Seismo Blog

Don't Get Scared, Get Prepared!

September 14, 2023

by Kelly Missett

September is National Preparedness Month – a national observance each fall that encourages and empowers people to prepare for natural disasters and other emergencies.

In the Pacific Northwest, one hazard that everyone should prepare for is earthquakes. Oregon and Washington face significant risk of experiencing an earthquake along the Cascadia Subduction Zone, a 600-mile-long fault off the coast of the Pacific Northwest. Subduction zone faults can produce the largest earthquakes on Earth, and the Cascadia Subduction Zone has a 37% chance of producing a M7.0 or greater earthquake in the next 50 years. Our region has an even higher likelihood of experiencing a deep intraplate earthquake similar to the one that hit Nisqually in 2001 - an 84% chance over the next 50 years. These earthquakes could cause extensive damage to homes and critical infrastructure and disrupt normal services like utilities and transportation for days, weeks, or even months.

National Preparedness Month gives all of us the opportunity to get prepared for these possibilities. You cannot control the likelihood that earthquakes and other natural disasters will occur. But you can control your risk, or the likelihood that you will be hurt or that your property will be damaged. Taking steps to get prepared will ensure that you are warned before a disaster occurs, stay safe while it unfolds, and have access to basic necessities and communication with loved ones after it ends.

Getting prepared for earthquakes and other natural disasters can seem like a daunting or expensive project. But there are many simple and inexpensive steps you can take. Read on to learn preparedness tips as well as what the PNSN is doing to get prepared.


Sign Up for Emergency Alerts

You can sign up for free emergency alerts that will notify you if a disaster is occurring in your town and keep you updated as it unfolds. Having multiple ways to receive alerts is important.

  • Double check your phone settings to ensure that emergency alerts are enabled. Both Android and Apple phones have settings that impact your ability to get automated emergency alerts from the Wireless Emergency Alert system. 
  • Sign up to receive additional emergency alerts from your local county government. In Oregon, sign up via the OR-Alert website. In Washington, visit your county government’s website to learn how to sign up for local alerts.
  • Download free apps powered by the ShakeAlert Earthquake Early Warning System that could provide advanced warning before you feel earthquake shaking. Visit our website to learn more about the ShakeAlert System and how you can receive alerts in your home and workplace.

Receiving an alert is only useful if you know how to respond. As soon as you receive an earthquake early warning alert you should Drop, Cover, Hold On. This action will reduce the likelihood that you will get injured due to falling or from an object falling on you.


Develop an Emergency Plan

Disasters can place people in scary and unpredictable situations. You can get separated from your loved ones or need to evacuate from your home. Creating an emergency plan empowers you to know ahead of time how you would respond in these situations.

  • In your emergency plan, think through where you would go if you needed to evacuate, as well as what you would do if you had to shelter in place for an extended period of time.
  • Because internet and cell service may be down, make plans with your family about when and where you would reunify.  Identify a friend or family member in a different region that can be a point of contact for check-ins in case local power, internet, or cell service is disrupted.  
  • It is important to consider and include elderly people, disabled people, pets, and children within your plan.
  • Be sure to discuss your emergency plan with your family, neighbors, and other loved ones.

Learn more tips about developing an emergency plan at Ready.gov


Build an Emergency Kit

It is important to build an emergency kit with a two-week supply of basic necessities like food, water, and medications. This will ensure you can survive even if access to critical goods and services becomes disrupted for several days and aid takes time to arrive. Additionally, having an emergency kit takes pressure off of emergency responders so they can prioritize life-threatening situations and help the most vulnerable.

  • Be sure to include special supplies for children, elderly people, and disabled people, and pets in your kit. Find specific recommendations from the Oregon Department of Emergency Management website
  • Purchasing supplies for an emergency kit does not have to be expensive. Even purchasing one extra box or can of non-perishable food each week can help. Learn more about building an emergency kit over 12 weeks from the Lane County, Oregon website.
  • It can also be helpful to build a go-bag with critical necessities for a few days, including important documents, in case you have to evacuate your home suddenly. Learn more about what to include in your go-bag from CalFire.

More information about what to include in your emergency kit can be found at www.ready.gov/kit


Go On a Home Hazards Hunt

Earthquake shaking can topple heavy furniture, knock books and dishes off shelves, shake pictures or televisions off of wall mounts, and damage your home’s structure. Doing a home hazards hunt can make you aware of these possible safety risks and provide an opportunity to mitigate them.

