurred as the result of shallow oblique-reverse faulting on or near the boundary between the Pacific and Australia plates in South Island, New Zealand. At the location of this earthquake, the Pacific plate moves to the west-southwest with respect to the Australia plate at a rate of approximately 40 mm/yr. The epicenter of the earthquake is about 30-45 km south-southeast of the main surface expression of the plate boundary in the region—the Hope Fault, part of the Marlborough Fault system that connects a subduction zone (the Hikurangi Trough) to the primary plate boundary in the South Island—the Alpine Fault. The plate boundary in the region of the earthquake is complex, involving a transition from subduction along the Hikurangi Trough to the east of the North Island, to transform faulting through the South Island. The shallow crustal region to the southeast of the Hope fault primarily involves thin-skinned shortening and fold and thrust belt tectonics. The size, depth (~25 km) and faulting orientation of the November 13 event suggest a larger, subduction-related structure, though the subduction zone interface is not thought to extend this far to the south. The complexity of the event, involving a main energy release delayed by about 40 s, combined with an early aftershock distribution extending about 150 km to the north-northeast of the mainshock, suggests the potential for triggered slip on the Pacific:Australia subduction zone interface.

While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Reverse-faulting events of the size of the November 13, 2016 earthquake are typically about 120x50 km (length x width). Modeling of this event implies slip occurred over an area about 200x60 km in size. Over the 9 hours following the M 7.8 mainshock, 25 aftershocks have occurred, ranging in size from M 4.5 to M 6.5 and extending from the region of the mainshock epicenter to about 160 km to the northeast.

The Pacific-Australia plate boundary in northern South Island has a history of large earthquakes both along the plate boundary proper and distributed around the plate boundary internal to the Australia and Pacific plates. The November 13th M 7.8 earthquake is the largest event in the region since an M 7.3 earthquake 100 km to the northwest in June 1929. That June 1929 earthquake occurred just 3 months after the March 1929 Arthur’s Pass strike-slip earthquake, 90 km to the west-southwest of the November 13th event. The Arthur’s Pass event caused damage but injured no one. The November 13, 2016 event is also about 100 km to the north of Christchurch, which was severely damaged by a series of large earthquakes in 2010-2015, including a M 7.0 to the west of Christchurch in September 2010, and a M 6.1 directly beneath the city in February 2011.

Because of the complexity of this plate boundary region, strain is being accommodated on many different structures of varying orientations, making it possible that more than one fault may be activated in this earthquake sequence.

Here's what's been happening over the past day. The 7.8 centre is the 4th one from the bottom left. Anyone see the surface expression of the fault that the quakes are following (I reckon)?

NZ quake 2.jpg

And the Shake map:

Shakemap.jpg
 
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That is not correct vis-a-vis energy. A two-point increase means the energy is around 1000 times greater.
 
You are not quite there yet. The 10x part of the scale does not relate to energy.

i do not have the numbers at hand, but I think the energy increase is 32x for each point of magnitude.

I lost family members to a quake in 1998, which is perhaps why it sticks in my head.

Best of wishes to people affected in New Zealand.
 
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Another earthquake has just hit NZ.

NEW Zealand has been rocked by an earthquake with a preliminary magnitude of 6.1 to 6.3.


The quake hit the nation’s North Island on Tuesday morning, centred off the coast almost 140km from Palmerston North at a depth of 37km.
 
The Pacific rim is a busy place :) This is all earthquakes over the past day - and an aftershock in Japan in the past hour.

World quakes.jpg
 
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