Hello, I am using google translate. I have a question, why earthquakes of the same magnitude and similar depth have such different intensities? These are two earthquakes, one in Chile and the other in Argentina, both have the same magnitude and similar depth, but the Argentine earthquake reached an intensity of between IX and X and the Chilean earthquake only reached a maximum intensity of VII. Why is that happening? by the type of soil? by topography?
As far as I understood sismology, seismic waves propagation (how they travel) and attenuation (how fast do they loose energy) are heavily dependent on geology.
Additionnaly, it’s very dependant on the distance to the epicenter. If there is nobody to testify close to the epicenter (like in a desert) where the highest intensities are expected, then the strongest intensity will not be reported: there is a sampling bias.
Yes, I think you are right that the shallow geology plays a role. Thick sedimentary basins - like what is likely to exist in Argentina - are known to amplify the shaking.
Also, the USGS ShakeMaps use a prediction, but are then refined by peak ground velocities/accelerations if any seismic stations exist nearby - this may affect the final shakemap result. The actual ShakeMap page for each earthquake should tell you what data went into the prediction.
Directivity effect may also be a cause of that difference… Dr. @shicks_seismo have also described well.
I also just noticed that these two events are from quite a long time ago (in the 1970s-1980s) so back then, the felt reports and seismic station distribution might have been a lot more sparse and subject to many uncertainties.
Depends on how these two intensities were calculated either from PGAs or from Number of reported damages/losses there could be many reason…
@Leonardo are you interested in these two earthquakes for a specific reason?
Indeed the fact that they are quite “dated” now can impact the precision of data.
It would be interested to see the actual felt reports!
@Yann many times I have noticed watching videos on the Internet that the earthquakes in Chile look less intense than other similar earthquakes in other countries, but that was just my impression. About a month ago there was an M 6.4 earthquake in the west of my country, Argentina, near the site of the 1977 earthquake (previous images). A friend from Chile saw a video of the earthquake in Argentina and told me that it seemed to be of a magnitude greater than 6.4, he told me that earthquakes of that magnitude in Chile do not feel as strong. Recently there was an earthquake in Japan of M 7.1, I asked my friend from Chile to see the videos of that earthquake and tell me what magnitude it seemed to him, and he told me that it seemed more than 8. He told me that earthquakes are very frequent in Chile but that Chileans are lucky that their earthquakes are deep and with distant epicenters and that is why they feel less intense, so I “confirmed” what I thought before, that in some areas of Chile earthquakes feel less intense than in other countries. So I started researching similar earthquakes in Chile and Argentina to see if there was any difference, and I found some examples in which earthquakes with an epicenter in the central west of Argentina feel a little more intense than similar earthquakes with an epicenter in Chile. , and the example with the biggest difference was the one that I uploaded previously in images.
I know that in Chile most strong earthquakes have their epicenter in the sea, the maximum intensity of Chilean earthquakes is rarely felt in continental territory, so I suppose that also has to do with the way in which Chileans feel earthquakes . The Chilean earthquake that I uploaded in images previously was the most similar to the 1977 Argentine earthquake that I found, the other earthquakes that I found were with an epicenter in the sea, and the intensity of those earthquakes in some cases does not exceed the intensity of 6 on land. But for example, many earthquakes in Japan also have their epicenter in the sea, and from what I read, in Japan they also feel a little more intense, so I guess the main cause is because of the type of soil, but I was not sure.
I do not think that the Argentine earthquake has been measured with the damages and losses, since in 1944 there was an earthquake of M 7 that killed approximately 10,000 people and destroyed 80% of the most populated city in the area. the 1977 earthquake killed less than 100 people and caused much less damage to cities, the 1944 earthquake was rated with a maximum intensity of 7 and the 1977 earthquake with a maximum intensity of 9/10, so I guess the earthquake 1977 was measured with the PGA.
this is the 1944 earthquake