  • Take 30 minutes to walk through your home. Imagine what would happen inside it during an earthquake. Make a list of these potential hazards as well as ideas for how to address them. This could include securely fastening heavy furniture and water heaters to walls, installing safety fasteners on cabinets and shelves, and removing pictures and mirrors from the wall above your bed.
  • A home hazards hunt is also an opportunity to learn how to turn off gas valves. This could prevent fires if gas lines break during an earthquake.
  • During your home hazards hunt, take a few extra minutes to list and photograph your possessions. If your home is damaged during an earthquake, this documentation can help you file insurance claims.
  • Earthquakes can also cause structural damage to older or poorly constructed homes. For example, homes can slide off their foundations and their cripple walls could collapse. Seismic retrofits can modify your homes to make them more resistant to this damage.

To learn more tips for conducting a home hazards hunt, download this PDF from the Washington Emergency Management Division website. To learn how retrofits can protect your home from earthquake damage, download this Homeowner’s Guide to Earthquakes from the Washington Geological Survey


The PNSN takes emergency preparedness seriously, and we have taken many similar steps to ensure our employees will be prepared if disaster strikes. The PNSN has a continuity of operations plan to guide our response in the event of a significant earthquake. This plan will ensure that employees can continue operating our seismic network and deliver timely information about the event to emergency managers and the public. We also maintain a supply of emergency food and water supplies at our Seattle headquarters, as well as smaller kits of emergency supplies in all employee vehicles. All PNSN employees are encouraged to enable their cell phones to receive earthquake early warning and other emergency alerts. We hope these examples will inspire you to take similar steps in your home and workplace.

Getting prepared will not only help keep you safe during an earthquake but also during other less perilous situations, such as a snowstorm that keeps you at home for several days. If you take action this month to get prepared, share it with us on social media! Tag your posts with #TakeControl123, #NPM2023, #BeReady, or #NationalPreparednessMonth.

 

Beast Quake (Taylor's Version) (From The Vault)

August 15, 2023

by Kyla Marczewski

A follow-up to the Taylor Swift 'fan-quake' events.
The many ways that the M3.9 earthquake can be visualized on PNSN.org all compiled in one place.

ShakeAlert 2nd Anniversary!

May 4, 2023

by McKenzie Carlson

Two years ago on May 4, 2021, public alerting of ShakeAlert earthquake early warning went live!

Pythia's Oasis

April 19, 2023

by McKenzie Carlson

You may have seen articles talking about the discovery of Pythia's Oasis, a seafloor seep off the coast of Oregon that is releasing fluid from the Cascadia Subduction Zone. There are a lot of misconceptions about this discovery, addressed here.
Medford 549c School District is now the first school district in Oregon to use ShakeAlert earthquake early warning technology. Medford 549c integrated ShakeAlert-powered alerts into the district’s intercom systems. When the ShakeAlert System detects an earthquake nearby, pre-programmed announcements will be automatically delivered in all district schools instructing students to Drop, Cover, and Hold On. By giving over 14,000 students and over 1,000 employees time to protect themselves, ShakeAlert could save lives during a Cascadia Subduction Zone earthquake.

Magnitude 4.4 event of Oct 7, 2022

October 7, 2022

by Steve Malone

A few observations and comments about the west central Oregon earthquake
Did you know that the world’s largest earthquake drill is happening soon? Get ready to Shake Out!
We highlight some of the contributions of our undergraduate summer interns. The variety of contributions they made give a peek into the many different corners of hazard-related science that PNSN and affiliated UW, UO and USGS researchers are involved in.
PNSN Director Harold Tobin and UW ESS grad students Anna and Peter spent a week doing fieldwork in Olympic National Park to investigate subduction zone rocks.
Distributed Acoustic Sensing (DAS) can use existing fiber-optic cables to monitor for earthquakes. A new research effort at UW and PNSN is exploring how.

The Fireball and the Explosion

April 7, 2022

by Paul Bodin

Former PNSN manager Paul Bodin follows up on a strange coincidence: an explosion on Orca Island preceding a fireball over the Cascades.

