Talk:San Andreas Fault

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Possibility of the fault breaking apart[edit]

it is scientifcally possible for the fault to 'break apart' isn't it. is there an estimate when this might happen?

Answer: probably in a million years or so...it moves like a CM a year

Also, the plates are not moving apart, they're moving sideways. -Will Beback 21:34, 29 May 2006 (UTC)

Is there an estimate of how many years it will take for Los Angeles to move so that it is parallel to San Francisco?

The arithmetic: 344 miles (554 km) divided by 1cm per year (given above) = 554 X 1000 X100 == 55.4 million years (roughly) - probably another 15-25my to separate into an island, a 2 or 3 hundred my to collide with Alaska. (This seems consistant with some numbers I saw on a Nova program, which I do not recall exactly). - Leonard G. 00:44, 29 September 2006 (UTC)
Addendum - rather than colliding with Alaska the island would likely be subducted into the Aleutian Trench. - Leonard G. (talk) 03:06, 11 August 2010 (UTC)

Added the new study from the journal Nature[edit]

Hey, everyone, I just added the new study by Yuri Fialko of Scripps Institution of Oceanography at the University of California, San Diego, which states that "the next big one" (magnitude 7.0 or greater) is "overdue" in southern California. Glad I live in Oklahoma. Take care. :) User:ProfessorPaul 01:14, 22 June 2006 (UTC)

Notice of availability of a new template[edit]

A new template is available, which can make for interesting reading in this article if properly used. The template, {{Days elapsed times factor}}, parses as follows:

{{ Days elapsed times factor | Year | Month | Day | Factor | Decimal places }}

This template calculates the amount of change since a given date. For instance, a template filled in like this…
{{Days elapsed times factor|1989|10|17|0.009582|0}}
…generates this numeric value, which automatically changes over time: 87

You might ask “Why is that nice?” Because one can use it to generate text like this:

The virtue of this is you don’t have to go back and periodically tweak the value. Every 3.4 months, the template will increment the value an additional centimeter. To accomplish this, one need code only as follows:

This strain rate of 33–37&nbsp;mm/year, which is about the speed at which a fingernail grows, may seem small. However, it quickly accumulates. Since the October 17, 1989 [[Loma Prieta earthquake]] roughly {{age|1989|4|18}}&nbsp;years ago, the Pacific plate has moved approximately <u>another {{Days elapsed times factor|1989|10|17|0.009582|0}}&nbsp;cm</u>!

In the above text, 182 days was subtracted off the {{age}} date template so the number of years would round to the nearest value. In the {{Days elapsed times factor}} template, I used the actual date of the earthquake. The 33–37 mm slip rate was averaged and scaled to a centimeter-based value of 3.5 cm/year, and divided by 365.25 to obtain a daily slip factor of 0.009582 centimeters per day. It is always wise to have at least 32 times more precision in a conversion factor, which explains the jump from 35 counts to 9582 counts.

Here’s the “live” slip rate, displayed with excess precision: 87.02372 cm

Sometime on March 22nd, the value (which as of this writing displays as 64.33355 cm and rounds down to 64 cm) will round up to 65 cm.

I will leave it to someone expert in the field of earthquakes to properly use this feature; I’m not sure as to what specific part of the San Andreas fault would be an appropriate place to reference a specific amount of strain.

Greg L (my talk) 00:39, 5 March 2008 (UTC)

Plate motion vectors[edit]

The North American Plate moves in a west to southwest motion as a whole. Nowhere does it move southEAST. That is only a relative motion along the fault line, relative to the Pacific Plate's motion. Tmangray (talk) 19:49, 4 April 2008 (UTC)

I was under the impression that both plates were moving in the same direction, just that one was moving faster than the other. Is this right? —Preceding unsigned comment added by 86.148.78.10 (talk) 23:53, 10 April 2008 (UTC)

