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Build Your Own Seismograph Ever wondered how a seismograph works? Here’s one that you can build yourself. It uses a horizontal swinging pendulum to detect earthquake waves and you can even display the results on a PC. By DAVE DOBESON* M OST AUSTRALIANS are thankful that we are not seriously affected by the large earthquakes and volcanoes that regularly devastate so many other parts of the world. However, few realise just how close we are to much of the tectonic action, or how easy it is to make your own amateur seismograph. The design described here can easily detect the half-dozen magnitude 7 quakes that occur around Australia each year. In fact, the author has observed three major quakes occurring “live” on the monitor, including one from El Salvador. Plate tectonics Before we take a look at the design of our seismograph, let’s first find 26  Silicon Chip out why major earthquakes occur. In particular, we need to have some understanding of “plate tectonics”. The basics are very simple – the crust of the earth is made up of about 20 major “plates” that “float” on semi-liquid layers underneath. In our region, the Australian-Indian plate (including the ocean floor out to NZ, Fiji, PNG, Indonesia, most of the Indian Ocean, and also India) is moving in a north-westerly direction by about 7cm each year. Over millions of years, India (which is at the leading edge of the plate) has “crashed” into Asia, forming the Himalayas. Earthquakes commonly occur at the boundaries of the plates, where they collide and produce stresses in the Earth’s crust . For example, deep ocean faults off the coast of Sumatra produced the magnitude 9.0 “Boxing Day Tsunami” earthquake last year and the related Niass 8.7 earthquake in April, 2005. Also associated with this plate are a number of volcanoes, including Krakotoa, which partly circle Australia from Indonesia, through PNG and down through NZ. Macquarie Island, situated half-way between Australia and Antarctica, had a magnitude 8.1 earthquake on Christmas Eve, the biggest in the world last year until the Boxing Day earthquake. Of course, many large earthquakes go unreported because they occur under the ocean or in sparsely populated areas and have no impact on humans. siliconchip.com.au This seismograph plot shows a magnitude 6.5 quake that occurred in PNG on April 11, 2005. A 6.8 quake near Noumea was detected only five hours later. The detector circuit used was the same as described here but the data logger was one of the types used in NSW high schools. The mechanical section of the seismograph uses parts that are readily available from a hardware store. It’s based on a swinging horizontal pendulum and movement is detected using a vane and light sensor circuit mounted at one end. If you look at the United States Geographical Survey (USGS) home page and click on “Recent Earthquakes” (to show the last seven days’ earthquakes for the US and the world), you will see that many of the larger earthquakes occur near the boundaries of our continental plate – see www.usgs.gov In addition, Geoscience Australia’s website (at www.ga.gov.au) has a table that gives information on recent earthquakes in Australia and significant worldwide quakes (just click on the “Recent Earthquakes” link). Both sites also have detailed information on the tectonic forces causing earthquakes, the design and operation of professional seismographs, records of historically significant quakes and links to records in other countries. siliconchip.com.au Another site that’s worth visiting is www.geonet.org.nz/drums – it shows “live” displays from seismographs around NZ. If your home-made seismograph detects a real earthquake, the event should also be reported within minutes by the above three sites. Designed for schools This do-it-yourself seismograph was originally described in “Scientific American” in 1979 and has been adapted for science teachers to build and use in the school laboratory – see http://science.uniserve.edu.au/school/ Seismograph Movements of the seismograph, which is basically a horizontal pendulum, are detected using a simple light sensor circuit. In operation, a metal vane attached to one end of the pendulum (or bar) partially blocks the light between a LED and an LDR (light-dependant resistor). However, when the bar moves (ie, during an earthquake), the amount of light falling on the LDR is modulated by the metal vane. This signal is then fed to a low-cost op amp