A Comparison of the February 28, 2001, Nisqually, Washington, and January 17, 1994, Northridge, California Earthquake

Feb 27, 2011

When large earthquakes happen in other areas of the world, SCEC is often asked questions such as, "What would the damage have been if it had happened in southern California?" or, "Why was there so much damage (in the other area) even though the magnitude of the earthquake was not that large?" After the recent earthquake in Washington state, SCEC was asked an unusual question: "Why was the damage in the Washington so much less than the damage caused by the 1994 Northridge earthquake, when their magnitudes were so similar-- 6.8 vs. 6.7?" This article presents a comparison of the geological aspects of the two earthquakes, which account for most of the differences in resulting damage.

The Puget Sound region of Washington state was jolted from its morning routine at 10:55 a.m. on Wednesday, February 28 by an magnitude 6.8 earthquake. In Seattle, Tacoma, and communities across the region, the ground shook for up to 40 seconds, rattling buildings and sending some occupants into a panic. A major earthquake had struck the area, centered just 18 kilometers (11 miles) northeast of Olympia, where the state capitol building sustained notable structural damage.

Soon after the shaking stopped, it became clear that the damage was less than might have been expected from an earthquake of this magnitude if it had occurred in a similarly populated region of southern California. This event, named the Nisqually earthquake for a river delta near its epicenter at the southern end of Puget Sound, was actually slightly larger than the Northridge earthquake. Yet Northridge, which had a magnitude 6.7, was the costliest disaster in U.S. history with an economic loss of $40 billion. Based on preliminary reports, it appears that the loss in the Nisqually earthquake will total only $2 billion. Likewise, only one death, a heart attack victim, was reported in the Seattle area, while 57 people lost their lives in the Northridge earthquake. Was this because the Seattle area was better prepared for earthquakes than the Los Angeles area? Or was it due to differences in the earthquakes themselves?



Hypocentral Depth

Much of the difference in the total damage caused by the Nisqually and the Northridge earthquakes can be attributed to the Nisqually earthquake's location -- not that of its epicenter, but rather, the depth of its hypocenter. The Northridge earthquake had a hypocentral depth of 18 kilometers (11 miles), deep for a California earthquake, but considered shallow compared to other regions. The Nisqually earthquake had a depth of some 52 kilometers (33 miles), meaning that no building experienced greater shaking than would be expected at a distance of 52 kilometers from a magnitude 6.8 earthquake. That may seem to be an obvious statement, but earthquake depth is often entirely overlooked in reports of an earthquake's effects; conversely, no one would be surprised if it were reported that a town at a distance of 18 kilometers from an earthquake's epicenter suffered greater losses than one located 52 kilometers away from the epicenter.



Fault Rupture

The fact that western Washington state can experience such deep earthquakes, while southern California cannot, is the direct result of the tectonic environment found in each region. The sense of fault slip for the Nisqually earthquake was what geologists call "normal," meaning that the ground above the tilted fault plane moved downward relative to the material beneath the plane. This type of motion is typically found in extensional tectonic environments -- places where the crust is being stretched and pulled apart. That may seem out of place in the Pacific Northwest, where the oceanic Juan de Fuca plate subducts beneath the continental North American plate, but that collision of plates creates the bending stresses necessary to cause extension within the Juan de Fuca plate, where the Nisqually earthquake occurred.



Ground motion

Ground-motion data from seismometers across Washington state were processed by the U.S. Geological Survey to create a shaking intensity map (ShakeMap). This map is compared in the figure on the right with a similar map from the Northridge earthquake. One can see that the total area of potentially damaging shaking (yellow and warmer colors) was fairly similar for the two events, but the area of high-intensity shaking for the Northridge earthquake, shown in red, was essentially absent for the Nisqually event.

The difference in these maps is primarily the result of the difference in hypocentral depths of the two earthquakes. Given that the Nisqually earthquake occurred 52 kilometers beneath the suface, someone at its epicenter would be at the same distance from the event as someone 45 kilometers away from the Northridge epicenter. Both of these locations have a similar color (yellow-orange) on the shakemaps.

This same pattern is reflected in the Community Internet Intensity Maps (aka "Did You Feel It?") for each earthquake. Reports from the Nisqually earthquake show a distribution much like that seen in the intensity map -- a large expanse of "very strong" shaking (Intensity VII), but no reports of severe or violent shaking (Intensities VIII or IX), even in those places nearest the epicenter. In the Northridge earthquake, residents in the epicentral region (and some areas farther afield) reported severe, even violent, shaking (Intensities VIII or IX), while communities farther away experienced more moderate intensities. (A public lecture on both ShakeMap and the Community Internet Intensity Maps will be given by David Wald on Thursday, March 15, at Caltech.)