The vectors of motion out of the MidAtlantic Ridge are westward to southwestward. I don't believe anyone has yet shown that the North American Plate changes its direction of motion intraplate. There is the still-unproven, but perhaps plausible idea that, assuming an asthenosphere-lithospheric traction force exists, the same mantle current carrying the Pacific Plate along also lies under the western part of the North American Plate, exerting some modification of its overall vector near the margin, and this might somehow be involved with the extensive faulting throughout the west as the underlying tractive force interacts with the southwesterly vector of the overlying plate. But so far, that's only a proposed hypothesis, I believe. Tmangray (talk) 17:23, 16 April 2008 (UTC)

July 2008 Movement and Reevaluation Of Length?[edit]

Yahoo! News reported on July 29,2008:

"As if the San Andreas Fault weren't long and menacing enough, newly found mud pots and mud volcanoes now suggest it extends another 18 miles, going under the Salton Sea and beyond, in the desert southeast of Palm Springs." Article titled "San Andreas Fault Longer Than Thought".

News Article: San Andreas Fault Longer Than Thought

This was after an earthquake estimated at 5.4 strikes near Los Angeles - new from AP:

"LOS ANGELES - A strong earthquake shook Southern California on Tuesday, causing buildings to sway and triggering some precautionary evacuations. There were no immediate reports of damage or injuries. The jolt was felt from Los Angeles to San Diego, and slightly in Las Vegas"

News: Strong quake shakes Southern California

We should updated the main page 'Notable earthquakes' section. —Preceding unsigned comment added by 69.109.221.3 (talk) 20:08, 29 July 2008 (UTC)

Photo says the San Andreas fault is "Clearly Visible"[edit]

This maybe so for a trained geologist. To me it just looks like some random photo of hilly terrain. Its not at all clear that it is a fault line at all. —Preceding unsigned comment added by 86.140.76.247 (talk) 10:03, 28 January 2009 (UTC)

The caption to the photo says 'in the valley floor' and I think that if you look there you can see a linear feature that presumably is the fault. OK, I know I'm a 'trained geologist' and I do agree that 'clearly visible' is overstating it a little, I'd prefer the term 'discernible'. In fact, lovely though this photo is, I think it's probably unnecessary for the article. Mikenorton (talk) 10:29, 28 January 2009 (UTC)

Why does this say the San Andreas Fault ends in Baja California Mexico?[edit]

The San Andreas Fault's southern segment does not go farther south than Salton Sea. This is even stated in this article. It is the Imperial Fault that starts below the southern end of the San Andreas Fault and goes into Mexico. Thanku4playing (talk) 15:12, 12 April 2010 (UTC)


Uh oh.....[edit]

I hear news going around that there is alot of stress on the southern part of the San Andreas fault and southern California is on high earthquake alert and we are to expect a big earthquake within 72 hours or more. Would this be worth noting in the article? It's no hoax because it's even on the internet and listed on a site called quakeprediction.com. In fact, an expert seismologist has found there is alot of stress on the fault that is likely to rupture. --Brats817 (talk) 02:52, 14 April 2010 (UTC)

Predicting earthquakes isn't like predicting thunderstorms. We are nowhere near capable of accurately predicting an imminent earthquake, if such precise prediction is possible at all. Keep in mind that geological timescales are much longer than, say, most meteorological phenomena; "soon" in geology can mean decades, centuries, or even millennia. In other words, anyone claiming to be able to predict an earthquake within a period of a few days is full of it. 74.178.168.216 (talk) 23:13, 21 April 2010 (UTC)
I think history has proven the folly of posting "Big earthquake within 72 hours" messages. 72.67.11.189 (talk) 01:37, 11 August 2010 (UTC)

Is there a correlation between Southern California earthquakes and Northern California earthquakes?[edit]

For example if Southern California has its big one, wouldn't we expect a Northern California to have its big one as well? Evidence includes several minor earthquakes and the 1989 Loma Prieta that Southern California experiences that Northern California then experiences a few hours or days after. In this context, I would ask if Southern California and Northern California should expect earthquakes due to the quake swarms in Baja?