Mystery Booms and Forensic Seismology

March 15, 2022

by Steve Malone

Strange night time booms heard in the San Juan Islands are investigated using seismology
Good news for people who have been waiting for a ShakeAlert app to come to their smartphones: As of today, the 322nd anniversary of the last Cascadia Big One, the MyShake app is now available in Washington state.
The recent tornado disaster in the Midwest was preceded by copious warnings, giving time for protective actions, yet the destruction and suffering caused was huge. What does that imply for earthquake early warning systems?
On Tuesday morning about 300 miles off the coast of Oregon, an intense swarm of moderate sized earthquakes began on the Blanco Fracture Zone. This is active, unusual, and interesting, but not concerning. We take a deep dive in this blog post.
Network Manager Emeritus Paul Bodin explains why and how PNSN is preparing to respond to the next earthquake before it happens.
Two PNSN field engineers share their experiences doing fieldwork on Mt. Olympus, and the significance of the OSD station.

Washington’s Unsafe Schools

September 20, 2021

by Jim Buck

A recent DNR report showed that many of Washington's schools are extremely vulnerable to earthquakes. Former State Representative Jim Buck argues that the state should be doing more to solve the problem.
A recent 41-day research cruise will give us a better view of the Cascadia Subduction Zone.
If an earthquake were to occur tomorrow somewhere in Washington or Oregon, how much warning time would you expect to get from ShakeAlert?

M 3.9 quake is the largest in a Mt. Hood swarm

June 7, 2021

by Renate Hartog

Mt. Hood in Oregon is experiencing another swarm of mostly small earthquakes on its south flank. The largest, a magnitude 3.9, was felt by some, but most of the others were too small.

A Space Junk Mystery - Maybe Explained

May 20, 2021

by Steve Malone

In late March a string of lights in the sky and seismic recordings marked the return of space junk over Washington.

ShakeAlert Rolling Out in Washington on May 4

April 30, 2021

by Gabriel Lotto

The West Coast’s earthquake early warning system will deliver alerts publicly through the Wireless Emergency Alert system and Android OS.

Tremor Catalog Update - again

March 10, 2021

by Aaron Wech

Tremor fans will see a change in the catalog. The last four years are now more robust and complete.

December 2017 Oregon Tremor Event

December 15, 2017

by Nancy Sackman

December 2017 Oregon Tremor Event

 

Over the past 9-10 days, it appears that tremor in central Oregon has picked up (Figure 1).  The last slow slip and tremor event was in February 2016, 22 months ago.  

 

Figure 1

Figure 1. Age progression of tremor in central Oregon for the past 9 days.  Earliest tremor locations start from 12/5/2017 and propagate roughly outward, clustering near Salem and Roseburg.  Last update was December 14, 2017.


 

Tremor is the release of seismic noise from slow slip along the interface of the Juan de Fuca and North American plates and lasts for several weeks to months.  This process is known as Episodic Tremor and Slip (ETS).  Slow slip happens down-dip of the locked zone (Figure 2).  The locked zone is where tectonic stress builds up until it releases in a great earthquake or megaquake.  The recurrence interval of slow slip and tremor varies at different regions along the Cascadia Subduction Zone.  

 

Figure 2

Figure 2. Cross section of the subducting Juan de Fuca Plate.  Figure from Vidale, J. and Houston H.  (2012) Slow slip:  A new kind of earthquake (Physics Today, 2012 pages 38-43).

 

The last ETS event in Cascadia started in February 2017 around the western edge of the Olympic Mountains.  The duration was approximately 35 days with a two-week quiescent period.  Prior ETS events in northern Washington/Vancouver Island area was approximately December 2015.  

 

The last ETS event in central Oregon was 2016 and lasted just over a week before it stopped on March 1, 2016.  

 

ETS events are still being studied to understand the processes about slow slip and megathrust earthquakes.  


More information about slow slip and tremor can be found here on the PNSN website.

December 2017 Oregon Tremor Event - Update

December 27, 2017

by Nancy Sackman

Tremor has continued in Oregon since the last post on December 15th.  Current tremor activity has been ongoing since about 12/5/2017 (figure 1).  

 

Figure 1

Figure 1. Age progression of tremor in central Oregon for the past two weeks.  Earliest tremor locations start from 12/5/2017 and propagate northerly and southerly.  Last update was December 26, 2017.

 

Since December 19th, tremor has now migrated northerly toward Portland and southerly toward Medford (figure 2).

 

Figure 2

Figure 2. Tremor activity from 12/19 to 12/26 showing progression in a northern and southerly direction.

