Cell Size and Scale
Here is a great illustration of the size of small things… Cell Size and Scale
(You drag the slider under the illustration to make things happen…)
A tip of the hat to Kathy and Craig for 1) finding it and 2) sharing it…
Magnitude 7.8 – VANUATU (and others)
Signals from four large earthquakes are embedded in the seismogram (northeast Ohio, USA) displayed above. They are
Time Magnitude Location
20:52:13, Oct 7 5.9 East of Severnaya Zemlya
21:41:14, Oct 7 6.7 Celebes Sea
22:03:15, Oct 7 7.8 Vanuatu
23:13:49, Oct 7 7.1 Vanuatu
A quick look at the extracted signals clearly shows the Russian surface waves. I think the 7.8 magnitude signal probably obliterates the one from the Celebes Sea which was quite deep and therefore lacking significant surface waves. The surface waves from the second Vanuatu earthquake are visible. Aside from the Celebes Sea earthquake the other three were relatively shallow.
Postscript:
The USGS just added another one: Magnitude 7.7 – SANTA CRUZ ISLANDS at 22:18:26 UTC
Magnitude 7.6 – SOUTHERN SUMATRA, INDONESIA
A Strong Magnitude 7.6 earthquake occurred this morning at 10:16:09 UTC Southern Sumatra. A tsunami watch statement was issued but appears to have been cancelled. The image shows the seismic signal for the Sumatran earthquake at the bottom. The Samoa Islands signal is still in the image at the top.
Magnitude 8.0 – SAMOA ISLANDS REGION
A Great Earthquake of Magnitude 8.0 occurred this afternoon, September 29, 2009 at 17:48:11 UTC in the Samoa Islands Region. A significant local tsunami was generated. The image is from my seismometer in Millersburg, Ohio, roughly 6960 miles away.
Magnitude 7.1 – IZU ISLANDS, JAPAN REGION
A Magnitude 7.1 Earthquake occurred this morning (10:55:56 UTC) off the coast of Japan. This is the extracted earthquake from my seismograph in Holmes County, Ohio.
This seismogram has several interesting features. The two largest peaks appear to be related to the S body waves as indicated by the arrival time table generated for my location by the USGS Arrival Time Calculator linked at the right and copied below. The first large peak is the direct S wave phase but due to the exact location of the hypocenter relative to my location and to its depth of 303 km, the ScS phase arrives at exactly the same time. The ScS phase is the reflection of the S phase off the core! The other tall peak is the SS body phase which is due to one reflection of the S phase off the earth’s surface!
So why do the S phase arrivals show so strongly in this seismogram? Notice that the station-to-eq azimuth in the table below is 327.1°. My seismograph is oriented to be most sensitive to east-west motion and the S phase is a transverse vibration. The first arrival was out of the northwest with the vibration transverse to that direction, ie roughly east-west.
The hypocenter was located at a depth of 303 km so there is a noticeable absence of surface waves when compared with earthquakes at shallower depths.
Here is the arrival time table:
DATE-(UTC)-TIME LAT LON DEPTH MAG Q COMMENTS 2009/08/09 10:55:56 33.14N 138.04E 303.1 7.1 US: IZU ISLANDS, JAPAN REGIO Expected 1s period body wave amplitude [ 1.02E+00 µm] [ 6.40E+00 µm/s] delta azimuth (degrees clockwise from north) (deg) eq-to-station station-to-eq 97.92 29.6 327.1 travel arrival time # code time(s) dy hr mn sec 1 Pdiff 781.30 0 11 8 57 2 pP 853.74 0 11 10 9 3 sP 885.48 0 11 10 41 4 PP 1024.18 0 11 13 0 5 PKiKP 1053.11 0 11 13 29 6 pPKiKP 1129.12 0 11 14 45 7 sPKiKP 1160.03 0 11 15 16 8 SKiKP 1238.36 0 11 16 34 9 SKSac 1388.76 0 11 19 4 10 SKKSac 1417.53 0 11 19 33 11 S 1440.19 0 11 19 56 12 ScS 1440.19 0 11 19 56 13 pSKSac 1491.85 0 11 20 47 14 sSKSac 1523.95 0 11 21 19 15 SP 1526.97 0 11 21 22 16 pS 1534.98 0 11 21 30 17 PS 1565.69 0 11 22 1 18 sS 1570.00 0 11 22 6 19 PKKPbc 1772.87 0 11 25 28 20 PKKPdf 1776.94 0 11 25 32 21 SS 1851.15 0 11 26 47 22 SKKPdf 1962.24 0 11 28 38 23 SKKPbc 1963.43 0 11 28 39 24 PKKSdf 1993.16 0 11 29 9 25 PKKSbc 1994.50 0 11 29 10 26 SKKSdf 2178.30 0 11 32 14 27 SKKSac 2183.63 0 11 32 19 28 P'P'df 2269.45 0 11 33 45 29 S'S'ac 3100.03 0 11 47 36 30 S'S'df 3104.71 0 11 47 40 31 LQ 2485.30 0 11 37 21 32 LR 2758.33 0 11 41 54
