Hello again! As promised earlier I will attempt to explain a more complicated topic to do with earthquakes. The topic: Focal Mechanisms!!
Here are three focal mechanisms for three different quakes in New Zealand over the past decade. But what do they mean? 1/n
Here are three focal mechanisms for three different quakes in New Zealand over the past decade. But what do they mean? 1/n
Earlier today I introduced the different types of faults - strike-slip, reverse, normal & oblique. Focal mechanisms - also known as beachball plots - graphically display the alignment (strike), inclination (dip) & type (slip/rake) of a fault in a quake 2/n
When a quake occur, the blocks of rock either side of the fault move past one another in different directions. As the seismic waves radiate out from the fault, different parts of the Earth are put into compression or dilation. Let's take a strike-slip earthquake as an example 3/n
On the 1st July 2017 a small earthquake struck the southern South Island, not far from the town of Wanaka. As the fault ruptured it sent out seismic waves all across New Zealand. These were picked up on the New Zealand National Seismograph Network - each dot is a seismograph! 4/n
These seismograph stations have several channels. These are different mini-instruments that record the arrival of seismic waves from different directions; generally these are vertical (Z), east (E) and north (N) - the picture displays 3 of 15 channels at one station in NZ 5/n
As a P-wave arrives - remember they are the first seismic wave to arrive - the instrument will record whether the rock where the station is has been stretched or compressed. This can be determined by whether the first tick (squiggle if you will) goes down or up 6/n
In the example of the Wanaka quake, four stations can show the areas which were compressed or stretched. To the west and to the east the stations show a downtick, or negative first polarity - these are stations MSZ (Milford Sound) and ODZ (Otahua Downs) 7/n
To the south and to the north-east (the station due north did not show this very well, so I used another!) the stations show an uptick, or positive polarity - these stations are SYZ (Scrubby Hill) and LBZ (Lake Benmore) 8/n
When other stations are used in different directions (azimuths) from the quake's location (for the azimuth this can be termed the epicentre) a picture of compressed and stretched areas of rock can be made. This is the barest bones of a focal mechanism 9/n
The reason is that an earthquake occurs at depth, so the direct path from source (several kilometres deep) to the station at the surface must be taken into account. This second measurement allows a point to be plotted in a circle with a "+"(compression) or "-" (tension) 10/n
Once lots of these + and - signs are plotted into a circle, you find quadrants which are negative or positive - areas which were in compression or in tension. Tension quadrants are generally left uncoloured, whilst compressed quadrants are coloured 11/n
In the case of the Wanaka quake, this determined that the quake was strike-slip. The focal mechanism indicated that the fault was either NW-SE in alignment or NE-SW in alignment - it can only narrow the causative fault down to two possibly solutions 12/n
As can be seen when this is superimposed onto a map, the focal mechanism matches the P-wave arrivals we found at the four stations. This proves that the earthquake was strike-slip 13/n
When a quake occurs on a reverse or normal fault two solutions with the same strike but different inclinations/dips are found. If the middle area on the focal mechanism is white then the fault responsible for the quake was normal; if the middle is coloured it was reverse 14/n
Oblique quakes show four quadrants, but unlike the strike-slip focal mechanism the lines (nodal planes - the possible fault planes on which the quake occurred) are not straight but curved. This example is an oblique normal quake 15/n
There is some more complexity to focal mechanisms: if a quake is complex (occurs on more than one fault with different fault types) then you can get different focal mechanisms for the same quake. These are both for the 2010 Darfield Earthquake in NZ 16/n
If you wish to find more information on how exactly focal mechanisms are produced view this fantastic @IRIS_EPO video: 17/17
