Synopsis

The proposed PISAGUA cruise aims to derive a detailed high-resolution model for the marine forearc in Northern Chile to image the forearc of an erosional subduction zone. One main target is to compare the forearc structure to the previous and ongoing accumulation of seismic moment. Therefore, the project composes an amphibious 2D and 3D refraction seismic experiment in Northern Chile offshore Taltal to investigate the control of incoming and overriding plate structures on earthquake rupture behavior. This subduction zone segment in the region offshore Taltal, where a mature seismic gap is located, ruptured in 1922 during a major earthquake and is poorly known from marine observations. The high-resolution structural 2D and 3D images obtained in this project, together with data from the seismic stations onshore, will be used to investigate the control of upper and lower plate structures on coupling and other rheological parameters. It is planned to complement the experiment with seismological stations onshore (Universidad de Concepción). Furthermore, we install novel drift-free pressure sensors (A-0-A type) capable of measuring water pressure changes without drift. This new kind of observation allows for resolving subsidence and sea-level changes with a precision better than 1 cm for more than five years.

Scientific Programme

The research project SO297 PISAGUA aims to address several specific scientific questions that have arisen from studies of subduction zones. The main objective of SO297 PISAGUA is to better understand the relationship between the structure and deformation of an erosive subduction zone. 

The three-dimensional model of the erosive subduction zone is obtained with 2D and 3D structural images from refraction seismic surveys. By comparing the high-resolution subsurface model with the coupling behavior of the continental margin, essential questions about the stress build-up and, thus, the origin of strong earthquakes can be derived.

The main scientific objectives of SO297:

- Creation of a three-dimensional and high-resolution velocity model of the current lithospheric architecture of an erosive subduction zone.

- Identify structural parameters that determine an erosive subduction zone's seismic and aseismic behavior. This is done by comparing structural information with other information such as slip models, seismicity, bathymetry and geodetic information. 

- Observations of down-dip and lateral structural variations and comparison changes of seismicity and deformation models.

- Mapping the geometry of the current deformation wedge and the geometry of the plate interface at depth to better assess the regional hazard caused by seismic activity and tsunamigenic processes.

- For the working area, only partial high-resolution water depths are known. During SO297, several regions will be mapped with multibeam echosounders for the first time.

- During SO297, we will install five novel pressure sensors. The time series measured from these drift-free pressure sensors can be converted into precise information about the water depths over several years. The main target of the pressure sensors is to infer long-term time series of sea level changes or uplift of the seafloor (e.g., in situ) for the first time.

Work Programme

The geophysical measurements during the SO297 cruise PISAGA are related to the following methods:

Refraction seismic with airguns and Ocean Bottom Seismometers (OBS)

Thirty ocean bottom seismometers (OBS) and 20 ocean bottom hydrophones (OBH) are used to record wide-angle reflections and re-flections from the airgun shots. Six OBS instruments are built for large water depths up to 8000 m. These instruments are deployed near the deep ocean trench down to 7500 m (Figure 1). All other instruments are rated for up to 6000 m water depth. The planned instruments from GEOMAR will be deployed first on two refraction profiles with 43 and 38 OBS, respectively (Figure 1). Profile P1 (43 OBS/OBH) trends perpendicular to the trench, is 137 nm long and related to the incoming Taltal ridge. Profile P2 (38 OBS/OBH) is 121 nm long and located along the Copiapó ridge, which is oriented slightly oblique to the trench and coast. Both profiles cover the incoming plate, the trench and the marine forearc and will be extended with land stations (Figure 1). Instruments along both profiles will be laid out, covered with airgun shots, and de-installed to obtain two 2D refraction profiles (Figure 1). We expect 6 (and 5.5) working days for profiles P1 (and P2), respectively.

Next, a 3D refraction experiment will take place on a large-scale areal deployment to im-age the entire marine forearc and incoming plate in three dimensions. The route during the 3D experiment is shown in Figure 1 with a yellow line. The distance for the installation of the 50 OBS/OBH of the 3D experiment is 752 nautical miles (about 5 days each) for installation and de-installation. The large-scale 3D working area will be traversed with air-guns in a grid fashion (Figure 1) to obtain a good velocity image of the subsurface in three dimensions. The shooting of the 3D experi-ment will require a significant part of the working days. The shooting track is 1592 nautical miles and will take about 12 days at 4-4.5 knots. A short streamer is deployed to record profiles over subsurface sedimentary structures for additional recording of reflection seismic dur-ing the wide-angle experiment.

Together with our Chilean collaborators (Uni-versidad de Concepción), we plan to extend the experiment onshore with up to 60 seismometers. Thirty short-period geo-phones will be borrowed from the Geophysi-cal Instrument Pool Potsdam (GIPP) and 30 stations will be provided by the Universidad de Concepción.

Visual observations ("MMO") will be con-ducted before and during airgun operations. Additional measures, such as a slow "ramp-up" of the airguns, will be implemented to minimize the impact on marine mammals (e.g., whales and dolphins).

Bathymetry and Parasound

During SO297, we will map the seafloor us-ing the EM122 multibeam system. Besides identifying suitable sites for the OBS (e.g., avoiding steep seafloor or canyons), the multibeam will provide accurate water depths. In particular, the multibeam coverage of the forearc offshore Taltal multibeam data is very sparse, and during SO297, we will facilitate the first accurate bathymetric map offshore Taltal. The parasound system will be used in regions of flat seafloor where we expect to measure coherent signals returning from the uppermost sediments.

Seismicity

The OBS will continuously register the seismic activity. OBS data from the marine forearc will detect seismicity down to small magnitudes. Due to the location of events below the network, accurate hypocentral parameters can be determined, including hypocentral depth. Seismicity contributes to identifying current deformation and faulting in the downgoing and overriding plate and will be used for the 3D inversion of a velocity model.

The land stations will be deployed in February 2023 and are expected to record about six months of natural seismicity. Overall, we expect an average of about one earthquake exceeding magnitude 2 per hour and about 1,000 microearthquakes for each month of recording during the land-station deployment.

Drift-free pressure sensors

We plan to install five drift-compensated pressure sensors (drift-free A-O-A measurement). Over five years, this novel technique allows for resolving slow vertical deformation or sea-level change with cm-precision. Knowledge of uplift or subsidence from the marine forearc will allow inferring information about the subduction zone's stress accumulation. Three pressure sensors from the University of Concepción will be installed on a tripod with a releaser. Two pressure sensors are from GEOMAR and will be mounted on an OBS frame.

Due to the large water depths and the rough morphology, we plan a controlled deployment (Figure 3) which has already been performed with other geodetic instruments. The transponders will be deployed on the deep submarine cable, with a release unit providing the link between the transponders and the cable. Two Posidonia units will be used on the cable to indicate the tripod touchdown on the sea bed. The distance between the units will not decrease until the unit reaches the seafloor. A ~20 m floating line above the buoyancy unit will be used to decouple the unit from the deep-sea wire at the seafloor to prevent stress on the instrument.