 

It is possible to take this a step further, and look at the ground accelerations recorded during each earthquake at locations with similar hypocentral distances. A plot of acceleration time histories can be seen at right. It shows the records of two strong-motion instruments from the Nisqually earthquake, each compared with a Northridge earthquake record made by a similar instrument from a similar hypocentral distance. Though it's impossible to draw sweeping conclusions from such a limited sample, you can at least that there are no radical differences in the accelerations experienced at similar hypocentral distances from these earthquakes.



Aftershocks

 

There was another striking difference between these two earthquakes that became apparent soon after the Nisqually earthquake struck. Whereas the Northridge earthquake was followed by hundreds of smaller earthquakes -- aftershocks -- within the first week, one week after the Nisqually earthquake only two aftershocks had been recorded. So far (and this likely won't change), the largest of the aftershocks to the Nisqually earthquake has been a magnitude 3.4 jolt that struck about 14 hours after the magnitude 6.8 mainshock. The largest Northridge aftershock, by comparison, measured magnitude 5.9, and five other first-week aftershocks exceeded magnitude 5. You can see these disparities in the graph at right. Moderate aftershocks are capable of causing further damage to weakened structures, so the lack of significant aftershocks was a factor in mitigating the total loss caused by the Nisqually earthquake.

It turns out that the dearth of Nisqually aftershocks came as no real surprise to seismologists. Previous large earthquakes at this depth in the subducting Juan de Fuca slab, in 1949 and 1965, were followed by very limited aftershock activity, so the Nisqually aftershock sequence, or lack thereof, is really quite typical. Likewise, the Northridge aftershock sequence was in no way unusual compared to the average activity of southern California aftershock sequences. Thus, the obvious difference between the two aftershock sequences reflects the underlying difference in the source of each mainshock -- one in the subducting oceanic slab beneath western Washington, the other in the comparatively shallow, brittle crust of southern California.

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Sound Seismic7543 15th Avenue NW, Seattle, WA 98117

Mary Ellen and I are both extremely pleased with the work that Matt and Richard did.  Please pass on my personal "thank you" to each of them. 

We were pleased with the amount of clean-up that was done at the end of each day. 

Even with your discovery of unforeseen conditions that necessitated additional framing, it was still done within a very reasonable time-frame.  My overall experience with Sound Seismic has been very positive and we will recommend your services to our circle of friends and family.

Again, thank you for a very "enjoyable" experience!

Mark and Mary Ellen Whitesell, Ballard, Seattle

 

 

I wish to praise SOUND SEISMIC for the earthquake retrofit they completed on my home. From the estimate to the finished job, Leif and his crew were professional, hard working, pleasant and exceptionally CLEAN. They handled the permit and inspections seamlessly. Office support from Elizabeth was equally professional, reliable and fast. I can't describe everything so I'll just say the whole experience was great.

Kristin Hill, Wallingford, Seattle

 

 

Sound Seismic was professional and they came when they said they were going to come. They came early and they called me to ask if it was okay to start the work earlier. They explained everything to me. It seemed like a reasonable price for the work because I had read other people’s descriptions on retrofitting on Angies list. The price seemed right in line with what I was expecting.
 
Sarah Morken, Tacoma
 
 
 

 

We recently had Sound Seismic do our earthquake retrofit. I cannot compliment enough the very professional job they did. Leif Jackson, the owner, sat with us for two hours answering questions and explaining how earthquakes work. He was completely above board, not promising more than he could deliver. He left interesting reading material so that we could evaluate and decide for ourselves. The most compelling was from FEMA warning people in the Puget Sound area to do everything they could to protect their homes because of our earthquake risk.

Once we were in contract, Leif sent our information to a structural engineer who drew up the plans. The bid was fair and complete. The crew showed up on the date the job started and completely it quickly. It looks great. Our new pony walls are gorgeous and the bolts and clamps very reassuring. City inspections were completed for the bolting and then for the finished job. This company is certified and high quality. Our house will be sticking around on the foundation. I encourage you to get over gambling about retrofitting and hire these guys. They are great.