circuit which, in turn, feeds into a data logger. Finally, the output of the data logger is fed to a computer to store, display and print the results. All high schools in NSW have edu- cational data loggers for use in experiments. Most of these units cost well over $1000 but a cheap, 4-channel, 10-bit serial data acquisition device (DI-194RS) from DATAQ in the US is available from Turnkey Solution for under $60 plus GST and delivery – see www.turnkey-solutions.com.au There’s an even cheaper way around this problem for the home enthusiast. A PICAXE-based A/D converter and a freeware graphing program called “StampPlot Lite” can do the same job for about $10.00 – provided you also have a PC. Building the seismograph OK, let’s take a look at the mechanical details of our seismograph and find out how it’s built. The seismograph described here is known as a “Lehman” or “Horizontal Pendulum” seismograph. It’s also called a “Swinging Gate Seismograph”, because the bar and its supporting wire look like an old-fashioned farm About The Author* Dave Dobeson is a science teacher at Turramurra High School and the University of Sydney Science Teacher Fellowship holder for 2005. September 2005  27 TOP PIVOT POINT (25-35CM ABOVE LOWER PIVOT POINT) This labelled photograph clearly shows how the Seismograph is built. This version uses a magnetic damper but liquid damping could also be used (see text and photos). Note that the light sensor and A/D converter unit shown here is an early prototype. TURNBUCKLE STEEL WIRE 1-2MM DIA. 2-3KG MASS DAMPER METAL VANE LIMITING BOLTS BAR: 5/16-INCH x 800MM THREADED STEEL ROD LIGHT-SENSOR & A/D CONVERTER CIRCUIT gate. The “hinges” (actually the pivot points) of the “gate” are not quite vertically aligned, with the top hinge just forward of the bottom hinge so that the “gate” will swing shut. In practice, this means that the horizontal pendulum (or bar) swings slowly back to its original resting position The accompanying photos show the basic set-up. As can be seen, it includes an 800mm-long 5/16-inch threaded steel rod that’s fitted with a 2-3kg mass at one end. The other end of the rod is ground to an edge and pivots on the end of a ½-inch bolt – this forms the lower pivot point. The supporting wire is attached to the rod at one end, just before the weights, and to a turnbuckle at the other end. This then pivots about 2530cm above the lower pivot. If we align the seismograph pivots so that the top pivot is less than 1mm forward of the bottom pivot, then the seismograph bar will always swing back to its central position and will have a natural period of about 5-10 seconds. However, if the pivots are exactly vertically aligned, there will be no restoring force and it will never swing back. We cannot move the top pivot too far forward though, otherwise the seismograph will be very insensitive. 28  Silicon Chip This unit is very sensitive to the mostly horizontal motion of earthquake “L-waves” but is insensitive to “P-waves” which are mostly vertical. Kiwis, because they are much closer to the action, might be able to detect P-waves if they use a spring instead of the wire. Perth, Tennant Creek and Yass also have small local quakes every few months, so you might like to experiment with a spring support if you live in these areas. By t he way, it’s important to remember that although we often talk about the bar (or pendulum) of the seismograph “swinging”, it’s really the room that moves during an earthquake. The bar, because of the inertia of a heavy mass attached to one end, initially stays still. In effect, the unit and its associated logger act as a low-pass filter which renders the unit insensitive to everyday events (footsteps, doors closing, passing traffic, etc). The accompanying photos show most of the construction details. The only critical dimension is that the top pivot must be less than 1mm in front of the lower pivot. As well as the wooden frame shown, the unit could be built into any strong cupboard, bookcase, shelf or even a strong, metal frame. In that case, the brackets and wooden frame would not be needed. Any type BOTTOM PIVOT POINT TILT ADJUSTMENT BOLTS RIGHT-ANGLE BRACKETS WITH DIAGONAL STAYS of metal rod could be used (as long as it’s strong enough) and the same goes for the mass at one end. Note that you will have to “re-zero” the seismograph for the first few weeks after building it, as the wire, brackets and wood flex under the strain. After that, it will be a matter of making routine