Monday, November 2, 2015

Links and commentary on earthquakes 11 km NE of Black Canyon City, Arizona (~11:30 pm local time; November 1, 2015)

At least 3 earthquakes (M3.2, M4.1, M4.0) occurred last night 11 km NE of Black Canyon City, Arizona (I am sorry to have missed being at a NASA meeting in Washington D.C.!). It was widely felt across central Arizona including greater Phoenix (USGS' Did You Feel Ithas >4800 responses by 3:30 am). USGS estimates 4.6M people were exposed to weak shaking.

The sequence began with a M3.2 at 8:59 pm (foreshock). The M4.1 mainshock occurred at 11:29 pm within 2 km of the foreshock. 20 minutes later and 4.6 km SSW the M4.0 aftershock occurred. An M3.1 event occurred in the area Fri May 08 2009. There are no mapped active faults nearby but the roughly north-south orientation is consistent with the bedrock geologic structure in the area and note the mapped active faults to the east (Horseshoe, Carefree, and Sugarloaf--all similar orientation

PDF of above map

Seismogram recorded at ASU processed by John D. West. PDF of above

Seismograms recorded at ASU processed by John D. West. PDF of above

There is a very cool large landslide near to Black Canyon City (see this post), but the earthquakes and landslides are not associated. I would think that the slide was shaken enough to move, and certainly the steep slopes of the region should have produced some rockfalls and other mass movements from this earthquake sequence.

Here are some links for the event:

Here are some links for background on Arizona earthquakes:


  1. Very nice report. Like the summary education on local geology and the qualities of the quake. Thank you

  2. Hi Dr. Arrowsmith.

    If I understand MMI correctly, the quake energy is proportional to the magnitude as 10^(3/2). So, a 0.1 delta in magnitude would be a 41% difference in energy, and 0.2 delta would correspond to a doubling of energy.

    The relative energies of the main quake and the aftershock, judged by integrating the amplitude vs time for each event, as well as the huge "Did you feel it?" difference in responses for each on USGS's website, seem to suggest that the difference in MMI between the two was greater than 0.1. Am I wrong, that these two quakes had to have been more than 0.1 magnitude different?

    Here's my own plot from a vertical sensor in my Tempe garage, paired with U of A's BHZ sensor on the second trace:
    The energy of the main quake seems to be well over double that of the aftershock.

    And here's my seismo (sub-hobby) website:

    Feeling this quake gave me a new respect for the instability of the inverted pendulum formed by the concrete tiles on my roof. I saw first hand the danger of that kind of structure in L'Aquila, Italy after their 2009 quake.

  3. So - I did a bit of homework. I have raw data, time-indexed 20 Hz sample rate in the form of SAC files, for both my own seismometer and the U of A's BHZ sensor. Tonight I looked at the Tucson data:

    TUC: Streckeisen STS-1VBB w/E300, Quanterra Q330 digitizer, 24 bit. (see )

    The nominal velocity sensitivity of the STS1 is 24V/cm/s, but it might not be, so I treat it as a first approx. The velocity data is first integrated to get displacement, and is then high-pass filtered with a 1.25 Hz corner 2-pole IIR filter to eliminate low-frequency artifacts.

    At this point, I use the M4.0 aftershock event as a reference, by varying the displacement sensitivity setting for the STS-1 until the computed MMI shows M4.0 - same as reported by USGS. That sensitivity is 16nm/LSB.

    Using the same sensitivity and integration followed by HPF, I look at the earlier, main quake. With the aftershock as a reference, the MI of the main quake is computed to be M4.6.

    M4.1 vs M4.0 is a 1.42x increase in energy.
    M4.6 vs M4.0 is a 7.9x increase in energy.

    I have this data:

    1. I personally felt the main quake in Tempe - the roof swayed and creaked for 20 seconds. I did not percieve the aftershock - as I was watching it happen on a live seismo trace (mine).

    2. The DYFI data shows 5542 responses in 175 zip codes for the main quake, versus 317 responses in 105 zip codes.

    3. A qualitative comparison of both station TUC and TLAZ (my station), show an integrated amplitude vs time difference of well over 2X for the main quake vs the aftershock.

    4. A *quantitative* comparison of the two quakes from Tucson’s data, in the form of binary SAC files, with its sensor type and sensitivity accounted for, and the digitizer properly identified, and using the aftershock as the M4.0 reference, gives the main quake MI=4.6.

    Data sets 1-4 all show a marked discrepancy with the reported energy main vs aftershock of 1.41x.

