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

Abstract

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 × 5 km and 20 km × 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 × 5 km grid squares vary from zero up to 2.34 ± 0.54 × 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 × 5 km grid squares integrated over an area of 80 km × 160 km shows the horizontal strain‐rate of the central Apennines is 1.18−0.04+0.12 × 10−8 yr−1 and −1.83−4.43+3.80 × 10−10 yr−1 parallel and perpendicular to the regional principal strain direction (043°–223° ± 1° ), associated with extension rates of ≤3.1−0.4+0.7 mm yr−1 if calculated in 5 km × 80 km boxes crossing the strike of the central Apennines. These strain‐rates are similar in direction to strain‐rates calculated using geodesy (over 126 yrs, 11 yrs and 5 yrs) and seismic moment summation (over 700 yrs); however, the magnitude is about 2.6 × less over a comparable area. The 102 yr strain‐rates are higher than 104 yr strain‐rates in some smaller areas (≈2000 km2, corresponding to polygons defined by geodesy campaigns and seismic moment summations) with the opposite situation in other areas where seismic moment release rates in large (>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 102 yr and 104 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.