In the Science Centre of the City Campus of the University of Auckland we record seismic waves with the TC1 seismometer. Routinely, our station AUCK records seismic waves from earthquakes in New Zealand and beyond. On January 20th, 2014, an earthquake occurred on the South side of the North Island, 15 km east of Ekatahuna. Here is a map of the epicentre, our station location, and the great-circle path between them.

On the left you can see 10 minutes of recordings, starting at the origin time of this earthquake. The green marker annotated with a Pn is the predicted arrival of the first wave traveling 4 degrees from the epicentre, 15 km east of Eketahuna, to Auckland. This prediction is based on a spherically symmetric model of the Earth, by Brian Kennett, and certainly seems to mark the start of minutes of vibrations in Auckland from this earthquake. In fact, if you look carefully you see that the wiggles after 10 minutes are still larger than before the first wave from this earthquake arrived. Larger earthquakes can make the Earth “ring” for many hours.

In the image on the right, we zoomed in on the first-arriving wave, almost exactly one minute after the earthquake originated. Now, you can see that the prediction is actually a few seconds before the arrival. This means the lithosphere under the North Island of New Zealand is a bit slower (~3% on this path) than the average on Earth. In general, a hotter lithosphere is slower than a cold one. This makes seismic waves traversing old, cold, continents relatively fast, and those sampling younger lithosphere like ours in New Zealand, relatively slow.

In general, it is these small travel time differences that provide images of the (deep) earth through a process called seismic tomography.

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:

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.

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.