Communicating Cascadia’s Earthquake Risk: How to Communicate Cascadia Subduction Zone Hazards


So I’m Tom Brocher, I’m with the US
Geological Survey in Menlo Park California, and I welcome this opportunity
and I’m very thankful for AGI to talk about how we might communicate Cascadia’s
subduction zone earthquake hazard and again we’re talking about an earthquake
that’s shown as occurring on this big pink patch on the map on the right or
subsections of that fault patch. So we’ve learned a lot of lessons from previous
earthquakes about how we communicate hazards, and one of these is to provide
the context of the of the hazard and the earthquake perhaps. In the Bay area we often
talk about the hazards in terms of what was experienced in the 1989
Loma Prieta earthquake, and in my presentation this morning I’ll be
talking about the Nisqually earthquake as a touchstone for people to get a
sense of what the Cascadia subduction zone earthquake might be like. We need to
be realistic when we talk about the hazard. There’s no need to make them larger
than they are. We need to be clear about what we know and what we don’t know and
make sure that everyone appreciates the fact that our knowledge is going to grow
over time. We need to communicate our our messaging with emergency
managers, engineers and public health officials so that we’re all speaking
with the same voice. If we have mixed messages that will lead to confusion and
confusion leads to inaction, and we need to make sure that we use very simple
language and we present the material in a variety of ways. If we want to
encourage preparedness one good approach is to show how preparedness has made a
difference in the past, and a couple of examples that I like to use are shown on
these illustrations on this figure. One shows a retrofit house. It turns out
there were two adjacent houses built the same way the same time with the same
materials that the same property owner bought in Santa Cruz, and he retrofit one
house before the 1989 Loma Prieta earthquake but he hadn’t retrofit the
other. The retrofit house did very well and sustained only a limited damage. The
unretrofit house was split apart in four sections and required extensive
rebuilding and in fact it had to be jacked up and placed on a new foundation. So a little bit of retrofitting can go a long way and we know that that works. The
other is drop cover and hold and we like to encourage that and we’ve just
had the annual ShakeOut exercise yesterday. It’s that most injuries that
result in earthquakes occur because people are pushed down by the earthquake
or fall over or things fall on top of them, so by getting onto the ground as
soon as possible and covering yourself you can hopefully avoid most of the
things that injure people. Also what works is to I found is that people like
to know what’s been done, what actions have been taken by governments and other
agencies to prepare for the earthquake. Once we tell people about the hazards
people want to know what they can do to prepare, so it’s important to tell them
that. And showing since we’re social animals showing pictures of people
preparing can be very effective in leading others to take action. So it’s
been mentioned by Jeff and Chris there have been about six large
subduction zone earthquakes in the past 50 years that we can learn from: three
have occurred in Chile, one in Alaska, one in Sumatra and one in Japan, and all
these have given us new lessons on how to survive earthquakes and tsunamis, how
to prepare for these hazards and have improved our building practices. And
after each of one of these earthquakes the US sends teams of geoscientists
and engineers to learn these lessons and recently there’s been some good news. The
the modern building codes that are in place in Japan and Chile have really
been effective in reducing the building damage in modern buildings. That’s really
good news. The biggest lesson probably from all these earthquakes and tsunamis
is that almost all the property damage and almost all the fatalities
result from the tsunamis that are produced by these earthquakes and the
submarine landslides. So, in terms of tsunami hazard mitigation, people need to know
that things are being done and so the hazard is being evaluated by tsunami
inundation maps. Once those are developed then evacuation routes can be
established, tsunami sirens can be put in place, training of the populations and
coastal communities is really important so that they know when the earthquake
