Tuesday, December 02, 2014

Tuesday Update - November 30 Oak Creek Canyon Earthquake

Geologists have developed clever ways to peer deep beneath the earth's surface and these tools are being utilized to get a clearer picture of the November 30 earthquake in Oak Creek Canyon. In addition, aftershocks have been pretty regular since the 10:57 PM main shock.

Courtesy Google Earth with annotations by Jeri Young Ben-Horin at AZGS - Updated December 3, 12:05 PM
The purple object represents the epicenter of the M3.5 foreshock on November 25
Here is a Google Image provided by colleagues that shows how the epicenters for the aftershocks have migrated northeast from the main shock epicenter. The main shock is depicted as the red dot in the lower left quadrant. Note four aftershock epicenters at top center. These are located just west of Interstate 17 near the Newman Park exit. These aftershocks range from 4.7M to 2.2M and are located between the Oak Creek Canyon and Munds faults (faults on this image are shown as very light white lines and the Munds fault parallels I-17 on its east side).

This is one of the tools geologists use to understand quake and fault plane mechanics. On the left you can read the various location parameters. These are obtained from seismic stations around the globe and give data on specific location (epicenter), depth of rupture (focus), magnitude (strength of jolt), and more. On the right is a moment tensor diagram, often called the beach ball diagram. These show the trend and angle of the rupture in the subsurface, as well determine whether a break is compressional or extensional.

The moment tensor here shows some relationships to the Oak Creek Canyon fault but with a focus depth of 6.4 miles, it would seem that the epicenter should have plotted farther east on the surface if it was the Oak Creek Canyon fault that ruptured. This is not the case and suggests that some subsidiary fault might have broken. One could argue if this negates the rupture being on the Oak Creek Canyon fault but remember that most faults are actually fault zones with multiple strands that can rupture when stressed.

Perhaps a previously unmapped or blind fault is what is responsible for this quake. (Blind faults are those that do not rupture the surface and so cannot be detected until they rupture). Time and the reading of the data will tell.

Meanwhile, here are some comments sent to me from those who felt the quake late Sunday night:

Robert,  Sedona: "Huge rock Falls at thunder Mountain in West Sedona"

Christina, Flagstaff: "Okay, we had the earthquake... it about knocked me out of bed... But for me the weirdest, wildest thing that also helped me confirm that it was indeed a quake was that all the crows called out right after it hit... You just don't hear birds make sounds at 11pm at night... Creepy... very creepy."

Chris, Sedona: "I was asleep upstairs in our bedroom, while my wife was awake on the first floor. Then it happened. Arousing me early in the night is difficult, but I felt the shake stirring awareness of picture frames toppling and my rock collection rattling in synchrony. My wife came running upstairs thinking I had fallen out of bed. Never having fallen out of bed, I would guess it would be less than a 4.7, but I assured her it was only a quake.  Sleep did not immediately return."

Chuck, Cheshire neighborhood in Flagstaff: "It really smacked the house. It woke me but not fully so. I heard the stuff fall off the shelves behind my head and thought hey that was a quake. Then fell back asleep. Got up this morning and found the stuff on the floor and thought oh yeah hey that’s right there was a quake last night forgot all about it. And a minute later they mentioned it on the radio. Cheshire is built on a bowl of jello."

Sheri, Oak Creek Canyon, (maybe 5 miles from the epicenter): "Indeed, I felt it, it woke my husband up. It got my heart racing. . . I thought a huge tree had crashed our house.  Lasted about 10 seconds. All the kitchen drawers opened up. I thought it was scary, it was felt miles around us. PS - I thought your blog was very interesting, articulate, easy to understand."

Brenda, Sedona: "Slept through it. Would loved to have felt it. Haven't experienced one yet. Our cat acted crazier than usual in the middle of the night. A little nightlight in our bedroom that hadn't worked for months unexpectedly came on sometime in the night."

Mike, Sedona: "The quake sent a shock wave through my house, but none of my rock hoo-doos in the yard were toppled!"

Ron: "Thank You! I appreciate the look into the geologic past of this region. We live in Jerome and slept through it but many felt it here."

Marlene, Sedona: "As to the earthquake….we both slept through it!  Didn’t even know about it until I got to the library this morning. I hope there’ll be a bit more on the news tonight."

Pete, Flagstaff: "My experience was a small shaking, then a series of small rolls to the east, then a snap to the west, followed by some diminishing rolls. No loud bang heard/felt by others. Funny, experiencing the P, S and surface waves. I am about 15 miles from the epicenter in Country Club. First earthquake in Arizona but growing up in Southern California, I am familiar with seismic events."

2 comments:

Stewart said...

Thanks Wayne for the updates. Our house in Kachina Village swayed for about 5 seconds but fortunately no visible damage. The quake was preceded by a loud explosion. What causes quakes to make loud noises?

jerijo said...

I hope this is not a duplicate. This is my third time trying to post.
I have updated the aftershock map with several more events. There have been at least 11 aftershocks, but not all of them are large enough to locate with enough certainty.
If you look at the map of my locations, you can see that I plot the mainshock NE of the USGS location. USGS does not use NAU's stations in their location estimations; therefore, I think their location is off a bit.
Also interesting to note, there was an M3.5 foreshock on 11-25-14.
Let me know if you'd like the updated map.

Love your Blog Wayne.
Jeri
AZGS