Refining rates of active crustal deformation in the upper plate of subduction zones, implied by geological and geodetic data: The E-dipping West Crati Fault, southern Italy.

We investigate crustal deformation within the upper plate of the Ionian Subduction Zone (ISZ) at different time scales by (i) refining geodetic rates of crustal extension from continuous Global Navigation Satellite System (GNSS) measurements and (ii) mapping sequence of Late Quaternary raised marine terraces tectonically deformed by the West Crati normal fault, in northern Ca-labria. This region experienced damaging earthquakes in 1184 (M 6.75) and 1854 (M 6.3), possi-bly on the E-dipping West Crati fault (WCF) which, however, is not unanimously considered to be a seismogenic source. We report geodetic measurements of extension and strain rates across-strike the E-dipping WCF and throughout the northern Calabria obtained by using veloci-ties from 18 permanent GNSS stations with series length longer than 4.5 years. These results sug-gest that crustal extension may be seismically accommodated in this region by a few normal faults. Furthermore, by applying a synchronous correlation approach we refine the chronology of understudied tectonically-deformed palaeoshorelines mapped on the footwall and along the strike of the WCF, facilitating calculation of the associated fault-controlled uplift rates. Raised Late Quaternary palaeoshorelines are preserved on the footwall of the WCF indicating that “re-gional” uplift, likely related to the deformation associated either with the subduction or mantle upwelling processes, is affected by local footwall uplift. We show that GIS-based elevations of Late Quaternary palaeoshorelines, as well as temporally constant uplift rates, vary along the strike of the WCF, implying normal faulting activity through time. This suggests that (i) the fault slip-rate governing seismic hazard has also been constant over the Late Quaternary, over multiple earthquake cycles and (ii) our geodetically-derived fault throw-rate for the WCF is likely a more than reasonable value to be used over longer time scales for an improved seismic hazard assess-ment. Overall, we emphasize the importance of mapping crustal deformation within the upper plate above subduction zones to avoid unreliable interpretations relating to the mechanism controlling regional uplift.

我们针对爱奥尼亚俯冲带（Ionian Subduction Zone, ISZ）上盘区域的地壳形变，在不同时间尺度下展开研究。具体通过两种路径开展工作：其一，基于连续全球导航卫星系统（Global Navigation Satellite System, GNSS）观测数据，优化地壳伸展的大地测量速率；其二，对卡拉布里亚北部受西克拉蒂正断层（West Crati normal fault）构造变形的晚第四纪抬升海蚀阶地序列进行填图。该区域曾在1184年（震级M 6.75）与1854年（震级M 6.3）发生破坏性地震，推测震源为东倾的西克拉蒂断层（West Crati fault, WCF），但该断层并未被学界一致认定为发震源。我们利用18个连续观测时长超4.5年的永久GNSS站点的速度数据，获取了横跨东倾西克拉蒂断层以及整个卡拉布里亚北部区域的伸展与应变速率大地测量结果。上述结果表明，该区域的地壳伸展可能通过少数正断层以地震活动的方式实现调节。此外，我们通过同步相关法，对西克拉蒂断层下盘及沿其走向所填绘的、研究程度较低的构造变形古海岸线的年代序列进行了优化，从而便于计算受断层控制的抬升速率。西克拉蒂断层下盘保存有晚第四纪抬升古海岸线，这意味着“区域”抬升——大概率与俯冲或地幔上涌相关的形变有关——会受到局部下盘抬升的影响。我们的研究显示，基于地理信息系统（GIS）获取的晚第四纪古海岸线高程，以及时间恒定的抬升速率，均沿西克拉蒂断层走向发生变化，这表明断层活动贯穿了整个晚第四纪时期。据此我们提出两点结论：其一，控制地震危险性的断层滑移速率在晚第四纪的多个地震周期内均保持恒定；其二，我们通过大地测量方法得到的西克拉蒂断层垂向滑移速率，完全可在更长时间尺度上用于优化地震危险性评估，具备充分合理性。总体而言，我们强调了对俯冲带上盘区域的地壳形变进行填图的重要性，以避免对区域抬升控制机制做出不可靠的解读。