The objective of this science cruise is to collect heat flow and seismic reflection data offshore the eastern margin of the North Island, New Zealand to develop a better understanding of this region, known as the Hikurangi subduction zone. The Hikurangi subduction zone is a convergent plate boundary, where the Pacific plate is being subducted beneath the Australian plate, which causes thrust faulting. Temperature, rheology, pore fluid pressures, and other properties influence the location and style of deformation along the subduction thrust, which includes earthquakes, aseismic creep, and slow slip earthquakes. The heat flow data collected during the cruise will illuminate the thermal structure of the subduction zone, while the high-resolution seismic reflection data will allow for a better understanding of the complex tectonics along the margin.
As the plates move past each other the motion between the plates can be manifested in a variety of ways. Slow continuous motion between the plates results in aseismic creep. Alternatively the plates can be stuck together (or “locked”). In this case strain energy is stored in the rocks during periods of interseismic coupling. Later, this energy can be released suddenly in an earthquake or more slowly in a slow slip earthquake. A slow slip earthquake is a slow earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. Because slip is slow, it is not felt or detected by seismometers but is recorded by strain-meters and GPS instruments.
During this research cruise we will conduct experiments along the northern and southern Hikurangi subduction zone outlined by the boxes in Figure 1. The fault slip behavior along the Hikurangi subduction zone dramatically varies along strike. In the northern field study area, offshore of Gisborne, fault slip behavior is characterized by repeating, shallow (< 15 km depth) slow slip earthquakes and aseismic creep. Conversely the fault slip behavior in the southern field area, offshore and southeast of Wellington, is locked at > 30km depth with slow slip earthquakes occurring at depths of 30-50km. We hope measurements collected during our cruise will provide insight into the reason(s) for the observed variations.
Thermal conditions along the fault will be determined by measurements of heat flow through the seafloor. Figure 2 shows the heat flow probe that will be used for these measurements. The design of this probe is analogous to a violin bow. The lance provides the mechanical robustness to withstand repeated insertions and withdrawals from the sediment, and the thermistor tube has the sensitivity needed to make highly accurate measurements. The probe is tethered to the ship with a strong cable and a heat flow measurement starts by plunging the probe into seafloor sediments under the force of gravity. Temperatures are recorded into a data logger within the weight stand that also houses electrical power and acoustic telemetry to monitor the instrument performance from the ship.
Heat flow is the product of the thermal gradient and thermal conductivity. Once the probe is in the sediments, the thermal gradient is measured by determining the temperature at each of the 11 thermistors in the thermistor string and the distance between the thermistors. Additionally, a heater wire extends along the thermistor string, which generates a short heat pulse. The way the heat decays lets us determine the thermal conductivity. These two measurements yield the heat flow.
For the seismic reflection data, a hydrophone streamer and small seismic sources are towed behind the ship. Signals from the seismic sources bounce off of geologic boundaries beneath the seafloor and are used to make an image of the sub-seafloor structure that is analogous to an ultrasound of the Earth. Trained marine biologists are on watch with powerful binoculars and night-vision goggles during seismic reflection experiments to ensure that no marine mammals are nearby while we are generating the seismic sources. Figure 3 shows a “bird” being attached to a hydrophone streamer as it enters the water. The birds control the depth of the streamer. Watch-standers watch the computer screens during the seismic operations to ensure that all sources fire as expected, all 48 channels receive data, and the streamer remains at the correct depth. The data are processed soon after acquisition to generate images similar to the image in Figure 4, which is from a similar cruise off Chile a few years ago.