Gerald P. Roberts

Spatial and temporal variations in growth rates along active normal fault systems: an example from The Lazio–Abruzzo Apennines, central Italy

Abstract The geometry, kinematics and rates of active extension in Lazio–Abruzzo, Italian Apennines, have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems. We present fault map traces, throws, throw-rates and slip-directions for 17 parallel, en echelon or end-on active normal faults whose 20–40 km lengths combine to form a soft-linked fault array ca. 155 km in length and ca. 55 km across strike. Throw-rates derived from observations of faulted late-glacial features and Holocene soils show that both maximum throw-rates and throw-rate gradients are greater on centrally-located faults along the strike of the array; total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike. When summed across strike, throw-rates are increasingly high towards the centre of the array relative to summed throws. We interpret the above to suggest that throw-rates have changed in the recent past (ca. 0.7 Ma) from spatially-random fault growth rates (initiating at 2.5–3.3 Ma) to growth rates that are greater on centrally-located faults. We interpret this as evidence for fault interaction producing throw-rate variations that drive throw profile readjustment on these crustal scale soft-linked faults. The results are used to discuss seismic hazards in the region, which are quantified in a second paper in this issue.

Bedrock channel adjustment to tectonic forcing: Implications for predicting river incision rates

We present detailed data of channel morphology for a river undergoing a transient response to active normal faulting where excellent constraints exist on spatial and temporal variations in fault slip rates. We show that traditional hydraulic scaling laws break down in this situation, and that channel widths become decoupled from drainage area upstream of the fault. Unit stream powers are ∼4 times higher than those predicted by current scaling paradigms and imply that incision rates for rivers responding to active tectonics may be significantly higher than those heretofore modeled. The loss of hydraulic scaling cannot be explained by increasing channel roughness and is an intrinsic response to tectonic forcing. We show that channel aspect ratio is a strongly nonlinear function of local slope and demonstrate that fault-induced adjustment of channel geometries has reset hillslope gradients. The results give new insight into how rivers maintain their course in the face of tectonic uplift and illustrate the first-order control the fluvial system exerts on the locus and magnitude of sediment supply to basins.

Constraining slip rates and spacings for active normal faults

Abstract Numerous observations of extensional provinces indicate that neighbouring faults commonly slip at different rates and, moreover, may be active over different time intervals. These published observations include variations in slip rate measured along-strike of a fault array or fault zone, as well as significant across-strike differences in the timing and rates of movement on faults that have a similar orientation with respect to the regional stress field. Here we review published examples from the western USA, the North Sea, and central Greece, and present new data from the Italian Apennines that support the idea that such variations are systematic and thus to some extent predictable. The basis for the prediction is that: (1) the way in which a fault grows is fundamentally controlled by the ratio of maximum displacement to length, and (2) the regional strain rate must remain approximately constant through time. We show how data on fault lengths and displacements can be used to model the observed patterns of long-term slip rate where measured values are sparse. Specifically, we estimate the magnitude of spatial variation in slip rate along-strike and relate it to the across-strike spacing between active faults.

The 2009 L’Aquila earthquake (central Italy): A source mechanism and implications for seismic hazard

An edited version of this paper was published by AGU. Copyright (2009) American Geophysical Union.

New constraints on sediment-flux-dependent river incision: Implications for extracting tectonic signals from river profiles

We present new field data from rivers draining across active normal faults that incise across the same lithology at the fault, have been subjected to similar climatic regimes and tectonic settings, and were perturbed by a well-documented increase in fault slip rate ca. 1 Ma. In spite of these similarities, the rivers exhibit markedly different long profiles and patterns of catchment incision. We use channel slope and hydraulic geometry data for each river to calculate bed shear stresses (τb), and show that there is no simple relationship between peak τb and the relative uplift rates across the faults, U , which differ by a factor of four. The long-term average sediment supply to each channel ( Q s), estimated from time-averaged catchment erosion rates, can explain the τb versus U data if bedload modulates bedrock incision rate, E , in a strongly nonlinear way. Together these field data allow us, for the first time, to evaluate theoretical predictions of the role of sediment on river profile evolution and to quantify the magnitude of the effect in natural systems.

Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy

The transient response of bedrock rivers to a drop in base level can be used to discriminate between competing fluvial erosion models. However, some recent studies of bedrock erosion conclude that transient river long profiles can be approximately characterized by a transport‐limited erosion model, while other authors suggest that a detachment‐limited model best explains their field data. The difference is thought to be due to the relative volume of sediment being fluxed through the fluvial system. Using a pragmatic approach, we address this debate by testing the ability of end‐member fluvial erosion models to reproduce the well‐documented evolution of three catchments in the central Apennines (Italy) which have been perturbed to various extents by an independently constrained increase in relative uplift rate. The transport‐limited model is unable to account for the catchments’response to the increase in uplift rate, consistent with the observed low rates of sediment supply to the channels. Instead, a detachment‐limited model with a threshold corresponding to the field‐derived median grain size of the sediment plus a slope‐dependent channel width satisfactorily reproduces the overall convex long profiles along the studied rivers. Importantly, we find that the prefactor in the hydraulic scaling relationship is uplift dependent, leading to landscapes responding faster the higher the uplift rate, consistent with field observations. We conclude that a slope‐ dependent channel width and an entrainment/erosion threshold are necessary ingredients when modeling landscape evolution or mapping the distribution of fluvial erosion rates in areas where the rate of sediment supply to channels is low.

