Tempe Goes Seismic

This page is experimental and provides a place where I can post data from a homemade seismometer. In development, artifacts are expected and the data is meant for personal use - to learn. As real events are detected I'll have a better idea of what the sensitivity is.

Details: The seismometer transducer is a torsion balance consisting of two copper pennies suspended between two Nd-Fe-B disk magnets by a thin vertical wire. Motion of copper in a magnetic field produces eddy currents; thus, the magnets provide passive damping. Transverse motion causes the pennies, which are suspended at an offset from their center, to twist the line. The motion is registered by a pair of photodiodes connected differentially to an amplifier/filter. Photos of the seismometer, and recordings of a few earthquakes which this instrument has detected are posted below the plot of current data.

Version 1 of this instrument went offline on the morning of 3/13/2010 , and was replaced by Version 2.0, a mechanical re-design, on Sunday 3/14. Details and notes about V1.0's evolution can be found here. This page documents the present design, V2.0. The circuit design of both versions is identical.

Seismometer current data, uploaded hourly

Observations:

1. 3/21/10 Sensitivity appears to be improved with the longer pendulum - 39 inches versus 9 inches in V1.0. But the sensitivity has come with a cost. It appears that any slow change in temperature sets up a thermal/convective gradient within the enclosure, with accompanying oscillation at a frequency of about 2 cycles per minute. Peak amplitude is about 5% of full-scale. This is the effective noise floor until I can find a way to reduce this artifact. Foam insulation at the top opening helps.

2. 04/06/10 M7.7 Sumatra quake recorded, distance 9400 mi., shows significantly improved sensitivity over the previous V1.0 mechanical design.

Pictures

The completed seismometer board. To the right you can see the IR emitter, magnet assembly and differential photodetector. From right to left, in order:
1) Photodiode transimpedance amplifier/filter (dc-coupled, abbreviated "TZA" in the dataplots)
2) Instrumentation amplifier and HPF
3) Sallen-Key LPF
4) SE-to-balanced output buffer to DAQ board.

The response characteristic is bandpass, with passband of 0.008 to 3 Hz.

Seismometer photo, in its enclosure. It's ready to be connected to the data acquisition board. The board is mounted on top of a 10 inch circular aluminum base plate, about 1/2 inch thick. Mount point screws are tapped and threaded, providing a convenient tilt adjust.

The pendulum wire, 5-mil dia tungsten, is suspended from the top of a rigid square aluminum pipe one meter long. Height adjustment is provided by a threaded set screw with a knob attached.

Seismometer enclosure. The DAQ board, USB-IP converter and WiFi access node are on top of the cabinet to the right.

The enclosure proper is a 10 inch spiral-wound cardboard cast form for concrete, 4 feet tall. Cost: about $12 at Lowe's hardware. It is thermally isolated on both ends with foam rubber.

Sumatra M7.7, 04/06/2010 This is the first large quake detected by the V2 seismometer. The M7.7 quake in Sumatra on 4/6/10, distance 9400 mi. made a response peak of 60% of full-scale. The distance record for V1.0 was the M8.0 quake in American Samoa in Sept 2009, a much weaker response at 5000 mi. distance.

The surface wave frequency is about 0.07 Hz - a very low frequency typical for the large quakes.

Prior to this quake, well over 50 aftershocks have been recorded to date from the M7.2 Baja California quake of 04/04/10, some as low as M2.0.

Travel Times M7.7 Sumatra 04/06/10 USGS seismic wave travel times for the M7.7 quake, 4/6/10.

Spain (Andalusia) M6.2, 04/12/10 UTC This M6.2 quake in southern Spain on 4/11/2010 is at the limit of sensitivity for detection. The pressure and shear wave components labeled P and S can only be seen in this zoom plot, superimposed against the instrument's own twice-per-minute thermal oscillation - a flaw which I still must correct (it's too sensitive to air motion, even in a closed box).

This is a milestone for me. This device was born because my wife Pam was in Italy when the M6.7 quake struck in L'Aquila in April 2009. My goal was to be able to detect that quake from that distance, and it appears now to have been achieved. Now on to curing the pesky thermal/convective oscillation - (which will likely not be easy if the required thermal isolation for the enclosure is too high).

Andalusia M6.2 Travel Times, 04/12/10 UTC USGS seismic wave travel times for the M6.2 Andalusia quake, 4/12/10 (UTC). The times for P and S agree with the recorded arrivals to within seconds.

Nomenclature for the phases of seismic waves is given here. The P waves are pressure (longitudinal) waves which travel in a linear path beneath the Earth's crust. The S waves are shear transverse waves which propagate at a slower speed than the P wave. LQ and LR are surface waves which propagate along the curvature of the Earth's crust - a longer path.

Papua New Guinea M6.9 07/18/10
The M6.9 at Papua New Guinea July 18, 2010, was one of three closely spaced quakes, and had the best definition from the garage detector. There are 4 identifiable phases: P is the initial pressure wave arrival, PKiKP is the same wave bouncing from the Earth's inner core boundary, S is the shear wave, and LQ is the surface wave. The surface waves are always lower in frequency, and typically the strongest signals from a distant quake. Distance 6920 miles.

Mindanao M7.6, 072310
This large M7.6 earthquake in the Philippines on July 23, 2010 was atypical in that its epicenter was 600km below the earth's surface. Little damage was reported. Note also that the normally strong surface waves LQ and LR are not prominent.