  • To answer your question, there is no correlation such as you suggest. The San Andreas fault is one of the boundaries between the North American tectonic plate and the Pacific plate. The two plates are moving alongside each other in opposite directions. San Francisco is on the southeast-moving North American plate; Los Angeles is on the northwest-moving Pacific plate; the two cities are thus moving toward each other, at the rate of about one inch per year. (They're literally "inching" toward each other.) Earthquakes happen along the fault because a particular section of one of the plates gets stuck on the other plate. Sooner or later, one or both of the plates gives way, and the section of plate that was stuck lurches forward more or less to where it would have been if it hadn't gotten stuck. That is what causes the earthquake, as the surface of the planet responds to the sudden movement deep underground. (The principle is similar to what happens when someone is standing on a stool that suddenly tips over; the "rupture" down below causes the "quake" above.) Any section of either tectonic plate, in either northern or southern California, can get stuck at any time; and subsequently rupture at any time. An earthquake along the southern California section of the San Andreas Fault can just as easily be followed by another southern California earthquake as by a northern California earthquake.
  • Besides which, here in San Francisco, we have about a dozen extremely minor earth quakes every day. So, there is clearly no correlation between northern and southern California earthquakes along the San Andreas Fault. It's just a random question of where the next piece of tectonic plate happens to get stuck, to build up pressure for a while, and then suddenly release the energy.

71.198.146.98 (talk) 03:31, 27 August 2011 (UTC)

Recently added section on future hazard[edit]

I reverted recent edits, because they broke up and deleted part of the section on evolution of the fault system. An additional paragraph was added but I'm not sure where it would fit best -

"Seismologists predict that a repeat of the 1906 San Francisco Earthquake on the Northern segment of the San Andreas Fault could kill 6,000 or more people in San Francisco, California and other areas, shatter and burn 45,000 structures in San Francisco, and cause hundreds of billions of dollars in property and economic damage in the worst case scenario, if the quake strikes at 2:00 p.m., during rush hour, since the fault runs right through San Francisco. [1]"

Perhaps we need a new section on future earthquake hazard? Mikenorton (talk) 23:32, 3 January 2011 (UTC)

Southern terminus? -- Expert Needed[edit]

This publication says the SA fault branches into several other faults starting at the Cajon Pass: http://pubs.usgs.gov/gip/earthq3/safaultgip.html. Then we have this map which indicates, vaguely, that the SA continues on down to the margin of the map: http://nationalatlas.gov/articles/geology/features/sanandreas.html. And here is more info: http://www.sci.sdsu.edu/salton/San%20AndreasFaultSyst.html. And here is a rather detailed publication: http://pubs.usgs.gov/pp/1990/1515/pp1515.pdf. (This last one indicates there are two parts to the "Indio Segment". page 7) The upshot of my comment is that "The" San Andreas Fault is poorly described in the article and giving it a distinct southern terminus lacks nuance. (Perhaps the article should be titled "San Andreas Fault Zone".) Accordingly, I'm going to tag the article as needing an expert. --S. Rich (talk) 19:52, 30 March 2011 (UTC)20:21, 30 March 2011 (UTC)