    I would love to be wrong about this - because then I would learn something. I have no interest at all in saying USGS was wrong or right. I do know that USGS sometimes fails to report quakes: on at least a half-dozen occasions I have seen a quake both on my TLAZ and on TUC, and those events did not produce a listing on the M1.0+ network detections. USGS does make mistakes. That does not concern me.

    In six years of recording quakes (hundreds - including one in Antarctica) just for curiosity and to learn, this is the first time I ever felt one. And then I didn’t feel the other.

    You and USGS have stated that 4.6 million people were exposed to mild shaking. How is it possible that, out of that number, I seem to be alone in asking you why I felt one, and not the other? That’s really astonishing. I do not understand incuriosity.

    I assume you have access to raw data. I have raw data (SAC files) both for TUC/BHZ and for my TLAZ station. I have screen captures from WinQuake showing MMI and other parameters, the STS-1 settings I used, and P and S-wave arrival times. I’ll happily send them if you would like.

    Steve Rector,
    Tempe, AZ

    1. This comment has been removed by the author.

    2. Correction on the sensitivity setting - the field was in cm, not m. so the setting was 0.16nm/LSB, or 1.6E-08cm/LSB

  4. I found a partial answer to my question: "Why did it seem more than M0.1 difference?" USGS computes magnitude statistically, from the closest stations distributed around the epicenter.

    For the M4.1, they had readings from 20 stations, with magnitude minimum M3.61 and max M4.66.

    For the M4.0, many more stations were on line (113), with minimum M3.03 and max M5.09.

    From this, I infer that the power traveling out from the epicenter is not equal in all directions (isotropic). I wonder if the main quake radiated more to the south than the aftershock did.

    1. USGS publishes a "moment tensor," this may answer if there was a difference in directivity between the two quakes.

    2. This comment has been removed by the author.

  5. I have input and questions related to the quake outlined above at 11:29 pm sunday. I heard and felt the quake for between 10-20 seconds. A rumble preceded a bang, then the shaking. Although it was reported as mild, my house creaked and crakedb as did the roof, items vibrated..this seems more than mild, and I wonder as did azstefano asks if the energy of the quake could have been greater to the south?

    1. Not sure if the owner of this blog answers questions, or has been too busy. I did find an answer for the main M4.1 quake: it was stronger to the south - specifically bearlin 160 degrees from the epicenter. Phoenix's East Valley falls on this line - as does Tucson.

      The USGS - for *some* quakes - publishes "moment tensor" parameters which give the strength and direction of the pressure and shear waves. For the M4.1 quake, I took these from USGS and used a script for Wolfram Mathematica from here:

      This showed that the direction of maximum shear force, propagating outward, was bearing 160 degrees. I posted a screen capture of the plot here:

      The arrow labeled "u" shows the direction of max shear wave strength.

      This explains why recorders along this bearing saw a much stronger main quake vs the aftershock. What would complete the explanation would be the moment tensor from the aftershock. Unfortunately, nobody has published it. The data strongly indicate that the max strength from the aftershock, was at a different bearing than the main quake.

      This pair of quakes (ignoring the foreshock 90 minutes earlier) seems to be a gift to educators - a teachable moment. There are many high schools with seismometers on the IRIS educational network - including a few in Phoenix. This interesting event is a case where students can learn what magnitudes actually mean - how they are determined, and why a single number may not explain where the energy went. I sure learned a few things!

      I hope this blog's owner will let us know a bit more about the first earthquake I ever felt, and the aftershock I didn't.

  6. Hello Ramon,
    Sorry you missed the shaking. I have refined the quake locations using all of the stations in AZ, including one I have offline via the DMC, and several of NAU's analog stations.
    The depth of the mainshock has a significant error associated with it, as do most of the quakes we locate; however the depth was more than twice that of the secondary 4.0 quake that occurred about 20minutes later.
    Here is a table of my locations:
    Mw Time -UTC Date Lat Long Depth (km)
    3.2 03:59.33 11-02-15 34.11016 -112.1460 <1.0
    4.1 06:29.66 11-02-15 34.23350 -112.12366 11.2
    4.0 06:49.32 11-02-15 34.17950 -112.08633 5.19

    Let me know if you want all of the error data.


  7. Tonight I checked the USGS archive for the former M4.1 and M4.0 quakes. The record of their magnitudes has been refined: they are now listed as M4.0 and M3.6, respectively.

    What was originally recorded implied a 40% difference in energy. The corrected magnitudes show a 400% difference, which agrees with my house, my garage seismometer, Tucson's VBB sensor and the DYFI delta in responses between the two events.

    Screen capture:

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  10. Replies
    1. Tectonic plates motion can cause earthquakes. Earthquakes activities arises these days. Quakes occur in the areas that have been trembled or not yet in areas prone to earthquakes. No one can really know when it will happen.