More FAQs on Slow Slip and Tremor

 

On our previous blog post, we briefly discussed what ETS (episodic tremor and slip) is.  Let’s go through a couple of more frequently asked questions.

 

1.What is tremor?

 

Tremor in the Cascadia Subduction Zone is the seismic noise of slow moving earthquake along the interface of the subducting Juan de Fuca Plate and the North American plates.  Compared to normal earthquakes, tremor has lower frequency energy and can last for minutes, hours or weeks.

 

2. What about volcanic tremor?

 

Tremor can also be volcanic.  But ETS is deep, non volcanic signatures that are a result of plate motion, not magmatic movement.

 

3. How deep are the tremors?

 

As it states on our website - “This is a topic of ongoing research.”  But research suggests that it occurs near the plate interface at approximately 30 - 40 km deep.   

 

4. What is the magnitude of tremor?

 

Tremor is probably made up of many tiny individual earthquake-like sources each with a "magnitude" of less than 1. Since tremor is an on-going continuous signal assigning a magnitude to it is never done.  


 

Check out the map on our web page:

 

https://pnsn.org/tremor

 

In the previous blog post about The M9 Project, we talked about how the Cascadia Subduction Zone can generate an M9.0 earthquake. However, our understanding of what an earthquake of this scale would actually look like is less advanced. While we have evidence of past earthquakes (e.g., native oral histories, tsunami records), we have no quantitative observations of how strong the shaking would be during a megathrust earthquake in the Pacific Northwest.

To address this problem, researchers with The M9 Project used 3D computer simulations to help understand what 50 different realizations of an M9.0 earthquake could look like in Cascadia. To create these simulations, The M9 Project researchers used multiple supercomputers: Stampede (University of Texas - Austin), Constance (Pacific Northwest National Lab), and Hyak (University of Washington). A single earthquake simulation took up to 46 hours to complete. If it was possible to run these earthquake models on a personal computer (many of which have a mere 2 processors, compared to the 576 processors used to run these simulations on a supercomputer), it would take about 522 days to complete one simulation.

 

Why are earthquake simulations important?

 

 

The unique properties of the Cascadia Subduction Zone prevents a side-by-side comparison between a future Cascadia earthquake, and other earthquakes that have occured around the world. For instance, an M9.0 earthquake in Japan, Chile, or Indonesia may look very different from an M9.0 in the Pacific Northwest.

 

Scientists have developed equations that can estimate the strength of ground shaking based on an earthquake’s magnitude and a specific location’s distance from the fault. However, these equations still rely on averages, and do not fully account for location specific 3-D effects (i.e., “How will seismic waves bounce around in the Seattle basin?”). Conversely, the supercomputer earthquake simulations, while still having some unknowns, can estimate shaking at every point in the Pacific Northwest, and are specific to the geologic conditions of the Cascadia Subduction Zone.

 

 

How are these earthquake simulations created?

Out of an infinite number of possibilities, 50 simulations of an M9.0 earthquake were run by The M9 Project team. The individual earthquake scenarios had a few important variations between them, that made each earthquake source unique:  (1) the hypocenter location (i.e., where the earthquake starts), (2) how far inland the rupture extends (i.e., how close the earthquake gets to major inland cities, such as Seattle), (3) the location of “sticky patches” on the fault, that generate the strongest ground shaking, and (4) the slip distribution on the fault (i.e., how far certain areas on the fault move during the earthquake).

 

Are certain earthquake scenarios “better” or “worse”?

The area affected by a megathrust earthquake is large enough that the outcome is going vary by location. A “best-case” scenario for one area in the Pacific Northwest could be a “worst-case” scenario somewhere else in the region.

One of the results of the computer simulations showed that when an M9.0 earthquake occurs on the Cascadia Subduction Zone, less violent shaking may be felt closer to the epicenter. This is because an earthquake on the Cascadia Subduction Zone will not occur at a single point -- instead, it will rupture a very large area. As the rupture moves along the fault, the seismic waves will start to “pile up,” similar to the Doppler Effect.

 

As the waves at the front of the rupture combine, their amplitudes get larger and create more violent ground motion. Therefore, locations closer to the hypocenter may receive less complex and destructive seismic waves than locations that are farther along the rupture and experience this “piling-up” of seismic energy.

