All posts by kvan637

A deep earthquake on the eastern margin of the Australian plate

Recently, we recorded seismic waves on our station AUCK from an Earthquake roughly 11 degrees to our North. This event is characterized by a strong P- and S-wave arrival as you can see in this figure:

fiji65_AUCKdata

Given the usual limitations of our (vertical) sensor when it comes to S-wave recordings, this is indicative of a very deep earthquake. The USGS estimates that this earthquake happened at a depth of 460 km. Now, under most places on Earth the rocks at those depths are too ductile to support the brittle breaking necessary for an earthquake, but in this case, the earthquake happened in — or on the boundary of — the brittle Pacific Plate subducted under the Australian Plate. Note that the epicentre of this event is about 500 km from the surface expression of the boundary between these plates. From the depth of the event and the offset to the plate boundary at the surface, we can estimate the angle of subduction may be around 45 degrees.

FIJI65_2014epicentre

The P- and S-wave markers are based on the average wave speed in the earth. In this case, they are a bit earlier than expected, because the subsurface between earthquake and the AUCK recording station is slower than average. As discussed previously, this is indicative of a young, warmer (and thus slower) lithosphere.

Furthermore, such deep earthquakes cause relatively little surface wave energy. The signal after the S-waves is likely a guided wave in the Pacific plate called a “leaky mode.” If you want to learn more about leaky modes in the Kermadecs, you should read this paper.

How to turn off ‘app nap’

On a mac, there is this clever tool to turn off (parts of) applications that the computer thinks you are not using. This tool is called ‘app nap’.

For jamaseis, this has to be turned off. The following video shows how:

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(Note: The initial part of the video shows how you can access the relevant settings by right clicking. You may need to hold down the control key on the keyboard while clicking to get to the contextual menus).

 

Correct magnet positions

There are three magnets suspended from the slinky in the TC1. The first two are centred in the coil. Motion of these magnets from earthquakes are responsible for the induced current in the coil. The third magnet is suspended in the copper tube,  and dampens the system via Lenz’s Law.

TC1_zoom.JPG
Correct magnet positions. Click on the image for a closer-up view.

The pair of magnets in the coil, and the single magnet in the copper pipe should not touch the coil and tube, respectively, and their tops should be flush with the coil/tube, as shown in the picture.

If the magnets are not in the right vertical position, you can adjust the magnets by threading the screw on the lid up or down. If you need even more vertical adjustment, you can bunch or release rungs clamped in the lid of TC1.

If the magnets are not in the right horizontal position, adjust the knobs on the legs.

 

Why is my station’s snapshot not up to date?

If the screenshot of your station is not updating here, consider the following:

  1. Is your computer still on? In principle, it can’t be off is power is available: the bios should be set that the machine comes on after a power failure.
  2. Is your computer on the network? For macs, you can troubleshoot a wireless connection here, or trouble shoot in general from System Preferences > Network > Assist Me > Diagnostics.
  3. Is Jamaseis still running? Jamaseis should be in your startup menu, so that if the computer comes (back) on, so does jamaseis.

If the answer to the three questions is “yes”, check your log file (under the window menu in jamaseis). If you see an error that you can fix, great. If not, send us the log file and restart jamaseis.

Is the TC1 set up right?

If you are unsure if the TC1 is balanced and set up correctly, visit this post