Sequential earthquakes in our country can often be explained by local sources with different magnitudes. So what is the reason for this?
After the 7.4 magnitude earthquake in Kahramanmaraş, different countries announced different earthquake magnitudes. The first statement of the Kandilli Observatory ‘7.4‘ while for the same earthquake from the USA ‘7.8‘ The explanation came.
So why do different sources explain different sizes? Reply, method used to measure earthquake magnitude lies.
Methods used to measure earthquake magnitude:
Earthquake magnitude is generally ‘ML (local size)‘ And ‘Mw (torque magnitude)‘ units are described. However, there are five different earthquake magnitude measurement methods in total, including these two:
- Local Size (ML): The method, named after the US earthquake scientist Charles Richter, is also known as the Richter Scale. In this method, the magnitude of the earthquake is calculated by measuring the sound waves. With this method, accurate measurement of severe earthquakes cannot be made.
- Moment Magnitude (Mw): Moment magnitude, which is shown as the most reliable measurement method by the Kandilli Observatory, is the most complicated method to calculate and is used for earthquakes with a magnitude above four.
- Time Dependent Size (Md): In this method, which is used for earthquakes smaller than five, it is measured by the vibration created by the shaking on the seismometer.
- Surface Wave Size (Ms): This method is used for earthquakes above gold. The determination of the size is carried out by measuring the waves propagating from the surface. The reliability of this method progresses in parallel with the increase in the distance measured.
- Body Wave Size (Mb): A method by which waves descending into the depths are measured.
The differences in the numerical value of the earthquake magnitude are also due to this:
Sources measuring earthquake magnitude offer their measurement using local magnitude or moment magnitude methods. For example, AFAD shares the moment magnitude for the earthquake magnitude, while the Kandilli Observatory shares the local magnitude in the first place.
The first measurements shared by the Kandilli Observatory are usually revised later. For example, in the 7.7-magnitude Kahramanmaraş earthquake that occurred on February 6, 2023, Kandilli revised the earthquake magnitude, which was declared as 7.4 in ML, to 7.7 in Mw.
Mw or more reliable ML?
Moment magnitude (Mw) was developed as a new method against the limitations of local magnitude (ML) and all other measurement methods. Especially in large earthquakes Mw is considered the most accurate measurement.
Kandilli Observatory also includes the following statements about the Mw used in its revisions:
“This size type is the most reliable relative to the others. In the scientific world, it is thought that if the moment magnitude for an earthquake can be calculated, other types of magnitudes are not needed. It is far more complicated to identify than any of them.”
In addition, Mw is in only a few observatories in the world and only for earthquakes above a certain magnitude is used.