In these two videos, notice how Seattle's mock seismogram has larger spikes (which denotes stronger ground motion) when the earthquake source is farther south, and the fault ruptures north.

BetterForSeattle_3DMovie

WorseForSeattle_3DMovie

This variation by location makes it virtually impossible to award a scenario the title “best-case” for the entire Pacific Northwest.

 

Can we do even better?

These computer simulations are the most accurate representations of what an M9.0 earthquake would look like in the Pacific Northwest. Unfortunately, there is a lot of variability in these calculations because there are still too many unknowns. An increase in seismic and GPS instrumentation throughout the Pacific Northwest, especially offshore, will help us identify more specifics about the Cascadia Subduction Zone and improve future computer simulations. For instance, we may be able to determine where “sticky patches” are located on the fault and obtain a more detailed image of the 3D structure of the subduction zone. Further constraining these variables in the computer simulations will ultimately help us refine our estimates of seismic hazards in the Pacific Northwest.

 

Special Thanks To

 

Dr. Erin Wirth, UW Affiliate Assistant Professor

 

 

M9 Simulation coverage from UW News

 

50 simulations of the 'Really Big One' show how a M9.0 Cascadia earthquake could play out - Hannah Hickey

 

Flickr Album

We are going to have The Big One. That’s a fact.

 

The final blog about The M9 Project is going to focus on you. What are you going to experience during a megathrust earthquake? How do we connect science and community? What should you do to be prepared?

 

The Next Stages of The M9 Project

 

Seismologists are not the only contributors to The M9 Project. Civil engineers, urban design and planners, statisticians, social scientists and public policy researchers also play a role in determining earthquake risk, safety measures, and public response to hazards.

 

Understanding the hazards and mechanisms of an earthquake is one thing, communicating effectively to the public is a completely different ball game. The next steps of The M9 Project focus on how we define and discuss hazards with communities.

 

For example, how does the way we design hazard maps affect how communities approach hazard planning? (See photo below) Or, how can hazards planning be steered towards rebuilding to community-specific values?

 

 

From assessing the utility of hazard maps (see image below), to hosting community planning workshops , The M9 Project’s research into “long term” preparedness- mitigation, response, and recovery focuses on how to best help you. We will discuss what to expect when the big one hits, as well as some resources so you can take steps to prepare .

 

Learning about Cascadia from other Large Earthquakes

 

The last megathrust earthquake on the Cascadia Subduction Zone occurred in 1700 AD, before written records were kept in the region. In addition to The M9 Project research at UW, we can also look to observations of other major earthquakes worldwide, to help us predict what The Big One may look like in the Pacific Northwest.

 

Ground Shaking

 

A magnitude 9 earthquake will generate very strong shaking for several minutes. The intensity, measured by the Modified Mercalli Intensity Scale (MMI), is determined by observations during an earthquake. Shaking tends to decrease farther away from the fault and will vary with local soil conditions, so intensity will vary by location. A more detailed description on intensity can be found here.

 

Below is a comparison of the shaking intensity from the 2001 Nisqually earthquake, compared to a hypothetical  M9.0 earthquake scenario. A megathrust earthquake will be felt over a much larger area, and generate stronger shaking.

 

 

 

You can find more about how magnitude and intensity are related here.

 

As part of The M9 Project, UW civil engineers are researching building response to strong ground shaking from a magnitude 9.0 earthquake in Seattle. This video from Kinetica Dynamics shows skyscrapers in Tokyo shaking from the 2011 M9.0 Japan earthquake.

 

 

Tsunami

 

Large earthquakes on a subduction zone are capable of generating large tsunamis. For example, the 2004 M9.1 Sumatra earthquake resulted in a 30+ meter high tsunami on the west coast of Sumatra (source). For more about tsunamis, visit our tsunami overview page.

 

This NOAA video models the tsunami from the 1700 Cascadia earthquake, which caused damage and loss of life as close as the west coast of North America, and as far away as Japan.  

 

 

We expect the next great Cascadia earthquake to be similar. As mentioned in our first M9 Project blog post, the record of past megathrust earthquakes can be found in muddy estuaries on the coast of the Pacific Northwest. In the layers of coast that have subsided and been filled again, there are bands of sand brought inland by tsunami waves, time and time again. Here is a article written by the American Museum of Natural History with more information on the Ghost Forests on the PNW.

