Dan Clark

Tectonic geomorphology of Australia

Abstract The Australian continent is actively deforming in response to far-field stresses generated by plate boundary interactions and buoyancy forces associated with mantle dynamics. On the largest scale (several 103 km), the submergence of the northern continental shelf is driven by dynamic topography caused by mantle downwelling along the Indo-Pacific subduction system and accentuated by a regionally elevated geoid. The emergence of the southern shelf is attributed to the progressive movement of Australia away from a dynamic topography low. On the intermediate scale (several 102 km), low-amplitude (c. 100–200 m) long-wavelength (c. 100–300 km) topographic undulations are driven by (1) anomalous, smaller-scale upper mantle convection, and/or (2) lithospheric-scale buckling associated with plate boundary tectonic forcing. On the smallest scale (101 km), fault-related deformation driven by partitioning of far-field stresses has modified surface topography at rates of up to c. 170 m Ma−1, generated more than 30–50% of the contemporary topographic relief between some of Australias highlands and adjacent piedmonts, and exerted a first-order control on long-term (104–106 a) bedrock erosion. Although Australia is often regarded as tectonically and geomorphologically quiescent, Neogene to Recent tectonically induced landscape evolution has occurred across the continent, with geomorphological expressions ranging from mild to dramatic.

Identification of Quaternary scarps in southwest and central west Western Australia using DEM-based hill shading: application to seismic hazard assessment and neotectonics

An examination of regionally extensive hill-shaded Shuttle Radar Topography Mission (SRTM) 90 m resolution and Department of Land Information (DLI) 10 m resolution digital elevation data, and discussions with government and industry geologists familiar with the local geology, has resulted in the identification of 38 previously unrecognized linear topographic scarps in the southwest and central west of Western Australia. I contend that most of these relate to Quaternary surface-rupturing earthquakes. If validated, this more than doubles the number of Quaternary fault scarps known from this area, bringing the total to 60. The newly recognized scarps average between 25 and 50 km in length and from ∼1.5 to 20 m in height. The geometric, recurrence and spatial attributes of these features make it possible to propose a model describing the causative seismicity. The model contends that uniform contractional strain in the ductile lithosphere manifests as localized, transient and recurrent brittle deformation in zones of pre-existing crustal weakness in the upper lithosphere. The data presented allow for ready calculation of the maximum probable magnitude earthquake for the southwest of Western Australia, and identify ‘earthquake-prone’ regions of interest to seismic hazard assessors.

The Challenges of Probabilistic Seismic‐Hazard Assessment in Stable Continental Interiors: An Australian Example

In stable continental regions (SCRs), the process of probabilistic seismic‐hazard assessment (PSHA) remains a scientific and technical challenge. In producing a new national hazard model for Australia, we developed several innovative techniques to address these challenges. The Australian seismic catalog is heterogeneous due to the variability between magnitude types and the sparse networks. To reduce the resulting high epistemic uncertainty in the recurrence parameters, a and b , the magnitudes of pre‐1990 earthquakes have been empirically corrected to account for changes in magnitude formulas around 1990. In addition, existing methods for estimating recurrence parameters (e.g., maximum likelihood estimation) were found to be unstable. To overcome this problem, a new method was developed that removes outlier earthquakes before applying a regression. The incorporation of a model of episodic seismicity into the new hazard model required deviation from the more conventional method of PSHA. The selection of the maximum earthquake magnitude M max is based on the analysis of surface ruptures from paleoearthquakes, with M max thought to vary between geological domains (e.g., 7.2–7.6 in nonextended SCR and 7.4–7.8 in extended SCR). The sensitivity of PSHA to M max, source zone boundary location, recurrence parameters, and ground‐motion prediction equations (GMPEs) was examined in this study. The hazard was found to be generally insensitive to M max in the estimated preferred magnitude range. The uncertainty in recurrence parameters was found to contribute a variation in hazard comparable to the epistemic uncertainty associated with the different GMPEs used in this study. For sites near source zone boundaries, a similar variation in hazard was observed by reasonable changes in the position of the boundaries. Aleatory variability and epistemic uncertainty in GMPEs are routinely incorporated in PSHAs, as is variation in M max. However, the uncertainties in recurrence parameters and source zone boundaries are generally given less attention.

