The earthquake yesterday morning, October 30 at 7.40, had a Richter magnitude 6.1 and a moment magnitude Mw 6.5.
The National Institute of Geophysics and Volcanology (INGV) uses both magnitudes: la Richter magnitude, also defined local magnitude ML, because it's very fast to calculate and is reliable enough for earthquakes up to magnitude 6; la moment magnitude Mw, because it provides a more accurate estimate of the energy released by the earthquake and, in particular, for the strongest ones.
The magnitude Richter MLit is calculated in a few minutes and is communicated to the Civil Protection Department after 2 minutes and to the population via the web and social media within 30 minutes (on average after 12 minutes).
The preliminary magnitude values, based on incomplete data but available just a few minutes after the earthquake, may differ from the definitive magnitude (up to around 0.5), but are essential for civil protection purposes. Over the next few minutes, they are updated with more accurate estimates as new data becomes available. The procedure used is reported above this blog Site guide cnt.rm.ingv.it with seismic events in real time.
Why Richter magnitude not good for strong earthquakes?
To understand this we must first explain how are the two magnitudes calculated?.
The magnitude Richter ML is obtained frommaximum amplitude of the oscillations recorded by a standard seismometer, Called Wood-Anderson, sensitive to seismic waves with a relatively high frequency of about 0.8 Hz.
This method works quite well for earthquakes with magnitudes below 6.0.
Earthquakes of greater magnitude, on the other hand, emit an important part of their energy at frequencies lower than 0.8 Hz, so the maximum amplitude measured on the seismographWood-Anderson it does not represent all the energy emitted by the earthquake.
For this reason earthquakes with moment magnitude Mw greater than 6 tend to have values of Richter magnitude ML very similar (see figure to the side). In these cases, therefore, the moment magnitude Mw must be used.
Per calculate the Mw a very long portion of the broadband seismograms must be analysed, in order to consider all the energy emitted and thus give a more realistic value. To do this you have to wait for the registration of all the seismic signal at the various (many) stations of the National Seismic Network and analyze them. This involves longer times, which are not compatible with civil protection purposes and with the public's desire for immediate information.
On our site cnt.rm.ingv.it it is possible to see everything the personnel do in the seismic room and how the locations and magnitudes change when new data arrives and new analyzes are possible.
Yesterday morning after 2 minutes we provided the Civil Protection Department by telephone with a first estimate of the value of the magnitude ML equal to 6.1, immediately specifying that the correct value would be higher and communicated shortly. This value was also made public on the INGV website 18 minutes after the seismic event.
In the minutes following the earthquake, a first estimate of the moment magnitude Mw obtaining a value of 6.5 which was reviewed and confirmed in the two hours following the event. The figure below shows some of the seismic stations used to calculate the seismic moment and relative magnitude.
The obtained value is well calculated because a lot of data has been used and the correspondence between observed and modeled data is very good. All the data from the seismometers of the INGV seismic networks and those connected are public and can be downloaded and used to calculate the hypocentral parameters, the magnitudes and other data.
The moment magnitude is also calculated by other international institutes. At present these are the estimates provided by various bodies:
| international institute | Calculated magnitude |
| INGV | 6.5 |
| USGS | 6.6 |
| GFZMore | 6.5 |
| CSEM | 6.5 |
As with any measurement of a physical parameter, the estimates are affected by uncertainties because they are made with different methods, with data from different seismic stations and with different models of the earth's crust. This explains the differences that sometimes characterize the estimates provided by the various bodies.
Another important aspect, already covered in one of our articles, is the link between thethe value of the magnitude and compensation for damage caused by earthquakes. As explicitly stated in the August 26th post the magnitude It is NOT used for compensation for damage caused by earthquakes; for this purpose in the past the values of intensity calculated on the basis of the Mercalli scale have been used (actually the Mercalli-Cancani-Sieberg scale).