Long endurance monitoring of tectonic motion

Subduction zones are tectonic plate collision boundaries where typically higher density oceanic crust is being pushed under continental crust. This is one of the most important processes in the evolution of the Earth’s morphology.

The challenge

Subduction zones comprise extremely large thrust faults, known as megathrusts, and are the source of the world’s most dangerous volcanoes and earthquakes. In between larger earthquakes, megathrusts comprise complex heterogeneous distributions of locking and therefore strain accumulation.

On land, GPS and laser observations enable precise geodetic measurements. Until recently, the inability to undertake cost-effective complementary observations subsea in the outermost subduction zone offshore, has been a critical flaw. This is where much of the elastic strain build-up and release occurs. Improving scientific understanding of the seafloor movement in these regions is an important basis for future seismic hazard assessment.

The solution

In response to this challenge, Sonardyne has worked with a number of research institutes, including GEOMAR, to supply networks of Autonomous Monitoring Transponders (AMTs ) – seabed instruments that are capable of taking hundreds of thousands of stable, highly precise geodetic observations, safely log the data and on command, wirelessly transmit it up to the surface.

Originally developed for the offshore industry to precisely measure vertical and horizontal seabed displacements caused by reservoir depletion, AMT is a long-life (up to five years depending configuration), deep-rated acoustic instrument fitted with high resolution pressure, sound velocity and temperature sensors. All of this is built around our 6G hardware platform and Wideband 2 digital signal technology.

AMTs run a fully automated logging regime gathering acoustic travel time (range) between neighbouring units, pressure, sound velocity, temperature and tilt data at intervals defined by the user. A passing AUV, vessel of opportunity, gateway buoy or unmanned surface platform can harvest data on demand and at any point, the user may amend the logging regime of any or all of the AMTs, using the bi-directional communications link.

GEOMAR have now deployed three AMT seabed arrays in Chilean seas. The largest array is the Geodetic Earthquake Observatory on the SEAfloor (GeoSEA) project, offshore northern Chile on the Nazca-South American plate boundary.

The last rupture resulting in a major earthquake at the array location was in 1877. This was identified as a seismic gap prior to the 2014 Iquique/ Pisagua 8.1 magnitude earthquake.

Nevertheless, the southern portion of the segment remains unbroken by recent earthquake activity and so, with the two plates converging at a rate of around 65 millimetres per year, new tension is being continuously built up. ‘Therefore the region is a focus site for seismologists to understand strain build-up prior to an earthquake,” says Professor Kopp

The GeoSEA array consists of 23 AMTs deployed from the German Research Vessel Sonne in late 2015 and comprises three sub-arrays that monitor different sections of the megathrust. The tectonic nature of the seabed gives rise to a variety of complex topographies.

Sonardyne’s in-house Survey Support Group worked closely with scientists from GEOMAR to plan the subarray layouts. Positioning of the AMTs were based on multibeam data collected during the preceding research leg. Precise placement in the order of a few tens of metres was required and in one case, an AMT had to be sited on a ledge on the side of a ridge that was only 50 metres wide and 150 metres long.

The first area is located on the middle continental slope between the main trench and the Chile coast and consists of eight transponders laid in pairs on a stairway-like feature of four topographic ridges at a depth of around 2,800 metres. The ridges, which are surface expressions of faults at depth, are approximately 100 metres high with around an 800 metre flat area between.

On the opposite (seaward) side of the trench from Chile, a further five AMTs were deployed in depths approaching 4,100 metres to monitor extension across plate bending related normal faults. Here, faulting of the Nazca Plate is caused by stretching of its surface as it is pushed downwards under the South American plate, and thus the fault lines are relatively new and active. The AMTs were laid at the intersection of multiple fault lines separating three or four blocks, with two being placed on the same block to provide a sound speed reference baseline.

The deepest area, located between 5,100 and 5,400 metres deep, is on the lower continental slope and comprises a circular pattern of eight AMTs surrounding two central instruments. This pattern provides a variety of short and long baselines to measure diffuse strain build-up in a highly faulted area made up of separate geological blocks, which are under high compression.

To cope with these extreme conditions, the AMTs used for the GeoSEA project are 6,000 metre-rated Lower Medium Frequency (LMF 14-19 kHz) omnidirectional units. Sampling at rates between one and a half and three hours, the GeoSEA AMTs are planned for an initial three and a half year deployment. However, the four metre high seabed frame in which each unit sits enables it to be easily removed by ROV, returned to the surface to allow its battery to be exchanged then placed back in the frame in the same position – giving scientists the option of extending the survey if required.

The results

Since initial deployment, data from the array has been recovered by a US research ship using a Sonardyne HPT 7000 dunking modem deployed over the side, as well as GEOMAR’s GeoSURF Wave Glider, equipped with a 6G acoustic communication module fitted in its hull.

“Overall, our experience with Sonardynes instruments is superb and our results have been beyond our expectations. It has been a pleasure for us to work with infrastructure that we can fully rely on.” Professor Kopp from GEOMAR commented.

The success of the AMT’s performance in the Nazca-South American plate boundary arrays has resulted in two smaller arrays in the Sea of Marmara off Turkey and on the submerged flanks of Mount Etna. Although the results of these surveys were yet to be published at the time of writing, scientists were excited by the new opportunities Sonardyne’s technology offers.

GEOMAR’s Doctor Dietrich Lange summarises that, “With this approach, we are taking a new path in earthquake research since previously, measurements of a few millimetres, were hardly possible.”