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SUMMARY Field observations and modelling indicate that elastic interaction between active faults can lead to variations in earthquake recurrence intervals measured on timescales of 102-104 yr. Fault geometry strongly influences the nature of the interaction between adjacent structures as it controls the spatial redistribution of stress when rupture occurs. In this paper, we use a previously published numerical model for elastic interaction between spontaneously growing faults to investigate the relationships between fault geometry, fault slip rate variations and the statistics of earthquake recurrence. These relationships develop and become systematic as a long-term consequence of stress redistribution in individual rupture events even though on short timescales earthquake activity appears to be stochastic. We characterize fault behaviour using the coefficient of variation (CV) of earthquake recurrence intervals and introduce a new measure, slip-rate variability (SRV) that takes into account the size and time ordering of slip events. CV generally increases when the strain is partitioned on more than one fault but the relationship between long-term fault slip rate (SRmean) and CV is poorly defined. In contrast, SRV increases systematically where faulting is more distributed and SRmean is lower. To first order, SRV is inversely proportional to SRmean. We also extract earthquake recurrence statistics and compare these to previously published probability density functions used in earthquake forecasting. The histograms of earthquake recurrence vary systematically as a function of fault geometry and are best characterized by a Weibull distribution with fitting parameters that vary from site to site along the fault array. We explain these phenomena in terms of a time-varying, geometrical control on stress loading of individual faults arising from the history of elastic interactions and compare our results with published data on SRV and earthquake recurrence along normal faults in New Zealand and in the Italian Apennines. Our results suggest that palaeoseismic data should be collected and analysed with structural geometry in mind and that information on SRV, CV and SRmean should be integrated with data from earthquake catalogues when evaluating seismic hazard. [ABSTRACT FROM AUTHOR]

The shallow subsurface structure of the 2009 April 6 Mw 6.3 L'Aquila earthquake surface rupture at Paganica has been investigated with ground penetrating radar to study how the surface rupture relates spatially to previous surface displacements during the Holocene and Pleistocene. The discontinuous surface rupture stepped between en-echelon/parallel faults within the overall fault zone that show clear Holocene/Pleistocene offsets in the top 10 m of the subsurface. Some portions of the fault zone that show clear Holocene offsets were not ruptured in 2009, having been bypassed as the rupture stepped across a relay zone onto a fault across strike. The slip vectors, defined by opening directions across surface cracks, indicate dip-slip normal movement, whose azimuth remained constant between 210° and 228° across the zone where the rupture stepped between faults. We interpret maximum vertical offsets of the base of the Holocene summed across strike to be 4.5 m, which if averaged over 15 kyr, gives a maximum throw-rate of 0.23-0.30 mm yr-1, consistent with throw-rates implied by vertical offsets of a layer whose age we assume to be ∼33 ka. This compares with published values of 0.4 mm yr-1 for a minimum slip rate implied by offsets of Middle Pleistocene tephras, and 0.24 mm yr-1 since 24.8 kyr from palaeoseismology. The Paganica Fault, although clearly an important active structure, is not slipping fast enough to accommodate all of the 3-5 mm yr-1 of extension across this sector of the Apennines; other neighbouring range-bounding active normal faults also have a role to play in the seismic hazard. [ABSTRACT FROM AUTHOR]

A 3-D fluid-flow model is used to investigate pore-pressure diffusion as a mechanism for reservoir-induced seismicity (RIS) at the Açu reservoir in NE Brazil. The Açu dam is a 34-m high earth-filled dam constructed in 1983 on an area of Precambrian shield. Seismic activity in this area has been monitored over a 10-yr period (1987–1997). The frequency of earthquakes clearly varies with seasonal fluctuations of the reservoir level. Information on the hydrological regime of the Açu reservoir (rainfall, vegetation, surface and subsurface storage, etc.) is used to set up a regional groundwater-flow model in order to obtain boundary conditions for a more detailed study of the area of seismic activity. To explain the observed time lag between maximum reservoir level and peak seismic activity we calculate the magnitude and timing of the maximum piezometric head in the depth range of observed seismic activity. By assuming that individual earthquake ruptures occur when the local piezometric head is at a maximum, values of bulk permeability,K, and storativity,S, are derived. If a 3-D homogenous subsurface permeability structure is assumed then the values ofKandSobtained are not self-consistent and are physically unrealistic. However, if a high-permeability fault is embedded into, and explicitly coupled with, the surrounding lower-permeability matrix, then our estimates of subsurface hydraulic properties agree well with other field and laboratory measurements. The inclusion of a discrete fault plane in the model is consistent with the results of high-resolution seismic monitoring using a local digital network of stations, which show that the earthquake hypocentres define a set of steeply dipping NE-striking fault planes beneath the reservoir. [ABSTRACT FROM AUTHOR]