One of my very important goals in life is to put as much distance between myself and major earthquakes as humanly possible. Hundreds of kilometers is generally a positive start. Thousands is even better. OTOH, this is tough to do in Anchorage, Alaska. So much for positive lifestyle choices and goals.
Friday, Nov. 30, Anchorage, Alaska suffered a major earthquake. The festivities kicked off at 0829 L. Most of us were at work. Schools were in session. Weather was not particularly frigid, with temperatures around 0 C. Overall a relatively nice day for this time of year.
The ground started shaking here in town. One of the things you start learning riding these things is to identify the difference in arrival time between the various waves generated by large earthquakes. The P & S waves arrive first, as they are fastest. The longer the period of time between the arrivals of P & S waves, generally the farther away the quake is. And if you can identify the P & S wave arrival, typically felt as sort of a buzzing (they are higher frequency than the surface waves) before the surface waves hit, the larger the quake is generally going to be,
Didn’t have any of that Friday, as we were sitting literally on top of the quake, just 12 km out of town to the north and 44 km deep. The surface waves arrived and quickly (within a second or two) ramped up to what felt like maximum amplitude. I rode the festivities out standing in a metal doorway frame.
The ground shook for a good 30 seconds, which felt like a week or two at the time. Time slows right down when you are having this much fun (/sarc for the “fun”). As the group of surface waves traveled south, the shaking abated and the ground kind of swayed for a while longer as things settled down. We were hit with the first major aftershock, a M5.7 a couple minutes after the main event. http://earthquake.alaska.edu/event/20419010/release
The following video was made in the State of Alaska courthouse downtown in Anchorage during the quake. Take a look at how far the pictures are swinging off the wall to the left of the courtroom:
As of this writing, there have been nearly 3,000 of them, with over 200 above M3.0+. I find I can generally feel anything above a M3.0+ at work or at home. Thankfully, they are settling down. More thankfully, so far, this event was not a foreshock. We will hope it remains that way.
Final note is the fundamental lack of lives lost in this event. The 2010 Haitian M7.0 earthquake took 100,000 – 316,000 lives depending on whose numbers you choose to believe. We are most fortunate. This is not luck. We made our own luck. There is no reason this can’t be done anywhere (or everywhere) else in the world.
Damage – Infrastructure
Lights in this part of town flickered briefly and went out in many parts of town as transformers tripped due to sloshing liquid in them. The electric utility got power back online pretty quickly. Water, sewer and natural gas utility service was generally maintained through the quake, though there were broken water mains in some parts of town. Many buildings (some but not all older buildings) had to contend with broken water pipes due to the quake. Phone and cell service did not drop, though they were very busy for the rest of the day.
I think I now know how earthquakes damage structures, especially the large quakes. Once you get to the maximum surface wave action, the motion over time simply beats the stuffing out of structures. A 30-second quake is bad enough. Consider what the M9.2 1964 Alaska earthquake did, moving for 270 long seconds. Surface wave amplitude in that one was around a meter total throw peak to trough.
Anchorage is built on glacial till, and there are a lot of wet, muddy, swampy parts in the region. This tends to allow earthquakes to liquefy wet soils underlying construction fills. Several portions of roads over and near these were the ones that failed as the ground underlying the fill partially liquefied. Onramp failures in Anchorage itself and partial failures of road shoulders north of Anchorage took place north of town. The section of Vine Road in the MatSu took place as the road crossed a swampy area. There are numerous road sections that sunk a bit and will have to be filled and brought back up to level grade.
One localism about roads is that road construction up here is generally a summertime event, normally April – September / early October. When the season is over, they shut down the asphalt plants throughout the course of the winter. In order to support the emergency road repairs, they had to restart the plants and it took a day or two to warm up.
Anchorage only has two roads out of town, the northbound Glenn Highway and the southbound Seward Highway. The Seward is a beautiful road that hugs rock cliffs for the first 100 km or so out of town. And it was closed for a while due to rockfalls on the roadway due to the earthquake.
