In March of this year, we Aucklanders felt something very unusual for us: earthquakes! They were my first earthquakes, and I’m glad that they were small ones. I imagine that the bigger ones would be quite scary.
They were located underneath and near Motutapu Island, as found by the GeoNet AVF monitoring network. As earthquakes go, they were quite small at magnitudes 3.1 and 3.9. But with Rangitoto Volcano next door, they caused many to wonder if we were about to experience a volcanic eruption. If you were one of them, well done, you–we do think that small earthquakes like these will be one of the first signs that we will get of an impending eruption. However, after calming down post-shake, there were a few reasons why I did not think that the earthquakes were signs of a new volcano forming in Auckland. Keep in mind that I am not a seismologist, but these were the thoughts that ran through my head afterward:
First and foremost, Auckland has many faults (don’t get non-Aucklanders started!) and tectonic earthquakes are completely normal for the area. As GeoNet’s webpage shows, we constantly have unfelt earthquakes. These just happened to be slightly larger, and therefore, felt by people. In Auckland, it is much, much more likely that any earthquakes you feel will be unrelated to volcanic activity. The ground will continue to shift and settle just like an old creaky house.
Secondly, I checked GeoNet, and they showed that the felt earthquakes in March were quite shallow, at 4 and 6 km deep. We would hope that volcano-related earthquakes would start occurring much deeper, so that we had time to prepare and evacuate. We think that the magma that forms Auckland’s volcanoes rises from a zone of melt about 80-100 km deep underneath Auckland. At that depth, the rocks surrounding the melt are hot, under a lot of pressure, and are therefore more flexible than in the crust, so the movement of the magma does not cause earthquakes. But when it hits the more brittle crust at about 30 km depth, it has to force its way up to the surface, breaking apart the rocks. When the magma reaches about 30 km, we would expect earthquakes to start being produced.
Another reason that made me think these were not volcano-related earthquakes is that they looked like they fell outside the current eastern boundary of the volcanic field, an old fault between Motutapu and Rangitoto Islands (described in this thesis). Thus far, no volcanoes have popped up east of that fault. Maybe it is harder for the magma to move through those particular rocks, we don’t know. That isn’t to say that it couldn’t happen, but it hasn’t so far.
Lastly (but not least!), seismologists can look at the seismograph trace that the earthquakes produce to tell what kind of earthquake it was. Magma breaking through crust forms a different-looking trace than two pieces of rock sliding past one another. I felt confident that the GeoNet and GNS folks on duty would set into action if earthquakes related to magma movement (called harmonic tremors) were being detected. Indeed, they later put out a statement that the earthquakes were normal old tectonic ones.
Now, if these earthquakes had been around 28 km deep, located in the middle of the field, and GeoNet detected a harmonic tremor, well…I definitely would have started thinking about what to do. I reviewed my emergency plan and supplies, anyway. The best time to do that is when things are calm, but this reminder that we’re in an active tectonic environment served its purpose as well!
These are just a few of the reasons why I did not think that these earthquakes were volcano-related in the minutes after the quakes, but to be doubly sure (and to gather more data for a project I’m doing), a GNS colleague and I visited Motutapu Island to see if we could ‘see’ the offending fault(s) at the surface using gas monitoring. But more on that in the next post!