adjustments every few months. Top pivot point The top “hinge” (or pivot point) is made by drilling a 5mm diameter hole about half-way through the outer section of a large, thick washer – ie, to make a “dimple”. Smaller washers and a nut are used to hold the large washer in position, while a nut and lockwasher behind the wooden upright panel lock the bolt in place. As shown in the photos, the hook at the end of the turnbuckle sits in this dimple, so that it can freely pivot. In operation, the turnbuckle adjusts the tilt of the bar and is set so that the bar is horizontal. The securing bolt can be screwed in or out to move the top pivot point relative to the bottom pivot. This is important for the overall functioning of the seismograph because it affects the natural period of the bar (ie, the time for one complete swing from the centre to one side, then back through the centre to the other side and finally siliconchip.com.au back to the centre again). A period of about five seconds seems to work best for my seismographs in Sydney. The pivot end of the 5/16-inch threaded rod is ground to a knife-edge and this sits vertically against the end of a ½-inch bolt. Wind a nut onto the rod before you cut and grind it, so that the thread is restored when the when the nut is removed. Be sure to use safety goggles when drilling, cutting or grinding metals – you only have one pair of eyes. Note that the lower mounting point must be directly below the upper mounting point. The best way to ensure this is to use a plum-bob made from fine fishing line and a lead sinker. The two rear-most vertical bolts that go through the support brackets are used for tilt adjustment – see photo. These both screw into threads that are tapped through the wooden base and the brackets (nuts under the wooden base will do) and each has a screwdriver slot cut into its end. This allows you to use a screwdriver to tilt the seismograph sideways and forwards or backwards, to alter the position of the bar and thus its period and sensitivity. The far end of the seismograph wooden frame has a single central support. A sheet of plywood or particleboard underneath will stop the three supports from sinking into the carpet when the unit is positioned on the floor. Swinging the weight Just about any mass of 2-3kg will provide sufficient inertia to initially keep the bar still during an earthquake, provided it doesn’t hang too far below the bar. A pair of 1.25kg barbell weights are ideal for the job. They cost less than $3 each from a sports store and come with a ready-made hole through the middle. This means they can be simply slipped over the end of the bar and clamped in position using nuts and washers on either side. Damping Once earthquake waves set the bar swinging, it will keep swinging for hours unless it is damped. Perfect damping would stop the bar with a few swings but in practice, under 2-3 minutes is OK. You can use either liquid or magnetic damping. For liquid damping, a 40 x 50mm plastic paddle dipped into a rectangular container of water will do the job. You can use a small bulldog clip to attach the paddle to the bar. The water will need topping up each week or so. Magnetic damping involves attaching one or two super magnets to the end of the bar using a U-shaped bracket. A thick sheet of aluminium or a coil of wire with the ends joined is then placed in the magnetic field. When the bar moves (ie, during an earthquake), current is induced into the aluminium or wire coil. This in turn produces a magnetic field that counters the magnets and so damps the motion of the bar. Discarded computer hard disks are a good source for super magnets but be careful – supermagnets are dangerous and the author has been badly cut when a pair decided to play “north attracts south with my hand in be- The hook at the end of the turnbuckle sits in a 5mm dimple that’s drilled into a large washer to form the top pivot point. The lower pivot point is formed by first grinding the end of the bar to a sharp edge. This sharp edge then rests vertically against the end of a 1/2-inch x 40mm-long bolt. tween”. They can also be a disaster if they get too close to your credit cards or a computer monitor! On the other hand, the good thing about magnetic damping is that once Above & right: these two views show the alternative damping methods for the swinging bar. Magnetic damping (above) uses a couple of super magnets and a coil of wire, while liquid damping (right) uses a 40 x 50mm plastic