The northbound Glenn had at least four road issues (bridges and roadbed) following the quake which seriously impacted daily commuter traffic into and out of town from the MatSu (50,000 vehicles each way).
The Anchorage International Airport closed for a while did halted inbound and outbound traffic. It reopened some hours after the quake when the facilities and runways were inspected. The audio file of the initial go around order from Approach Control can be found here:
With all that excitement, Anchorage briefly became an island as the only traffic in and out would have had to flow through the Port of Anchorage. This event will re-ignite a decades-long argument over a second bridge for northbound commuter traffic and a second route (third bridge) for southbound traffic.
Damage – Structures
While most of the big things rode the quake quite well, there was substantial damage to structures and things in those structures that will have to be addressed in the days, weeks and months to come. Things are standing, but have been rearranged slightly at best, substantially at worst.
Local home construction is typically based on a wooden frame with painted sheetrock inside. The wood is used because it is flexible. Sheetrock is not all that flexible and tends to crack and evolve off painted sheetrock mud as the flexing opens cracks that need to be repaired.
Local businesses generally did pretty well, though the liquor stores (lots of glass bottles) were particularly hard hit. Grocery stores had a lot of goods on shelves wind up on the floor.
One of the things a large earthquake does is open cabinet doors and cabinet drawers, throwing the contents on the floor or counter from some height above. This does not go so well with glassware, crystal, dishes and other breakables.
Local schools were hit pretty hard, with at least three closed pending repairs. There is some concrete block used in the older school buildings, which does not play well with extended shaking of large earthquakes. Local schools were cancelled for the week following the quake to assess structural damage before the kids start attending again.
Local contractors will be busy for the next few years doing repairs large and small.
One of the things that seemed to help us survive the quake were fairly stringent building codes that have incrementally improved structural survivability of newer buildings and structures, as the newer building tended to do better than the older ones. Local media and politicians were particularly proud of themselves for this in early public statements. The problem is that building codes almost always increase the cost of building. They are also a slam-dunk vehicle for local politicians to generate fawning media coverage and as such easy to ratchet up to unreasonable levels. Sooner or later you end up having to answer the economic and public safety question: What are reasonable and more importantly, affordable construction standards in an earthquake prone area?
The following is a computer simulation of how waves from the earthquake traveled.
Aftershocks have been going hot and heavy since the earthquake on 11/30. As of this writing, there have been 211 M3+ aftershocks. M3 is generally the dividing line between the ability to feel an earthquake or not feeling it. There have been 5 above M5+. All of those took place within the first 24 hours following the quake. As time goes on, the activity is tailing off with the quakes slowly becoming fewer and farther between.
USGS has a forecast page on aftershocks for the event: https://earthquake.usgs.gov/earthquakes/eventpage/ak20419010/oaf/commentary
Aftershocks, some in the M4+ range continue as I write this on 12/06 at 1250 L.
We are fortunate that the main quake was not (yet) a foreshock.
Liquefaction and slope failure was widely observed, particularly on muddy, swampy, saturated ground or on slopes right next to those sorts of flat areas.
There was at least one report of a pet going absolutely nuts the day before the earthquake. These sorts of reports are not uncommon before large quakes.
I ran across one report of earthquake lights in the mountains north of the MatSu the night before the quake. There was an associated video which I was not able to verify and is no longer available.
Many thanks to local outdoor writer Craig Medred for the pointer to a 2010 paper by Wong et al that nicely describes the seismic hazard in Anchorage. Much of this section will use the results of that paper. https://craigmedred.news/2018/12/05/preparation/
The 11/30/18 quake is described as an instraslab quake in the Pacific Plate underlying Anchorage. The boundary between the two plates lies some 32 km below Anchorage.
