In 4 August 1885, 06:30 local time, a strong earthquake (reported intensities up to VI–VII modified Mercalli intensity [MMI]) was felt in the French Guiana, causing slight damage. Recently discovered newspaper records show that this event was also felt as far as Georgetown (British Guyana), Belém, and several other localities along the Amazon River toward Manaus (Brazil). The distribution of intensities and the radius of the felt area indicate a magnitude around Mw 6.9, which makes it the largest known earthquake in the stable continental region of South America, since the nineteenth century. The epicenter, determined with four different attenuation relations, lies onshore near the border between the French Guiana and Brazil, although an epicenter offshore in the continental slope cannot be ruled out with 95% confidence. The epicenter (03.4° N, 52.9°W±100 km⁠) likely lies in the Transamazonian (2.2–2.0 Ga) geochronological province in the Guyana shield of the Amazon craton. No nearby failed rift is known onshore near the epicenter, which would place this event in the ∼30% class of nonextended stable continental crust. Other nearby smaller earthquakes (both historical and instrumental) with magnitudes up to mb 5.2, indicate a cluster of seismicity in the region of the 1885 earthquake, possibly delineating an onshore seismic zone separate from the sparse seismicity along the continental shelf. This large midplate earthquake will likely affect future reevaluations of seismic hazard in midplate South America.

中文翻译：

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1885年法属圭亚那-巴西边界的M 6.9级地震：南美洲19世纪最大的中板事件

1885年8月4日当地时间06:30，法属圭亚那感受到了强烈的地震（据报导，烈度达到VI-VII修正的Mercalli强度[MMI]），造成了轻微破坏。最近发现的报纸记录显示，甚至在乔治敦（不列颠圭亚那），贝伦和沿亚马孙河往马瑙斯（巴西）的其他几个地方也感受到了这一事件。强度分布和毡状区域的半径表明震级为6.9 Mw，这使其成为自19世纪以来南美稳定大陆地区最大的已知地震。以四个不同的衰减关系确定的震中位于法属圭亚那和巴西之间边界附近的陆上，尽管不能以95％的置信度排除大陆斜坡上的震中。震中（03.4°N，52。9°W±100km⁠）可能位于亚马逊克拉通圭亚那盾构的Transamazonian（2.2–2.0 Ga）年代学省。震中附近在陆上没有附近的裂谷，这将使这一事件发生在约30％的未扩展稳定大陆壳中。附近的其他较小地震（包括历史地震和仪器地震）的震级都高达5.2 mb以上，表明1885年地震区域发生了一系列地震活动，可能划定了与大陆架上稀疏地震活动区分开的陆上地震带。这次中板大地震可能会影响南美中板未来的地震危险性重新评估。这将使这一事件发生在约30％的未扩展稳定大陆壳中。附近的其他较小地震（包括历史地震和仪器地震）的震级都高达5.2 mb以上，表明1885年地震区域发生了一系列地震活动，可能划定了与大陆架上稀疏地震活动区分开的陆上地震带。这次中板大地震可能会影响南美中板未来的地震危险性重新评估。这将使这一事件发生在约30％的未扩展稳定大陆壳中。附近的其他较小地震（包括历史地震和仪器地震）的震级都高达5.2 mb以上，表明1885年地震区域发生了一系列地震活动，可能划定了与大陆架上稀疏地震活动区分开的陆上地震带。这次中板大地震可能会影响南美中板未来的地震危险性重新评估。