paddle dipped into a rectangular container of water. siliconchip.com.au September 2005  29 This side-on view clearly shows the tilt adjustment bolts. These are set so that the base is perfectly horizontal (both east-west and north-south), so that the pivot points are in the same vertical plane. The turnbuckle is then adjusted so that the bar is also horizontal. TILT ADJUSTMENT BOLTS you get it right, it stays right. Old aquarium air pumps have coils of fine wire, which can be used for magnetic damping if the ends of the wires are joined together. A 400g coil of 0.7mm enamelled wire with the ends joined together and a super magnet that moves inside the coil gives almost perfect damping. Use your multimeter to check that the winding hasn’t burnt out before using the coil. The perfect location for your seismograph is on a concrete block that’s set into bedrock at the bottom of a sealed mine shaft! If you don’t have access to a mine shaft(!), the seismograph should be installed in a closed room or cupboard, or in a strong bookcase surrounded by a Perspex cover (to prevent air movement over the unit). Circuit details Many different seismograph detector and A/D (analog-to-digital) converter circuits are available on the net. The best-known site is called the Public Seismic Network at www. psn.quake.net (in California). It has designs that go from pens writing on rolls of paper to very complex circuits with low-noise op amps, 16-bit A/D converters and damping using negative feedback. By contrast, the circuit used here is quite simple – see Fig.1. As previously stated, it’s based on a light sensor circuit that’s interrupted by a metal vane attached to the end of the bar. In practice, the unit is set up so that the vane normally blocks about half the light from the LED to its LDR. The light detector and its associated op amp circuit is exactly the same as the one designed for use with school data loggers. The logger output is simply taken from the output of IC1, as shown. Alternatively, you can add your own data logger, based on A/D converter stage IC2 (a PICAXE-08M). In greater detail, power for the circuit comes from a 9V DC plugpack supply. Diode D1 provides reverse polarity protection, while the associated 100W resistor and 470mF capacitor provide decoupling and ripple filtering. The filtered DC rail is used to power LED1 via a 1kW current limiting resistor. The LDR and its associated 10kW resistor effectively form a voltage divider across this supply rail, the voltage at their junction varying according to the resistance of the LDR. This in turn depends on the amount of light reaching it from the LED. The output from the LDR is fed to the inverting (pin 2) input of op amp IC1 (741) via two back-to-back 470mF capacitors. These capacitors block the DC component at the output of the LDR while allowing signal fluctuations to be fed to the op amp. They also block any slow variations in the LDR signal due to thermal variations in the room. IC1 functions as an inverting amplifier stage. Its non-inverting input (pin 3) is biased to half-supply using two In the prototype, the LED & the LDR were brought out through holes in the case, with the vane sitting between them – see above. By contrast, in the final version, the LED & LDR are inside the case and the vane rides in a slot. The vane is positioned so that it normally “shadows” about half the LED body. 30  Silicon Chip siliconchip.com.au 10kW resistors, while its gain can be varied from 0-10 using potentiometer VR1, which is in the feedback loop. Note that although the circuit shows a 741 op amp, you could also use an OP27 device for improved accuracy. IC1’s output appears at pin 6 and is fed to a voltage divider consisting of two 3.3kW resistors. The top of this divider (ie, at pin 6) can be used to directly drive an external data logger. Alternatively, the divider output (at the junction of the resistors) can be used to provide a nominal 0-5V signal, which may be required by some loggers. Pin 6 of IC1 also drives trimpot VR2 and this is used to set the maximum signal level into pin 3 of IC2 (to about 4V). IC2 is programmed to function as an A/D converter, using the simple program shown in the accompanying panel (more on this later). Its output is taken from pin 7 (P0) and fed to pin 2 of DBF9 socket CON2. This socket is in turn connected to the serial port of a PC, to provide the alternative data logger. The PICAXE-08M is programmed via pin 3 of the DBF9 socket. The incoming data signal