The closest significant fault to the epicenter is the Castle Mountain Fault, a strike – slip fault line to the west. It is not considered to be a significant contributor to the earthquake hazard in Anchorage. There are 12 potential active faults in the vicinity of Anchorage, Cook Inlet and westward to Castle Mountain. While these may contribute to the overall seismic hazard of the region, there is simply not enough data to characterize that hazard. These faults are not thought to be major contributors to earthquake hazard in Anchorage.
There are several causes for earthquakes in the Alaskan subduction zone. These are bending moment normal fault events in the Pacific Plate near and seaward of the Aleutian trench (the M7.9 Gulf of Alaska quake of Jan. 23, 2018 was one of these), megathrust earthquakes with a maximum depth of 35 – 40 km (the M9.2 Great Alaska Earthquake of 1964 was the most recent local example), intraslab quakes within the down going slab to depths of 150 km (the July 2015 M6.4 Iliamna, the Jan 2016 M7.1 Ilimana quakes, and within the North American Plate.
The Great Alaska Earthquake of 1964 was one of the most violent earthquakes on record and the second or third largest ever recorded. Rupture was about 100 km east of Anchorage with a displacement of about 20 m. There were around 130 deaths in state due to the earthquake, landslides, subsea landslides and associated tsunamis. USGS research suggests that average recurrence for the Prince William Sound segment of the megathrust portion is perhaps 650 years. The southwestern part of the zone near Kodiak Island ruptures separately and more frequently.
Intraslab earthquakes dominate the regional earthquake hazard near Anchorage, with at least 15 M6.5 or greater earthquakes within 200 km of Anchorage since 1898. These have been relatively deep at 30 – 120 km.
Since 2000, we here in Anchorage have felt at least five significant earthquakes, four in the last three years:
- Nov 2002 M7.9 Denali Fault earthquake. Located 327 km north of Anchorage. Quake was felt in town, though not strongly. Quake was a shallow, strike slip rupture of 300 km of the Denali Fault. There was a M6.7 foreshock 10 days before the quake. https://pubs.usgs.gov/fs/old.2003/fs014-03/
- July 2015 M6.4 Iliamna earthquake. Located 220 SW Anchorage. Strongly felt locally. Deep quake at 117 km. Subducting Pacific Plate ruptured. https://earthquake.alaska.edu/earthquakes/notable/2015-m64-iliamna-earthquake
- Jan 2016 M7.1 Old Iliamna earthquake. Located 261 km SW Anchorage. Up until last week, this was the strongest earthquake I ever rode through. Deep earthquake at 129 km. Subducting Pacific Plate ruptured. https://earthquake.alaska.edu/m71-iniskin-earthquake-evolving-content
- Jan 2018 M 7.9 Gulf of Alaska. Located 577 km south of Anchorage. Strongly felt in Anchorage. This was an intraplate strike-slip rupture. Generated Tsumani warnings for Kodiak, 180 km to the west of the epicenter. https://www.usgs.gov/news/january-23-2018-m79-gulf-alaska-earthquake-and-tsunami
Last week’s quake has been described as similar to the 2001 Nisqually earthquake near Seattle, which caused $1 – 4 billion in damage, one heart attack death, and hundreds of injuries.
Final cheery thought about large earthquakes is that reoccurrence times work out a lot like predicted volcanic eruption frequencies, in other words, they don’t. While the average time between Great Earthquakes in the Prince William Sound – Kodiak megathrust segment has been around 650 years, there is another megathrust segment farther along the Aleutians that triggered massive quakes in a short period of time. The 1957 M9.1 Andreanoff Island earthquake was followed in 1986 by a M7.9 with the epicenter in the same location. There were only 29 years between the two massive quakes.
At the top level, it is good to be alive with family, friends and neighbors intact an unharmed. Damage to buildings and contents can all be repaired over time, and it will. This earthquake is a reminder of the forces at work underneath the place we chose to live. We understand that choice and embrace it. Our job is to continue the incremental work of hardening ourselves, where we live, and where we visit, shop and work so that they can ride out the next one intact and alive.