Seismo-Magnitude Clustering is Imposed by Structural Constraint and Shear Stress
Authors: Xiong Q. , Gossett D. , Brudzinski M. R. , Lin Q. , Hampton J. C. 
Published: September 2023

ABSTRACT
The clustering phenomenon in seismology draws significant attention from geophysicists as well as the public at large. The reason for this is rooted in the fundamental interest of reliable earthquake prediction; or otherwise, if earthquake prediction is possible. The investigations of earthquake spatial-magnitude clustering, especially if earthquake magnitudes are clustering within a controlling geological structure(s), are however, still of active debate. In this paper, we first present several discoveries on the magnitude clustering phenomenon via the interrogation of the laboratory acoustic emission (AE); after that, we shortly display the accompanying instigations on field-scale catalogs intrigued by the laboratory interrogation. In laboratory, the clustering of AE event magnitude can only unconditionally exist under shear stress; the tensile stress will remove the magnitude clustering phenomenon. This confirmation from laboratory interrogation intrigues the field investigation on several natural and anthropogenic catalogs crossing the North American continent. As the consequence that pure tensile stress condition can hardly exist in field-scale, magnitude clustering phenomenon for field-scale catalogs is found to be prominent crossing scales from the scale of hundreds of meters to the scale of hundreds of kilometers.

INTRODUCTION
The clustering phenomenon in space and time is a well-recognized feature of earthquakes. There are prominent examples of earthquakes as being spatial clustering, i.e. the aftershocks around a mainshock, and temporal clustering, i.e. the Omori-Utsu decay in the temporal productivity after a mainshock (Felzer & Brodsky, 2006; Utsu, Ogata, & Matsu’ura, 1995). However, the existence of clustering in earthquake magnitudes is still a matter of active debate. The earthquake magnitudes were thought to be independent until a set of studies reported magnitude correlations between sequential catalogued earthquakes (A. Corral, 2004; A. Corral, 2006; Lippiello, Godano, & de Arcangelis, 2007). Questions on these results had also been proposed by other researchers as the magnitude clustering observation could be influenced by catalog incompleteness (Davidsen & Green, 2011; Davidsen, Kwiatek, & Dresen, 2012). The reason for the extensive investigations on earthquake clustering phenomenon is rooted in the fundamental interest of reliable earthquake prediction; or otherwise, to address the question if earthquake prediction is really possible (Davidsen & Green, 2011). If magnitude clustering does exist, it can have practical meanings for the short-term earthquake forecasting. For instance, if similar magnitude events are clustered in aftershock sequences (Field et al., 2017; Nandan, Ouillon, & Sornette, 2019; Nichols & Schoenberg, 2014; Spassiani & Sebastiani, 2016). Specifically, contemporary forecasting approaches typically utilize the epidemic-type aftershock sequence (ETAS) methodology for simulating seismicity. This method has incorporated with the spatial and temporal clustering but has not included any form of magnitude clustering (Field et al., 2017; Hardebeck, 2013; Ogata, 1988).