Articolo ben strutturato, ricco di informazioni, che consentono di avere un idea chiara sia del contesto geologico strutturale, che della storia sismotettonica dell'area del sisma.
Thanks for this. I’ve always been curious about the 1755 earthquake because there just doesn’t appear to be a thrust fault in the region with a large enough area to generate the ‘observed’ quake and tsunami magnitude. Multiple simultaneous thrusts does make sense.
I'm a retired O&G geologist. A few years ago I worked on industry seismic surveys close to the epicentre of this earthquake. It sure isn't your ordinary passive margin! The structural grain is dominated by old (mainly Jurassic) NNE trending half grabens, linked to the opening of the Atlantic, with main faults mainly dipping to the WNW and that show quite large vertical displacements, in the order of a few kilometres. These half grabens were then reactivated in a transpressive regime, forming narrow and elongated anticlines, during the late Cretaceous (?) / Eocene (the age of the collision in the Alps...). Most of them seem to be inactive since then, but others show hints of possible, relatively recent (Pliocene or later), reactivation again in a transpressive regime. This picture seems to fit quite nicely with the focal mechanism of this earthquake.
Thanks! Yes, a similar structural setting has been proposed for the 1909 earthquake near Lisbon - a steep Mesozoic normal fault reactivated as a thrust.
Gràcia et al. (2003, https://doi.org/10.1130/0091-7613(2003)031%3C0083:MAFOPN%3E2.0.CO;2) shows a few line drawings of seismic profiles similar to what you describe, but a bit south of the epicenter. Do you know of any more recent papers on the topic, or coverage within the epicentral area itself? It would be really interesting to be able to match up the event with some candidate structures.
I gave a look at the paper you mentioned. The area covered in it is more to the South and West compared to the area I was working on. The style of at least one of the drawings (figure 3A) is similar to what I was describing. What these drawings are missing is the strong transpressive event that occurred in the Late Cretaceous/Eocene, which is quite evident in the area I worked on.
Interesting! Maybe this earthquake will encourage people to go back and take a look at these surveys. There's so much room around the world to apply older seismic imaging to seismic hazards - it's too bad that so many of them are difficult to access.
I contacted people I know from GALP. They told me that the seismic survey dataset should be accessible by asking the relevant authority and gave me some details on how to do it. If you are interested, please tell me where I could reach you privately and I will send you the information. Having worked on this area I am quite interested in better understanding its dynamics.
That's great! We aren't really in a position to take this on, though, unfortunately - that kind of long-term project is a bit outside our current scope. However, if we get any inquiries, maybe I could pass them to you? I suspect that there will be some interest among researchers in Portugal following the earthquake (although who knows if they will find this discussion).
I am not aware of recent papers about the geology of the area around this epicentre. There are however as I mentioned good seismic surveys just to the south of the epicentre (and maybe even at least partially covering it). Don't believe they are publicly available, but asking the relevant Portuguese authority may be worthwhile. Not sure of the current name of the relevant authority though: they restructured their bureaucracy at least once since I worked on the area. The surveys were operated, if I am not mistaken, by GALP, the Portuguese O&G company. As far as I am aware there are no more O&G activities in Portugal so maybe they would be open about making at least some seismic lines available for a study...
I am curious why earthquake reports rarely say how long an earthquake lasted. The magnitude of the quake and how long it lasted both are indicators how how damaging it might be.
Engineers for a while estimated duration, but a good definition is elusive. They used the interval when 5% to 95% (or 10% to 90%) of the energy was released, if I recall correctly, which of course has little relation to how long people felt it. Duration was mostly a measure by which to contrast the prolonging of motion in basins compared to nearby hard rock sites.
Hi Deborah, I'm also not sure why shaking duration isn't more commonly reported! Maybe it's because there can be differences in shaking duration in different areas for a single earthquake, especially when the seismic waves are amplified and resonate within sedimentary basins. So maybe it is not reported because it is spatially variable. Or maybe it is because it is hard to define a quantitative measure of shaking duration, while it is easier to feel it personally. However, the shaking duration is clearly related to the rupture duration, and it should be obvious that a M4 earthquake should have much shorter shaking than a M8 earthquake. People certainly report shaking duration as a major part of their felt experience. I'll ask our colleagues at the USGS if they have thought about this. I would really love to see maps of shaking duration, by some definition, for different earthquakes. Thanks for the really interesting comment!
The idea of “cascading” quakes along multi-fault systems (San Andreas and Cascadia) is sufficient to trigger insomnia.
Articolo ben strutturato, ricco di informazioni, che consentono di avere un idea chiara sia del contesto geologico strutturale, che della storia sismotettonica dell'area del sisma.
