Wiggles and more wiggles

For the past 3 years, I’ve been looking at ocean bottom seismometer data off the east coast of New Zealand’s North Island, and I always wondered about the logistics behind my dataset of earthquakes. The earthquakes I work on are located along the margin where the Pacific plate is subducting beneath the Australian plate, creating mountains, earthquakes, and tsunami risks for New Zealand. Usually, land-based (onshore) seismometers are used to locate the underwater (offshore) earthquakes, however this locations are not as accurate as if the seismometers where located right on top of the seismic events. Using ocean bottom seismometers has the big advantage of being located closer to seismic events, therefore having more appropriate hypocenter locations. This allows seismologists to have a better understanding of the subduction process and associated seismic risk.

It turns out that deploying ocean bottom seismometers is a huge task that includes multiple people; from the crew members: bridge, engineers, and cook teams, to the science teams: the Apply-to-Sail students, WHOI team, and K12 teachers. This experience has exceeded all my expectations. I imagined a boring and repetitive process, but every single station has its own challenges: the bathymetry indicating a rough or steep relief so we have to move somewhere close by with a more flat and soft bathymetry, be sure the temperature sensors are the ideal for specific depths, fill the sheets with station information and log it in our physical and digital forms, and a large etcetera (see Natalia’s post ‘Halfway Point’).

When the data will become available in a year or so (after the seismometers are collected by a second ship), the information from every single station will look like the figure below. The black line going up and down in the three panels is what we call a wiggle. The horizontal axis is time in seconds and the vertical axes register the displacement of the seismometer in terms of voltage. The upper and middle panel are the displacement in the horizontal axes, i.e. the displacement relative to the North and to the East, respectively. The lower panel registers the displacement in the vertical axes. These waveforms are from a magnitude 2.9 earthquake recorded in an ocean bottom seismometer in New Zealand and the ‘P’ and ‘S’ red labels show the arrival of the two main wave phases that allow us to locate an earthquake. In region like the Gulf of Alaska we expect thousands of earthquakes or microearthquakes (magnitudes lower than 4) that would only be possible to register (and locate) with ocean bottom seismometers – the ones we are deploying right now! Like in New Zealand, these seismometers will allow the geological community have a better understanding of the subduction process in this region and lead to future research in earthquake hazard forecast.

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After this experience, I really value all the effort that the science crew did for the deployment and recovery of the data that I am currently working on. For the future seismologists that are going to work with this Gulf of Alaska data, I want to say that we did our best to make sure the seismometers were meticulously deployed and I’m sure the recovery crew will be equally careful to collect the year-long log of wiggles from the more than 70 stations deployed by the 1st and 2nd legs. Good luck!

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