EQL is mainly flashes of light or glows associated with earthquakes. Most show a white spherical ground-based core surrounded by radiating blue-white light, fading the further it is from the core. There are now thought to be numerous valid records throughout history, but mechanisms are still being understood.
The light is usually co-seismic. Though there have been some reports of pre-quake light they are usually only seconds before. There have been a few reports of low-intensity pre-seismic glows mostly seen over hills at night. EQ Light has been seen emerging from the sea.
They are usually to be differentiated from explosions in the electrical grid, ie., transformers. Transformers do explode and catch fire during earthquakes but the event lasts much longer, smoke and fire are usually associated and the colour and shape is described very differently by observers. They are also much more contained and localised. Often there are no reports of destroyed transformers.
The light appears to emerge from the ground. Though some observers have reported orbs and flashes of light in the clouds these appear mainly to be reflections of what's happening at ground level because there are no known weather events at the time.
The EQ Light displays of blue-white light are commonly of a radius of 10s of meters, but have been as large as several kms in diameter - eg Mt Ahir inTurkey. Some EQ Light - the white central orb - was described in Christchurch in 2010-2011 as being up to 200m in diameter. Obviously their brilliance in a dark scene can make earthquake light appear larger.
EQ Light does not necesssarily occur around earthquake epicentres. More typically EQ Light displays are generated fairly close to observers who may be up to 100s of miles from a large EQ epicentre. Typically a major fault or tectonic movement ripples into associated fault networks. each locally releasing stress and creating local light.
There is no radioactivity associated with these events and no-one has reported direct electric shocks from being in or near a flash. Observers have talked of "fuzzy feelings", the air feeling electric, and getting shocks from touching metal and random surfaces around the time of a large earthquake but these are not from earthquake light.
Obviously earthquake light is most visible when it happens at night. Though earthquakes obviously occur in daylight, associated earthquake light is rarely reported because it is rarely observed.
Many flashes may be observed during a quake - randomly, all over an area. The larger flashes may coincide with regional power failures.
The most accepted explanation is that stresses on underground rock (usually silicates or quartz) during earthquakes are so great that they break chemical bonds, releasing electric charge that immediately separates or ionises. The negative electron (charge) remains trapped underground but an enormous wave of positive charge is released to the earth's surface. There it combines with nitrogen in the atmosphere, creating the white colour seen in the flashes, and then almost instantly returns to earth recombining with separated negative underground charge to reform into the original material. The white light, by a process called Raleigh scattering, becomes blue. All this happens in less than a second but the flashes can vary in size depending on the amount of separated charge generated.
What does the science say? Watch a laboratory demonstration of this process.
It is most common to find EQ Light in areas of quartz or in silicates. These comprise most of the world's rock types. Other rock types often contain microscopic quartz crystals - as in New Zealand - which has very little quartz and is largely "greywacke" - a kind of "dirty" sandstone made up of feldspar, volcanic rock fragments, silt and clay, but does contain angular quartz grains. The full story is not clear because NZ has produced spectacular displays of EQLight, and one of the world's largest EQ Light displays was over Mt Ahir in Turkey - which is limestone. But asphalt, sedimentary rocks and gravels can contain quartz and could host small amounts of EQ light.