A team of INGV researchers has developed a new method to evaluate the depth of past earthquakes based on the effects of recent earthquakes
A study conducted by researchers from the National Institute of Geophysics and Volcanology (INGV) has developed a new method that allows the depth of past earthquakes to be assessed using only the so-called "macroseismic intensities", i.e. the description of the effects that the earthquake produced in a certain place according to the Mercalli scale.
"With the same magnitude, the depth of an earthquake determines drastic differences in the violence of the seismic shaking and in its distribution", explains Paola Sbarra, INGV researcher and author of the article, "if there was very clear evidence of this on 21 August 2017, when an earthquake of magnitude 4.0, which would have caused only a lot of fear if it had had the typical depth of Apennine seismicity - about 10 km - instead occurred at a depth of 1 km below Casamicciola, in Ischia, causing collapses and much damage, albeit in an area of very limited dimensions”.
The technologies and instruments available today make it possible to estimate with great precision both the epicentral coordinates of earthquakes and their depth. However, much of what is known about the seismicity of any area of the globe depends on historical earthquakes, i.e. earthquakes that occurred in the pre-instrumental era. In a historic earthquake, the estimated intensities for individual locations can be considered as pieces of a puzzle: if they are few, it is not possible to apply the analytical methods available today to evaluate the depth of the earthquake, and the puzzle remains incomplete.
The innovative method developed by the INGV team has made it possible to calibrate a relationship that links the depth of an earthquake to the distribution of macroseismic intensity. To do this, twenty earthquakes that occurred between 1983 and 2019 were studied, for which data relating both to the instrumental depth and to a large number of observations on the intensity with which each single event was felt by the population was available. different distances from the epicenter: earthquakes which, taken together, represent a sort of modern seismological "Rosetta stone".
In fact, for about ten years, thanks to the collaboration of citizens, it has been possible to count on an in-depth knowledge of the effects of recent earthquakes, which INGV collects with the site http://www.haisentitoilterremoto.it/, through which anyone can submit their observations on a specific seismic event: this is the first seismological application of the "crowdsourcing" mechanism, made increasingly effective today by the widespread use of smartphones.
“Knowing the depth of historical earthquakes is essential to be able to correctly estimate the magnitude of each event of the past and to associate it with a specific seismogenic fault: a particularly critical step in regions where the same epicentral location can correspond to faults located at different depths, such as takes place in the northern Apennines and in the Po Valley", concludes Paola Sbarra, "the new method makes it possible to describe in greater detail the faults capable of generating future earthquakes and, therefore, to effectively predict the geographical distribution of the expected shaking".
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Figure 1 - Map of the study area, showing all earthquakes of M> 3.5 that occurred between 1985 and 2019 (empty circles: data from INGV). Earthquakes of known depth, used to build the Rosetta Stone, are indicated with blue dots; past earthquakes for which we have calculated the depth are indicated with red dots. The orange boxes are the surface projections of the seismogenic sources present in the INGV DISS database. The Quaternary deposits of the Po Valley are highlighted in dark gray. The box on the top left shows a schematic section of the Northern Apennines.
Figure 2 - Mercalli intensity attenuation curves (MCS) as a function of distance from the epicenter for three earthquakes that occurred in 2012 in our study area. The three earthquakes, well recorded by the INGV seismic network and well characterized in terms of effects, had very different depths (green 6 km, blue 29 km, red 72 km). This characteristic is reflected in the decay of intensity as a function of distance: rapid for the earthquake at a depth of 6 km, much slower for the earthquake at a depth of 72 km. Through this key it was possible to evaluate the depth of numerous pre-instrumental earthquakes. It should be noted that over a distance of about 50 km the attenuation of the intensity becomes very similar for the three earthquakes.