Earthquakes in New Zealand--why?

Location of today's earthquake in New Zealand
from this site. Note that the earthquake was on the South Island,
whereas Wellington is on the southwestern tip of the North Island.

New Zealand has again been shaken by a series of earthquakes. The largest, at magnitude 6.5-6.6 struck Friday, and was followed by several smaller aftershocks with magnitude about 5.  Although these are "moderate" sized-earthquakes, they are capable of causing widespread damage if near population centers. Fortunately, these were not and, also fortunately, New Zealand has strict building codes. Even so, chimneys collapsed, roofs caved in, and a bridge collapsed on a major state highway. The earthquake was 94 km west of Wellington, the capital, at 10 km depth was fairly shallow. Bill Fry, a seismologist with GNS in Wellington said that the quake was similar to a 6.4 tremor that struck in the same area on July 21, and appears to be a continuation of a sequence that started with some of magnitude in the high 7's.

Tectonic setting of New Zealand
from Wiki here   

     The most powerful earthquake in New Zealand's recent recorded history, a magnitude 8.2, struck Wellington in 1855, although there are stories of earthquakes in the Maori legends. Wellington sits on the coast and was thrust upward so far by the quake that the shoreline receded 200 meters. Here is a great site with details of many of the major earthquakes.
     New Zealand has a wide variety of active geologic phenomena--earthquakes, volcanic eruptions, geothermal areas, and landslides--because it sits at the boundary of the Australian and Pacific Plates. In the north (see graphic) the Pacific Plate is subducted under the Australian Plate, but in the south, the reverse happens: the Australian Plate is subducted under the Pacific Plate. These two subduction zones are connected by the Alpine Fault that runs along much of the west coast of the south island of New Zealand. Subduction rates are high--tens of millimeters per year, but so are erosion rates. The mountains rise about 10 mm/year, but are eroded down at about the same rate. The combination produces some of the most beautiful mountains in the world on the south island. The Alpine Fault is considered to be at high risk of producing a major earthquake in the next 40 years (see GNS).
    Recent activity has been in the vicinity of the Marlborough fault system and, in particular, the M7.1 Canterbury (2010) and Christchurch (2011) earthquakes were on relatively minor faults. The fault system was named after the 1848 M7.5 earthquake centered in the Marlborough district of the South Island, a quake that produced substantial damage in the Wellington area as well. The European population of Wellington was approximately 4500 at the time; only 3 people died. Because stone and brick buildings suffered much more severe damage than wooden ones, for a time many buildings in the area were constructed of wood. But, after only 25-30 years, the institutional memory was lost and stone and brick buildings returned, partially encouraged by concerns about fire.  At the beginning of the 20th century the country seemed calm, and the New Zealand Official Yearbook included the comment: "earthquakes in New Zealand are rather a matter of scientific interest than a subject for alarm." Quote from this source. The population of Wellington in 2012 was 385,600.

Coal Mine Explosion in New Zealand

The entrance to the Pike River Coal mine.  Photo/NZPA
published by the nzherald.co.nz, Nov. 19, 2010

On November 19, 1 man was killed and 27 miners are missing due to an explosion in the Pike River coal mine on the south island of New Zealand.  The mine has been under development since the 1970's, and is excavating from the largest hard-coking coal deposit in New Zealand. It is in an environmentally sensitive area, and has been lauded as an exemplary development in a sensitive area.  The tunnel, seen at the left, goes nearly 2.5 km nearly horizontally into the mountain.  The trapped miners may only be a few hundred feet underground.

From this Wiki site

Dangers in coal mines arise from at least three factors: the presence of both methane and carbon dioxide gases, and coal dust.  In this particular area of New Zealand, active faults produce a seismic risk as well.  Other causes of accidents are rock bursts, collapse of natural or artificial pillars, flooding, malfunctioning of equipment, and improper use of explosives in the mining process.  Methane gas is nasty stuff: an asphyxiant, flammable and potentially explosive.

The combustion of methane, CH4, is exothermic.  When combined with two O2 molecules, it decomposes to form CO2 (gas) and 2 H2O (liquid) molecules, releasing 891 kJ of energy. It is both flammable and explosive.  When mixed with oxygen and nitrogen in air in certain mixtures (orange region on the ternary diagram to the right), it will explode. The blue line represents air. If there is less than about 5% methane in the mixture it is too lean to explode (the LEL point on the diagram), and if there is more than about 15%, it is too rich to explode (the UEL point).

The process of mining coal generates an enormous amount of coal dust.  When high concentrations of this are suspended in air, and there is even a small ignition source, the nearly instantaneous reaction of the fine coal particles with oxygen can produce an explosion.  The ignition source does not need to be a flame; it can be an electrostatic discharge, friction, or sparks from machinery.  Dust explosions are not restricted to coal mines, but have occurred around grain silos, flour mills, and metal works (aluminum, titanium).  They are intentionally part of thermobaric weapons; see my older post here.