NetQuakes
The American Geophysical Union held their Fall Meeting in San Francisco this week. My wife heard a report on some of the happenings of interest to the general public on the radio this afternoon. One of the reports described the NetQuakes program being undertaken by the U.S. Geological Survey.
The NetQuakes program asks volunteers to host small seismographs at their homes in order to increase the number of seismographs in the San Francisco Bay area. These appear to be strong motion sensors based on triaxial accelerometers and record data only when triggered by an acceleration of 0.25% g. The triggered waveforms are sent to the USGS via wifi through the vounteer’s internet connection and are immediately analyzed for possible earthquake events. The triggered waveforms are also displayed on a page off of the NetQuakes homepage.
As I was exploring the NetQuakes site, I noticed that several, but not all the stations showed a coherent signal at about 10:16:30 on Dec 18, 2009. I clicked on on one, J021-NC. Then I went the USGS Earthquake Hazards page to look for small California earthquakes at that time. I looked first at the listing for Magnitude 3+ events but didn’t find any at the proper time, which I should have expected from the limited number of stations reporting the event. When I looked at the Magnitude 1+ list I found a Magnitude 1.8 earthquake just a few miles from the J021-NC station. The station names can be determined by clicking on the location map at the location indicators…pretty cool and it should give them very fast earthquake severity and location information.
Help Needed… Aircraft Spectrum Analysis
Unknown Spectral Feature - Air Tractor Fixed Wing Aircraft
My search for Doppler shifted sounds has resulted in a number of recordings of aircraft. As I studied those recordings for the Doppler shift, I noticed that there was also another feature in the spectrograms that I didn’t understand. There is a shallow bowl shaped feature centered on the time of closest approach, ie when the aircraft was roughly overhead. The “bowl” seems to be formed by harmonic content in the signal. The frequency increases not only as the aircraft is approaching, as one would expect for a Doppler shift, but also as it moves away. The two spectrograms accompanying this post, for the Air Tractor fixed wing and the Chinook helicopter, show the feature pretty clearly.
I have thought about refraction due to the temperature lapse rate as a function of altitude but the equations don’t show any frequency dependence. I considered source directivity but I see it in two very different aircraft. If you listen carefully to an airplane or helicopter flying over you can hear a frequency component that is increasing as the craft flies away from you…it seems to be a real phenomenon. I am stumped!
Please comment if you know what causes this feature or if you have a suggestion as to what it might be.
Thanks.
Unknown Spectral Feature - Chinook Helicopter
Magnitude 6.6 – QUEEN CHARLOTTE ISLANDS REGION
A strong Magnitude 6.6 earthquake struck the Queen Charolette Islands region off the west coast of Canada at 15:30:46 UTC on November 17, 2009. Because it was shallow, the surface waves were quite strong and were clipped on my seismograph here in northeatern Ohio. The two types of surface waves, the Love and Rayleigh waves, are also very well delineated in their arrivals, which is often not the case.
The arrival time information from the USGS is included below:
DATE-(UTC)-TIME LAT LON DEPTH MAG Q COMMENTS
2009/11/17 15:30:46 52.15N 131.38W 11.6 6.6 US: QUEEN CHARLOTTE ISLANDS
Expected 20s period surface wave amplitude [ 1.07E+02 µm] [ 3.36E+01 µm/s]
Expected 1s period body wave amplitude [ 8.99E-01 µm] [ 5.65E+00 µm/s]
delta azimuth (degrees clockwise from north)
(deg) eq-to-station station-to-eq
35.41 89.1 306.0
travel arrival time
# code time(s) dy hr mn sec
1 P 415.68 0 15 37 41
2 pP 419.26 0 15 37 45
3 sP 420.80 0 15 37 46
4 PnPn 494.28 0 15 39 0
5 PP 496.11 0 15 39 2
6 PnPn 498.58 0 15 39 4
7 PnPn 498.59 0 15 39 4
8 PP 514.92 0 15 39 20
9 PcP 565.21 0 15 40 11
10 S 751.10 0 15 43 17
11 pS 755.35 0 15 43 21
12 sS 757.22 0 15 43 23
13 ScP 790.89 0 15 43 56
14 PcS 792.36 0 15 43 58
15 SnSn 891.40 0 15 45 37
16 SS 906.01 0 15 45 52
17 SS 939.25 0 15 46 25
18 PKiKP 1006.43 0 15 47 32
19 pPKiKP 1010.42 0 15 47 36
20 sPKiKP 1011.88 0 15 47 37
21 ScS 1035.38 0 15 48 1
22 SKiKP 1217.87 0 15 51 3
23 PKKPdf 1901.07 0 16 2 27
24 SKKPdf 2112.52 0 16 5 58
25 PKKSdf 2113.98 0 16 5 59
26 SKKSdf 2325.36 0 16 9 31
27 P'P'df 2407.79 0 16 10 53
28 P'P'ab 2498.68 0 16 12 24
29 S'S'df 3259.19 0 16 25 5
30 LQ 898.62 0 15 45 44
31 LR 997.34 0 15 47 23
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
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