Mindanao Mag 7.6 Quake Audio recording at 250x speed
This is an audio file of the Mindanao M7.6 quake with a superimposed image of the 22 second waveform made by playing the quake datafile at a sample rate of 3072 Hz, (about 250 times the actual sample rate). It sounds a bit like rolling thunder.

Ecuador M6.9, 08/12/10 Ecuador M6.9 quake, Aug 12, 2010, 0454 PDT, distance 3360 miles. This quake occurred in a sparsely populated area east of the Andes, facing the Amazon tropical forest.

P = initial body wave. S = transverse shear wave. sPKiKP = the same shear wave reflected off of the earth's inner core. LQ and LR are the surface waves.

An audio recording of this event, compressing 48 minutes into 12 seconds, is here.

Notes:

1. Another example of a torsion balance seismometer may be found here .

2. A torsion pendulum is a special case of a compound pendulum. The two main handles on the resonant frequency are the moment of inertia of the weight, and the torsion constant of the wire (inversely proportional to the wire cross-sectional area).

3. A schematic diagram for this design is here. The seismometer is completely powered off of the 5V USB bus from the Labjack U12 12-bit data acquisition board. Supply current consumption is under 100 mA. Note: U3A should be an OP284 and not an MC33202, since low offset is needed in the feedback path. U1 and U3 have been changed to OP284, which has greatly reduced 1/f noise in the seismic frequency range down to 0.01 Hz.

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	The completed seismometer board. To the right you can see the IR emitter, magnet assembly and differential photodetector. From right to left, in order: 1) Photodiode transimpedance amplifier/filter (dc-coupled, abbreviated "TZA" in the dataplots) 2) Instrumentation amplifier and HPF 3) Sallen-Key LPF 4) SE-to-balanced output buffer to DAQ board. The response characteristic is bandpass, with passband of 0.008 to 3 Hz.
	Seismometer photo, in its enclosure. It's ready to be connected to the data acquisition board. The board is mounted on top of a 10 inch circular aluminum base plate, about 1/2 inch thick. Mount point screws are tapped and threaded, providing a convenient tilt adjust. The pendulum wire, 5-mil dia tungsten, is suspended from the top of a rigid square aluminum pipe one meter long. Height adjustment is provided by a threaded set screw with a knob attached.
	Seismometer enclosure. The DAQ board, USB-IP converter and WiFi access node are on top of the cabinet to the right. The enclosure proper is a 10 inch spiral-wound cardboard cast form for concrete, 4 feet tall. Cost: about $12 at Lowe's hardware. It is thermally isolated on both ends with foam rubber.
	This is the first large quake detected by the V2 seismometer. The M7.7 quake in Sumatra on 4/6/10, distance 9400 mi. made a response peak of 60% of full-scale. The distance record for V1.0 was the M8.0 quake in American Samoa in Sept 2009, a much weaker response at 5000 mi. distance. The surface wave frequency is about 0.07 Hz - a very low frequency typical for the large quakes. Prior to this quake, well over 50 aftershocks have been recorded to date from the M7.2 Baja California quake of 04/04/10, some as low as M2.0.
	USGS seismic wave travel times for the M7.7 quake, 4/6/10.
	This M6.2 quake in southern Spain on 4/11/2010 is at the limit of sensitivity for detection. The pressure and shear wave components labeled P and S can only be seen in this zoom plot, superimposed against the instrument's own twice-per-minute thermal oscillation - a flaw which I still must correct (it's too sensitive to air motion, even in a closed box). This is a milestone for me. This device was born because my wife Pam was in Italy when the M6.7 quake struck in L'Aquila in April 2009. My goal was to be able to detect that quake from that distance, and it appears now to have been achieved. Now on to curing the pesky thermal/convective oscillation - (which will likely not be easy if the required thermal isolation for the enclosure is too high).
	USGS seismic wave travel times for the M6.2 Andalusia quake, 4/12/10 (UTC). The times for P and S agree with the recorded arrivals to within seconds. Nomenclature for the phases of seismic waves is given here. The P waves are pressure (longitudinal) waves which travel in a linear path beneath the Earth's crust. The S waves are shear transverse waves which propagate at a slower speed than the P wave. LQ and LR are surface waves which propagate along the curvature of the Earth's crust - a longer path.
	The M6.9 at Papua New Guinea July 18, 2010, was one of three closely spaced quakes, and had the best definition from the garage detector. There are 4 identifiable phases: P is the initial pressure wave arrival, PKiKP is the same wave bouncing from the Earth's inner core boundary, S is the shear wave, and LQ is the surface wave. The surface waves are always lower in frequency, and typically the strongest signals from a distant quake. Distance 6920 miles.
	This large M7.6 earthquake in the Philippines on July 23, 2010 was atypical in that its epicenter was 600km below the earth's surface. Little damage was reported. Note also that the normally strong surface waves LQ and LR are not prominent.
Mindanao Mag 7.6 Quake Audio recording at 250x speed	This is an audio file of the Mindanao M7.6 quake with a superimposed image of the 22 second waveform made by playing the quake datafile at a sample rate of 3072 Hz, (about 250 times the actual sample rate). It sounds a bit like rolling thunder.
	Ecuador M6.9 quake, Aug 12, 2010, 0454 PDT, distance 3360 miles. This quake occurred in a sparsely populated area east of the Andes, facing the Amazon tropical forest. P = initial body wave. S = transverse shear wave. sPKiKP = the same shear wave reflected off of the earth's inner core. LQ and LR are the surface waves. An audio recording of this event, compressing 48 minutes into 12 seconds, is here.