A document by Dan Sandiford. Click on the document to view its contents.

A document by Dan Sandiford. Click on the document to view its contents.

The coupling between subducted slabs and trailing plates is often conceptualised in terms of a net in-plane force. If a significant fraction of upper-mantle slab buoyancy (e.g. ~ 25%) were transferred in this manner, a net in-plane force on the order of 5-10 TN/m would be typical of the trailing plates. Results from a numerical subduction model are presented here which question both the magnitude and-perhaps more profoundly-the mode of force transmission. In this model the subducting plate (SP) driving force is predominantly supplied by differences in gravitational potential energy (GPE). The GPE associated with plate downbending (flexural topography) provides about half the total driving force. The magnitude of the trench GPE is related to the amplitude of topography, but is mediated by the internal stress distributions associated with bending. Above the elastic core, the stress is Andersonian and vertical normal stresses are lithostatic. This implies horizontal gradients in the vertical normal stress, across columns of different elevation in the outer slope. The bulk of the trench GPE arises from this upper, extensional section the lithosphere. Vertical shear stress (and horizontal gradients thereof) are concentrated in the elastic core of the slab, where principal stresses rotate through 90 degrees. In this region, horizontal gradients in vertical normal stress rapidly diminish; they fully equilibrate at about twice the neutral plane depth. For the deepest trenches on Earth, these relationships imply trench GPE of up to about 5 TN/m. The model demonstrates that mantle slabs can drive plate tectonics simply through downbending, where the predominant mode of slab-plate coupling is via the vertical shear force and bending moment. 

A document by Dan Sandiford. Click on the document to view its contents.

Seafloor spreading at slow rates can be accommodated on large-offset oceanic detachment faults (ODFs), that exhume lower crustal and mantle rocks in footwall domes termed oceanic core complexes (OCCs). Footwall rock experiences large rotation during exhumation, yet important aspects of the kinematics - particularly the relative roles of rigid block rotation and flexure - are not clearly understood. Using a high-resolution numerical model, we explore the exhumation kinematics in the footwall beneath an emergent ODF/OCC. A key feature of the models is that footwall motion is dominated by solid rotation, accommodated by the concave-down ODF. This is attributed to a system behaviour in which the accumulation of distributed plastic strain is minimized. A consequence of these kinematics is that curvature measured along the ODF is representative of a neutral stress configuration, rather than a ‘bent’ one. Instead, it is in the subsequent process of ‘apparent unbending’ that significant flexural stresses are developed in the model footwall. The brittle strain associated with apparent unbending is produced dominantly in extension, beneath the OCC, consistent with earthquake clustering observed in the Trans-Atlantic Geotraverse at the Mid-Atlantic Ridge.

SUMMARY This report summarises the development of a new Probabilistic Seismic Hazard Analysis (PSHA) for Victoria called the Victorian Earthquake Hazard Map (VEHM). PSHA provides forecasts of the strength of shaking in any given time (return period). The primary inputs are historical seismicity catalogues, paleoseismic (active fault) data, and ground-motion prediction equations. A key component in the development of the Victorian Earthquake Hazard Map was the integration of new geophysics data derived from deployments of Australian Geophysical Observing System seismometers in Victoria with a variety of publicly available datasets including seismicity catalogues, geophysical imagery and geological mapping. This has resulted in the development of a new dataset that constrains the models presented in the VEHM and is also is provided as a stand-alone resource for both reference and future analysis. The VEHM provides a Victorian-focussed earthquake hazard estimation tool that offers an alternative to the nationally focussed 2012 Australian Earthquake Hazard Map . The major difference between the two maps is the inclusion of active fault location and slip estimates in the VEHM. There is a significant difference in hazard estimation between the two maps (even without including fault-related seismicity) due primarily to differences in seismicity-analysis. These issues are described in the discussion section of this report, again resulting in a higher fidelity result in the VEHM. These differences make the VEHM a more conservative hazard model. The VEHM currently exists as a series of online resources to help assist those in engineering, planning, disaster management. This is a dynamic dataset and the inputs will continue to be refined as new constraints are included and the map is made compatible with the Global Earthquake Model (GEM) software, due for release in late 2014. The VEHM was funded through the Natural Disaster Resilience Grants Scheme. The NDRGS is a grant program funded by the Commonwealth Attorney-General’s Department under the National Partnership Agreement on Natural Disaster Resilience signed by the Prime Minister and Premier. The purpose of the National Partnership Agreement is to contribute towards implementation of the National Strategy for Disaster Resilience, supporting projects leading to the following outcomes: 1. reduced risk from the impact of disasters and 2. appropriate emergency management, including volunteer, capability and capacity consistent with the State’s risk profile.