Emily Elliott, Queen Anne

 

The crew kept me informed of their progress and plans all the time.  They told me what their plans were, and delivered every time.  Very satisfying knowing I could count on them doing what they promised - when they promised it!

Martin Muller, Olympic Manor, Seattle

 

 

My experience with Sound Seismic was very good. They were upfront about the work that needed to be done, about the cost and about the specific changes that would be required. In general, they had a high level of professionalism. They were on schedule, did a good job and cleaned up afterwards. They actually returned calls, which seems to be rare with contractors. I haven’t had other work of this sort done before. They did a really good job overall. They were very good about scheduling and estimates, and did quality work. They were one of the few providers in the area who would do the work - I thought the price was reasonable. I would use them again in the future.
 
Karl Bystrom, Bellevue
 
 
 

 

Sound Seismic was the most professional group of contractors/employees I have ever dealt with. They were knowledgable, dependable, and skilled. The project lead was obviously well trained and knew what he was doing, was able to effectively manage and direct his crew and subcontractors. A quality job. Discovered an area behind my walls were my house was not up to code. Worked well will me to correct the problem and professional and reasonable about the change order.
 
Grateful homeowner, Mercer Island
 
 

The Sound Seismic crew arrived on time and got started on this structurally engineered and contracted job. It was estimated to take approximately one week to complete this work. The job was completed in 4 days with great professionalism and dedication to the work completed. The work passed a city inspection with no additional work required. The quality of work is excellent with great attention to detail.

Leon Young, Issaquah

 

Based in Ballard, the 2 man crew of Sound Seismic did a very clean, professional job of installing the earthquake retrofit. I was very happy with quality of the work and the way the crew kept the site clean and organized every day. They stick to a firm 8-hour work day for their employees and offer health benefits, a rarity in the contract world, it would seem. So their bid was not the lowest, however was comparable to a number of other firms, within 5-10%. Very punctual and professional in all coordination and in handling the 2 inspections reqd. by the city. No surprises or additional costs. I would recommend this contractor overall.

Jennifer Dovey, Broadview, Seattle

 

 

Sound Seismic did such a thorough job. Looking at their work lets me sleep better at night knowing my house is now very unlikely to shift off its foundations in an earthquake.

P. Kiraly, Madrona, Seattle

 

 

We thank you for the very professional work done by all of your people that we have had contact with.  The crew working here were outstanding, and we were charmed by your lead carpenter, Matt. 

Our basement has not been this clean in 70 years (the house is 70 years old)!

Thanks again,

the Pearsons

Bryant neighborhood, Seattle

 

 

We had a wonderful experience with Sound Seismic.  After the devastating earthquake in Japan my husband and I were quite worried about our 3 story brick home and decided to retrofit our basement as a way to ease our anxiety and protect our family.  

Sound Seismic's estimate was more reasonable than any of the others we got and we felt very confident knowing that Leif and his team have done many retrofits, including our friend's basement.  Leif was able to explain the process to me, answer my questions, and gave me the estimate within an hour of coming to the house.  

The hardest part of the process was moving our furniture and boxes out of the way to prepare the basement - after that things just moved along smoothly.  I had been expecting huge holes in the walls and dust everywhere but the work was really tidy and not nearly as disruptive as I had feared.  My kids and I were able to stay in the house the whole time, only playing out in the backyard a couple times when the banging was loud.  The baby was even able to take naps in the midst of the demolition!  

Matt was very professional and friendly, he always arrived on time and he communicated well, checking in with us at the end of each day to show us the progress of the work.  He was able to finish the work a half-day sooner than planned and there were no hidden costs or extra changes when the invoice arrived.  

I would highly recommend Sound Seismic to anyone -- they were fantastic!

Rachel Opel, Ballard, Seattle

 

 

"Leif

Thanks for the great work on the house - you've done an outstanding job of prompt, low-impact work - and leaving the job site in excellent shape. Appreciate the attention to detail."

 

Wayne Dodge, Ravenna, Seattle

 

"I wanted to follow up and thank you for your good communication, strong planning and flexibility.

Matt was great. Very professional, low key, friendly and efficient."

 

Urs Koenig, Madrona, Seattle

They did an excellent job. The whole experience from start to finish was excellent. When they came out they told us what would need to be done, how it would be done and how much it would cost. We set a date and they came. They did the work exactly as they said they would. There were no surprises. The communication with the contractor throughout the process was excellent. The crewmember that came out to do the work did a fantastic job of cleanup afterwards. It was just excellent from start to finish. Because of the work they did we were able to get earthquake insurance, which we had not been able to qualify for previously. We would use them again and we would recommend them to any of our friends.
 
