1906 San Andreas quake revisited - methods ancient & modern Using a combination of the most

1906 San Andreas quake revisited - methods ancient & modernUsing a combination of the most recent geophysical observations of the 1906 San Andreas fault rupture and the very oldest historical information on the quake, collected in the days immediately following it, researchers have identified the exact location of a portion of the fault, re-drawing the geological map of the area.The magnitude 7.8 earthquake that hit San Francisco at 5:12 am 18th April 1906 is one of the most significant ever, since it was the first such event that was subsequently subjected to detailed geological examination and survey. Although it preceded the discovery of plate tectonics by a half century, the use of photography and geological surveys carried out in the days, months and years after the quake mean that useful data remain through to today on the geological nature of the event. Measurements of strain rebound after the earthquake led to our understanding today of the nature of the earthquake cycle, for example.An analysis published this week in the Bulletin of the Seismological Society of America by Chester and Robert Wrucke (of Portola Valley) with Ted Sayre examines new evidence for the details of the trace of the rupture through Portola Valley, 70 km southeast of San Francisco and 30 km west of San Jose. A century ago it was an area that professors, students and photographers from Stanford University had easy access to, and they recorded the damage produced by the event in 1906.Some of the most recent geophysical observations of the area have been made using the LiDAR (Light Detection and Ranging) technique made available through the EarthScope programme. Combining Lidar data, field observations and original 1906 photographs the researchers have reinterpreted the earlier results and located the 1906 trace precisely.The original survey was conducted by J C Banner, geology professor at Stanford, and concluded that the earthquake ruptured through Portola Valley along a single fault trace, but the trace was mapped with unintentional errors in its location.Subsequent mapping, later in the 20th century, altered the location of the fault and suggested that the earthquake ruptured along multiple traces in the area.Going back to Banner’s original description, the original photographs, hand-drawn field slips and a geological notebook compiled back in 1906, and comparing the original data with today’s Lidar images, the new study demonstrates that there was no step-over in the fault rupture through the town.The correct fault position has been determined, which will help guide building in the area in the future. But the study also demonstrates the benefit both of careful records of original observational data, a skill still highly prized in geology graduates today, as well as the insights offered by the latest techniques such as Lidar.~SATR (on twitter @sim0nredfern)Image: 1906 photo of the San Andreas fault rupture tracing across Alpine Rd, Portola Valley.http://www.bssaonline.org/content/103/4/2404.abstracthttp://www.geopoem.com/2013/08/forensics-of-1906-san-andreas-quake.html -- source link

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