Aftershocks and Background Seismicity in Tangshan and the Rest of North China

Abstract

At 3:42 a.m. (UTC+8) on July 28, 1976, a magnitude (M s ) 7.8 earthquake hit Tangshan, an industrial city in North China just 150 km east of Beijing, causing a surface rupture >47-km long (Figure 1) (Guo et al., 2017). About 15 h later, a second shock, of M s 7.1, occurred ∼40 km to the east-northeast on a different fault and ruptured surface >6-km long (Guo et al., 2017). These two events, together referred to as the Great Tangshan earthquake, obliterated the city of Tangshan and killed >240,000 people, making it the deadliest earthquake of the 20th century (Chen et al., 1988). More than 40 years have passed, but the memory of the devastation remains fresh. Thus, a series of moderate (M ≥ 4.5) earthquakes in recent years near the 1976 Tangshan earthquake rupture zone, including an M s 5.1 event on July 12, 2020, have raised social concern and scientific debate: are these events aftershocks of the Great Tangshan earthquake? Or are they indicators of stress buildup for future large earthquakes in Tangshan or somewhere in North China? North China, or the geologically defined North China block, includes the North China Plain and the mountain ranges and the Ordos Plateau to the west. It is an Archean craton that was reactivated during the

Long‐Lived Aftershocks in the New Madrid seismic Zone and the Rest of Stable North America

“…We also considered the ranges of magnitudes of the historic mainshocks in the New Madrid, Charleston, and Charlevoix seismic zones (Tables S1-S3 in Supporting Information S1) in our analysis. We used the nearest-neighbor (NN) method to distinguish aftershocks from background seismicity (Baiesi & Paczuski, 2004;Chen et al, 2021;Zaliapin & Ben-Zion, 2016;Zaliapin et al, 2008). As in all declustering approaches, background events are assumed to be independent from each other, while aftershocks are linked to their mainshocks.…”

“…The NND ( η ) of earthquakes in real catalogs usually shows a bimodal distribution (Figure 2a) (Chen et al., 2021; Peresan & Gentili, 2018; Zaliapin & Ben‐Zion, 2013). The same pattern shows for synthetic catalogs predicted by the ETAS models that mix self‐excited aftershock series with a homogeneous Poissonian background field (Zaliapin et al., 2008).…”

“…In this study, we used the nearest‐neighbor (NN) method (Baiesi & Paczuski, 2004; Zaliapin et al., 2008) to identify long‐lived aftershocks. In comparison with other declustering methods, the NN method is stable to catalog incompleteness and does not assume the underlying physical processes and the associated parameters (Chen et al., 2021; Zaliapin & Ben‐Zion, 2013), which are not well constrained for earthquakes in stable North America. We focus on three regions with large historic earthquakes: the New Madrid, Charleston, and Charlevoix seismic zones (Figure 1).…”

“…Some workers have suggested that the present‐day seismicity near the epicenters of large historic earthquakes in stable North America consist of long‐lived aftershocks. Supporting evidence includes the apparent consistency between the Omori‐law of decay of aftershocks and current seismicity rate in the New Madrid, Charleston, and Charlevoix seismic zones (Ebel et al., 2000), dynamic modeling of postseismic frictional afterslip and current crustal deformation in the New Madrid seismic zone (NMSZ) (Boyd et al., 2015), long duration of aftershock sequences in intraplate settings (Chen et al., 2021; Liu & Wang, 2012; S. Stein & Liu, 2009; Toda & Stein, 2018) that is consistent with the rate‐and‐state friction laws (aftershock duration is inversely proportional to the stressing rate) (Dieterich, 1994), geodynamic modeling that indicates that large intraplate earthquakes may dominate local strain energy for thousands of years (Li et al., 2007), and common seismic characteristics shared by the aftershock sequence of the 2011 Virginia earthquake and modern seismicity near the epicenter of the 1886 Charleston earthquake: concentration of modern seismicity where the Coulomb stress may have been increased by the mainshock (Chapman et al., 2016).…”

“…sequences in intraplate settings (Chen et al, 2021;Liu & Wang, 2012;S. Stein & Liu, 2009;Toda & Stein, 2018) that is consistent with the rate-and-state friction laws (aftershock duration is inversely proportional to the stressing rate) (Dieterich, 1994), geodynamic modeling that indicates that large intraplate earthquakes may dominate local strain energy for thousands of years (Li et al, 2007), and common seismic characteristics shared by the aftershock sequence of the 2011 Virginia earthquake and modern seismicity near the epicenter of the 1886 Charleston earthquake: concentration of modern seismicity where the Coulomb stress may have been increased by the mainshock (Chapman et al, 2016).…”