Variation in fault-slip directions along active and segmented normal fault systems

Abstract Local fault-slip directions show a systematic variation over distances of 20–35 km along the active and segmented Gulf of Corinth normal fault system, central Greece. Where fault throws are large (several km) close to the centres of fault segment map traces, local fault-slip directions parallel the regional N-S (±20°) slip-vector azimuth interpreted from earthquake focal mechanisms. However, where fault throws are small (tens of metres) close to fault segment boundaries, local fault-slip directions are oriented ∼NE-SW when measured close to the western ends of fault segments, and ∼NW-SE close to the eastern ends of fault segments. Local fault-slip directions change by ∼90° across fault segment boundaries. It is suggested that fault-slip directions close to fault segment boundaries record local strain patterns at fault tips so that care should be taken when using this information during attempts to infer regional stress trajectories.

Comparison of earthquake strains over 102 and 104 year timescales: Insights into variability in the seismic cycle in the central Apennines, Italy

In order to study the existence of possible deficits or surpluses of geodetic and earthquake strain in the Lazio-Abruzzo region of the central Apennines compared to 15 +/- 3 kyrs multi seismic cycle strain-rates, horizontal strain-rates are calculated in 5 km x 5 km and 20 km x 20 km grid squares using slip-vectors from striated faults and offsets of Late Pleistocene-Holocene landforms and sediments. Strain-rates calculated over 15 +/- 3 kyrs within 5 km x 5 km grid squares vary from zero up to 2.34 +/- 0.54 x 10(-7) yr(-1) and resolve variations in strain orientations and magnitudes along the strike of individual faults. Surface strain-rates over a time period of 15 +/- 3 kyrs from 5 km x 5 km grid squares integrated over an area of 80 km x 160 km shows the horizontal strain-rate of the central Apennines is 1.18(-0.04)(+0.12)x10(-8) yr(-1) and -1.83(-4.43)(+3.80) x 10(-10) yr(-1) parallel and perpendicular to the regional principal strain direction (043 degrees-223 degrees+/-1 degrees), associated with extension rates of Ms 6.0) magnitude historical earthquakes have been reported to be as low as zero. This demonstrates the importance of comparing the exact same areas and that strain-rates vary spatially on the length scale of individual faults and on a timescale between 10(2) yr and 10(4) yr in the central Apennines. We use these results to produce a fault specific earthquake recurrence interval map and discuss the regional deformation related to plate boundary and sub-crustal forces, temporal earthquake clustering and the natural variability of the seismic cycle.

Fault-slip directions in central and southern Greece measured from striated and corrugated fault planes: Comparison with focal mechanism and geodetic data

Fault-slip directions recorded by outcropping striated and corrugated fault planes in central and southern Greece have been measured for comparison with extension directions derived from focal mechanism and Global Positioning System (GPS) data for the last ∼100 years to test how far back in time velocity fields and deformation dynamics derived from the latter data sets can be extrapolated. The fault-slip data have been collected from the basin-bounding faults to Plio-Pleistocene to recent extensional basins and include data from arrays of footwall faults formed during the early stages of fault growth. We show that the orientation of the inferred stress field varies along faults and earthquake ruptures, so we use only slip-directions from the centers of faults, where dip-slip motion occurs, to constrain regionally significant extension directions. The fault-slip directions for the Peloponnese and Gulfs of Evia and Corinth are statistically different at the 99% confidence level but statistically the same as those implied by earthquake focal mechanisms for each region at the 99% confidence level; they are also qualitatively similar to the principal strain axes derived from GPS studies. Extension directions derived from fault-slip data are 043–047° for the southern Peloponnese, 353° for the Gulf of Corinth, and 015–014° for the Gulf of Evia. Extension on active normal faults in the two latter areas appears to grade into strike-slip along the North Anatolian Fault through a gradual change in fault-slip directions and fault strikes. To reconcile the above with 5° Myr−1 clockwise rotations suggested for the area, we suggest that the faults considered formed during a single phase of extension. The deformation and formation of the normal fault systems examined must have been sufficiently rapid and recent for rotations about vertical axes to have been unable to disperse the fault-slip directions from the extension directions implied by focal mechanisms and GPS data. Thus, in central and southern Greece the velocity fields derived from focal mechanism and GPS data may help explain the dynamics of the deformation over longer time periods than the ∼100 years over which they were measured; this may include the entire deformation history of the fault systems considered, a time period that may exceed 1–2 Myr.

The geometry, kinematics and rates of deformation within an en echelon normal fault segment boundary, central Italy

Abstract The geometry, kinematics and rates of deformation have been investigated within an en echelon fault segment boundary between two left-stepping major normal faults in the central Apennines, Italy. Examination of faulted post-glacial sediments and geomorphic features attributed to glacial retreat (18 ka) reveals that the two major faults dominate the recent slip, with rates that are 5–10 times greater than the other faults. At the centres of the major faults, slip is parallel to the regional extension direction (NE–SW) defined by focal mechanisms and borehole breakouts. Slip-vector azimuths defined by striations on faults in the stepover zone indicate NE–SW extension on NW–SE faults, sub-parallel to the regional extension direction, together with significant and contemporaneous NW–SE extension, along the strike of the Apennines, on ESE–WNW and ENE–WSW faults. Thus, distributed three-dimensional strain occurs in the stepover between the two major bounding faults. Extension rates summed across the stepover are 0.68 mm/y parallel to the regional extension, compared to ∼2 mm/y at the centre of one of the major faults. Extension rates along-strike of the fault zone are 0.14 mm/y within the stepover and zero at the centres of the two major bounding faults. The above information is used to discuss how extension is partitioned between different structures during the growth and linkage of normal faults.