Abstract
Large pre-historical earthquakes leave traces in the geological and geomorphological record, such as primary and secondary surface ruptures and mass movements, which are the only means to estimate their magnitudes. These environmental earthquake effects (EEEs) can be calibrated using recent seismic events and the Environmental Seismic Intensity Scale (ESI2007). We apply the ESI2007 scale to the 1992 MS7.3 Suusamyr Earthquake in the Kyrgyz Tien Shan, because similar studies are sparse in that area and geological setting, and because this earthquake was very peculiar in its primary surface rupture pattern. We analyze literature data on primary and secondary earthquake effects and add our own observations from fieldwork. We show that the ESI2007 distribution differs somewhat from traditional intensity assessments (MSK (Medvedev-Sponheuer-Karnik) and MM (Modified Mercalli)), because of the sparse population in the epicentral area and the spatial distribution of primary and secondary EEEs. However, the ESI2007 scale captures a similar overall pattern of the intensity distribution. We then explore how uncertainties in the identification of primary surface ruptures influence the results of the ESI2007 assignment. Our results highlight the applicability of the ESI2007 scale, even in earthquakes with complex and unusual primary surface rupture patterns.
Highlights
Paleo-earthquake magnitudes are usually estimated from the lengths of mapped primary surface ruptures and the single-event offsets of geomorphological markers
Primary surface rupture length, and average/maximum offset allow calculating magnitudes (e.g., References [1,2]). Those relationships are derived from recent surface-rupturing earthquakes with well-constrained magnitudes. Using this approach in paleoseismological studies comes with several limitations: (i) It can only be applied to earthquakes with primary surface ruptures
One approach to overcome the problem of estimating paleo-earthquake magnitudes based solely on primary surface ruptures is the application of the Environmental Seismic Intensity Scale, ESI2007 [8,9,10]
Summary
Paleo-earthquake magnitudes are usually estimated from the lengths of mapped primary surface ruptures and the single-event offsets of geomorphological markers. One approach to overcome the problem of estimating paleo-earthquake magnitudes based solely on primary surface ruptures is the application of the Environmental Seismic Intensity Scale, ESI2007 [8,9,10]. The conversion of ESI2007 intensities to magnitudes, can only be based on a large set of modern case studies, which are currently being collected in the Earthquake Environmental Effects (EEE) catalogue hosted by ISPRA [11]. This study contributes to extending the entries for central Asia and compressional tectonic settings; (ii) the Suusamyr Earthquake was special in its primary surface rupture pattern, and the exercise of applying the ESI2007 scale to it thereby points out a general problem; (iii) the earthquake occurred in a sparsely populated area, which hampers the application of traditional intensity scales that mainly focus on damage to human-made infrastructure; and (iv) we document secondary cracks of the Suusamyr Earthquake with a high-resolution digital elevation model (DEM) computed from drone aerial imagery and the Structure-from-Motion (SfM) technique [7,12,13]. MM and MSK intensities and the implications of our study for the application of the ESI2007 scale on paleo-earthquake in general
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