I think earthquakes are allowed to be devastating, cataclysmic, and so on. Earthquakes taking down Boundary protections would certainly explain why a Rome with a powerful and active Cult of Mercury still lost Pompeii and Herculaneum, for example, if the volcanic eruption was accompanied by an earthquake as so many are.
If it is actually moving a boundry (as opposed to shaking it) that is a pretty massive earthquake. Also not sure it is technically an attempt unless there was a will directing the attack. However it does make the aegis vulnerable to a mundane with a large hammer, or seige weapons depending on what your boundary is made of.
A boundary for a type 1 T: Boundary spell does not affect the spell effect after the spell is cast.
Moving a boundary for a type 2 T: Boundary spell does indeed cancel the spell.
Failed attempts at moving a boundary of course do not affect it - also not for magical purposes.
I discussed this briefly at my gaming table, and it came out that there's an important class of ... things that are intuitively Boundaries, but that are not intrinsecally immobile: those that move with the cycles of nature.
The example that came out during play was the snowline of a (sufficiently tall) mountain. This can go up and down considerably with the turning of the seasons - but it's the "natural" Boundary for many folklore rituals.
Then, islands immediately came to mind. I am not talking of magical wandering islands like Delos (from SE). Just plain mundane islands, particularly in the ocean, that see the tides go up and down - so that dry land, particularly if very "flat", shrinks and expands. While you could define the Boundary of the island as, say, the high-tide boundary, it's much much more natural to say the Boundary of the island is where the land falls into the sea.
This is even truer of river islands, where the water level rises and falls e.g. with the rain, because there is no such thing as a high-tide line. An example would be one of the sites suggested for a Covenant in GotF p.123, Pfalz island. In fact, many of these islands can grow, shrink, and shift considerably over longer timeframes as water deposits sediment, or erodes the land.
Many other "slowly moving" boundaries come to mind. Forests. Cities. Kingdoms. Now, many of these admittedly do not move significantly during the timeframe of D:Year. But T:Boundary is a valid target for many magics, exotic or partially integrated in Hermetic Magic (see e.g. the Merinita Mysteries) that can last significantly longer than a Year. It seems wrong to me that a curse on a forest can be broken just by letting the forest grow.
A clarification/reworking of Boundary that makes it work "as intended" (at least for me) would be something along the following lines:
Boundary: the spell affects everything within a locale, that must have and maintain a clear identity (for example a particular forest, lake, or city) and a clear perimeter demarcated by a natural or man-made boundary (for example the edge of the forest, the shore of the lake, or the walls of the city). Moving this boundary alters the area affected by the spell insofar as it shifts the perimeter of that particular locale. Thus a spell affecting everything within the walls of a city would expand as the city rebuilds its walls to accomodate growth, but magically transporting those same walls to surround a village in a neighbouring duchy would not make the spell affect the village.
This could be reworked instead (if slightly less effectively) into the passage at the end of the text on containers, replacing "Boundaries are, by their nature, immobile...things that do not move relative to the earth do not move at all.", as follows:
A Boundary concretely represents the perimeter of a locale with a clear identity, such as a forest, a lake, or a city. Moving a Boundary alters the area affected by a spell insofar as it shifts the perimeter of the locale. Thus a spell affecting everything within the walls of a city would expand as the city rebuilds its walls to accomodate growth, but magically transporting those same walls to surround a village in a neighbouring duchy would not make the spell affect the village.
I think this wording works well with forests, glaciers, and islands; as well as with cities, kingdoms and similar human constructs. It allows Delos as a persistent Boundary. It also clears all doubts that a moving regio can, in fact, be a persistent Boundary. It may, or may not allow a ship as a Boundary depending on whether you consider that a locale - this can be clarified further, or left for troupes to decide - but if it is a Boundary, it is a persistent one. It does avoid OneShot's "Aegis of Assault" problem, and related problems stemming from a quick fence raised just to cast a precisely and "unnaturally" shaped T:Boundary spell. Last but not least, it keeps all existing spells working, and in fact makes them work as they should, removing some potential abuses.
If a tent can flex in the wind without being destroyed as a Room, I think the tide can go in and out without destroying an island as a Boundary. Similarly for a growing forest, as long as it doesn't get bigger than the size modifiers on the original curse.
It is probably worth editing so that one of the examples is a Boundary, however.
I did a bit of rewriting:
Being Inside a Container
The standard Ars Magica rules use an intuitive definition of "inside" when determining whether something is within a container target. A person standing in the centre of a 1 pace diameter circle is inside the circle; someone who happens to be standing directly above it three floors higher is not. A person standing on the battlements of a castle is within the Structure, but a crow flying over the walls is not. If your troupe needs a more precise definition, you can work something out, and nothing should break.
Persistence of Containers
The standard Ars Magica rules use an intuitive definition of whether a container still exists to be the target of a spell. If the same container still exists, then the target still exists. An island does not cease to exist when the tide rises or falls, unless a causeway joins it to the mainland at low tide. A room does not cease to exist when the door is opened, but a room inside a tent does cease to exist when the tent is packed away. Containers can move if they are still the same thing while, and after, moving. Note that certain choices of container make spells vulnerable: if a Room is just a tent, the spell can be ended by collapsing the tent, while if a Boundary is just a fence, the spell can be ended by pulling down any part of the fence. Containers can get larger or smaller, but spells do end if the container becomes too big for the spell to affect.
Circles and Boundaries require some extra comment. A circle is a different circle if its size changes, or if it is transferred to a different surface. Circles are very vulnerable, which offsets the power of Ring Duration and Circle Target. If a Boundary is nothing more than the thing marking the border, then it is as vulnerable as a circle, and cannot be moved, altered, or expanded. If a Boundary is something like "the waterline of this island" or "the wall around this covenant", then it is much less vulnerable, and may change with natural changes in the thing.
My Saga uses the outer boundary of the Regio (a thicket of trees and bushes following its outer edge, butted up against mist shrouded water). Since Regio can naturally change in size, any rolled growth for the year would break it under the old ruling while this version would not.
I am sure my Saga is not unique in using the outer boundary of a Regio as the Boundary Target for their Aegis.
I just wanted to say it is just so much better now: clear, concise, thought-provoking, effective!
I think that looks very good overall. I think it makes things quite clear and quite playable.
That works much better.
I like it.