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<span style="margin: 0px; font-weight: bold; font-size: 11pt; font-family: Times, "Times New Roman", serif; color: rgb(200, 38, 19); text-indent: 0.5in;">The Chilean Earthquakes of 1822 and 1906:</span><span style="margin: 0px; font-size: 11pt; font-family: Times, "Times New Roman", serif; color: rgb(200, 38, 19);"> </span><span style="margin:0px;font-family:Times;color:rgb(200, 38, 19)"><b><span style="margin: 0px; font-size: 11pt; font-family: Times, "Times New Roman", serif; color: rgb(200, 38, 19);">Searching
for Pre-Geodetic Evidence of Land Level Change in Historical Nautical Charts to Constrain Rupture Models</span></b></span><span style="margin: 0px; font-family: Times; color: rgb(200, 38, 19);"><b> </b></span><span style="margin: 0px; font-family: Times; color: rgb(200, 38, 19);">
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<font color="#c82613" face="Times"><span style="color: rgb(200, 38, 19);"><b>Rob Wesson—USGS Emeritus</b></span></font></div>
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<span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">While the megathrust zone along the coast of Chile is well-known for producing giant earthquakes with massive local and Pacific-wide tsunamis, including those of 1960 and
2010,</span><span style="margin:0px;font-size:11pt;font-family:Times"><span style="margin: 0px; color: rgb(200, 38, 19);"> </span></span><span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">the section of the megathrust
just off the coast from the city of Valparaiso—that is, the section closest to the most populous region of Chile—remains enigmatic. Historical records suggest that this section produced a massive earthquake in 1730 (M</span><span style="margin:0px;font-size:11pt;font-family:Times;color:rgb(200, 38, 19)"><sub><span style="margin: 0px; color: rgb(200, 38, 19);">w</span></sub></span><span style="margin:0px;font-size:11pt;font-family:Times"><span style="color: rgb(200, 38, 19);"> </span></span><span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">9.1
to 9.3) with a rupture zone 600-800 km long and large local and Pacific-wide tsunamis. Since 1730, earthquakes in 1822 (M</span><span style="margin:0px;font-size:11pt;font-family:Times;color:rgb(200, 38, 19)"><sub><span style="margin: 0px; color: rgb(200, 38, 19);">w</span></sub></span><span style="margin:0px;font-size:11pt;font-family:Times"><span style="color: rgb(200, 38, 19);"> </span></span><span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">8.5?),
1906 (M</span><span style="margin:0px;font-size:11pt;font-family:Times;color:rgb(200, 38, 19)"><sub><span style="margin: 0px; color: rgb(200, 38, 19);">w</span></sub></span><span style="margin:0px;font-size:11pt;font-family:Times"><span style="color: rgb(200, 38, 19);"> </span></span><span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">8.2),
and 1985 (M</span><span style="margin:0px;font-size:11pt;font-family:Times;color:rgb(200, 38, 19)"><sub><span style="margin: 0px; color: rgb(200, 38, 19);">w</span></sub></span><span style="margin:0px;font-size:11pt;font-family:Times"><span style="color: rgb(200, 38, 19);"> </span></span><span style="margin: 0px; font-size: 11pt; font-family: Times; color: rgb(200, 38, 19);">8.0)
have produced only modest local tsunamis, leading to the hypothesis that the ruptures during these earthquakes were somewhat deeper and involved less, if any, slip on the shallower, seaward portions of the megathrust. Historical evidence for land level change
associated with these earthquakes offers the possibility of constraining the location and depth of their ruptures. Historical nautical charts with soundings showing water depth in Valparaiso and Quintero Bays extend as far back as 1790. Analysis of these charts,
particularly the oldest ones, presents a host of challenges, especially georeferencing. Possible sedimentation in the bays must also be considered. Notwithstanding all the resulting uncertainties, the nautical charts at Valparaiso provide reasonably strong
support for one to two meters of uplift at Valparaiso in 1906. They also offer suggestive evidence of an uplift of a similar size in 1822 and of a somewhat surprising lack of rebound in the following decades.</span></p>
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<span style="margin:0px;color:rgb(95, 95, 95);opacity:0.36">________________________________________________________________________________</span></div>
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<span style="font-size: 13.3333px;"><i>We look forward to seeing you tomorrow,</i></span></div>
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<span style="font-size: 13.3333px;"><i>The GHSC Seminar Committee: Oliver Boyd, Francis Rengers, and Josh Rigler</i></span></div>
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