 

Liquefaction

 

For liquefaction to occur, three things must happen. (1) Young, loose and grainy soil (2) needs to be saturated with water, and (3) experience strong ground shaking. The USGS, in coordination with California Geological Survey, give a summary of liquefaction and its effects here.

 

This video from the 2011 M9.0 Japan earthquake shows dramatic cracks in the ground, as well as liquefaction.

 

 

Our webpage on liquefaction includes video of the 2011 Christchurch earthquake, as well as links to liquefaction hazard maps for Washington and Oregon.

 

Landslides

 

Strong shaking can increase susceptibility to landslides.

 

This blog from the American Geophysical Union details some of the significant landslides in Paupa New Guniea that were triggered by a M7.5 earthquake on February 25th, 2011.

 

The Pacific Northwest is susceptible to landslides due to seasonal conditions, and strong shaking will increase landslide risk. Here are some resources from the states of Washington and Oregon.

 

Building Damage and Fires

 

The 1989 Loma Prieta earthquake caused widespread damage to infrastructure, such as the collapse of the Cypress Viaduct in Oakland, and fires in the San Francisco area.

 

Major cities in the Pacific Northwest would be just as susceptible to fire damage, and the construction of the Cypress Viaduct bears a striking resemblance to Seattle’s Alaskan Way Viaduct.

 

 

So, What Can I Do?

 

In order to prepare effectively, it is important to be aware of all earthquake-related hazards, such as the ones listed above. The following websites are great resources for earthquake hazards and preparedness in the Pacific Northwest. We encourage you to know your risks, be prepared, mitigate against hazards, respond safely, and enagae in holistic recovery planning.

 

In the event of an earthquake, don't forget to drop, cover and hold!

 

Special Thanks To

 

Dr. Erin Wirth, Affiliate Professor, University of Washington

Lan T. Nguyen, Doctoral Student, Interdisciplinary Urban Design and Planning, University of Washington

 

 

MSH Anniversary Media Round-Up

May 18, 2018

by Elizabeth Urban

On May 18th, 1980, at 8:32 AM, the landscape in Southwestern Washington was forever changed by an explosive eruption of Mount St. Helens. This was the most deadly volcanic event in US history. 

Mount St. Helens is part of the Cascade Range, a chain of volcanoes from British Columbia to Northern California. The PNSN and the Cascades Volcano Observatory cooperatively operate 21 seismometers on or near Mount St. Helens, the most historically active volcano in the Cascade Range.

Seismogram of May 18th from one of our seismic stations. 

Main PNSN Page on Mount St. Helens

Main CVO Page on Mount St. Helens

 

On the anniversary of the eruption of Mount St. Helens, earth science gets its day in the spotlight. Here's a collection of news stories about the eruption and what we have learned about volcanoes since 1980.

38 years later: What's changed since the Mount St. Helens Eruption? 
- Q13 News 

Features an interview with PNSN DIrector Emeritus Steve Malone

 

Mount St. Helens: Remembering the deadliest U.S. eruption 38 years later
- USA Today, King 5 News

Summary of events on May 18th, 1980 with photo galleries

Lessons Learned: Mount St. Helens to Kilauea
- King 5 News

Interview filmed in the PNSN Seismology Lab with Director Emeritus Steve Malone

 

Remembering Mt. St. Helens as Cascade event looms
- KOIN 6

Event overview, brief discussion of Cascade hazards, and dispels concern of a Kilauea-triggered event in the Cascades.

Photos: The Mt. St. Helens eruption of 1980
- KOIN 6

Includes an interview with CVO Scientist-In-Charge Seth Moran

 

Scientists Reflect on the Catastrophic 1980 Mount St. Helens Eruption
- Ashley Williams, AccuWeather

Details about the activity that led to the eruption, how life in the MSH area fared, and another Steve Malone feature. 

 

How Dangerous are the Northwest's Volcanoes?
-KUOW, Oregon Public Broadcasting

Interview with CVO Scientist-In-Charge Seth Moran, discusses Oregon volcano hazards

A US Geological Survey seismic hazard map for the city of Seattle.