Magnitude 6.9 – GULF OF CALIFORNIA
Four significant earthquakes in the Gulf of California are embedded in the seismogram above. The first was a Magnitude 5.8 event occuring at 17:55:24 UTC followed by the main Magnitude 6.9 event at 17:59:57 UTC. Most of the signal on the left side of the image is due to the larger event. Subsequent events with magnitude 5.0 at 18:33:34 and magnitude 5.9 at 18:40:51 are also included in the signal. The large surface waves of the 5.9 magnitude can be seen at the lower right of the image.
The USGS link above describes the area of the earthquakes as being at the boundary of the Pacific and North American tectonic plates. The Pacific plate is moving northwestward relative to the North American plate at the rate of 45 mm/yr (1.77 inches/yr). There are multiple faults in the crust in the region caused by that motion. Some of these are located in small spreading basin areas and give rise to what are called normal and reverse thrust faults where land on one side of fault is displaced upward or downward relative to the land on the other side. There are also faults in which the land on one side of the fault moves sideways relative to other side, moving in the direction or “strike” of the fault. The motion detected by the USGS seismometry indicates that the magnitude 6.9 earthquake was of the latter type.
Magnitude 7.8 – OFF WEST COAST OF THE SOUTH ISLAND, N.Z.
A Magnitude 7.8 earthquake occurred today off the South Island of New Zealand. This is the extracted signal from my seismograph in Holmes County, OH.
The focus of this earthquake is in the “core shadow” zone for my location. The simple P or S body waves are “blocked” by the core but remnants of them which have been reflected or refracted by interactions at the surface, the outer liquid core, and the inner solid core do appear as what are called “core phases” and surface reflections. The arrival times become difficult to interpret, with the arrival of many core phases occurring within small time periods. The PP phase that does show clearly is the reflection of the P phase off the earth’s surface. Here is a partial list of phases (from the USGS Arrival Time Calculator linked at the right hand side of the page) that may or may not be visible in the signature. The nomenclature for these phase can be found here.
DATE-(UTC)-TIME LAT LON DEPTH MAG Q COMMENTS
2009/07/15 09:22:32 45.72S 166.64E 35.0 7.8 US: OFF W. COAST OF S. ISLAN
Expected 20s period surface wave amplitude [ 1.93E+02 µm] [ 6.07E+01 µm/s]
delta azimuth (degrees clockwise from north)
(deg) eq-to-station station-to-eq
131.12 70.2 239.9
travel arrival time
# code time(s) dy hr mn sec
1 Pdiff 959.31 0 9 38 31
2 pPdiff 970.47 0 9 38 42
3 sPdiff 974.74 0 9 38 46
4 PKPdf 1148.12 0 9 41 40
5 PKiKP 1148.89 0 9 41 40
6 pPKPdf 1159.57 0 9 41 51
7 pPKiKP 1160.33 0 9 41 52
8 sPKPdf 1163.78 0 9 41 55
9 sPKiKP 1164.54 0 9 41 56
10 PP 1285.42 0 9 43 57
11 SKPbc 1348.99 0 9 45 0
12 SKPab 1349.60 0 9 45 1
13 PKSbc 1353.24 0 9 45 5
14 PKSab 1353.87 0 9 45 5
15 SKPdf 1360.56 0 9 45 12
16 SKiKP 1362.01 0 9 45 14
17 PKSdf 1364.77 0 9 45 16
18 SKSac 1574.64 0 9 48 46
19 SKSdf 1577.01 0 9 48 49
20 pSKSac 1590.24 0 9 49 2
21 pSKSdf 1592.67 0 9 49 4
22 sSKSac 1594.47 0 9 49 6
23 sSKSdf 1596.88 0 9 49 8
24 SKKSac 1697.61 0 9 50 49
25 PKKPdf 1746.34 0 9 51 38
26 Sdiff 1771.91 0 9 52 3
27 pSdiff 1786.63 0 9 52 18
28 sSdiff 1791.11 0 9 52 23
29 SP 1894.18 0 9 54 6
Magnitude 7.1 – OFFSHORE HONDURAS
A strong 7.1 magnitude earthquake off the coast of Honduras as it registered here in Holmes County, Ohio. Helicorder gain is set to 20, the same as for the screenshot in the Seismicity! post below. Epicenter was 80 miles NNE of La Ceiba, Honduras. A basin wide tsunami was not predicted but there is a local tsunami watch in effect.