is fed to pin 2 (SER IN) of the IC via a voltage divider consisting of 22kW and 10kW resistors. Power for IC2 is supplied via 3-terminal regulator REG1. This provides a regulated +5V rail to pin 1. Building the circuit Building the circuit is easy since all the parts are mounted on a small PC board coded 04105091. Fig.2 shows the assembly details. Note that REG1 and the PICAXE (IC2) are required only if you don’t already have a data logger. If you do have a logger, these parts can simply be left out, along with the DB9F socket, trimpot VR2, the 100nF capacitor and the 22kW and 10kW voltage divider resistors from pin 2 of IC2. Begin by installing the re- Par t s Lis t 1 PC board, code 04105091, 123 x 57mm 1 9V DC plugpack 1 2.1mm DC power socket, to suit plugpack (CON1) 1 DB9F connector, PC mount 1 plastic utility box, 130 x 67 x 44mm (UB-3 size) 4 9mm-long untapped spacers 4 M3 x 15mm machine screws 4 M3 nuts 3 PC stakes 1 serial computer cable (see text) 2 8-pin IC sockets 1 100kW linear potentiometer (VR1); Jaycar Cat. RP-8518 1 5kW horizontal trimpot (VR2) 1 Light Dependent Resistor (LDR1) 1 3-way pin header 4 10kW 1 100W 1 3.3kW Plus 1 x 10kW or 1 x 3.3kW or 1 x 1kW to match LDR resistance – see text Mechanical Parts 1 800mm-long x 5/16-inch threaded steel rod 5 5/16-inch nuts and washers to suit rod 1 50mm-long x 1/4-inch bolt 3 1/4-inch nuts and washers 1 40mm-long x 1/2-inch bolt 1 1/2-inch nut and washers 1 3/8-inch washer 1 1-metre length 1-2mm diameter steel wire 2 bull-dog clips to suit 1 2-2.5kg mass (eg, 2 x 1.25kg barbell weights) 1 piece of thin aluminium sheet (to interrupt light beam) 1 50 x 50mm piece of aluminium or rigid plastic for paddle (see text) 2 small bolts & nuts to fasten paddle to bulldog clips 2 braced right-angle brackets, 250 x 250mm 8 1/4-inch x 40mm bolts, nuts & washers 3 5/16-inch x 100mm roundhead bolts, nuts & washers 1 wooden base, 900 x 250 x 20mm 1 wooden back, 400 x 250 x 20mm Semiconductors 1 741 or OP27 op amp (IC1) 1 PICAXE-08M microcontroller (IC2) 1 7805 3-terminal regulator (REG1) 1 1N4004 diode (D1) 1 red or white high-brightness LED (LED1) Capacitors 3 470mF 25V electrolytic 1 100nF MKT (code 104 or 100n) Resistors (0.25W, 1%) 1 22kW 2 1kW sistors and capacitors. Table 1 shows the resistor colour codes but it’s also a good idea to check each resistor using a digital multimeter before soldering them into circuit, just to make sure. Follow these parts with diode D1, the two IC sockets (don’t install the ICs yet) and trimpot VR2. Take care to ensure that D1 and the electrolytic capacitors go in the right way around. LED1 can go in next. Bend its lead down through 90° close to its body before installing it at full lead length on the PC board – ie, the centre of the LED should be about 22mm above the PC board (see photo). Again, take care to ensure that it’s oriented correctly. That done, you can install the LDR but there’s just one wrinkle here. The 10kW resistor shown in series with the LDR on Fig.1 is correct for most LDRs. However, some LDRs have a Table 1: Resistor Colour Codes o o o o o siliconchip.com.au No. 1 4   2   1 Value 22kW 10kW 1kW 100W 4-Band Code (1%) red red orange brown brown black orange brown brown black red brown brown black brown brown 5-Band Code (1%) red red black red brown brown black black red brown brown black black brown brown brown black black black brown September 2005  31 REG1 7805 100 LED1 K  OUT 100nF 10k LDR1 K 470F 9V DC IN 1 6 470F 10k VANE ON SEISMIC MASS CON1 SERIAL OUTPUT CON2 DB9F SENSITIVITY VR1 100k A 470F 25V 7805 GND GND A  D1 1N4004 IN +5V OUT IN 2 7 3 IC1 741 4 6 VR2 5k 4 3.3k 10k* (SEE TEXT) 1k 3 Vdd P0 P1 P3 2 3 IC2 5 PICAXE P2 -08M SER 2 IN P4 Vss 5 22k 8 H L E 10k 7 3.3k 10k LED SC  2005 LOGGER OUTPUT SIMPLE SEISMOGRAPH 1N4004 A K K A 22k IC2 PICAXE 3.3k IC1 741 5 470F 470F E L H 5k 10k VR2 3.3k 470F 2 3 100nF 10k K CON2 LDR1 10k A LED1 (SLOT IN BOX ABOVE) CON1 REG1 7805 10k 100 5002 © 1k 9V DC IN D1 19050140 (BEND LEADS SO LED FACES LDR1) 10k 1N4004 Fig.1: the circuit uses a light detector based on LED1 & LDR1 to detect movement of an interrupter vane placed between them. The resulting signal is then amplified by IC1 and fed to the logger output. IC1 also drives IC2, a PICAXE-08M chip programmed to function as an A/D converter. Its output can then be fed to the serial input of a PC, to provide an alternative data logger. DB9F VR1 100k LOGGER OUT Fig.2: install the parts on the PC board as shown here, making sure that all polarised parts are correctly oriented. IC2, REG1, VR2 and CON2 can be left out if you already have an external data logger. 