Dawn C. Martindale

1. The TenBroek and Magrudes Letters: The Local Interpretations of the Great 1857 Fort Tejon, California Earthquake Dawn C. Martindale

2. Acknowledgements • Research Team • Geologists • Jim Evans • Historian • Dawn Martindale • Funding • Grant from the United States Geological Survey, Earthquake Hazards Reduction program

3. 1857 Fort Tejon EarthquakeBackground • One of 10 largest earthquakes in known U.S. history • Presently, estimated to be near 8.0-8.1 magnitude. • Struck primarily along San Andreas Fault, osculating motion • 2-3 Foreshocks between 4-6 A.M. • Main Shock at approx 8:30 A.M. • New results pending current analysis

4. Motivation for Reexamination • Previous Examinations • D. Agnew and K. Sieh, 1978 • Last in-depth study • 2002-2004 Martindale & Evans Study • Integration of history and geology to locate new materials • Result in update of earthquake • Insight into nature of shaking, including possible future threats

6. Methodology of Project • Implement historical research methods to integrate with geological analysis techniques • Review various collections • Newspapers, Journals, Correspondence, and other archival materials • Analysis • Preliminary and Data-based • Examine individual felt reports per area and catalog individually to create an composite of the area • Human perception by geographical site

7. Mercalli Index Assignment • “Branches of trees were broken off, and large oaks fell to the ground.” -Stockton Daily Argus, 16 Jan 1857 MMI: VIII “trees shaken strongly, branches broken off”

9. 1884 Bear Lake Utah Earthquake, Intensity Map

10. P.G.S. TenBroeck and W. T. Magrudes Letters

11. Accounts from Fort Tejon • Recorded Event • Lieutenant Colonel B.L. Beal • Major General John E. Wool • Thomas Turner Fauntleroy, Commanding Officer of 1st Dragoons • Assist General D.R. Janco • Assist Surgeon, P.G.S. TenBroeck • Table Analysis • Quartermaster William Thomas Magrudes • 12 letters (all include tables) • Mr. Cannady, rider sent from area to other locations • Common account reported in various newspapers

12. William Thomas Magrudes, Quartermaster • Wrote series of letters to Major W.W. Mackall, Department of the Pacific • Letters brief and include notation of felt shocks from September 1857 to February 1858 • Several letters include letters and tables from TenBroeck. • Information on life and duties of Magrudes continues to be gathered.

13. Colonel E. F. Beale’s Home (photo courtesy of Fort Tejon State Park Museum) “Since my last report I have the honor to report rather a heavy shock Of an earthquake at this post on the 23 ult at 1:15 p.m. No effect Was produced.” -3 Jan 1858 “I have the honor to report the following shocks of earthquake at this Post.” Lists five earthquakes” heavy to slight shock and time. -12 Nov 1857

14. Peter Gerard Stuyvesant TenBroeck, Assistant Surgeon • Wrote letters to Col. Thomas Fountberry, 1st MS Dragoons and Brig. Gen Thoshawson, Surgeon General • Detailed letters of structural damage and human perception • Included extensive tables that record daily aftershocks for at least six months • Additional tables are included in Magrudes letters

15. Background of TenBroeck • Born 8 October 1822 to Reverend Petrus S. TenBroeck and Lucretia L. Cutler. • Claims to be from Maine while stationed at Fort Tejon. • Assistant Surgeon at Fort Tejon, California. • Rise to rank of Major, Surgeon by end of Civil War. • Probably served for either Maine or New York. • Married Mary Octavia Wood after the war. No children. • Died 19 December 1867 • Character: • Letters reveal a detailed individual inquisitive about world around him. Fort Tejon, ca 1855 (Map courtesy of Fort Tejon State Park)

16. Felt Reports from TenBroeck19 January 1857

17. “Our quarters here are much injured, some of them being Completely ruined, but the peculiar make of the roofs, have Saved the most of them, from utter destruction. “

18. “My hospital is a wreak, and my sick have been in a hospital tent since the first Shock. I had just put my last received medicines Upon the shelves of the Surgery, and the Earthquaque has made a sad mixture of them….[I] was unable to keep my feet, being thrown upon my face. And men were thrown from the top to the bottom of the Hospital stairs.”

19. “Just above the garrison oak trees, 8-10 feet in diameter were snapped off near the ground. One thing I remarked during the Shock, that the trees about me instead of bending in the direction of the vibration, seemed to bow themselves into a common centre.”

20. TenBroeck Table, 3 July 1857

21. Record aftershocks noticed by himself or hospital staff only • Oscillating shocks primarily, with an occasional vertical one. • Provides geographical range of earthquakes since main shock and the continual diminishing circle of aftershocks. • “No perceptible dimination in either frequency or intensity. The only difference is, that we have become accustomed to them, and they do not affect us as at first.”

22. Conclusion • Results of TenBroeck and Magrudes’ Letters • Provide unique local accounts • Tables and charts • Detailed information not found in other sources • Human perception and adaptation to natural disasters • Insight into present-day attitudes • Earthquakes, tornados, etc. • Possible danger resulting for commonality of event • Preliminary results update Agnew and Sieh (1978) study • Upgrade rating to possible X-XI (MMI). • New quantifications into the nature of shaking • Continued research will quantify ground acceleration (PVGA/more than 1G)

23. Future Objectives • Obtain remaining letters in collection to include in final analysis • Publish in-depth examination of earthquake affects in region and overall.