Background: The Challenges of Predicting Seismic Hazard

We all want to know when the next big earthquake will happen. But because we can’t predict earthquakes, the next best thing we can do is figure out how strong shaking could be during future quakes. To do this, earth scientists can reconstruct an area’s historic earthquake record using geologic markers and any available written accounts, which allows them to constrain recurrence and magnitude ranges for quakes on local faults. Scientists then combine these estimates with what we know about an area’s surface geology to predict how often and how strong the ground will shake during an earthquake. This sort of analysis is the basis for the US’ National Seismic Hazard Mapping Project, and it is critical for making sure we build structures that can withstand earthquake shaking.

Predicting how an earthquake will shake an area is not a trivial task, however.  Besides the challenges of figuring out the historical earthquake record from sparse geologic markers, understanding how conditions at a given location will affect earthquake shaking is very difficult. In Seattle, for example, there are many interesting geologic features that change and amplify ground motion during earthquakes; large areas of water-logged, artificial fill under Pioneer Square and SODO have amplified ground motion and even liquefied during historic earthquakes; the deep Seattle Basin, which contains thick layers of relatively soft sediments, traps seismic waves and “sloshes” like a big bowl of jelly in some earthquakes. Taking these effects into consideration during seismic hazard analysis is important, but requires a detailed understanding of the local geology and how it affects seismic waves.

 

The hills are alive with the sound of … earthquakes?

A geographic component that is not often considered during seismic hazard analysis is topography. As seismic waves from an earthquake propagate through the earth, they interact with subsurface geology, like faults and basins; however, the waves also interact with surface features, like hills, cliffs, and valleys. Depending on the wavelength of the seismic waves and the direction they’re traveling, the shaking felt on these features can be much stronger (or weaker!) than in surrounding flat areas. This amplification happens because these features scatter and trap seismic energy; they can also experience a sort of structural “resonance”, similar to the way buildings and bridges can. Studies of ground motion from the 2009 L’Aquila and 2010 Haiti earthquakes [1, 2] have shown that topography significantly amplified ground motion at certain locations, sometimes shaking twice as strong as surrounding areas!

With these findings in mind, we are interested in seeing how topography might affect earthquake shaking here in Seattle. While the city doesn’t have any particularly tall or prominent ridges, it does have many steep bluffs and cliffs overlooking Elliott Bay and Puget Sound. We know from past earthquakes that these features are prone to land-sliding, and we would like to know how topographic amplification might contribute to that hazard, as well as to the general safety of structures built on the bluffs.

 

How will the 70-90m tall bluffs around the city behave in an earthquake?

 

If you live inside the blue box, please consider hosting a station!

How will things shake out in West Seattle?

So, as part of a graduate-student-led research project, we are looking for volunteers in West Seattle to host seismometers for a small experiment. In this project, portable seismometers will be placed along and down the bluffs facing Puget Sound. Over the course of a few hours, these seismometers will record the ambient vibrations caused by ocean waves, weather systems, and even car traffic. Together, these sources create what’s known as ambient seismic noise. By comparing recordings of this noise from points along the bluff, we can understand how the topography amplifies ground shaking during earthquakes.

For our experiment, we are looking for hosts in West Seattle living near the bluffs along Alki. Ideally, hosts would live on or between Sunset Ave. SW and Alki Ave. SW. We also need a few sites in the neighborhood away from the sea bluff (see the map below). The experiment itself will last just 1 day, and only requires access to your yard, where we will plop-down a coffee-can sized seismometer for a few hours. The experiment will take place some time between early October and late November.

So, if you are a citizen scientist who would like to help us better understand how earthquakes rattle the hills around our city, and you live on or near the Sound-facing bluffs in West Seattle, please consider filling out the form linked below!

If you have any questions about the experiment, you can contact the project lead Ian Stone at ipstone@uw.edu

Link to Sign-up Form: https://forms.gle/vfSBgpZUapbKs5eo7
 

References

[1] Massa, M., Lovati, S., D’Alema, E., Ferretti, G., and M. Bakavoli, 2010. An experimental approach for estimating seismic amplification effects at the top of a ridge, and the implication for ground-motion predictions: The case of Narni, Central Italy. Bul. Seis. Soc. Amer., 100 (6) 3020-3034.

[2] Hough, S. E., Altidor, J. R., Anglade, D., Given, D., Janvier, M. G., Maharrey, J. Z., Meremonte, M., Mildor, B. S. L., Prepetit, C., and A. Yong, 2010. Localized damage caused by topographic amplification during the 2010 M 7.0 Haiti earthquake. Nature Geoscience, 3. 778-782.