04105091 © 2005 Fig.3: this is the full-size etching pattern for the PC board. 32  Silicon Chip lower resistance than others in the presence of light and you may have to adjust the value of the series resistor accordingly. That’s easy to do – just measure the resistance of the LDR in a brightly lit room and use a series resistor that’s about the same value. The value isn’t all that critical. In practice, you can buy 1kW, 3.3kW and 10kW resistors and use the one that’s closest to the measured LDR value. The LDR is mounted in similar fashion to the LED – ie, bend its leads down through 90° before installing it. It should be mounted so that its face is siliconchip.com.au This view shows the fully assembled PC board. Note the arrangement for the LED & the LDR. directly aligned with the LED. Regulator REG1 is mounted with its metal tab flat against the PC board. To so this, bend its leads downwards by 90° about 5mm from its body, then secure it to the board using a 3M x 6mm machine screw and nut before soldering its leads. There’s no need for a heatsink, as it supplies just a few milliamps to IC2. The board assembly can now be completed by fitting CON1, CON2, potentiometer VR1 and a 3-pin header for the external logger interface. Serial cable options A standard serial cable is used to connect the PC board to the computer (if you’re using a PC as the data logger). There are several options here. First, you could go out and buy a serial cable but that’s the expensive way of doing things. It’s far better to scrounge a cable instead. For example, if you have an old modem (left over from your dial-up days), you can use its serial cable (you did keep it, didn’t you?) to connect to the PC. Another possibility is to use a serial cable from a discarded mouse. Just cut the cable off close to the mouse, then strip the wires back and use a multimeter to identify which lead goes to which pin on the socket – you need to use the leads that go to pins 2, 3 & 5 (the rest can be trimmed off). These leads can then be soldered directly to three PC stakes mounted at the appropriate points on the PC board. As a bonus, you don’t need the siliconchip.com.au Above: a slot is cut into one end of the case to provide access for the metal vane that’s attached to the seisomograph bar. on-board DBF9 socket, which means you can save even more money. Checks & adjustments Before fitting the two ICs, it’s necessary to make several voltage checks. First, connect a 9V DC plugpack supply and switch on. The LED should immediately come on. If necessary, adjust it so that it shines directly on the LDR. Next, use a digital multimeter to check the voltages on IC1’s socket pins. Pin 7 should be at the supply voltage (about 9V, depending on the plugpack), pin 2 should change when the light to LDR is suddenly inter- rupted and pin 3 should be at half supply voltage. That done, check for +5V on pin 1 of IC2’s socket and for 0V on pins 2, 3, 7 & 8. If it all checks out so far, disconnect the plugpack and install IC1 (but not IC2). You now have to adjust trimpot VR2 so that the voltage on pin 3 of IC2 can never exceed 5V. This is done as follows: (1) Connect a clip lead across the two back-to-back 470mF capacitors (ie, short them out); (2) Set both VR1 and VR2 to their midrange positions; (3) Place a piece of thick cardboard (or other opaque object) between the LED September 2005  33 Tectonic Plates, Earthquake Waves & The Richter Scale ”An earthquake is the way the Earth relieves its stress by transferring it to the people who live on it.” – Dr Lucy Jones, USGS. E ARTHQUAKES occur when adjacent blocks of the Earth’s crust slide past each other along a fracture we call a fault line. Most active faults are located near the boundaries of the Earth’s tectonic plates. These plates move in several ways: (1) they can slide past each other; (2) they can move away from each other (diverge); or (3) they can move towards each other (converge). For example, the west coast of New Zealand’s South Island – which is at the eastern edge of the AustralianIndian plate – moves north along the Alpine Fault. This movement is relative to the eastern side of the island, which is part of the Pacific plate. This area experiences several magnitude five quakes every year, as well as much larger but less frequent earthquakes. Plate divergence generally occurs at mid-ocean ridges