J.P. Faure Walker

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.

Orogen-scale uplift in the central Italian Apennines drives episodic behaviour of earthquake faults

Many areas of the Earth’s crust deform by distributed extensional faulting and complex fault interactions are often observed. Geodetic data generally indicate a simpler picture of continuum deformation over decades but relating this behaviour to earthquake occurrence over centuries, given numerous potentially active faults, remains a global problem in hazard assessment. We address this challenge for an array of seismogenic faults in the central Italian Apennines, where crustal extension and devastating earthquakes occur in response to regional surface uplift. We constrain fault slip-rates since ~18 ka using variations in cosmogenic 36Cl measured on bedrock scarps, mapped using LiDAR and ground penetrating radar, and compare these rates to those inferred from geodesy. The 36Cl data reveal that individual faults typically accumulate meters of displacement relatively rapidly over several thousand years, separated by similar length time intervals when slip-rates are much lower, and activity shifts between faults across strike. Our rates agree with continuum deformation rates when averaged over long spatial or temporal scales (104 yr; 102 km) but over shorter timescales most of the deformation may be accommodated by <30% of the across-strike fault array. We attribute the shifts in activity to temporal variations in the mechanical work of faulting.

A 667 year record of coseismic and interseismic Coulomb stress changes in central Italy reveals the role of fault interaction in controlling irregular earthquake recurrence intervals

Current studies of fault interaction lack sufficiently long earthquake records and measurements of fault slip-rates over multiple seismic cycles to fully investigate the effects of interseismic loading and co-seismic stress changes on the surrounding fault network. We model elastic interactions between 97 faults from 30 earthquakes since 1349 AD in central Italy to investigate the relative importance of co-seismic stress changes versus interseismic stress accumulation for earthquake occurrence and fault interaction. This region has an exceptionally long, 667-year record of historical earthquakes and detailed constraints on the locations and slip-rates of its active normal faults. Of 21 earthquakes since 1654, 20 events occurred on faults where combined co-seismic and interseismic loading stresses were positive even though ~20% of all faults are in ‘stress shadows’ at any one time. Furthermore, the Coulomb stress on the faults that experience earthquakes is statistically different from a random sequence of earthquakes in the region. We show how co-seismic Coulomb stress changes can alter earthquake inter-event times by c.103 years, and fault length controls the intensity of this effect. Static Coulomb stress changes cause greater inter-event perturbations on shorter faults in areas characterized by lower strain (or slip) rates. The exceptional duration and number of earthquakes we model enable us to demonstrate the importance of combining long earthquake records with detailed knowledge of fault geometries, slip-rates and kinematics to understand the impact of stress changes in complex networks of active faults.