Ake Fagereng

Bio: Ake Fagereng is an academic researcher from Cardiff University. The author has contributed to research in topics: Slip (materials science) & Fault (geology). The author has an hindex of 27, co-authored 91 publications receiving 2051 citations. Previous affiliations of Ake Fagereng include University of Cape Town & University of Otago.

Mélange rheology and seismic style

Abstract: Shear displacements in crustal fault zones are accommodated by a range of seismic styles, including standard earthquakes, non-volcanic tremor, and continuous and transitory aseismic slip. Subduction channel shear zones, containing highly sheared, fluid-saturated trench-fill sediments intermingled with fragments of oceanic crust, are commonly inferred to occur along active subduction megathrusts. If this interpretation is correct, these plate boundary faults are not discrete planes, but may resemble the melange shear zones commonly found in exhumed subduction-related rock assemblages. Melange deformation appears to depend critically on the ratio of competent to incompetent material, with shear surfaces localized along lithological contacts or within competent domains, while matrix flow accommodates shearing by distributed strain. If the style of strain and/or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip, the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones, and along comparable mixed-lithology fault zones.

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

Abstract: The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrust's seismogenic zone.

Shear veins observed within anisotropic fabric at high angles to the maximum compressive stress

Abstract: Some faults seem to slip at unusually high angles (>45°) relative to the orientation of the greatest principal compressive stress1, 2, 3, 4, 5. This implies that these faults are extremely weak compared with the surrounding rock6. Laboratory friction experiments and theoretical models suggest that the weakness may result from slip on a pre-existing frictionally weak surface7, 8, 9, weakening from chemical reactions10, elevated fluid pressure11, 12, 13 or dissolution–precipitation creep14, 15. Here we describe shear veins within the Chrystalls Beach accretionary melange, New Zealand. The melange is a highly sheared assemblage of relatively competent rock within a cleaved, anisotropic mudstone matrix. The orientation of the shear veins—compared with the direction of hydrothermal extension veins that formed contemporaneously—indicates that they were active at an angle of 80°±5° to the greatest principal compressive stress. We show that the shear veins developed incrementally along the cleavage planes of the matrix. Thus, we suggest that episodic slip was facilitated by the anisotropic internal fabric, in a fluid-overpressured, heterogeneous shear zone. A similar mechanism may accommodate shear at high angles to the greatest principal compressive stress in a range of tectonic settings. We therefore conclude that incremental slip along a pre-existing planar fabric, coupled to high fluid pressure and dissolution–precipitation creep, may explain active slip on severely misoriented faults.

An Explanation of Episodic Tremor and Slow Slip Constrained by Crack-Seal Veins and Viscous Shear in Subduction Mélange

Abstract: Episodic tremor and slow slip (ETS) occurs in the transition zone between the locked seismogenic zone and the deeper, stably sliding zone. Actual mechanisms of ETS are enigmatic, caused by lack of geological observations and limited spatial resolution of geophysical information from the ETS source. We report that quartz‐filled, crack‐seal shear and extension veins in subduction melange record repeated low‐angle thrust‐sense frictional sliding and tensile fracturing at near‐lithostatic fluid pressures. Crack‐seal veins were coeval with viscous shear zones that accommodated deformation by pressure solution creep. The minimum time interval between thrusting events, determined from a kinetic model of quartz precipitation in shear veins, was less than a few years. This short recurrence time of low‐angle brittle thrusting at near‐lithostatic fluid overpressures within viscous shear zones may be explained by frequent release of accumulated strain by ETS.

Slow slip source characterized by lithological and geometric heterogeneity

Abstract: Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.

Population and Housing Census

Abstract: The questionnaires from the field were received, checked and stored by the data processing personnel. They checked: 1. The completeness of the questionnaires 2. The correct bubbling 3. The correct number of questionnaires per household, if total males + total females > 8 as the questionnaire ONLY accommodated maximum of 8 household members. 4. The reference number appears in all the 10 pages of the questionnaires.

The Mechanics Of Earthquakes And Faulting

Abstract: • Stability of steady frictional sliding, inertia and the quasi-static limit • work on ps 4; see course web site

Hydrogeology and Mechanics of Subduction Zone Forearcs: Fluid Flow and Pore Pressure

Abstract: At subduction zones, fluid flow, pore pressure, and tectonic processes are tightly interconnected. Excess pore pressure is driven by tectonic loading and fluids released by mineral dehydration, and it has profound effects on fault and earthquake mechanics through its control on effective stress. The egress of these overpressured fluids, which is in part governed by the presence of permeable fault zones, is a primary mechanism of volatile and solute transport to the oceans. Recent field measurements, new constraints gained from laboratory studies, and numerical modeling efforts have led to a greatly improved understanding of these coupled processes. Here, we summarize the current state of knowledge of fluid flow and pore pressure in subduction forearcs, and focus on recent advances that have quantified permeability architecture, fluxes, the nature and timing of transience, and pressure distribution, thus providing new insights into the connections between fluid, metamorphic, mechanical, and fault slip proc...

National Seismic Hazard Model for New Zealand: 2010 Update

Abstract: A team of earthquake geologists, seismologists, and engineering seis- mologists has collectively produced an update of the national probabilistic seismic hazard (PSH) model for New Zealand (National Seismic Hazard Model, or NSHM). The new NSHM supersedes the earlier NSHM published in 2002 and used as the hazard basis for the New Zealand Loadings Standard and numerous other end-user applica- tions. The new NSHM incorporates a fault source model that has been updated with over 200 new onshore and offshore fault sources and utilizes new New Zealand-based and international scaling relationships for the parameterization of the faults. The dis- tributed seismicity model has also been updated to include post-1997 seismicity data, a new seismicity regionalization, and improved methodology for calculation of the seismicity parameters. Probabilistic seismic hazard maps produced from the new NSHM show a similar pattern of hazard to the earlier model at the national scale, but there are some significant reductions and increases in hazard at the regional scale. The national-scale differences between the new and earlier NSHM appear less than those seen between much earlier national models, indicating that some degree of consis- tency has been achieved in the national-scale pattern of hazard estimates, at least for return periods of 475 years and greater. Online Material: Table of fault source parameters for the 2010 national seismic- hazard model.

Do subducting seamounts generate or stop large earthquakes

Abstract: Seamount subduction is a common process in subduction zone tectonics. Contradicting a widely held expectation that subducting seamounts generate large earthquakes, seamounts subduct largely aseismically, producing numerous small earthquakes. On rare occasions when they do produce relatively large events, the ruptures tend to be complex, suggesting multiple rupture patches or faults. We explain that the seismogenic behavior of these seamounts is controlled by the development and evolution of an adjacent fracture network during subduction and cannot be described using the frictional behavior of a single fault. The complex structure and heterogeneous stresses of this network provide a favorable condition for aseismic creep and small earthquakes but an unfavorable condition for the generation and propagation of large ruptures.