such as the Atlantic’s, which rises above sea-level to form Iceland’s central rift valley. Convergence occurs at “subduction zones” like the one that caused Aceh’s Boxing Day earthquake. Here, the northern edge of the AustralianIndian plate is descending under Indonesia, which is part of the Eurasian Plate. While most active faults are located near plate margins, about 10% of active faults occur well away from the plate margins. The earthquakes generated in these locations are known as intra-plate earthquakes and are mostly thought to occur either as a response to stress transmitted through the plate from its interaction with adjacent plates or from thermal equilibration, which can cause con- and the LDR (to block the light); (4) Reapply power and check the voltage at pin 6 of IC1. It should be about 1V less than the supply rail; (5) Monitor the voltage at pin 3 of IC2’s socket and adjust VR2 for a reading of 4V (or slightly less). 34  Silicon Chip traction as the plate cools down or expansion as the plate warms up. The Northern Territory’s Tennant Creek fault is a world-famous example of one of these intra-plate structures and generates a number of generally small earthquakes each year. Several types of vibrations are generated as blocks of rock grind past each other during an earthquake and these propagate around and through the planet as different types of earthquake waves. The fastest (and the first to arrive) are “Primary” or P-waves, which are longitudinal compressional waves that propagate at speeds of 1.5-8km/s (depending on rock density). The next fastest are the “Secondary” or S-waves which are shear waves (or transverse waves) and these propagate at speeds of about 3.2-4.8km/s. Both P and S-waves move through the body of the planet and are refracted and reflected as they encounter rock density and composition changes. However, S-waves cannot propagate through the liquid part of the Earth’s core. In fact, it was by examining the geographic pattern of P-waves and S-waves that led to the formulation of the core-mantle-crust model of the Earth. The slowest waves are surface waves, which propagate at speeds of about 2-5km/s. There are two types of surface waves: Rayleigh and Love (L) waves. It’s the shear and surface waves that generally cause the damage associated with earthquakes. By measuring the time gap between the arrival of the P and S waves, it’s possible to calculate how far away the earthquake was from the seismograph. This is roughly 500km for every minute between their arrival. The location of the epicentre is determined by a form of “triangulation”. To do this, a circle corresponding to the calculated distance is drawn Once that’s done, disconnect the plugpack and install the PICACE-08M, with its notch facing to the left – see Fig.2. Final assembly The PC board is designed to fit around at least three different seismograph locations on a map of the region. Where the circles intersect is the likely epicentre. Most earthquakes occur at depths of less than 100km. P waves have higher frequencies and are best detected with a “Short Period (one second or less) Vertical Seismograph”, while S, L and Rayleigh waves have lower frequencies and are best detected by a “Long Period (10 seconds or longer) Seismograph”, such as the design described here. Professional seismic stations have short, long and wideband seismographs mounted northsouth, east-west and also vertically, with both low and high-sensitivity detectors. Analysis and filtering of the seismic patterns allows the arrival of each type of wave to be determined from the mixture of P, S, L and Rayleigh waves, reflections (PP and SS waves), refracted waves and alternative path surface waves. Our seismograph with a 1-second (or 10 second) sample rate, will probably only detect S waves and the much larger displacement L waves and Rayleigh waves. If you live very close to the action, such as in NZ or PNG, you might also detect P waves. The Richter value, devised by Charles Richter in 1935, is basically a logarithmic measuring scale. It’s calculated according to the largest ground motion waves that are detected 100km from the epicentre of the earthquake. Because the scale is logarithmic, a magnitude 7 earthquake has 10 times the ground motion (and more than 30 times the energy) of a magnitude 6 quake. The Aceh Earthquake measured 9.0 on the Richter scale and released many thousands of times more energy than the 5.6 Newcastle earthquake of 1989. inside a standard