The Sun and the Solar Cycle
In order to for us to establish a link between solar activity and earthquakes, we must first briefly examine the nature of the Sun. The Sun is a G-type yellow-dwarf that contains over 99% of the mass of the Solar System. The Sun contains hot plasma that is primarily made up of hydrogen and helium. The Sun is powered by nuclear fusion and temperatures at its center can reach millions of degrees Celsius.
The Sun also has intense magnetic fields which extend far out into space. A process known as differential rotation occurs within the sun where the Sun spins faster at the equator than at the poles. This causes the magnetic field lines of the Sun to twist and erupt from the surface as magnetic field loops causing sunspots and solar prominences. The process is also responsible for the Sun's 11-year cycle, called the solar cycle, where greater activity of the Sun is observed as the magnetic poles reverse themselves. The solar cycle has a major impact on the Earth's climate as well as the oceans and is thought to trigger earthquakes as well.
Effect of Solar Events on Global Seismicity
It is known that during times of increased solar activity, electrojet activity, electric currents flowing in the E region of the ionosphere of the Earth, also increases high up in the Earth's atmosphere. The increase in electrojet activity causes magnetic pulses that are irregular. What is speculated to happen is that the magnetic pulses affect the Earth and the oceans by generating currents within the Earth's interior. Within the Earth are fault zones, areas where majority of earthquake activity occurs. In a region of the Earth that is already under stress, where pressure between the plates is increasing, the currents induced by the magnetic pulses directly affects the fault zones causing micro-fracturing and further weakening of the faults. Weakening of the fault zones causes them to crack releasing the pressure between the plates causing earthquakes.
Another way in which the faults could be affected by the currents is by the release of trapped gas, which would open up large cavities causing the fault zone to become unstable, hence resulting in an earthquake. Solar activity can also induce these currents by creating Kelvin-Helmholtz instabilities over the magnetic field lines, which can cause fault zones to weaken.
Effect of Sunspots on Global Seismicity
Sunspots are cool dark regions on the surface of the sun caused by extreme magnetic activity. The 11-year cycle of the sun results in increasing sunspots at first with a gradual decline towards the end of the cycle. Sunspots have long been known to affect the climate on Earth. Now, a link to earthquake activity on Earth has also been established. It has been observed that during periods of solar minima or low sunspot activity greater number of earthquakes occur on Earth. Why this is still remains a mystery.
The hypothesis that is postulated is as follows: Electrical discharges cause sunspots with intense magnetic fields and the same discharges interact with the ionosphere, causing increased electrical activity. The ionosphere acts like a conducting plate while the Earth, the other conducting plate. Movement of charges occurs as voltage on one of the plates changes. Rocks within the Earth become semi-conducting in nature as a result of the movement of charges and this causes an electrical breakdown within the rock, releasing energy in the form of an earthquake.
Relationship between Solar Flares and Earthquakes
What has been observed that earthquakes in the past have been preceded by a solar flare. But not all solar flares are followed by earthquakes. A solar flare is a huge outburst of energy on the surface of the sun caused by the magnetic fields of the sun. They release highly charged particles and radiation that bombard the Earth and can disrupt communication satellites and also cause damage to living cells. Solar flares can also cause geomagnetic storms by interacting with the Earth's magnetosphere. During periods of increased solar activity there is also an increase in solar flare activity.
The theory of how solar flares cause earthquakes is that the charged particles released interact with the Earth's magnetosphere and produce ring currents, which are electrical currents caused by trapped charged particles in the magnetosphere, The ring currents are thought to speed up the tectonic plates at the fault zones, thus causing earthquakes. However, it must be noted that even though more solar flares occur during times of increased solar activity, the overall number of earthquakes is less during times of greater solar activity. Not every solar flare results in an earthquake.
When Does Solar Activity Increase?
When does solar activity increase? The answer to that is not as straightforward as one might expect. Observations indicate that the 20th century has seen an increase in the total energy output by the Sun. In contrast, the past few decades have not seen a rise in solar activity as it has stabilized. However, the temperature on Earth continues to rise. In general, during the 11-year cycle of the Sun the most amount of solar activity occurs during solar maximum and the least amount of solar activity occurs during solar minimum. One thing is for certain, however, that the Sun has a major impact on the Earth's climate, its oceans and can even affect the interior of the Earth.
Image Credit: Wikimedia Commons/NikoLang/HansBernhard/NASA