Patterns of Reconstruction at Pompeii

Book XV, chapter 22 of the Annals of Tacitus reports that "An earthquake ... demolished a large part of Pompeii" [Tacitus 1895, p. 295]. The date was February 5, 62 AD, however little else is recorded about the effects of the earthquake. The events of the 79 eruption of Vesuvius can be reconstructed in much more detail, based on the letters of Pliny the Younger which thoroughly chronicle the events of that spanned from approximately 1:00 PM August 24 into the daylight hours of the next day. In addition, undisturbed the volcanic deposits which remain immediately outside the walls of ancient Pompeii give information about the nature and sequence of eruption events [Sigurdsson 1985, p. 350].

There were two basic types of eruption in 79: pumice fallout, and pyroclastic flow. Pumice fallout produced a rain of lightweight pumice pellets, which accumulated to a depth of 2.5 meters. The significant structural effect of the fallout is the potential to collapse floors and roofs. Pyroclastic flow, by contrast, is a fast-moving mix of volcanic dust and ash which has the potential to topple masonry walls; the damaging potential of pyroclastic flow is discussed in more detail in chapter 3.

Pliny's letters reveal that eruption phenomena were not the only threat to buildings in 79, there was also significant seismic ground motion associated with the eruption. In the city of Stabiae, approximately 7 kilometers south of Pompeii, ground tremors on the evening of the 24th motivated residents to move outside [Sigurdsson 1982, p. 45]. In Misenum, approximately 35 kilometers from Pompeii, tremors on the morning of the 25th were strong enough to move vehicles even with stones blocking their wheels [Sigurdsson 1982, p. 46].

Thus, prior to the eruption, there are three significant events that may have caused damage to the Macellum: ground shaking in 62; pyroclastic flow in 79; and ground shaking in 79. From the perspective of damage and repair, the key point is that damage occurring in 62 may have been repaired by ancients, but damage occurring in 79 certainly was not. There is also the possibility that there may have been other earthquakes in the interval between 62 and 79. While not discounting this possibility, any such earthquakes will be conceptually grouped together in this study and treated as a single event.

2.2 Events after the excavation

It is also important to consider the potential for events following the excavation of the Forum in the 18th and 19th centuries. Although it is known that eruptions of Vesuvius since that time have not struck Pompeii, there have been several earthquakes affecting the Pompeii region. Since Pompeii was a nearly-vacant ruin for many decades following the excavation, it is possible that there was unrecorded seismic damage which has since been repaired. To assess the likelihood of this possibility, it is useful to examine two types of evidence: the historic record of earthquakes in the region since 1800, and the state of the ruins itself, since the proportions of currently undamaged masonry elements can be used as a kind of barometer to gauge the maximum shaking which the element has experienced.

Considering the historic record, since 1800 there have been four earthquakes which have caused significant shaking in Pompeii [Postpischl 1985], occurring in 1805, 1857, 1930, and 1980. Earthquake magnitude is commonly reported to the public in terms of the Richter scale. This scale measures the total energy released by the earthquake, but does not measure the level of damage at a particular location, which generally declines with increasing distance from the earthquake center. The level of damage at a particular location is called earthquake intensity, and is commonly measured by the Mercalli scale: a twelve-level scale which measures intensity based on observable effects rather than precise measurements¹. Intensity is often used to assess historic earthquakes, since instrumented measurements are not available. Table 2.1 lists the Mercalli intensities for the significant earthquakes which have struck Pompeii since 1800.

Name	Date	Intensity (Mercalli scale)²
Branello	July 26, 1805	VI
Basilicata	December 16, 1857	VI
Irpinia	July 23, 1930	VI-VII
Irpinia	November 23, 1980	VII

Table 2.1: Mercalli intensities of significant earthquakes affecting Pompeii since 1800

It is important to note that for all but the 1980 Irpinia earthquake, the intensity levels listed above are not based on direct observation of effects at Pompeii, but rather by interpolating intensities observed and recorded at nearby locations. The record shows that the strongest shaking is level VII, a level defined as causing cracking in good quality unreinforced masonry and damage in low quality masonry, along with falling of loose stones and architectural elements [Newmark 1971, p. 586]; in other words, little damage to stout structural elements in good condition.

To confirm the indication of the historical record, it is useful to examine the proportions of undamaged masonry elements currently observable in Pompeii. Free standing columns are particularly illustrative, since it follows from physics that the aspect ratio of a column (the height divided by the diameter) determines the level of ground acceleration required to overturn it. To a first approximation, the ground acceleration expressed as a fraction of gravity (e.g. one tenth g) required to topple a column is equal to the inverse of the column's aspect ratio, so that at least one-tenth g is required to topple a column with an aspect ratio of 10. Similarly, a ground acceleration of at least 20 percent g would be required to topple a column with an aspect ratio of 5³.

A Mercalli intensity of VII corresponds to a ground acceleration of roughly 0.15 g, fifteen percent of gravity [Wakabayashi 1986, p. 9]. Fifteen percent corresponds to an aspect ration of 6.67. Therefore, based on the historical record, one would expect that columns with an aspect ratio less than 6.67 should be standing, and observation finds supporting examples. Figure 2.1 shows photographs of two columns located on Via del Foro, just north of the Forum. The left photo shows the columns as observed in 1997, and the right photo includes annotation which graphically illustrates the column proportions using stacked squares.

Figure 2.1: Columns at the Via del Foro. The aspect ratio is approximately 5.9 for the column with plaster (in the foreground), and approximately 6.3 for the column without plaster

The column on the right has an aspect ratio of approximately 6.3, corresponding to an overturning acceleration of about 16 percent g; it's lack of plaster coating makes it more slender than the column on the left, whose aspect ratio is approximately 5.9, corresponding to an overturning acceleration of about 17 percent g. The fact that these columns are standing is consistent with the assertion that Pompeii has not experienced ground accelerations greater than 15 percent g. Similar evidence can be found in columns at the Marina gate, as shown in figure 2.2.

Figure 2.2: Columns at the Marina Gate. The width-to-height ratio is approximately 1:5.9. Note the visible lean in the foremost column.

The three standing columns each has an aspect ratio of approximately 5.2, corresponding to an overturning acceleration of 19 percent g. Note that the leftmost column is leaning visibly, which reduces the acceleration required to overturn it toward the leaning direction.

Of course, the condition of these columns does not mean there has been no seismic damage since the excavation. The 1980 earthquake damaged many walls, and severely tilted others [Triches 1982, p. 75], however this damage does not contradict the conclusion that ground motions have not exceeded 0.15 g. Many walls at Pompeii have height-to-thickness ratios as high as 8 or 9, and many are tilted with foundation settlement or deteriorated with age. It is credible that an earthquake with accelerations of 0.12 or less could damage such walls, while leaving the cited columns standing.

2.3 Summary

Reviewing the historic record both before the eruption and after the excavation reveals that major damage in the building is likely the result of one of the following three events:

Since the excavation, eruptions have not affected the city, and the intensity of earthquakes has been limited to damaging only vulnerable elements.

2. Identifying Potential Damage Events

2.1 Events up to the 79 Eruption

2.2 Events after the excavation

2.3 Summary

Chapter 2 Notes