Evacuated!

After this earthquake.... there was a tsunami warning and I got evacuated!

What an exciting Saturday night! After spending the day cleaning the place I am moving out of, my husband and I went down into town to have dinner and meet some friends. After dinner while we were waiting I got a phone call from another friend who lives on Oahu saying that there was a tsunami warning in place for the Hawaiian Islands. You know you are a geologist when... your first question is "where was the earthquake!?" followed by "How big was it!?".

By the time my friends showed up at our appointed meeting place (a hotel that is closing this week that we were going to enjoy for one last time) the tsunami sirens had started going off. What do 2 oceanographers and 2 geologists do? Well run down to the beach to take before pictures of course! Ok, we didn't have to go that far we were already next to the beach. But we took some long exposures of the beach to get a before and watched the insane amount of traffic until we got kicked out by the cops. Now where I live in Hawaii to get out of the evacuation zone you pretty much walk up one block.

Our before picture, I still need to go take an after picture...

At home I am at 1,500 ft and my lanai looks out over the ocean. We could see the lights of all the boats that went out last night in preparation of the tsunami, and hear the siren. We had live streaming news (from Oahu really nothing on Hawaii Island) and a few webcams open, and were constantly refreshing the PTWC page for more info. And then it hit. Or we assumed that is when it hit. The tsunami sirens just kept going and going and going. And then it was silent. After that the news started coming in that the first was was smaller that forecast (but I say better than bigger right?).

The whole night I was thinking about the events in Italy (which I haven't even written about yet) and the relationship between the public, civil defense agencies and scientists in general. In Italy, an event (EQ) that was larger than anticipated happened, here the event (tsunami) was smaller than anticipated. In both cases there is always bitching which leads to a damned if you do and damned if you don't environment. I am always impressed with the individuals that after these events stand in front of the media like the guys from PTWC did last night and take a beating. Good job guys! In my mind with the limited data you had you did an excellent job! So a big MAHALO from me! :)

Overall a very exciting night. But enough personal story and on to the earthquake. The second I got my hands on a smart phone last night and saw where the earthquake was I got excited. REALLY excited. Between the Juan de Fuca subduction zone and the Alaska Subduction zone? How cool is that!? I have lived along the west coast and in Alaska for years and know about the potential for earthquakes along the whole coast but never expected to see one there! Once I got home, it being me I was on my computer on the usgs site poking through everything that was up.

The first thing I did was look at the moment tensor solution. What type of quake was it? Being passivingly familiar with the area I know one thing for sure: this section of the plate boundary is transform... this means the event is likely a strike slip. This fact actually puzzled me for a while last night, if its a strike slip why is there a tsunami warning and why are they saying its going to be big and why are they evacuating everyone?

When I got my hands on my friends phone I saw this:

HOLY NOT A STRIKE SLIP!  

That explained the tsunami warning! This event turned out to be a M7.7 (!) at a depth of ~17.5 km and a thrust event at that. There was a tsunami warning and evacuation that went out for all of Hawaii. Last night before the waves hit we were hearing 3-5ft and to get up NOW. It did not end up being that big, but it made for a very exciting night!

This morning I did more poking. I started looking at the historical seismicity maps because, well I haven't really heard of much in this area. However, turns out there has been some seismic activity in this area in the past!

This is all the earthquakes from 1900 to present in the area. This is a map of reported earthquakes since 1900. This next map however totally shocked me. 

This is a map of M7 and larger earthquakes in the region since 1900. LOOK AT THAT! There is another M8 in the region! Well it is time to put my on detective hat and find out about that one! 

Here is the plan.

find this other large event

compare the events

gain knowledge.

WIN

As with most inquiries into earthquakes like this, a good place to start is the summery on the USGS EQ page for the event. There I found this trivia: 

This region of the Pacific:North America plate boundary has hosted 7 earthquakes of magnitude 6 or greater over the past 40 years – the largest of which was a M 6.6 earthquake in 2009, 80 km to the south east of the 2012 earthquake. In 1949, a M 8.1 earthquake occurred closer to the Pacific:North America plate boundary, likely as a result of strike-slip faulting, approximately 100 km northwest of the October 28th earthquake, near the northern extent of Haida Gwaii region (formerly Queen Charlotte Islands).

Well isn't that interesting? There have been 7 events M6+ and in 1949 there was a M8.1! This event is reported to have been on the Queen Charlotte fault system (and a strike slip event) which is exactly what I thought this one was (and turned out not to be). 

According to the Natural Resources Canada website 

The shaking was so severe on the Haida Gwaii that cows were knocked off their feet, and a geologist with the Geological Survey of Canada working on the north end of Graham Island could not stand up. Chimneys toppled, and an oil tank at Cumshewa Inlet collapsed. In Terrace, on the adjacent mainland, cars were bounced around, and standing on the street was described as "like being on the heaving deck of a ship at sea". In Prince Rupert, windows were shattered and buildings swayed. 

So about that 2009 M6.6 event? What type of event is that? I found the usgs earthquakes page on the event and that event was also a strike slip event, like the 1949 event. On this page I found a historic moment tensor map. 

So there you have it. There is one thrust event on this map, and the majority are strike slip. I say that means our earthquake last night is quite special and I am looking forward to reading all the papers that it produces!