Toward comparable relative locations between the mainshock slip and aftershocks via empirical approaches

Abstract

The relative locations between mainshocks and their aftershocks have long been studied to characterize the mainshock-aftershock relationships, yet these comparisons may be subjected to biases inherited from various sources, such as the analysis method, data, and model parameters. Here, we perform both a relocation analysis of interplate events to obtain accurate relative centroid locations and a slip inversion analysis of the mainshock slip relative to the relocated events, with some of the relocated events used as empirical Green's functions, to retrieve the spatiotemporal slip features of the mainshock relative to all of the relocated events. We perform these analyses on the large ( 6.0 ≤ M W ≤ 7.3 ) interplate earthquakes that occurred near four giant ( M W ≥ 8.5 ) megathrust earthquakes: the 2007 M W 8.5 Bengkulu (Indonesia), 2005 M W 8.6 Nias (Indonesia), 2010 M W 8.8 Maule (Chile), and 2011 M W 9.1 Tohoku-Oki (Japan) earthquakes. Most of the spatiotemporal slip features of the mainshocks are consistently recovered using different empirical Green's functions. We qualitatively and quantitatively demonstrate that the large interplate aftershocks within 5 years of the four analyzed mainshocks are largely located on the periphery or outside of the large-slip regions of these four giant megathrust earthquakes.

Hypocenter Hotspots Illuminated Using a New Cross‐Correlation‐Based Hypocenter and Centroid Relocation Method

“…However, other events have propagated in different directions from their slip direction, such as the large events in the Naka region that initiated around the central hotspot, which grew approximately perpendicular to the slip direction (Figure 5b), and the diverse rupture propagation directions of the large events in the Tsukuba region (Figure 8b). To obtain such directivity information, other approaches such as relative source time function measurements via empirical Green's functions (Abercrombie et al., 2017, 2020; Ammon et al., 1993; Folesky et al., 2018) or amplitude measurements (Boatwright, 2007; Kane et al., 2013) could also be applied. More detailed information regarding rupture processes can also be obtained via slip inversion analysis of the larger events, which would provide more details of the slip growth history, as was done by Okuda and Ide (2018b) on some of the events in the Naka and Kushiro regions.…”

“…This enables us to compare the relative hypocenter and centroid locations under the same unified coordinate, which yields information on rupture development without the requirement of a more detailed, event‐specific analysis, such as a slip inversion analysis. This approach provides a different perspective on rupture directivity, thereby yielding directivity information simply through high‐resolution hypocenter and centroid locations, without requiring approaches such as relative source time function measurements through empirical Green's functions (Abercrombie et al., 2017, 2020; Ammon et al., 1993; Folesky et al., 2018), or by amplitude measurements (Boatwright, 2007; Kane et al., 2013). We apply this new approach to three study regions in the Tohoku‐Hokkaido subduction zone, Japan (Figure 1), all of which contain repeating earthquakes grouped by Igarashi (2020), and two of which contain large repeating earthquakes that have been studied by Okuda and Ide (2018b).…”

“…This approach has been extended to jointly determine the hypocenter and centroid locations using picking information for the hypocenter locations and cross‐correlation information for the centroid locations (Uchida et al., 2012; Waldhauser & Ellsworth, 2000). Another scheme utilizing cross‐correlation is the network correlation coefficient (NCC) method, which has been applied to better determine the relative locations of low‐frequency earthquakes (Ohta & Ide, 2008, 2011), and also regional (Okuda & Ide, 2018a, 2018b) and teleseismic (Chang & Ide, 2019, 2020) earthquakes. Advances in such relocation methods could reduce the centroid uncertainties by as much as 1–2 orders of magnitude (e.g., Schaff et al., 2002), whereas the hypocenter uncertainties may still be up to several hundreds of meters in practical applications (e.g., Uchida et al., 2012), presumably due to uncertainties in the onset phase picking, which are influenced by the event magnitude and signal‐to‐noise ratio conditions of the waveforms.…”

Mainshock and Aftershock Sequence Simulation in Geometrically Complex Fault Zones

“…Many observations show lower aftershock productivity in large slip areas in the mainshock (Beroza & Zoback, 1993; Chang & Ide, 2020; Das & Henry, 2003; Wetzler et al., 2018). For example, Wetzler et al.…”

“…Many observations show lower aftershock productivity in large slip areas in the mainshock (Beroza & Zoback, 1993;Chang & Ide, 2020;Das & Henry, 2003;Wetzler et al, 2018). For example, Wetzler et al (2018) compiled the spatial distribution of aftershocks for a large number of subduction zone earthquakes and statistically found that aftershocks are clustered around the edges of the coseismic slip.…”