It is often said and there is a widespread misconception that the "Balcones Fault" is "inactive", meaning that it no longer moves. First, the idea that it is only one fault is inaccurate; it is more correctly a "fault zone" made up of probably millions of smaller, local faults, some mere meters long, others well up on the order of dozens of kilometers. And they move; the seismograph says so. Consider that there are many pre-existing joints and faults and that would otherwise appear to be 'inactive'. These joints are filled with mud which dissolves the surfaces of these rocks. As the rocks are dissolved their center of gravity and their contacts with adjoining rocks changes or goes away, or rains wash away or introduce clay or other material. Rains and drouths will alter the mud and moisture content. Seasonal heating and cooling of the near-surface rocks will affect their stability. The rock's immediate environment changes over time and it must adjust itself accordingly. It moves. And that movement, if large enough, will register on a seismograph; it's a mini-earthquake. There are thousands of them every day along the length of the (seemingly inactive) Balcones Fault Zone.
It is entirely possible, and even likely, that the slumping going on in San Antonio is a mass of rock spalling along a joint or fault after a lower side-supporting member has been removed through natural processes or quarrying and the recent rains have washed out and/or lubricated any clay binders in the joint. --Ediger on Mon, Jan 25, 2010 at 11:38 PM, SS <[email protected]> wrote: > The neighborhood was probably built on a landfill...lol. > > That's what happens when you cut corners...I hope Centex gets sued into > bankruptcy. > > > -----Original Message----- > From: Geary Schindel [mailto:[email protected]] > Sent: Monday, January 25, 2010 12:48 PM > To: [email protected]; [email protected] > Cc: [email protected] > Subject: RE: [Texascavers] San Antonio sinkholes.. > > Folks, > > This is not a sinkhole but a classic hill slope failure. Quite impressive > as slips go. > > Geary > > -----Original Message----- > From: [email protected] [mailto:[email protected]] > Sent: Monday, January 25, 2010 12:32 PM > To: [email protected] > Cc: [email protected] > Subject: [Texascavers] San Antonio sinkholes.. > > > "Our backyard has shifted about 10 feet, and everything is pushed closer to > our house," said Sara Koenig, 23. > > She and her husband, 27, left early Sunday to study and to volunteer at a > local hospital, respectively. She said neighbors called her at 11 a.m. to > report their back fence had collapsed but her two Chihuahuas were safe. > > > > Near helotes. > > http://www.mysanantonio.com/news/82553087.html > > > --------------------------------------------------------------------- > Visit our website: http://texascavers.com > To unsubscribe, e-mail: [email protected] > For additional commands, e-mail: [email protected] > > > > --------------------------------------------------------------------- > Visit our website: http://texascavers.com > To unsubscribe, e-mail: [email protected] > For additional commands, e-mail: [email protected] > >
