Propagation:
Propagation is the spread of a crack in a weak layer from an
initial fracture location. A fracture can propagate at about
the speed of sound, which makes it possible for huge slabs of
snow to release from a mountainside instantaneously. The propagation
potential of a particular slab and weak layer dictates how large
an avalanche may become once triggered, and also determines
if it’s possible to trigger avalanches from flatter terrain
connected to steeper slopes.
Whether a localized crack propagates or not, or how far the
propagation will proceed, depends on several factors including:
• Snow structure
• Stored elastic energy in the snowpack
• Snow stability
• Spatial variability of the weak layer and slab
For instance, if a skilled avalanche worker digs several snow
profiles on a test slope and finds easy compression tests and
rutschblock tests, high quality shears, a persistent weak layer
with a critical combination grain type, grain size and hardness
differences between the slab and the weak layer, plus they find
those same conditions in several snow profiles on the same slope,
they can safely conclude that the snowpack can both initiate
and propagate a fracture. In other words, avoid all similar
slopes steep enough to slide.
The tricky part is that these factors change through space and
time. Here is an example that we notice in many areas of North
America: Typically, during and right after the storm we see
widespread soft slab natural avalanche activity on all kinds
of slopes, even small slopes with a lot of compressive support
and even in relatively thick trees, but the fractures typically
don't propagate very far. Since the slab is soft, we can also
trigger avalanches easily, and they usually fracture at our
feet or snowmobile instead of above us
But as time passes, the slab settles and gains strength. With
a persistent weak layer, the slab typically gains strength much
more quickly than the weak layer beneath it. Thus, over time,
we usually see fewer avalanches and they are less sensitive
to triggers, but the ones we do see will occur on large, open
slopes without anchors, the fractures will occur above us instead
of at our feet, and because of the stiffer slab, the fractures
will propagate much longer distances. With very fragile, persistent,
weak layers such as surface hoar and faceted snow, it's not
unusual for fractures to propagate around corners and over ridges,
or to trigger the slopes "remotely" or “sympathetically”
meaning from a distance away. This is partly because the stiffer
slab transmits energy farther and can store more elastic energy.
Avalanche books tell us that most avalanches occur during or
immediately following storms, which is true, but especially
with persistent weak layers such as surface hoar, avalanche
danger to people is sometimes worse after a storm than during
a storm--after the slab has had a chance to stiffen up. Most
avalanche professionals I know would rather deal with the slab
while it's still soft because the hazard is both more obvious
and easier to manage.
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