Abstract
ShakeMap is the tool to evaluate the ground motion effect of earthquakes in vast areas. It is useful to delimit the zones where the shaking is expected to have been most significant, for civil defense rapid response. From the earthquake engineering point of view, it can be used to infer the seismic actions on the built environment to calibrate vulnerability models or to define the reconstruction policies based on observed damage vs shaking. In the case of long-lasting seismic sequences, it can be useful to develop ShakeMap envelopes, that is, maps of the largest ground intensity among those from the ShakeMap of (selected) events of a seismic sequence, to delimit areas where the effects of the whole sequence have been of structural engineering relevance. This study introduces ShakeMap envelopes and discusses them for the central Italy 2016–2017 seismic sequence. The specific goals of the study are: (i) to compare the envelopes and the ShakeMap of the main events of the sequence to make the case for sequence-based maps; (ii) to quantify the exceedance of design seismic actions based on the envelopes; (iii) to make envelopes available for further studies and the reconstruction planning; (iv) to gather insights on the (repeated) exceedance of design seismic actions at some sites. Results, which include considerations of uncertainty in ShakeMap, show that the sequence caused exceedance of design hazard in thousands of square kilometers. The most relevant effects of the sequence are, as expected, due to the mainshock, yet seismic actions larger than those enforced by the code for structural design are found also around the epicenters of the smaller magnitude events. At some locations, the succession of ground-shaking that has excited structures, provides insights on structural damage accumulation that has likely taken place; something that is not accounted for explicitly in modern seismic design. The envelopes developed are available as supplemental material.
Highlights
In Italy, ShakeMap (Wald et al 1999) is primarily used for emergency management
The shaking intensity is evaluated automatically in near-real-time after the national seismic monitoring network has recorded an earthquake of magnitude equal to or larger than three; when manually processed and quality-checked waveforms are available, ground motion intensities from ShakeMap are revised
Between August 2016 and January 2017, seven earthquakes with magnitude ranging from 5.0 to 5.8 and two with magnitude equal to 6.0 and 6.5 occurred in a relatively small region surrounding the villages of Accumoli, Amatrice, Norcia and Visso, that is, the 2016–2017 central Italy seismic sequence
Summary
In Italy, ShakeMap (Wald et al 1999) is primarily used for emergency management. The shaking intensity is evaluated automatically in near-real-time after the national seismic monitoring network has recorded an earthquake of magnitude equal to or larger than three; when manually processed and quality-checked waveforms are available, ground motion intensities from ShakeMap are revised. It is the case of the yellow shaded areas at the north of Visso in the leftmost panels, where accelerations in terms of PGA, Sa(T = 1 s) and Sa(T = 3 s) for both the events M6.0 and M6.5 are lower than those provided by the envelopes, which partly motivates the interest in investigating sequence-based ShakeMap rather than those event-specific.
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