Rivers carry sediment (soil and other debris) into the ocean, and
sediment collects on the continental shelf and continental slope,
which slopes into deeper water. More and more
material builds up on the the continental shelf sea floor until it becomes unstable and slides down the
continental slope, much like an avalanche, in what is called a turbidity
current. 
The
resulting layer of sediment this current deposits on the sea floor is
called a turbidite.
A number of events can potentially trigger turbidity currents. These
events include tsunamis, storm induced waves, slope failures, and earthquakes.
The turbidite record strongly suggests the latter — coastal Washington
and Oregon experienced strong coast-wide shaking typical of a large
subduction zone earthquake.
Core
samples taken from various drainage channels off-shore were studied by Gary B. Griggs, and all samples showed that 13 turbidites had been
deposited off coastal Washington and Oregon since the eruption of Mount
Mazama (a well-dated geologic event that produced a highly visible layer within all the samples) John Adams suggested that turbidity
currents originating from different locations occurred simultaneously during great subduction zone earthquakes.
When simultaneous turbidity currents from different side channels merge, the main channel
can be expected to show a single large turbidite.
If the turbidites orignated at different times in the side channels, the
main channel would record each separate turbidite event,
The consistent number of turbidites
in core samples from the side and main channels indicate that the turbidity currents were likely
caused at the same time and by the same event.
Large storms are an unlikely source of a coast- wide event because
these storms produce waves not much larger than smaller, more common
storms. If common and rare storms produce waves that are approximately
the same magnitude, the turbidite record should reflect more than 13
events in the last 5,000 years.
The 1964 Alaska earthquake generated the most recent damaging tsunami
that struck the Oregon-Washington coast. Although this earthquake is
one of the largest seismic events of the 20th century, it did not produce
any recorded turbidites. If this large tsunami did not trigger
a turbidity current, it is highly unlikely the turbidite record reflects
the occurrence of tsunamis.
In a slope failure, so much sediment develops on the inclined continental slope
that it slips, much like an avalanche triggered by excessive snowfall.
When enough sediment accumulates at a given point on a coastal slope,
slope failure will occur. This underwater avalanche can cause turbidity
currents to spread sediment throughout the underwater seachannels. Although
these kinds of currents are likely to occur given enough time, the different
rates of sedimentation and inclination of coastal regions make the synchronized
turbidity currents implied shown in the core samples unlikely.
Cascadia zone earthquakes, on the other hand, prove to provide enough
force and affect a large enough region of coast to have caused the turbidites
in the core samples. Subduction zone earthquakes are cyclical and have
large recurrence intervals, as do turbidity currents. Radiocarbon dating
of each turbidite in Adams' core samples show a recurrence interval
of about 590 years, closely matching the interval of coastal subsidence
observed in coastal Washington.
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