Abstract
Determining the hypocentral depth of pre-instrumental earthquakes is a long-standing geophysical issue that still awaits to be elucidated. Using very well documented recent earthquakes we found that the depth of crustal and upper-mantle events correlates well with the slope of the first 50 km of their intensity attenuation curve, regardless of their magnitude. We used this observation to build a magnitude-independent method for calculating the depth of selected historical and early-instrumental earthquakes of northern Italy based on their macroseismic intensity field. Our method relies on both standard intensity data and questionnaire-based data for 20 earthquakes, encompassing a relatively large range of magnitude (Mw 4.0–5.8) and depth (3.0–72.4 km), that occurred in Northern Italy between 1983 and 2019. We then used the method to estimate the depth of 20 older earthquakes that occurred in the same region between 1570 and 1972. Knowing the approximate depth of historical earthquakes is crucial for assigning them to the relevant seismogenic source, especially where seismogenic faults occur at different depths, allowing for a better characterisation of the region’s seismotectonic setting. Knowing the focal depth also allows recalculating the equivalent magnitude, which turns out to be consistently larger for deeper events, suggesting a reassessment of the local seismic hazard.
Highlights
Determining the hypocentral depth of pre-instrumental earthquakes is a long-standing geophysical issue that still awaits to be elucidated
The method we propose rests on three fundamental pillars: (a) it relies only on the slope of the attenuation curve, not on the absolute intensity levels attained by any given earthquake: as such, it is fully independent of its magnitude; (b) it is based on individual Macroseismic Data Points (MDPs), not on isoseismals: as such, it eliminates the subjectivity relating to the style of isoseismal drawing and to the associated geographic uncertainties; (c) it is primarily based on data gathered through the web-based questionnaires collected in the HSIT database (Hai Sentito Il Terremoto, or Did You Feel the Earthquake; http://www.haisentitoilterremoto.it/): a nationwide, community-based effort for collecting information from people who felt an earthquake and for creating large and homogeneous pseudo-intensity datasets[36]
Using a set of recent, well-recorded crustal and upper-mantle earthquakes we found that their depth correlates well with the slope of the first 50 km of the curve describing the decay of macroseismic intensity with epicentral distance
Summary
Determining the hypocentral depth of pre-instrumental earthquakes is a long-standing geophysical issue that still awaits to be elucidated. Using very well documented recent earthquakes we found that the depth of crustal and upper-mantle events correlates well with the slope of the first 50 km of their intensity attenuation curve, regardless of their magnitude. We used this observation to build a magnitude-independent method for calculating the depth of selected historical and early-instrumental earthquakes of northern Italy based on their macroseismic intensity field. Our method relies on both standard intensity data and questionnaire-based data for 20 earthquakes, encompassing a relatively large range of magnitude (Mw 4.0–5.8) and depth (3.0–72.4 km), that occurred in Northern Italy between 1983 and 2019. Its southern portion hosts the external buried thrusts of the chain, organised in a complex system of arched fronts overlain by a variable thickness of terrigenous sediments[13]
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