Seismicity!
For the last few weeks on our bicycle rides along the Holmes County Trail we have noticed, first, a lot of survey markers and flags, and second, a little later, a small tracked drilling rig. The first day I saw the rig, I also saw an Amish farmer plowing a nearby field. When I came back on my return trip, the farmer was giving his horses a rest and had walked over to the Trail. I stopped and asked him what all the drilling and survey work was for. They were running a seismic exploration for deep oil deposits.
Then late last week I began seeing strong impulsive signals showing up on my seismograph. Sharp impulsive signals are indicative of local disturbances as their higher frequency components die out quickly with distance (making them no longer sharp and impulsive). These signals showed up around 8:00 pm or early in the morning. I assume that these were the seismic sources being detonated and were done at that time to minimize the number of people on the Trail when they set off the buried charges. Another one is showing up this evening as I write.
Then yesterday there were two distant earthquakes which registered on the display too, one in the Kermadec Island region with a magnitude of 6.4 and another near Kodiak Island at a magnitude of 5.9.
So I share a seismogram with natural and manmade seismicity in the same view.
The Analysis of Sound — Helicopter
Last week my wife and I were bicycling on the Holmes County Trail north of Millersburg. A little over a mile from the Depot, the Trail crosses a large open field that stretches for about a mile. About half way across, I looked up to see a twin rotor Chinook helicopter coming toward me just a few hundred feet above the ground (I later discovered that my data and little snippets of my recollection do not support the low altitude. See the pages linked below). I have wanted to get a recording of an aircraft to see what information I can pull from the sound. Here was my chance. I have a Canon Powershot G10 camera that has a fine audio recorder built into it. I stopped the bike and fumbled around to get the camera out of my jacket. I turned it on and started punching buttons to make it record. All the time the aircraft was bearing down on me. I didn’t have a lot of time to observe it but I did get a recording as the helicopter went right over head. It was a perfect pass.
I got about 22 seconds of sound. When I listen to the recording it sounds like a helicopter going over. It’s pretty good but I was disappointed when I looked at the spectrogram in Raven Lite. I had hoped for a distinctive Doppler signature brimming with information but it just wasn’t there. I let it rest for a day or two.
Then I listened again. As the aircraft passed overhead and receded away from me, the thumping sound of rotors intensified. When I zoomed in on that part of the time domain waveform I could easily see pairs of impulses. Ahhh! I think there is some information here! I calculated the interval between a couple of pairs and calculated the frequency. This must be the blade passing frequency. The twin impulses are from some phasing phenomenon of the twin rotors. I had been so focused on getting the recording that I didn’t count the number of blades on the rotor. My brother-in-law is a helicopter pilot and instructor in Florida so I sent an email to him with some information and questions.
A little research on the internet in the meantime showed that a Chinook has three blades per rotor and the rotor speed is 225 rpm. I divided the frequency I calculated by three and multiplied by 60 to get the speed…206 rpm. A little low but I assume that the speed changes like it does on a car for instance. Then I got a reply from my brother-in-law. He had forwarded my questions to a friend of his who flies Chinooks. He said the speed is 225 rpm. Evidently the rotor speed is kept pretty constant. Then I realized that the machine was moving away from me…it was Doppler shifted to a lower frequency.
As soon as I have a little time, I’ll add another page ( done ) in the sound analysis section describing this recording and the operating information I am able to pull from it. It should be interesting to see how much I can learn about this helicopter from a 22 second audio recording.
