Wednesday, January 25, 2017

Historical records help scientists pinpoint 1904 earthquake

By learning about strong earthquakes from the past, we can determine where and when they will occur in the future. Geophysics professor Carl Tape has put this theory to work, using historical instrument readings and records to learn more about a little-known 1904 earthquake.
“Every earthquake is a different story,” Tape said. “Characterizing them is a focus of my research.”
On Aug. 27, 1904 a large earthquake of about 7.4 in magnitude, struck somewhere in the Interior of Alaska. Until recently, there were no known written accounts of shaking from the earthquake. Tape and his collaborators have discovered five previously undiscovered accounts with details about the earthquake.
In the Interior, people might not be prepared for a large earthquake as there have not been any major ones in recent history. Understanding the 1904 earthquake could help people be better prepared for such an event today, according to Tape.
Josh Hartman - Color Instrumental   Data.jpg
When an earthquake occurs, it produces two different waves through the ground, “P” and “S” waves. The graphic shows the relationship between the epicenter of the earthquake in Alaska and seismology stations that recorded the earthquake around the world. The graphic on the left shows the relation for P waves and the right does so for S waves. In total the earthquake was recorded by 45 different stations in 1904. Image courtesy of Tape.
Earthquakes occur when two tectonic plates—the large masses of land that make up the ground—occasionally get stuck while sliding against each other. Eventually these stuck plates “slip” and release a lot of force, causing an earthquake.
“In the last few decades, we have stations that allow us to locate earthquakes fairly well. When you go back further in time like 1904, it gets harder,” Tape said.
Usually to learn about earthquakes from the past, scientists can look for evidence such as scars on rocks from faults, damaged structures (like chimneys) or evidence of tsunamis caused by earthquakes.
To understand the 1904 earthquake, Tape and other scientists combed through historical documents trying to find any “felt reports” or written records describing the earthquake. They were able to find five documents, including one from the diary of James Wickersham, who secured land that now makes up the UAF campus.
After compiling these felt reports and making an estimate for where the epicenter—the origin—of the earthquake was, they looked at instrumental data.
“I couldn’t believe it when I started looking at this earthquake that there was actual instrumental data to work with,” Tape said.
Josh Hartman - Felt Reports.jpg
“Felt reports” are historical documents describing how strong an earthquake was. Carl Tape and his colleagues looked through many documents including these two 1904 newspaper clippings from Rampart (Yukon Valley News) and St. Michael (Nome Semi-Weekly News) respectively. Images from microfilm collection at the UAF Rasmuson Library, courtesy of Carl Tape.
In 1904, the world had very recently started recording information about earthquakes and there were enough instruments to accurately record a large earthquake anywhere on Earth, according to Tape.
Using data recorded in 1904 by 45 stations around the world, Tape and other scientists were able to come up with a more accurate location for the epicenter. The team was also able to determine when the earthquake occurred to within 10 seconds. This surprised other seismology researchers, Tape said.
He had an activity for his presentation to show uncertainty.
Josh Hartman - Color James Wickersham.jpg
Earthquake at 12 oclock. Non. From S.W. to N.E. and quite strong for several minutes — no damage but everybody ran outdoors. Busy closing up court matters + nothing left that I can force to be finished.” Reads a felt report in the personal diary of US Federal Judge James Wickersham in Fairbanks 1904. Image courtesy of the James Wickersham Papers Manuscript Collection, Alaska State Library.
“Raise your hand if your clock says it’s 7:02 right now. What about 7:03?” Tape asked the room. “Alright, we already have an uncertainty of sixty seconds of our clocks in this room.”
With modern instruments, an earthquake can be timed to within a tenth of a second to when it started, according to Tape.
Tape will be presenting his team’s research in the first Science For Alaska lecture on Tuesday, Jan. 31 at 7 p.m. in the Raven Landing Center, 1222 Cowles Street.
The lecture series will continue at 7 p.m. every Tuesday until March 7 at that location.
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