Which Earthquakes are Expected to Exceed the Design Spectra?

Abstract

Extended recording coverage of contemporary seismic events allows a comparison of observed seismic actions with their counterparts used for design. Said comparison shows actions systematically exceeding design spectra. This paper discusses: (1) that considered exceedances can be anticipated by the probabilistic seismic hazard on the basis of which design actions are determined, (2) exceedances of elastic design actions are expected for earthquakes occurring close to the site even if their magnitude is far from the maximum magnitude considered in the hazard assessment, and (3) design spectra are likely to be exceeded in epicentral areas of earthquakes that occur frequently in the region where the code is enforced, but rarely occur close to the site under consideration. In fact, code-mandated protection against these earthquakes is factually warranted by the rarity with which they are expected to occur near the structure and other safety margins implicit to earthquake-resistant design. All these issues, addressed with reference to Italy, are discussed with the intent not to criticize the way spectra are determined, but rather to raise awareness and give a probabilistic measure about what to factually expect from state-of-the-art design at a national level.

Peak‐over‐threshold: Quantifying ground motion beyond design

“…Conversely, it can be appreciated from the middle panel that if earthquakes with magnitude larger than M6 occur, for example, within 15 km from the site, the probability of exceeding sa T r = 475 is from 0.4 to almost 1 (ie, exceedance is almost certain for these scenarios). This, as extensively discussed in Iervolino et al, 6 shows why exceedance of the design spectra is expected in the epicentral areas of earthquakes of magnitude that would be considered relatively moderate otherwise, as systematically observed. 21…”

“…Hazard terms for the 1‐second spectral acceleration with 475‐year exceedance return period in L'Aquila. Left: occurrence rates per magnitude and distance bins; center: probability of exceedance from the GMPE; right: rates of earthquakes exceeding the threshold from specific magnitude‐distance bins [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Hazard terms for the 1-second spectral acceleration with 475-year exceedance return period in L'Aquila. Left: occurrence rates per magnitude and distance bins; center: probability of exceedance from the GMPE; right: rates of earthquakes exceeding the threshold from specific magnitude-distance bins 6 It can be seen from the left panel that the maximum magnitude of earthquakes that can occur on the sources affecting the hazard of L'Aquila is larger than M7. However, the rates of earthquakes larger than M6 are negligible compared with those of magnitude below M6.…”

“…For example, would an earthquake of magnitude about six to occur anywhere in Italy, a site within 5 km would likely see its peak ground acceleration (PGA) with T r = 475 years exceeded. 6 This happens also in regions where the maximum magnitude considered in the hazard assessment is about 7.5. The consequence of engineering interest is that it is systematically expected that the structures in the epicentral areas of moderate-to-high earthquakes must withstand seismic actions larger than those accounted for by design.…”

“…Moreover, it has been demonstrated that exceedance of design spectra is likely‐to‐almost‐certain in the epicentral area of earthquakes of moderate‐to‐high magnitude, especially when the hazard assessment is based on seismic source zones. For example, would an earthquake of magnitude about six to occur anywhere in Italy, a site within 5 km would likely see its peak ground acceleration (PGA) with T r = 475 years exceeded . This happens also in regions where the maximum magnitude considered in the hazard assessment is about 7.5.…”