occurs that’s their tsunami alert and that they need to evacuate as soon as
it’s safe to do so. Usually that’s when the shaking stops. Vertical evacuation
structures are being looked at as a solution for people who have long
distances to travel before they can make it out of safely outside of inundation
zones, and one is currently being constructed in West Port Washington as
part of a newly built elementary school, and you can see a picture of that
in the lower lower right. And as Jeff was mentioning avoidance of the hazard
altogether by land use planning and zoning might be another tool in our
tools chests to mitigate tsunamis, but I want to return to the earthquake shaking
hazard for a while and I want to make sure everyone’s aware that the Cascadia
subduction zone earthquake has been included in the USGS national seismic
hazard map since 1996, and in the building codes since about 2000. And that both the national seismic hazard map and the building codes are
updated about every six years to incorporate the latest science
into local resilience. Now as I said before and I want to emphasize in both
the recent Japanese and Chilean earthquake, subduction zone earthquakes
similar building codes have been very effective in preventing significant
building damage. Other tools for hastening and planning for these events
are scenario maps and I’ll show some examples of these. And the USGS is
also performing supercomputer simulations of what a subduction zone
earthquake might look like and the strong ground motions that it will generate
and these include realistic geological models of the crust along the I-5
corridor and elsewhere along the coast. And these all show that the ground
motions in the I-5 corridor will be lower than that they are along the coast
and I’m showing you here a scenario ShakeMap for a magnitude 9 – a wall to
wall rupture of the entire subduction zone – for Cascadia. And so the right
colors in this scenario map correspond to the highest levels of shaking which
are intensities seven or eight or so. Now the little inset to the left shows you
an actual shaking map that was made from observations from the 2000 Nisqually
earthquake which was a magnitude 6.8 and one of the points I would like to make
is that the shaking levels from this Nisqually earthquake in the Puget
lowland are comparable to what we expect from the Cascadia subduction zone
earthquake in Seattle but all along not only Seattle but all along the urban I-5
corridor. So the shaking levels per se are not expected to be any higher
than the Nisqually earthquake type levels. Now it’s true that the duration
of shaking is going to be longer and that a much larger area is going to be
impacted, but the levels of shaking are something that we’ve seen before and the
and this may not this is kind of a surprising result from these scenarios
but the good news for us is that the shaking levels are reduced by the fact
that the earthquake is primarily offshore and is located at some depth. So
that helps us reduce the shaking levels that we can expect. If we look at a
smaller earthquake similar to some of the smaller patches that Chris showed in
his presentation say a patch that might produce a magnitude 8.3 earthquake, we
see that this is still true that those shaking levels along the I-5 urban
corridor are about the same as we would we have seen in the Nisqually
earthquake. Now Chris alluded to the fact that there are a lot of uncertainties
and knowns about what the next subduction zone earthquake is going to be, how big
it’s going to be, where it’s going to start, and how far down towards the
coast it’s going to rupture, and the advantage of the USGS national
seismic hazard maps is that they incorporate all these unknowns and
uncertainties as different possibilities and they also including the possibility
that one of these magnitude 8.3 earthquake along the subduction zone
could occur anywhere on on the subduction zone. We also have tools in place and
we’re developing them more to mitigate aftershocks and we haven’t talked about
aftershocks very much yet but we’ve experienced in these prior large
subduction zone earthquakes that they are going to be very large and numerous
magnitude 6.7 aftershocks that will begin immediately after the earthquake,
They are going to be widespread they’re going to occur along the main plane that
produce the earthquake. They will occur within the oceanic crust and some of
these will occur in the crust of the coastal ranges.
