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As previously noted, we’ve had a few earthquakes. A feature of the period after a large quake is that there are many aftershocks. The name aftershock gives the wrong idea. Aftershock means earthquake and we have had over 7000, up to magnitude 6.3, close to our city.
The frequent occurrence of nearby aftershocks was an opportunity to get some measurements of our own and learn more about seismology and what was happening under our city.
Here’s a magnitude 5.4 quake that we captured in the evening of 21 June. The quake was centred about 12km away.
The three graphs show the ground acceleration in all three dimensions. The Y-axis is north-south and the Z-axis is vertical acceleration. The units of acceleration are expressed as a percentage of Earth’s gravity. The peak acceleration we experienced in this quake was about 9%g, about 880mm/s2.
Looking at the Z-axis graph, you can clearly see the higher frequency P-wave, arrive just over a second before the lower frequency S-wave. The difference in propagation speed between the P and S waves is about 8km/s.
I used a Fourier transform to extract the constituent frequencies of the shaking, from the accelerometer data. The top graph shows strong peaks in the three to five Hertz range for the S-wave. The P-wave around 20Hz is clear in the Z-axis plot.
Fourier transform - x-axis
Fourier transform - z-axis
You can see the accelerations at other points in the city on the shaking map on the GeoNet website. First zoom in the central Christchurch, then select PGA from the Add Instrument Shaking drop-down menu. Put your mouse over the square icons to see the ground acceleration. Close to the quake, the accelerations reached 60%g, strong enough to cause damage.
How was it done
There are many designs for amateur seismographs. The best known are from Scientific American articles, Lehman seismograph from 1979 and the Shackleford-Gundersen design from 1975. Both these instruments involve a fair amount of mechanical fabrication and electronics.
So to get something working quickly we used a Bosch micro-machined accelerometer chip, which which can record nearby quakes with strong ground acceleration, i.e. like those we have been experiencing. This type of instrument is classed as a strong motion accelerograph.
The instrument is built around an Arduino Mega2560, using microSD storage. A ChronoDot provides timekeeping. An LCD display and a couple of LED’s provide status information.
Accelerometer
The Bosch BMA180 has the best performance of all the devices I tried. It is a digital sensor with a high sample rate, SPI and I2C interfaces and built-in low-pass filters. The seventy page datasheet is a little intimidating.
I also tried a Freescale MMA7260Q and Analog Devices ADXL345 accelerometers. Both devices had lower sample rates and higher noise floors than the Bosch device. The ADXL345 would be adequate for this application. These sensors are all MEMS technology.
The BMA180 is powered from the Arduino 3.3V pin and connected directly to the I2C pins (pins 20 and 21).
The program uses a timer to trigger an interrupt to read the accelerometer at regular intervals. A sample rate of 200Hz is more than adequate for seismic signals, which are below 20-30Hz.
The main limitation of MEMS accelerometers is the noise level. A magnitude 3.7 quake within about 15km is the lower limit for getting a good signal.
Data capture
The original plan was to transmit the data direct to a PC, via ethernet. However, given that mains power failures often accompany stronger quakes, this perhaps wasn’t the best possible solution.
The Arduino Ethernet shield also has built-in microSD card slot. This can easily write the accelerometer data to the microSD card faster than it is captured.
When an event is detected the data in the ring buffer is written to the microSD card and the program keeps writing for several seconds after the end of the event.
A ring buffer is a common pattern for capturing data. Readings are added to the end of the buffer until it is full. At that point the oldest reading is overwritten. In this program, the accelerometer is read continuously and data is stored in a ring buffer 399 points long. When an event is triggered the main program loop writes data from the buffer to the microSD card, as long as the recording continues. A standard Arduino doesn’t have enough RAM for this size ring buffer, hence the Mega2560.
Using a ring buffer with a timer ensures that the samples are read at even intervals regardless of the timing of writing data to the microSD card. Since there are already 399 samples in the buffer at the start of the event, the recording includes data for two seconds before the event was detected.
