Geobulletin alpha

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Author:	Stéphane Baize
Date:	2016-04-18 00:41:00
Blog:	Stephane on Blogger Earthquakes, geology and related topics... but not only
URL:	http://stephaneonblogger.blogspot.com/2016/04/sightseeing-along-1906-san-francisco.html

Summary:

This earthquake is one if the most known event in the short history of seismology and geology of earthquakes. Even if it has not been the first event to be surveyed in terms of geological effects, it is a crucial landmark in earthquake geology.The NW-SE Tomales Bay, along the San Andreas Fault.As stated by the USGS page dedicated to this quake, "its importance comes more from the wealth scientific knowledge derived from it than from its sheer size". Since 1906, several stronger crustal and continental earthquakes occurred, with larger surface ruptures (e.g. M7.8 Kunlun quake with 450 km of surface rupture). The San Francisco earthquake effects were investigated in an extensive way by a commission of scientists, led by Pr Lawson and including in particular the USGS geologist G.K. Gilbert. The studies were compiled in the Lawson report (1908) which is still today a reference document for geologists, seismologists and engineers. From the observations reported in this document, significant scientific results emerged, such as that faulting causes the earthquakes, such as the Reid's theory of "elastic rebound" (formulated from levelling measurements analysis), or the impact of surface geology on seismic motion and damages.For relevant information on the quake, I recommend the USGS event page and the USGS page showing the significance of this quake in earthquake sciences; as well as the paper by M. L. Zoback (2006), GSA Today, vol. 16. As well, the California Geological Survey website provides the database of intensity observations for the 1906 quake (among others). In 2005, Anthony Lomax proposed a probabilistic relocation of the hypocentral area few kms offshore San Francisco (see his dedicated webpage and his BSSA paper "A reanalysis of the hypocentral location and related observations for the Great California Earthquake).In December 2014, I could enjoy -with my colleague Oona- a visit around the notorious fault section in the Point Reyes area, between Bolinas Bay and Inverness Ridge. This is the area where the famous picture of the 1906 surface rupture was shot by G. K. Gilbert (see below).Turistic map of the Point Reyes National Parkin OlemaPerhaps the most famous and the earliest picture of an earthquake surface rupture, taken by G.K. Glibert close to the Point Reyes Station at the southern end of the Tomales Bay. Biologist Alice Eastwood provides the scale of this large "mole track".Wonderful place: a view of the NW-SE trending Tomales Bay (looking to SE), from Pierce Point Ranch area on the Inverness Ridge. The 1906 Surface Rupture of the San Andreas Fault is running somewhere between the Bay shorelines. Close to Olema, an Earthquake Recreation Area has been set up. A short Earthquake Trail guides you to outstanding places where you can appreciate the surface rupture effects.Close to Olema, the Bear Valley Visitor Center and Earthquake Trail guides you to the surface rupture trace. It is here marked by blue poles. Note the Shaffer Ranch in the distance.At the same spot, the offset (18 feet=5-6 m) of the fence has been preserved.A NW view from Highway 1 of the Stinson beach on Bolinas Bay and the Bolinas Lagoon to the right. The 1906 Surface Rupture was recognized and pictured (by Gilbert, 1907) along the western shore of the lagoon

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California