UB3 utility case. It’s mounted on the lid on four 9mm untapped spacers and secured using M3 x 15mm long screws and nuts. That done, you have to make a cutout in one end of the case to provide clearance for the DBF9 socket (CON2) siliconchip.com.au and the pot shaft. This cutout measures 45mm long x 12mm high and is about 12mm from the lip of the base. Alternatively, if you’re not using CON2, the serial cable can be run through a small hole in the case and secured using a small cable tie. The same applies if you are connecting an external logger to the 3-pin header. You also need a hole directly in-line with the DC power socket (CON1). This is horizontally centred 17mm from the lip of the case and should be drilled and reamed to 8mm. Finally, a slot must be cut in the case in line with the light sensor to provide access for the vane that’s attached to the bar. This slot should be positioned 37mm from the end of the case and can be about 4mm wide. The unit can then be assembled into the case and attached to the base of the seismograph. Position the vane so that it normally blocks about half the light between the LED and the LDR. Programming the PICAXE To program the PICAXE, you first have to download the free “Programming Editor” from www.rev-ed.co.uk/ picaxe That done, connect the board to your computer via the serial cable (this should be done with the computer off) and download the simple program shown in Listing 1 into the PICAXE chip. If you increase the logging interval to 10 seconds by changing line 5 to “wait 10”, you can keep a continuous seismograph record for up to a week. You could also hang a piezo transducer off the PICAXE and add an “Alarm” loop to the program to warn you if b1 exceeds a certain value. Once the program is loaded and running in the PICAXE-08M (check by looking at the “debug” screen), you must close down the PICAXE Programming Editor to free the COM Port, so that the StampPlot Lite program can use it. StampPlot Lite is available free from www.selmaware.com Fig.3: this simulated plot of an earthquake was produced during final sensitivity tests of the seismograph. A gentle puff of air aimed towards the seismograph masses from two metres away produced the first “earthquake” waves, while similar puffs from one metre gave the full scale deflection. (3). Click on “Connect” and “Plot Data” – the program should immediately begin to graph the values sent by the PICAXE-08M. You can test this by blowing on the bar from a distance of about one metre. Adjust the sensitivity control (VR1) for full-scale deflection. The “action” near the bottom of the screen indicates that data is being collected. (4). Set the maximum number of points to 200,000 or higher. (5). A “Time Span” of 400 seconds will show each swing of the bar during testing but increasing this to 25,600 will let you see most of a night’s recording. Australia is normally a long way from the action and different types of earthquake waves will continue to arrive for more than an hour after a distant quake. (6). Click on “Save data to file” so the program saves the data as a .txt file. siliconchip.com.au Acknowledgement: thanks to Dr Tom Hubble of the University of Sydney for his geological knowledge and neighbours Jo and Manfred for computing and design assistance. Program Listing 1 Using StampPlot Lite StampPlot Lite is the logging program. Once it’s installed, you need to carry out the following steps: (1). Set the COM port so that it’s the same as the port that connects to the PICAXE. (2). Change the Baud rate to 4800. (7). Click on “Clear min/max on reset” and you will be able to see if any values have been detected that are significantly above the background line (ie, an earthquake) and when this occurred (approximately). If you deselect “Connect” and “Plot Data” to stop the recording, you can look back at stored parts of the graph by moving the bar next to “Enable Shift”. The running graph can be seen on the screen and “.txt” values can be exported to Excel and graphed. (8). Click on “Time Stamp” so that Excel will show “Time” on the graphs. Good luck and I hope that the Earth SC moves for you. main: readadc10 4, b1 debug b1 sertxd (#b1,cr,lf) wait 1 goto main 'makes an A-D conversion of the value at input 4 and sends to b1 'allows you to see the value at b1 on the Picaxe debug screen 'sends the value of b1 out to the StampPlot Lite program 'sets the time gap in seconds between readings 'makes the program loop back to the start September 2005  35