La Palma - grid

English About LastQuake
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We have received many questions about the seismic activity in La Palma island displayed on our maps. Indeed many of you reporder this grid effect

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[Picture and caption from a user, doubting it can be natural]

Others wondered about tsunami risk or if it meant that these earthquakes were not caused by unatural sources.

Indeed, when looking at the map at this size, it looks very unusual. Actually, this “grid” of earthquake locations is an artefact, due to the fact that the EMSC rounds longitude and latitude coordinates to two decimal points.

We take this occasion to explain why the EMSC map displayed a grid pattern of earthquake locations in La Palma island. If you look at the longitude and latitude data provided by EMSC (Earthquakes - Earthquake today - Latest Earthquakes in the World - EMSC), you will see that there are two decimal points in each value, resulting in a less-granular, less-defined dataset.
As a way of example, Google Earth uses 6 decimal points for the lon/lat couples. This allows for exponentially more granularity, which results in a higher-definition display.
In tight earthquake clusters like the ones occurring in La Palma, when you zoom way in, the 2 decimal point approximation creates an artefact and the earthquake locations are displayed like a grid.

This artefact is not unique to La Palma. It happens everywhere on our maps, whenever there is a huge zoom on a very small area. In La Palma, the artefact is pronounced because of the very small size of the island.

IGN, the authoritative source for seismic information in Spain, does not round the coordinates, and the seismicity does not look like a grid on their map:
Visualizador Terremotos Próximos.

We recommend consulting the local sources for tight seismicity like the one we are observing in La Palma.

Instituto Geográfico Nacional, IGN
Website: Instituto Geográfico Nacional
Twitter: @IGN_Sismologia)

Instituto Volcanológico de Canarias
Website: https://www.involcan.org/
Twitter: @involcan)

We are sorry for this misunderstanding and are glad of this opportunity to exchange with you and better explain our services.

In short … yes this activity is natural, and no there is not going to be a mega tsunami :heart:

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Very helpful !

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what does cya stand for? my dad is a geologist and seattle city planner and says he was never very good at the English language, but a lot of fancy words being used here.

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Thank you this information- very helpful

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Hello Laure,

Just a question…

Why not switch to a third (or more) decimal system ?

Thanks for all.

Louis.

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That’s good question Louis, and that’s exactly what I wondered yesterday: why can’t we have more decimals…
My colleague @jul got me some answers yesterday so I’ll let him explain, it’ll probably be more precise :slight_smile:

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Dear @SismoLaure

In my humble opinion, you have one of the most comprehensive databases available on the web.

A little effort on the decimals (most of the time, you have the data, but elsewhere on the site … by searching for it carefully …)

And another to try to lower the threshold of magnitudes in certain regions.

If you ran out of arguments … here are some …

Good luck !

Louis.

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We are fully aware of the grid problem. This issue has been raised several times in the last month. We are sorry for the misunderstanding it causes.
The reason is that due to the age of our software, we use formatted files that allow us to store only 2 digits which is enough in 99% percent of the case considering the precision of the earthquake locations. Unfortunately in very narrow earthquake swarm, this is clearly not enough and shows this strange grid pattern.

We are currently working on upgrading our real time system which will drop the decimal problem, all decimals provided by every contributors will be kept (usually up to 4 digits).

regards,

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@jul

Great challenge… waiting for the changes !..

Thanks in advance.

L.

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@jul

And in a final effort, change the magnitudes to two decimal … As a Christmas present …

…under the tree

:evergreen_tree: :evergreen_tree: :evergreen_tree: :evergreen_tree: :evergreen_tree:

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Here’s a fact checkig article for those of you who may be interested: Fact check: La Palma earthquake grid represents natural quakes

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As a last explanation of this “phenomenon”…

In the “For seismologists” part of the site, in the “Latest data contributions” sub menu, we found data with more decimals, like this:

https://www.emsc-csem.org/Earthquake/alert/?id=ZMBRW;MAD&date=2021-11-09

DATA_TYPE BULLETIN IMS1.0:short with ISF2.0 extensions
Bulletin from IGN, generated 2021-11-09 03:54:27
EVENT 021vylsz Canary Islands, Spain Region
   Date       Time        Err   RMS Latitude Longitude  Smaj  Smin  Az Depth   Err Ndef Nsta Gap  mdist  Mdist Qual   Author      OrigID
2021/11/09 03:50:11.77   0.83  0.28  28.5619  -17.8309   6.9   4.1  79  10.7   4.4   26   13 112   0.02   0.26 m i ke IGN       021vylsz

And on the public site, we have only two decimals for coordinates…

Magnitude	ML 2.7
Region	CANARY ISLANDS, SPAIN REGION
Date time	2021-11-09 03:50:11.7 UTC
Location	28.56 N ; 17.83 W
Depth	11 km

In this world, where some are ready to find conspiracy everywhere, precision of values and terms is an excellent way to tell the truth…

The “swarm” seems to reduce its rate this week-end, but the La Palma volcano has not said its last words…

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