* Astronomy

Blobrana · L

RE: 1700 Cascadia earthquake

Blobrana · L

Penn Research Helps Paint Finer Picture of Massive 1700 Earthquake

In 1700, a massive earthquake struck the west coast of North America. Though it was powerful enough to cause a tsunami as far as Japan, a lack of local documentation has made studying this historic event challenging.
Now, researchers from the University of Pennsylvania have helped unlock this geological mystery using a fossil-based technique. Their work provides a finer-grained portrait of this earthquake and the changes in coastal land level it produced, enabling modellers to better prepare for future events.
The Cascadia Subduction Zone runs along the Pacific Northwest coast of the United States to Vancouver Island in Canada. This major fault line is capable of producing megathrust earthquakes 9.0 or higher, though, due to a dearth of observations or historical records, this trait was only discovered within the last several decades from geology records. The Lewis and Clark expedition did not make the first extensive surveys of the region until more than 100 years later, and contemporaneous aboriginal accounts were scarce and incomplete.
The 1700 Cascadia event was better documented in Japan than in the Americas. Records of the "orphan tsunami" - so named because its "parent" earthquake was too far away to be felt - gave earth scientists hints that this subduction zone was capable of such massive seismic activity. Geological studies provided information about the earthquake, but many critical details remained lost to history.
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Blobrana · L

Title: Heterogeneous rupture in the great Cascadia earthquake of 1700 inferred from coastal subsidence estimates
Authors: Pei-Ling Wang, Simon E. Engelhart, Kelin Wang, Andrea D. Hawkes, Benjamin P. Horton, Alan R. Nelson, Robert C. Witter

Past earthquake rupture models used to explain paleoseismic estimates of coastal subsidence during the great A.D. 1700 Cascadia earthquake have assumed a uniform slip distribution along the megathrust. Here we infer heterogeneous slip for the Cascadia margin in A.D. 1700 that is analogous to slip distributions during instrumentally recorded great subduction earthquakes worldwide. The assumption of uniform distribution in previous rupture models was due partly to the large uncertainties of then available paleoseismic data used to constrain the models. In this work, we use more precise estimates of subsidence in 1700 from detailed tidal microfossil studies. We develop a 3-D elastic dislocation model that allows the slip to vary both along strike and in the dip direction. Despite uncertainties in the updip and downdip slip extensions, the more precise subsidence estimates are best explained by a model with along-strike slip heterogeneity, with multiple patches of high-moment release separated by areas of low-moment release. For example, in A.D. 1700, there was very little slip near Alsea Bay, Oregon (~44.4°N), an area that coincides with a segment boundary previously suggested on the basis of gravity anomalies. A probable subducting seamount in this area may be responsible for impeding rupture during great earthquakes. Our results highlight the need for more precise, high-quality estimates of subsidence or uplift during prehistoric earthquakes from the coasts of southern British Columbia, northern Washington (north of 47°N), southernmost Oregon, and northern California (south of 43°N), where slip distributions of prehistoric earthquakes are poorly constrained.

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Spoiler

Spoiler

Blobrana · L

The 1700 Cascadia earthquake was a magnitude 8.7 to 9.2 megathrust earthquake that occurred in the Cascadia subduction zone on January 26, 1700. The earthquake involved the Juan de Fuca Plate underlying the Pacific Ocean, from mid-Vancouver Island in British Columbia, Canada, south along the Pacific Northwest coast as far as northern California, USA. The length of the fault rupture was about 1,000 kilometres with an average slip of 20 metres.
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