A series of consecutive readings above the noise threshold triggers recording and it continues for several seconds after the signal drops below the threshold. Once the event is complete, the file is closed and another one opened.
Power
When there is a powerful earthquake the electrical power distribution system shuts down to protect itself. Battery backup ensures continuous recording even if mains power fails.
The Sparkfun combination LiPoly charger and boost converter makes this easy. The board supplies power to the instrument and charges the LiPoly battery. If the power fails, the boost converter on the board supplies regulated 5V to the Arduino board. Power is wired directly to the 5V pin on the Arduino.
The 1200 mAh battery can run the unit for 5-6 hours. More if the LCD backlight is disconnected.
Data Processing
I used Microsoft Excel to process the comma-delimited text files from the microSD cards and make the graphs. The Excel Analysis Toolpack includes a Fast Fourier Transform function.
More things to try
- look for ways of reducing accelerometer noise
- add web server to the instrument to remotely read status and download data
- automated processing
- alternate sensors, e.g a geophone or one of the designs from Scientific American
Parts
- Arduino Mega2560
- Arduino Ethernet Shield (or any other shield or breakout with an SPI interface to a microSD card).
- Macetech Chronodot.
- Bosch BMA180 accelerometer on a Sparkfun breakout board.
- Sparkfun LiPoly charger/boost converter and 1200mAh LiPoly battery.
- Adafruit protoshield with mini breadboard.
- Adafruit breadboard wires.
- Red and green LED with 100 ohm resistors
- PCB pushbutton switch
- 16×2 LCD character display with Adafruot I2C/SPI adapter – SPI mode.
- 100uF 16V capacitor
- 5v main power adapter with microUSB connecter
The Arduino sketch can be downloaded, or cloned from the bitbucket repository https://bitbucket.org/johnmccombs/seismograph_bma180
We live in Christchurch, New Zealand, and there have been two significant earthquakes recently. One in September last year, and another in February of this year. The February earthquake was rather more damaging – it resulted in 181 deaths and many more injuries. A large number of homes have been damaged beyond repair and life is hard for many in the city who have been without water supply, electriciy or sewerage.
The magnitude 6.3 (Feb 2011) event caused more damage than the 7.1 (Sep 2010), because it was close to the city – which means that a greater percentage of the total energy released hit Christchurch itself. Part of the central city is still deemed a no-access “red-zone”, and half of the commercial buildings will be demolished.
More technical detail is available from Geological and Nuclear Sciences here, a good collection of photos by Ross Becker here, and a video by Civil Defence here. A more cheery view, of how things used to be, is here.
Crane about to demolish a house on our street, with chimneys and a balcony
The September quake happened in the middle of the night. Since the power went off and there were no street lights, we thought we would traipse outside to look at the stars, as you do, and were rather suprised to find our courtyard area flooded and muddy. In the morning we found extensive amounts of sand in various parts of the yard. These are called ‘sand boils’ or ‘sand volcanoes’. They occur where water and sand mix and are subjected to pressure from an earthquake. The water is forced up through a gap in the overlying soil and it carries the sand/silt with it.
Sand and silt that has come up through the cracked driveway
Sand Volcano
Then in February, we were taking a moment outside, after the initial quake, with camera in hand. So when one of the several major after-shocks caused liquefaction right in front us, we were able to capture a little of it.
Where it was not very deep, the mud dried into interesting patterns. In most places however it just stayed in big piles that had to be shoveled out.
Dried mud - where it was not very thick
The sand and silt from liquefaction has been a major problem for the waste water system too, which is still not fully functional. This little ‘poo-bot’ was inspecting the sewerage system in our street a week or so ago:
Robot, for looking at the sewer
LATERAL SPREAD
We had some lateral spread – essentially the banks eroding towards the river. Our neighbours, right next to the river, had the floor plate of their house split in two.
One of the cracks, parallel to the river bank, at Mona Vale. About 1 m deep.
We are still getting significant aftershocks. There was a magnitude 5.5 this morning.
Chaos - it's not just a theory... 