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Exploring the Origin of Temporal b-Value Variation: Insights From the 2016/17 Central Italy Sequence

Description: 

As explored in various previous studies, the 2016/17 central Italy earthquake sequence exhibited notable spatiotemporal variations of the Gutenberg–Richter b-value. These variations have been linked to stress changes (Gulia and Wiemer 2019), fault rheology (Collettini et al. 2022), or the general complexity/heterogeneity of the fault system (Herrmann et al. 2022). Here, we reanalyse the sequence focusing on the spatio-temporal evolution of seismicity close to the Norcia mainshock (M_w 6.5). Specifically, we apply a combination of three algorithms: DBSCAN for performing event clustering, OPTICS for analyzing spatially nested dense zones within clusters, and PCA for inferring the planar geometry of those zones as fault surfaces. Before the Norcia event, we identified two clusters. One of them exhibited nested dense zones, and we could infer a near-vertical fault plane (dip of 63°) for the largest one; for the other cluster we inferred an overall horizontal fault plane orientation (dip of ~10°). After Norcia, we selected seismicity close to each of those structures associated with a fault plane. We analyze the magnitude-frequency distributions and b-values of the clusters and structures of interest. To reduce potential biases from short-term aftershock incompleteness, we excluded affected periods from the analyses.

We found that the MFD and b-value of in the structures of interest are temporally constant. In other words, the b-value depends on the structure. Our interpretation of this observation is that temporal b-value variations over the whole sequence might only be due to different structures being active (at various intensity) over time, i.e., spatial variations of seismicity. Our findings therefore highlight the role of distinct fault segments and structural heterogeneity in b-value analysis. Notably, higher b-values were associated with deeper, sub-horizontal seismicity, consistent with previous findings (Herrmann et al. 2022) This finding reiterates that spatially isolating seismically active clusters allows for a meaningful resolution of the fine-scaled variation of the b-value.

Session: Building and Decoding High-resolution Earthquake Catalogs With Statistical and Machine-learning Tools - I

Type: Oral

Date: 4/15/2025

Presentation Time: 05:15 PM (local time)

Presenting Author: Paola

Student Presenter: Yes

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