For that reason they can cause additional damage. It can hamper rescue
operations, and they can also take a psychological toll on people because
they will be ongoing for months following the main shock. The USGS
routinely issues aftershock forecasts after large earthquakes talking about
the numbers and magnitudes of earthquakes to expect, and after the 1989
Loma Prieta earthquake, the USGS set up a system to issue real-time aftershock
alerts to rescuers working on a collapsed freeway, and that’s based on earthquake
early warning where when we have an earthquake it produces two different
types of earthquake waves. The first wave is not very damaging and it’s followed
by the more damaging waves shown in red here. We can using sensors that are on
the ground which I think of we can think of as trip wires, the sensors can detect
the earthquake, determine its location or its epicenter, and then estimate its
magnitude, and it can relay this information forward of the earthquake
before the damaging earthquake waves arrive. And we’re currently partnered
with the University of Washington, the Cniversity California Berkeley and
Caltech to develop a prototype prototype earthquake early warning
system called ShakeAlert. A similar system was in place in Japan for their
magnitude 9 2011 Tohoku earthquake and it worked in that event. ShakeAlert will
provide up to a few minutes of warning for a Cascadia zone earthquake. The
farther one is from the epicenter the more warning one will get. And it has
many uses but among those uses is that it can be used to provide aftershock
alerts which can help reduce some anxiety and it can certainly be used to
inform rescue operations that are in partially collapsed buildings. As Chris
alluded to there will be a new coastline along the Pacific Ocean
after this earthquake and that’s because there’s going to be an instant and
permanent lowering of the coastline of three to six feet allowing daily tides
to reach in much further into low-lying areas. So one can think of this as an
instant sea-level rise as well of three to six feet and so this will have an
instant flooding hazard will result as as well as a longer-term coastal erosion
effect. And this slide the background of this slide shows Brian Atwater, USGS
geologists and some of the dead trees that died because the coastline went
down three to six feet and killed the trees back in 1700. As both
Chris and and Jeff alluded to, we know we have vulnerabilities along the
subduction zone. We have a lot of built although we have very good building
codes in place now there are a lot of buildings that were built before these
codes, and so these are some of these are vulnerable. The most vulnerable of these
are unreinforced masonry buildings, buildings that have structurally weak
first stories, buildings that are older and built in soft soils and soft
deposits and some of our taller buildings in sedimentary basins which
will shake at the frequencies that the sedimentary basins will shake. Just
so you know both the cities of San Francisco and Los Angeles have passed
ordinances requiring mandating the retrofit of some of these most
vulnerable buildings so that those cities are more resilient to the
earthquakes that they are expecting. We also have significant tsunami evacuation
challenges as Jeff alluded to. This figure on the bottom shows a figure from
a recent USGS authored report that shows the communities number
up to 73 from Washington, Oregon and California – showing the numbers of residents
as a column and the color of the column or parts of the column reflect whether
it’s possible for those residents to evacuate tp higher ground at a slow
walk, a fast walk, or if it’s not possible. And so they’re different communities
along the coast. Most of the communities are very small and they can be evacuated
at a slow walk, but there are a few that have higher numbers of residents up to
12,000 or so, and some of these residents cannot walk their way to
safety. So we have different groups and different needs for tsunami evacuation
of planning and preparedness. I’d like to review very briefly, and this is one of
my last slides I think, an example of hazard assessment that led to a very
successful mitigation and this is an example of the Alaska oil pipeline which
was constructed in the early 1970s and as the planning for the pipeline was in
place they realized the pipeline would cross that Denali fault, which is a major
strike-slip earthquake fault very much like the San Andreas Fault and so
geologists went out to investigate the fault and they determined that the fault
produced large earthquakes about every hundred years and that the ground
shifted during these earthquakes about 15 to 20 feet. So the engineering
solution to that that hazard was to place the pipeline on the surface on
teflon skids with the theory that when the earthquake happened the ground was
shift underneath the pipeline and the pipeline would stay in place and that’s
exactly what happened. In the 2002 Denali earthquake which was a magnitude 7.9, one
of the largest earthquakes in US history the pipeline performed very well. It
didn’t spill a drop of oil and pipeline operations continued very
rapidly. So that’s a major success story, and it shows that these hazards can be
mitigated if we know about them in advance. And that’s really my the last
thought I’d like to leave you with is that you know we’re very fortunate in
the Cascadia that we’ve recognized the hazard before the earthquake has
happened. We can think of many examples in human history where this was not the
case, like Pompeii, Krakatoa, the 1906 San Francisco earthquake where the hazard was
not recognized and it couldn’t be mitigated. So here we have the hazard
recognized so now we can mitigate it. And to kind of reiterate what Chris said
along much of the Cascadia subduction zone at least the interval between large
back-to-back wall-to-wall earthquakes is something like every 500 years. It’s
about a one in ten chance in the next 50 years of a magnitude nine. Now to get
some context to that estimate the odds of a repeat of a magnitude 6.8 type
Nisqually earthquake are about 8 to 10 times higher. So if we prepare for a
Nisqually-type earthquake that preparedness will help us for the coming
Cascadia event as well. So I think that’s my last slide, and thank you for your
attention.

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