The TC-1 seismometers in the Ru network are continuously recording small vibrations of the Earth. When these vibrations are seen on a seismogram they are called ‘noise’.  Recently, we have been extracting useful information from the noise recorded by  TC-1 seismometers in the Ru network using a technique called seismic interferometry.

Noise can be thought of as a collection of seismic waves of different frequencies all stacked on top of each other to create a somewhat random signal–the same as sound noise, but with a much lower, inaudible average frequency.  Because New Zealand is an island country surrounded by large oceans, a significant component of the low frequency noise comes from the interaction of the ocean with the seafloor and the land. Seismic interferometry analyses this noise and reveals evidence for surface waves generated by such a source which propagate through the Earth.

The stations best suited for this analysis are those in coastal regions which form a near straight line between coasts when connected, such as those at Oratia District School (ODS), Bayview Primary School (BYVW), and Wentworth Private School (WWPS). The coastal environment of the Tasman Sea on the west of New Zealand is known to have higher energy than that on the east coast, so surface waves generated by the ocean-land interaction on the west coast which propagate east should be recorded in the seismic noise. And that’s exactly what the seismic interferometry with this data has revealed!

The figure above shows the same as what you would see when looking at a normal seismogram, with time on the x-axis and amplitude of the signal on the y-axis. However,  on the positive side of the x-axis, this plot shows Wentworth’s recording of an “earthquake” which occurred at 0 seconds at Oratia (and vice versa for a negative lag time). Of course, an earthquake never actually happened, but the cross-correlation uses the recorded noise over a long period to show what the signal might look like if an earthquake did occur (this is why ODS1 is the “virtual” source). The large peak at a time of about +15 to +25 s shows that a large component of the noise recorded at Wentworth takes 15-25 seconds to travel the distance from Oratia. The distance between the stations is 36.75 km, so waves which take 15-25 s to travel between Oratia and Wentworth propagate at about 1.5-2.5 km/s, a typical velocity for surface waves! A region of higher amplitude at -15 s to -25 s probably indicates surface waves travelling in the opposite direction, from the east coast to the west coast.
Similar results are obtained by cross-correlating the data between Oratia and Bayview, and Bayview and Wentworth. Since Bayview is at approximately the halfway point of the distance between Oratia and Wentworth, these two cross-correlations show the large peak at about half the lag time.

While seismic interferometry can be used by seismologists to investigate properties of the subsurface (e.g. seismic wave velocities), our positive results demonstrate that data from the TC-1 seismometers in the Ru network is capable of being analysed in the same way as data from dedicated research-grade seismometers.

Thanks to all the station managers who have sent us their data, including those whose stations are not mentioned here (it’s a work in progress!).

This Arduino ‘weather station’ acts as an example of the functionality of the Arduino microcontroller (which is used in the TC-1). It was presented at the recent Ru workshop as an “Arduemo” (!?) and is a nice little project for those with little or no experience in wiring or coding the Arduino. Here is a picture showing the layout of the circuit:

The temperature/humidity and pressure sensors are Arduino-compatible modules from freetronics, while the anemometer is ‘home-made’ from two ball bearings in a brass casing with a steel shaft and windcups (see specifications). The anemometer sensor is a magnetic reed switch from a bike computer, so the Arduino simply watches for when the switch is opened and closed. A closer look at the individual components and wiring of the circuit can be found here.

This project is very flexible and could be modified to include many more sensors. My version of the Arduino sketch and a Python script to display the data on a computer are just one example of how the data can be read and processed.

Hundreds more Arduino examples and a huge number of helpful Arduino tips can be found at the Arduino playground: http://playground.arduino.cc/

See this map at http://ru.auckland.ac.nz/the-network/.

The code

Insert the following “inline” where you want the map to appear. At the time of writing, the code was still in flux, but what you see here will be a functional subset of what may be available later.

<!-- rū map -->
<div id="ru_canvas"></div>
<style type="text/css">
@import url("https://nzseis.phy.auckland.ac.nz/map/css/ru_mapem.css");
</style>
<script src="https://maps.googleapis.com/maps/api/js"></script><script src="https://nzseis.phy.auckland.ac.nz/last50/js/ru_stations.js"></script><script src="https://nzseis.phy.auckland.ac.nz/map/geonet.php?mode=js"></script><script src="https://nzseis.phy.auckland.ac.nz/map/usgs.php?mode=js"></script><script src="https://nzseis.phy.auckland.ac.nz/map/js/ru_mapfunctions.js"></script><script src="https://nzseis.phy.auckland.ac.nz/map/js/ru_legend.js"></script><script src="https://nzseis.phy.auckland.ac.nz/map/js/ru_mapinitem.js"></script><script type="text/javascript">
// <![CDATA[
var ru_options = {
"centreonstation": false,
"width": "640px",
"height": "640px",
"zoom": 5,
"iconscale": 1,
"fontscale": "100%",
"showusgs": true,
"showgeonet": true,
"showlegend": true,
"lat": -41.0,
"lon": 174.0,
"canvas": document.getElementById('ru_canvas'),
};
RU_ICON_SCALE = ru_options.iconscale;
RU_FONT_SCALE = ru_options.fontscale;
google.maps.event.addDomListener(window, 'load', ru_initialize());
// ]]>
</script>
<!-- /rū map -->

A number of parameters can be configured to change the way the map is displayed -these are wrapped up in a javascript object (ru_options).

The paramaters

centreonstation
false or the short name of a station in the Rū family. If false, then the map will centre on the latitude and longitude specified using “lat” and “lon”. If a station is specified, then the map will centre on the latitude and longitude recorded for that station.

width and height
String literals that are assigned to the respective style properties of the map canvas. You could specify a dimension in pixels, a percentage, or whatever units CSS allows. The posts associated with stations in this web site, use “474px” for height and width.
“100%” (with appropriate styling of the canvas element) could be used to fill a viewport.

zoom
An integer between 1 and 20, representing the zoom steps the Google Maps API allows. 5 or 6 is good for a view of the whole of New Zealand, 17 is good for a view of a building in its context.

iconscale
A positive and non-zero decimal value used to vary the scale of the icons used as map markers. All markers are scaled by this value which can be used to make the map more legible at different sizes.

fontscale
A string literal added as a font-size style property to the legend (if it is visible). This can be used to adjust how the legend looks at different sizes.

showusgs, showgeonet and showlegend
True or false. These give you control of the visibility of the various information layers available.

lat and lon
A Latitude and longitude that will be used to centre the map if centreonstation is false. The default provided is near the centre of New Zealand (-41.0, 174.0).

canvas
Used to pass the element used as the map canvas to the initialisation function -if you name the container differently (ie. not ru_canvas), then change the id inside the getElementById expression.