We present a magnitude-dependent aftershock model for the eastern Intermountain West (IMW) that can be used to inform and improve reliability of initial U.S. Geological Survey (USGS) operational aftershock forecasts (OAFs). The model is derived after considering recent M ??? 5.0 aftershock sequences in the eastern IMW, and careful identification of M < 5.0 aftershock sequences from regional earthquake catalogs (western Montana, Utah). We propose a workflow for M < 5.0 earthquake sequences that consists of: (1) spatiotemporal analysis of magnitude of completeness, (2) application of multiple declustering???clustering algorithms, (3) discrimination between mainshock???aftershock sequences (MS-AS) and earthquake swarms, and (4) synthesizing the results to determine the final list of MS-AS. In addition, we examined separately aftershock sequences with 5.0 ??? M < 6.0 and M ??? 6.0 and find a notable difference in earthquake productivity. Our results show that the proposed model for IMW differs from the one used in the USGS OAF, and highlights the need for region-specific aftershock modeling. This study validates the hypothesis that earthquake sequences following large surface-faulting earthquakes in the eastern IMW, and perhaps generally in the Basin and Range Province, behave fundamentally differently than those following moderate-size earthquakes.
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