For historical earthquakes, the spatial distributions of macroseismic intensity reports are commonly used to estimate the event locations. The methods to locate historical earthquakes assume that the highest seismic intensity shows the best estimate of the location of the earthquake. Uncertainties in the locations estimated from macroseismic data can be due to an uneven geographic distribution of sites with intensity reports, variations in intensities due to local soil conditions, ambiguous historical reports, and earthquake directivity effects. Additional constraint on the location of a historical earthquake can come from places where most aftershocks were felt, because these localities may have been closest to the fault on which the mainshock took place. Examples of estimated earthquake locations based on aftershocks are those of the 1727 MLg 5.6 earthquake in northeastern Massachusetts, the MLg 5.7 earthquake in Maine, and the 1755 MLg 6.2 earthquake offshore of Cape Ann, Massachusetts. In all of these cases, the earthquake locations based on the aftershock data are somewhat different from previous locations derived from the macroseismic intensities alone. Uncertainties with this method include identifying aftershocks in historical accounts and the possibility that smaller events that are reported following a strong earthquake are not on or near the mainshock rupture. Even so, evidence of possible aftershock activity may help constrain the location of that mainshock. Because aftershocks of strong earthquakes (⁠M≥7⁠) can last months to years, archival research for aftershocks must be carried out with a somewhat different mindset than that for a mainshock.

中文翻译：

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使用余震帮助定位历史地震

对于历史地震，通常使用宏观地震烈度报告的空间分布来估计事件的位置。定位历史地震的方法假定最高地震烈度显示出对地震位置的最佳估计。根据大地震数据估算的位置不确定性可能是由于强度报告站点的地理分布不均匀，局部土壤条件导致的强度变化，历史报告不明确以及地震方向性影响。对历史地震发生地点的其他限制可能来自感觉到大多数余震的地方，因为这些地点可能最接近发生主震的断层。根据余震估算的地震位置的示例是1727 MLg 5。马萨诸塞州东北部发生了6次地震，缅因州发生MLg 5.7级地震，马萨诸塞州安普角附近的1755 MLg 6.2级地震。在所有这些情况下，基于余震数据的地震位置与仅根据大地震烈度得出的先前位置有所不同。这种方法的不确定性包括确定历史记录中的余震，以及发生强震后报告的较小事件不在主震破裂上或附近的可能性。即使这样，可能发生余震活动的证据也可能有助于限制该主震的位置。由于强地震（M≥7⁠）的余震可能持续数月至数年，因此对于余震的档案研究必须以与主震不同的思维方式进行。