Fascinating! Thanks for yet another informative blog.
Thanks for this. I’ve always been curious about the 1755 earthquake because there just doesn’t appear to be a thrust fault in the region with a large enough area to generate the ‘observed’ quake and tsunami magnitude. Multiple simultaneous thrusts does make sense.
I'm a retired O&G geologist. A few years ago I worked on industry seismic surveys close to the epicentre of this earthquake. It sure isn't your ordinary passive margin! The structural grain is dominated by old (mainly Jurassic) NNE trending half grabens, linked to the opening of the Atlantic, with main faults mainly dipping to the WNW and that show quite large vertical displacements, in the order of a few kilometres. These half grabens were then reactivated in a transpressive regime, forming narrow and elongated anticlines, during the late Cretaceous (?) / Eocene (the age of the collision in the Alps...). Most of them seem to be inactive since then, but others show hints of possible, relatively recent (Pliocene or later), reactivation again in a transpressive regime. This picture seems to fit quite nicely with the focal mechanism of this earthquake.
Thanks! Yes, a similar structural setting has been proposed for the 1909 earthquake near Lisbon - a steep Mesozoic normal fault reactivated as a thrust.
Gràcia et al. (2003, https://doi.org/10.1130/0091-7613(2003)031%3C0083:MAFOPN%3E2.0.CO;2) shows a few line drawings of seismic profiles similar to what you describe, but a bit south of the epicenter. Do you know of any more recent papers on the topic, or coverage within the epicentral area itself? It would be really interesting to be able to match up the event with some candidate structures.
I gave a look at the paper you mentioned. The area covered in it is more to the South and West compared to the area I was working on. The style of at least one of the drawings (figure 3A) is similar to what I was describing. What these drawings are missing is the strong transpressive event that occurred in the Late Cretaceous/Eocene, which is quite evident in the area I worked on.
Interesting! Maybe this earthquake will encourage people to go back and take a look at these surveys. There's so much room around the world to apply older seismic imaging to seismic hazards - it's too bad that so many of them are difficult to access.
I contacted people I know from GALP. They told me that the seismic survey dataset should be accessible by asking the relevant authority and gave me some details on how to do it. If you are interested, please tell me where I could reach you privately and I will send you the information. Having worked on this area I am quite interested in better understanding its dynamics.
That's great! We aren't really in a position to take this on, though, unfortunately - that kind of long-term project is a bit outside our current scope. However, if we get any inquiries, maybe I could pass them to you? I suspect that there will be some interest among researchers in Portugal following the earthquake (although who knows if they will find this discussion).
Sure! Thanks
I agree. They should become available, at least for researchers, after a reasonable amount of years
I am not aware of recent papers about the geology of the area around this epicentre. There are however as I mentioned good seismic surveys just to the south of the epicentre (and maybe even at least partially covering it). Don't believe they are publicly available, but asking the relevant Portuguese authority may be worthwhile. Not sure of the current name of the relevant authority though: they restructured their bureaucracy at least once since I worked on the area. The surveys were operated, if I am not mistaken, by GALP, the Portuguese O&G company. As far as I am aware there are no more O&G activities in Portugal so maybe they would be open about making at least some seismic lines available for a study...
I fob that in April 2018 at the AAPG congress in Lisbon P. Sala gave a presentation using the seismic data set I was talking about. Here you can find the abstract: https://www.searchanddiscovery.com/abstracts/html/2018/lisbon.90325/abstracts/29631991.html
Couldn't find the actual presentation though.
I am curious why earthquake reports rarely say how long an earthquake lasted. The magnitude of the quake and how long it lasted both are indicators how how damaging it might be.
Engineers for a while estimated duration, but a good definition is elusive. They used the interval when 5% to 95% (or 10% to 90%) of the energy was released, if I recall correctly, which of course has little relation to how long people felt it. Duration was mostly a measure by which to contrast the prolonging of motion in basins compared to nearby hard rock sites.
Hi Deborah, I'm also not sure why shaking duration isn't more commonly reported! Maybe it's because there can be differences in shaking duration in different areas for a single earthquake, especially when the seismic waves are amplified and resonate within sedimentary basins. So maybe it is not reported because it is spatially variable. Or maybe it is because it is hard to define a quantitative measure of shaking duration, while it is easier to feel it personally. However, the shaking duration is clearly related to the rupture duration, and it should be obvious that a M4 earthquake should have much shorter shaking than a M8 earthquake. People certainly report shaking duration as a major part of their felt experience. I'll ask our colleagues at the USGS if they have thought about this. I would really love to see maps of shaking duration, by some definition, for different earthquakes. Thanks for the really interesting comment!