Physical Acoustics Lab

Tsunamis can be generated by earthquakes near subduction zones. Here, the white gutter represents one tectonic plate with water — an ocean — on top. The wooden board is a plate subducting under the “gutter plate.” Friction between the two plates, pulls the gutter down until the friction is overcome by the strength of the gutter. At that point, the gutter plate rebounds, throwing water up. This is the start of our wave, as you can see in the video:

In the video below, you see a wave travel from one side to the other in shallow water in a 30-cm square baking tray that is only a few centimeters high.

For our lab in ENVPHYS100, you can answer some of the following questions:
– What is the wave speed?
– What is your estimate of the wavelength?
– If the wavelength is much less than the water depth, then the wave speed equals the square root of the product of gravity and water depth. Can you estimate the water depth in the tray?

1. Now, break out your own baking tray, and collect data to test the shallow water wave equation v \approx \sqrt{g h} (format!)
2. Considering the full water wave speed equation with a deep and a shallow water term. Discuss this in terms of Tsunami. For example, what happens to the Tsunami wave
as it approaches the coast?
3. In the previous you focused on the wave speed of a tsunami. If you now remember Mila’s lecture on conservation of energy, discuss the wave height of a Tsunami from deep in the ocean to the coast.
4. Joining (changes in) wave speed and height, draw me your best Tsunami, as it makes landfall.

A new publication led by Jonathan Simpson appeared in Geophysical Research Letters, this month.  Laser ultrasonic measurements show that the wave speed in Alpine Fault rocks decreases for waves with a larger amplitude, as shown below. The closer to the fault, the more the wave speed decreases with amplitude. The observation of “rock weakening” contributes to the bigger topic of when and how earthquakes happen, in particular to so-called “triggered” earthquakes.

In December 2020, the AGU meeting was held virtually, and Jonathan won a best student presentation in the seismology section of the meeting with a presentation that blended a traditional “zoom-style” presentation with a tour of the setup of his laser-ultrasound experiments on nonlinear rock properties under in situ conditions. These nonlinear rock properties may play an important role in triggered earthquakes, or even the propagation of an earthquake fault rupture.

We would like to congratulate Caitlin Smith and Jonathan Simpson with their brand-new BSc (science scholar) and MSc degree, respectively. It has been such a pleasure to work with them in our lab, that we are very happy  they both will continue research in the Physical Acoustics Lab in pursuit of a BSc Honours (Caitlin) and PhD (Jonathan) degree!

After a successful PhD defense, Josiah is making his next move. And it is a big one! We wish him and his family all the best in Warshaw, Poland, where he will be starting work at the seismic sensors group at Astrocent, in November 2020. Astrocent is a part of the Advanced Virgo Experiment and the emerging Einstein Telescope (ET) project detecting gravitational waves from space. Obtaining high quality gravity waveform data requires ultrasensitive sensors and the monitoring of and compensation for seismic noise. Seismic noise not only shakes the test masses in the interferometers, but is also a source of the Newtonian – or gravity gradient – noise. This type of noise is due to fluctuations of the local gravity by seismic or sound waves in the medium surrounding the detector. Work on the mitigation of this type of noise is the primary goal of the group. His work will involve designing and setting up seismic networks, characterizing seismic noise fields, optimizing the performance of seismic sensors for monitoring the future sites, and perhaps some site-scale seismic tomography.

Josiah has been a member of the PAL throughout very successful MSc and PhD projects, and we are extremely proud of his accomplishments. All the best, Josiah!

Today, a new paper came out in Science showing that in many places around the globe the noise levels on seismic stations dropped dramatically after lockdown measures to combat COVID-19 went in effect. The study was led by colleagues in Brussels, but Kasper’s involvement kept him busy when our campus was closed, and the result includes an example of how Auckland was one of the places where seismic noise reduced by approximately 50% of its normal levels. This quiet period gives us a data set of the best quality to learn more about the Auckland Volcanic Field and the network that is installed to monitor it.

Seismic noise in Auckland compared to mobility data from Google show strong correlations with the amount of human activity (particularly with the activities associated with trips to the grocery and pharmacy)

This morning Josiah defended his PhD thesis titled “Multi-Component Ambient Seismic Noise Tomography of the Auckland Volcanic Field” (AVF). Josiah gave an excellent presentation and fielded the questions from the examiners with confidence.The outcome of the defense was the recommendation to award Josiah the PhD degree, pending minor corrections.

Josiah’s research culminated in the first three-dimensional structural model of the AVF, extending from the surface to ~25km depth: a worthy accomplishment in its own right, and one that is an important addition to the existing literature. Josiah’s thesis provides another piece of the puzzle that is the AVF.

Josiah has been a member of the PAL, having completed his MSc with us previously. We are proud to call him our “pal”, and wish him well in the next phases of his career. Congratulations to Josiah!

Josiah pictured 2nd from the right, with James, Jonathan, Shreya and Kasper at the GSNZ meeting in Josiah’s backyard of Hamilton.

Seismometers around the world mainly act as ears to the ground to learn about the Earth’s subsurface structure and dynamics. Still, seismic waveforms record more than earthquakes and volcanic activity. Wind and ocean waves generate seismic noise, while seismographs in the vicinity of cities record the occasional rock concert, rugby and football game.

In the battle against the COVID-19 pandemic, seismic stations have recorded a reduction in anthropogenic noise sources. In New Zealand that has been reported by Geonet and the Ru educational seismic network, for example. In general, nights are quieter than days, and weekends are quieter than weekdays. However, after the closing of schools and workplaces, noise levels during weekdays is equivalent to nights and weekends.

In recent days, seismologists have seen seismic noise levels pick up again in cities around the globe. Some countries have officially reduced the restrictions on regular daily lives, but in other places there is seismic evidence of society slowly running out of patience with the strict rules of lock-downs. In the figure below, you can see this does not appear the case for New Zealand: data from a seismometer in a borehole in Herne Bay shows the noise levels in the city of Auckland remain much lower than before the lock-down. Kia Kaha, Tamaki Makaurau!

Brilliant work in the MSc theses of Jonathan and Pat resulted in the publication of a new Geophysical Research Letter manuscript, titled Constraining microfractures in foliated Alpine Fault rocks with laser ultrasonics.

Jonathan extended our capabilities in laser ultrasonic measurements to rocks under in situ conditions, while Pat was able to numerically model wave propagation of fractured rocks. As a result, this paper examines the amount and orientations of fractures in rocks, affected by the tectonic processes associated with the Alpine Fault. These new findings may help explain the dynamics, as well as the seismic imaging of the Alpine Fault.

On Monday, James Clarke defended his PhD thesis titled “Laboratory and Numerical Experiments to Infer the Effects of Fluids on Volcano Seismicity.” The examiner and other attendees were impressed by the clear introductory presentation and the excellent handling of the questions from the examiners. James passed with flying colours! His advisers Ludmila Adam and Kasper van Wijk are very proud of the research James has been able to do over these past years. The three of them would like to thank Dr. Joel Sarout and his team in the Rock Properties group at CSIRO for hosting James for his important measurements on The relation between viscosity and acoustic emissions as a laboratory analogue for volcano seismicity. They also thank the New Zealand Earthquake Commission, and the Dodd Walls Centre for financial support to James during his thesis. Mila, Kasper and the rest of the PALs will miss having James around, but wish him all the best in the next phases of his career.