Estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale. This study uses the tephra from the caldera forming eruption of Mount Mazama (Crater Lake, OR, USA) to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude. We combine > 250 Mazama tephra occurrences, reported by a range of disciplines (including archaeology, paleoclimatology and volcanology), with new field studies to better understand the extent of the distal tephra. We find that the Mazama tephra has been remobilised to varying degrees over the past 7000 years, so each tephra locality was appraised for the likelihood that it records primary tephra fallout. We designated 45 of the distal (> 100 km from source) tephra sites as suitable for use in the production of isopachs using a spline fitting method. The new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic Mazama eruption to ~ 176 km 3 (~ 61 km 3 dense-rock equivalent; DRE). This study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit. Interpreting remobilised deposits can provide insight into post-eruptive processes that could potentially pose secondary hazards following large magnitude eruptions. We also show that in some circumstances, remobilised deposits preserve important physical properties such as grain size.

中文翻译：

---

Mazama 等厚线的不确定性来源以及对大规模喷发造成的远端火山灰沉积物的解释

估计火山灰喷发面积和大规模喷发量对于解释这种规模的喷发造成的危害至关重要。本研究使用来自马扎马山（美国俄勒冈州火山口湖）火山口形成喷发的火山灰来展示在处理大型史前火山灰沉积物时面临的挑战，并概述了确定喷发量和规模所需的方法。我们将一系列学科（包括考古学、古气候学和火山学）报告的 > 250 个 Mazama 火山灰事件与新的实地研究相结合，以更好地了解远端火山灰的范围。我们发现 Mazama tephra 在过去 7000 年中已不同程度地重新活动，因此每个 tephra 地点都被评估为记录主要 tephra 沉降物的可能性。我们指定了 45 个远端（距源头 100 公里以上）的火山灰点适合使用样条拟合方法生产等厚线。然后将新的远端等厚线与近端沉降数据和来自先前研究的 ignimbrite 体积估计值相结合，以将估计的大量喷发体积从高潮 Mazama 喷发修改为 ~ 176 km 3 （~ 61 km 3 致密岩石当量；DRE）。这项研究证明了从一系列学科中整理 tephra 地点的重要性，甚至重新移动的沉积物也提供了关于沉积物范围的宝贵信息。解释再活动的沉积物可以深入了解可能在大规模喷发后造成二次危害的喷发后过程。我们还表明，在某些情况下，