Institutskolloquium

Marine terraces as records of offshore fault activity: evidence from the Sea of Japan coast

The 2024 Noto Peninsula earthquake (Mw 7.5), which occurred along an offshore active fault on the back-arc side of Japan, highlighted both the importance and the challenges of understanding faults located beneath the seabed. This event caused up to 5.2 metres of coseismic uplift along the coast of the Noto Peninsula, leading to the formation of a new marine terrace.

The age and elevation of marine terraces provide valuable records for reconstructing the long-term activity of offshore faults. In this seminar, the outcomes of the DFG Temari Project, “Tectonic and wave controls on the generation and preservation of marine terraces”, are presented. The project was conducted on the Noto Peninsula and Sado Island, located to the north-east of Noto. The study demonstrates how the geomorphology and luminescence chronology of marine terraces can be used to reveal the long-term tectonic history of regions affected by offshore active faults.

Photo: Northern coastline of the Noto Peninsula. The extent of uplift caused by the 2024 earthquake is visible from dried calcareous algae, which turned white after being exposed above sea level.

The shallow megathrust offshore central Chile (~32°–34°S) has not ruptured coseismically since 1730, despite repeated earthquakes on deeper segments beneath the coast. This behavior indicates strong downdip segmentation and defines a mature tsunami gap along Chile's most densely populated coast. The margin has been influenced by the quasi-stationary collision of the Juan Fernández Ridge (JFR) since ~12–13 Ma. Here, we study whether this long-lived ridge–margin interaction explains the observed segmentation using controlled-source seismic data across the ridge–trench collision zone. P-wave velocity models image a broad low-velocity zone (2.0–5.0 km/s) extending from the trench to ~40–60 km landward, sharply bounded by higher velocities (>6.0 km/s) associated with the Coastal Cordillera basement. This reduced-velocity zone reflects dominant accretion in the syn-collision sector and subduction erosion in the post-collision sector. Near the southern edge of the active collision zone, an additional low-velocity zone is imaged near the San Antonio Canyon, coincident with the Teniente–San Antonio fault system, forming a mechanically weak domain between the upper plate and the subducting slab beneath the forearc basin. South of the JFR collision zone, this weak domain thins markedly, indicating re-established upper-plate rigidity. Depth-migrated reflection images reveal splay faults rooted in the decollement and reaching the seafloor, indicating potential pathways for shallow coseismic slip and tsunami generation. Despite these indicators, geodetic models show strong shallow locking. We propose that the rigid Coastal Cordillera backstop mechanically decouples shallow weak domains from deeper locked segments, explaining both high coupling and long recurrence of shallow tsunamigenic rupture.
 

Commissioning a geothermal heat or an electrical power plant

All three known geothermal German resources, the north German basin, the Upper Rhine Graben (URG) and the Molasse basin, are water or brine dominated reservoirs, differing in fluid chemistry and gas content. Beside targeting the connection to the downhole resource, the well planning and the well-drilling activities are the main challenges in a geothermal project. The daily operational cost of a deep drilling rig is expensive.

In deep geothermal wells, several casing sizes are installed and cemented to create several barriers, while the hot fluid is produced from depth or cold fluid is reinjected at depth after the heat has been extracted at the surface. Most of the German geothermal projects comprise a so-called doublet, i.e., one well is used for production and another well is used for reinjection.

After a geothermal well has been drilled to depth, hydraulic tests are performed to gain the productivity and/or injectivity value of the well while the drilling rig is still on site. Therefore, steady state conditions cannot always be reached and the hydraulic results need to be extrapolated, if no further long-term circulation test will be performed, after the rig has been released.

In all German projects, the water needs to be artificially lifted with a downhole-pump system and sometimes an additional reinjection pump is used to increase the pumping pressure for injection. The production pump is the master and the reinjection pump is the slave in the thermal brine cycle. Only during commissioning, it can be proven, whether the installed pump curves are matching the reservoir long-term trends of the wells, while pump failures, well integrity losses and any other damage of the installed equipment need to be avoided. The experience in commissioning geothermal heat and power plants with and without a reinjection pump will be reported on three different examples, Landau, Insheim (URG), and Polling (Molasse).

* Bestec, Bismarckstraße 19, 76870 Kandel, Germany (https://www.bestec-for-nature.com/)