George Murphy, Northgate, Seattle
 
 
 

Sound Seismic gave a very competitive bid, clearly explained everything they were going to do, accomodated my schedule, and produced a very high quality outcome.  I highly recommend them.

Gary Johnson, Wallingford, Seattle

 

 

Dear Leif,

Matt and Richard did a great job!  It was a pleasure to meet them. And as soon as I met Matt, I knew that everything would be done 'right.' 

They took extra care at every level: and left everything better than ever.  They were able to remove the siding without cutting it, which is great, too.

I am glad that I got to see how substantial the anchors, fasteners, etc. are - and how robustly the plywood panels are nailed in. ( It was hard for me to tell from the blueprints alone.)

I am very happy with all aspects of the plan and the work, Leif.  

 

David Rosenbaum, Sand Point, Seattle

 

 

All in all, we felt the project went extremely well.  Given the number of earthquakes worldwide of late, we are pleased to have had the work done.  We’d rather not have it tested, but we know we are in as good a position as possible if the ground starts shaking!

Barbara E., Capital Hill, Seattle

 

 

I wanted to let you know what a great job that your crew did on our earthquake retrofit.  They were very knowledgeable about the whole process and answered any questions we had without hesitation. Their work was excellent and they finished the job quickly.  I have done my share of remodeling in the past and I can say that they cleaned up after each day’s work more thoroughly than anyone I’ve ever encountered.  They took obvious pride in the retrofit work they did but also in the cleanup. They saw that as an integral part of the Sound Seismic service.  All in all, it was a fast and painless experience.

 

Thanks for assigning those fine young men to our job.

 

Paul Gustie, Fremont

 

 

Sound Seismic performed an engineered retrofit on our house built in 1928.  I've never had a better experience working with a contractor - thoroughly professional (truly experienced and knowledgeable in their craft), they were also excellent communicators throughout the engagement.  And the cleanup afterwards was amazing.  I would recommend them without reservation!

George Murphy, Northgate, Seattle

 

 

I wanted to thank you for the wonderful job you did retrofitting my home in South Seattle. I was aware that it would be a challenge and it proved to offer more problems than we expected. You met them all and did a great job correcting the rot and damage that happened over the years since 1927, when the house was built.

 

When I arrived home yesterday the basement was clean and organized. It was the first time I did not have to spend hours cleaning up after a contractor. Your lead carpenter and his assistant were a great team. Thanks for all your efforts.

 

Regards,

Patrick Murphy, Mount Baker

 

 

I can't think of a single way you could improve the service of your company.  Yours is one of the VERY best companies I've worked with on this home.

Jeannie Macri, West Seattle

 

 

Sound Seismic completed a earthquake retrofit on our house. Their explanation about what to expect and what we'd get was clear and straightforward; the work was completed as described, as scheduled and exactly on time; clean-up: spotless.

 

Harriet B., Ballard

 

 

"The experience with Sound Seismic was possibly the finest I have had in my dealings with any of my trade contractors. Your crew was knowledgable, well informed, hard working, tidy, as well as a pleasure to have on my job."

Jesse Lee Honor, Project Manager, Northwest Homecrafters

 

 

Thank you very much for your excellent work on retrofitting our house.  We appreciated your careful oversight of the project, ongoing contact with the engineering firm, attention to detail, and clean and timely work.  You really rose to the challenges presented by "this old house."

Mike E., Capital Hill, Seattle

 

 

Sound Seismic exhibited a great combination of skill, hard work, and professionalism.  You treated our home and our project as though you were doing it on your own house.

Linda and Rick T. Kirkland, WA

 

 

Everything looks good. Thanks for the all the work your guys did, very professional.

Cheers,

Stuart Jamieson, Montlake

 

 

Sound Seismic did an excellent job of the seismic retrofit on my two-story house, which entailed quite a lot of siding work as I don't have a basement. The workmen were punctual, neat and meticulous. I feel safer because of their work.

Pippa Kiraly., Madrona

 

 

Just wanted to send you a note to express how pleased we are with the work your firm recently completed.  You did exactly what you said you would and the quality of the work is first rate.  Also, please tell Matt how much we enjoyed him.  Thanks,

Gary Johnson, Wallingford

 

 

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