The Hyden fault scarp, Western Australia: paleoseismic evidence for repeated Quaternary displacement in an intracratonic setting

We present new paleoseismicity data for the 30 km long and 2.5 m high Hyden fault scarp in Western Australia, which, when combined with the results of previous research, provides the most extensive record of surface-rupturing earthquakes yet assembled for an ‘active’ Australian intracratonic fault. The data indicate that four to five surface-rupturing earthquakes have occurred on the Hyden Fault during the Quaternary (E1, ca 20 ka; E2, ca 55 – 50 ka; E3, ca 100 – 90 ka; and two events E4 and E5, >200 ka). Activity is episodic, with single seismic cycle slip rates varying from 0.03 mm/y to <0.01 mm/y. Paleo-earthquake magnitudes are estimated to have been in the order of M w 6.8. The identification of a similar fault scarp immediately northwest of the Hyden scarp, and of two airphoto lineaments to the west of the Hyden scarp, indicates that strain is distributed among a family of faults in this region. The presence of multiple nearby active faults suggests that the recurrence of severe ground shaking in the Hyden region is more frequent than indicated by the paleoseismic data presented here.

Fracture systems in granite pavements of the eastern Pilbara Craton, Western Australia: indicators of neotectonic activity?

Continental Australia is characterised by high levels of seismic activity in comparison with intracratonic areas worldwide. However, the link between earthquake events and earthquake-related geomorphology in Australia remains poorly understood for all except the largest events, because landscape impact unambiguously attributable to seismic activity is difficult to recognise. In this context, we describe several unusual fracture systems of possible tectonic origin that transect granite pavements in the Archaean eastern Pilbara Craton of Western Australia. Occurring at four localities (Gallery Hill, North Shaw, Mulgandinnah Hill and Muccan) separated by up to 150 km, the fracture systems typically range up to 100 m in length and 20 m in width, locally offset pavement surfaces by up to 15 cm vertically, and expose uniformly fresh-looking rock. At one locality (Muccan), the fractures cross-cut two out of three generations of aboriginal petroglyphs etched into the pavement surface, which suggests that fracture formation occurred both recently and rapidly. All four localities are characterised by extensional structures (tension fractures and dilated joints) striking 020–040°, and three preserve compressional structures (steeply dipping reverse faults at Gallery Hill and North Shaw; A-tent crestal fractures at Mulgandinnah Hill) trending 100–135°. The strongly correlated alignments of the fracture systems militate against a purely weathering-controlled origin, and the observed pattern is compatible with fracture formation in a single East Pilbara-wide stress field, dominated by pure shear and characterised by northeast-southwest- to north-northeast-south-southwest-directed horizontal compression. This orientation is consistent with that derived by spatial averaging of existing stress orientation data from northwest Australia. These preliminary results have exciting implications for the inexpensive field-based determination of regional stress orientation, and for the use of granitic landforms in probabilistic seismic hazard assessment and the identification of earthquake-prone areas.

