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To Intensity and Beyond: On the Limits of the Conventional Macroseismic Intensity Scale

Description: 

Observed earthquake shaking is increasingly well-characterized with instrumental recordings and systematically interpreted macroseismic data. Ground motion models are determined for not only peak ground acceleration but also peak ground velocity and longer period spectral ordinates. Visualization, however, still commonly relies on the conventional intensity scale, which provides an intuitive way to depict shaking severity. Intensity-based characterizations are moreover used for applications ranging from assessment of early warning performance to estimation of earthquake losses. It has been noted that intensity values ≥ 8 are rare in compilations of both traditional and internet-based intensities. Among the compilation of Did You Feel It? (DYFI) intensities for California earthquakes since 1999, for example, out of over 700,000 intensities estimated by DYFI, intensities ≥ 8 number in the 100s (Chauffer et al., 2025). Theories have been advanced to explain the paucity of observations, including a doughnut effect whereby reports are not submitted from areas where damage was severe (Bossu et al., 2018), and the inability of the DYFI questionnaire to capture sufficiently detailed information to evaluate structural damage. Considering the aftermath of recent damaging earthquakes including 2015 Gorkha, Nepal, and 2019 Ridgecrest, California, events, I suggest that intensity values ≥ 8 are limited in part because population centers tend to cluster in areas underlain by sediments, where pervasive nonlinear sediment response deamplifies shaking at frequences that would be damaging to vernacular small (1-2 story) structures. Strong long-period shaking can damage taller structures, break buried pipelines, and throw people to the ground, but such effects may not correlate well with conventional macroseismic effects. The challenge for characterizing strong ground motion may not only be to better estimate high intensities, but to move away from the conventional intensity scale, which fails to capture shaking effects that are strongly frequency-dependent at frequencies of engineering concern.

Session: Macroseismic Intensity: Past, Present and Future [Poster]

Type: Poster

Date: 4/17/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Susan

Student Presenter: No

Invited Presentation: 

Poster Number: 19

Authors