The locations of aftershocks are often observed to be on the same fault plane as the mainshock and used as proxies for its rupture area. Recent developments in earthquake relocation techniques have led to great improvements in the accuracy of earthquake locations, offering an unprecedented opportunity to quantify both the aftershock distribution and the mainshock rupture area. In this study, we design a consistent approach to calculate the area enclosed by aftershocks of 12 Mw≥5.4 mainshocks in California, normalized by the mainshock rupture area derived from slip contours. We also investigate the Coulomb stress change from mainshock slip and compare it with the aftershock zone. We find that overall, the ratios of aftershock zone area to mainshock rupture area, hereinafter referred to as “aftershock ratio”, lie within a range of 0.5–5.4, with most values being larger than 1. Using different slip‐inversion models for the same mainshock can have a large impact on the results, but the ratios estimated from both the relocated catalogs and Advanced National Seismic System catalog have similar patterns. The aftershock ratios based on relocated catalogs of southern California fall between 0.5 and 4.3, whereas they exhibit a wider range from 1 to 5.4 for northern California. Aftershock ratios for the early aftershock window (within one‐day) show a similar range but of smaller values than using the entire aftershock duration, and we propose that continuing afterslip could contribute to the expanding aftershock zone area following several mainshocks. Our results show that areas with positive Coulomb stress change scale with aftershock zone areas, and spatial distribution of aftershocks represents stress release from mainshock rupture and continuing postseismic slip.

中文翻译：

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余震带地区是否是主震破裂区的良好代表？

通常观察到余震的位置与主震在同一断层平面上，并用作其破裂区域的代理。地震搬迁技术的最新发展大大提高了地震定位的准确性，为量化余震分布和主震破裂区域提供了前所未有的机会。在这项研究中，我们设计了一种一致的方法来计算加利福尼亚州12Mw≥5.4主震的余震所包围的面积，并通过从滑动轮廓得出的主震破裂面积进行归一化。我们还研究了主震滑移引起的库仑应力变化，并将其与余震区进行了比较。我们发现，总体而言，余震带面积与主震破裂面积之比（以下称为“余震比”）在0.5-5.4的范围内，大多数值都大于1。对于相同的主震，使用不同的滑移反演模型可能会对结果产生很大的影响，但是从重新定位的目录和“国家地震系统高级目录”估算出的比率具有相似的模式。根据重新布置的南加州目录的余震比率介于0.5到4.3之间，而北加州的余震比率显示范围从1到5.4。与使用整个余震持续时间相比，早期余震窗口（一天之内）的余震比率显示出相似的范围，但值较小，我们建议持续的余震可能会导致几次主震后扩大的余震带面积。我们的结果表明，库仑应力为正值的区域随余震带区域的变化而变化，