A Revised Paleo‐Earthquake Chronology on the Southeast Reelfoot Rift Margin near Memphis, Tennessee

A trench was excavated across the southeastern Reelfoot rift margin for paleoseismic research purposes and for the 2011 Seismological Society of America national meeting field trip. The trench was parallel to and 6 m southwest of the Oldham trench described in 2006. In this 2011 trench, faulted alluvial fan stratigraphy and liquefaction deposits less than 4000 yr old were exposed. The trench revealed three tectonic deformation events. The first event (graben formation) and the second event (sand blow, minor faulting, and injection of sand dikes) both post-date a paleosol circa 4000 yr B.P. and pre-date a surficial colluvial soil deposit circa 2000 yr B.P. The third event (minor shallow liquefaction and surface deformation) post-dates a 2000-year-old surface colluvial soil. These age constraints allow this third event to be attributed to the 1811-1812 New Madrid earthquakes or a remotely triggered earthquake related to the 1811-1812 earthquake sequence. Although we cannot rule out that the deformation revealed in this trench was caused by earthquakes on other faults in the New Madrid seismic zone, our favored interpretation is that the deformation was caused by rupture on an underlying fault at the base of the Mississippi River bluff that was previously imaged in a shallow reflection profile near this site.

The Cadell Fault, southeastern Australia: a record of temporally clustered morphogenic seismicity in a low-strain intraplate region

Abstract The Cadell Fault, found in stable continental region (SCR) crust in southeastern Australia, provides a record of temporally clustered morphogenic earthquakes spanning much of the Cenozoic. The slip rate, averaged over perhaps as many as five complete seismic cycles in the period 70–20 ka, is c. 0.4–0.5 mm a−1, compared with an average rate of c. 0.005–0.01 mm a−1 over the period spanning the late Miocene to Recent. If full length rupture of the 80 km long feature is assumed, the average recurrence for Mw 7.3–7.5 earthquake events on the Cadell Fault in the period 70–20 ka is c. 8 kyr. About 20 kyr, representing more than two average seismic cycles, have lapsed since the most recent morphogenic seismic event on the fault. It might therefore be speculated that this fault has relapsed into a quiescent period. Episodic rupture behaviour on the Cadell Fault, and nearby faults in Phanerozoic SCR crust in eastern Australia, might be controlled by their linkage into major crustal fault systems at depth, in apparent contrast with the style of deformation in non-extended Precambrian SCR crust. Periods of strain localization on these major crustal fault systems, effectively turning deforming regions ‘on’ and ‘off’, might be influenced by changes in distant plate boundary forces. If proved, this would have profound consequences for how the occurrence of large earthquakes is assessed in Australia, as the fundamental assumption of morphogenic earthquakes occurring as a result of the progressive build-up of strain, and thus being in some way predictable in their periodicity, is not satisfied. Documenting such fault behaviour in SCR crust assists in conceptualizing the points critical to understanding the hazards posed by SCR faults worldwide. Supplementary material: Seismic reflection and refraction survey parameters are available at: http://www.geolsoc.org.uk/SUP18869

Paleoseismology of the Mount Narryer fault zone, Western Australia: A multistrand intraplate fault system

Our paleoseismological study of faults and fault-related folds comprising the Mount Narryer fault zone reveals a mid- to late Quaternary history of repeated morphogenic earthquakes that have influenced the planform and course of the Murchison, Roderick, and Sanford Rivers, Western Australia. The dominant style of deformation involves folding of near-surface sediments overlying discrete basement faults. Carbon-14, optically stimulated luminescence, and in situ–produced 10 Be constrain the timing of the events and late Quaternary slip rates associated with fault propagation folds in tectonically uplifted and deformed alluvial channel deposits. A flight of five inset fluvial terraces is preserved where the Murchison River flows across the Roderick River fault. These terraces, which we infer to be coseismic, are consistent with at least four late Quaternary seismic events on the order of moment magnitude (M w ) 7.1 within the last ~240 k.y. Secondary shears expressed on the folds indicate a component of dextral strike-slip displacement. Quaternary slip rates on the underlying faults range from 0.01 to 0.07 mm yr –1 , with a total slip rate for the zone between 0.04 and 0.11 mm yr –1 . These rates are intermediate to those in the adjacent Mesozoic basin (>0.1 mm yr –1 ) and Precambrian craton ( –1 ) and so provide insight into how tectonic strain is partitioned and transferred across a craton margin.