The SA bends southeast north of Los Angeles at its "Big Bend" where it intersects the Pleito thrust fault. You can see a map about half way down this page. Due to this bend, a huge amount of fracturing occurs, with the result that there are many faults running under southern California, as shown on the map. The east-west Transverse Ranges were created by folding and faulting from this bend in the plate boundaries. Farther east, the fault curves southward, and actually splits into three faults: the San Andreas on the east side running into the Salton Sea, the San Jacinto Fault Zone, and the Elsinore Fault Zone. The latter two created the fault-block mountains of Orange, Riverside, San Diego, and Imperial Counties. The San Andreas proper ends on the south side of the Salton Sea. At this point, the Gulf of California Rift Zone begins (although a last cracking spur of the San Andreas exists as the San Hills fault). From this point, the plate boundaries are no longer solely a transform fault. They are now a spreading/transform fault plate boundary, which is actually the northernmost part of the giant East Pacific Rise mid-ocean ridge. The first spreading zone of the Gulf of California Rift Zone is right on the southeast shore of the Salton Sea. It's followed by a short strike-slip fault, then another spreading center in the middle of Imperial County near Brawley. The next strike-slip fault is the fairly long Imperial Fault Zone that crosses into Mexico. Following a third spreading zone, you have the Cerro Prieto Fault that runs out into the Gulf of California. The whole centerline of the Gulf is like this... spreading and transform boundaries in a zig-zag. The Gulf is widening, and the Baja California peninsula is moving northwestwards.
Some sources for the southernmost areas of California:
  • Rise and Fall of San Diego: 150 Million Years of History Recorded in Sedimentary Rocks by Patrick L. Abbott
  • Geology of San Diego County: Legacy of the Land by Steven G. Spear, Diane Burns and Lowell Lindsay
  • Geology of the Elsinore Fault Zone, San Diego Region, San Diego Association of Geologists
  • Geology of Anza-Borrego: Edge of Creation by Paul Remeika
Parsa talk 22:55, 26 March 2012 (UTC)
See also Brawley Seismic Zone, Salton Sink geology section and Cerro Prieto. Vsmith (talk) 01:27, 27 March 2012 (UTC)

Discussion at Category talk:Seismic faults of California[edit]

I have opened a discussion at Category talk:Seismic faults of California on whether should be some integrated treatment of all these faults, rather than the current piecemeal approach. - J. Johnson (JJ) (talk) 00:10, 1 June 2011 (UTC)

Re-evaluation of San Andreas Fault System[edit]

The San Andreas Fault became an important geologic concept over 100 years ago. The San Andreas Fault has been divided into 3 segments - southern, central, and northern.The southern segment occurs in a separate structural province, south of the "Big Bend" and the Garlock Fault. Abundant surface, subsurface and geophysical data is available and the presence of strike-slip faults is documented. Re-evaluation is not necessary at this time although geologic problems exist. The northern segment is poorly documented and will not be considered. The central segment has still not been examined in detailed geologic studies. A recent study failed to find evidence of assumed strike-slip faults. The geologic history and three dimensional analyses of geologic maps indicate that a compressive structural regime exists with mini-terranes impinging on a growing continent. The boundary faults of these mini-terranes were lined up on small scale maps to give the impression of a through going continuous fault.

One hundred and twenty two geologic quadrangles were evaluated in central California in the Coast Ranges. All quadrangles were located generally to the east of the San Andreas Fault, but a few did include the fault. Geometric and structural principles were applied in re-mapping these quadrangles. Hundreds of cross sections were constructed to determine the basic structural form of the region. Some rather surprising conclusions were reached. The Hayward and Calaveras Faults are not strike-slip faults. Each fault zone was aligned with a specific terrane, with faults not extending beyond the boundaries of the mini-terranes. The Hayward Fault is the leading edge of the semi-rigid Franciscan Bay block, which impinges on the Great Valley Sequence. A minimum of 35,000' of vertical displacement occurs at the fault. The Calaveras fault represents several different fault segments squeezed out of a highly deformed syncline. Correlations were made at scattered points along the syncline and separate faults were connected on small scale maps to give the impression that there was one continuous fault.

The same practice of "connecting the dots" has been applied to the San Andreas system. Only reconnaisance geologic maps are available for portions of the fault system,and cross sections do not exist to determine the three dimensional nature of the boundary. Re-evaluation of the fault is critical for plate tectonic theory, textbook revision, and for accurate assessment of earthquake risk.

Ron Crane Ron Crane (talk) 21:34, 19 February 2012 (UTC)

It's not clear what you would like to see changed in the article as it stands. Without a source for statements such as "The Hayward and Calaveras Faults are not strike-slip faults", you are unlikely to persuade editors to change the current article. Mikenorton (talk) 22:32, 19 February 2012 (UTC)
It's just soapboxing. I recommend closing. ~ J. Johnson (JJ) (talk) 00:08, 25 February 2014 (UTC)

Use of newspapers as sources.[edit]

A recent citation of a newspaper regarding the probable effects of an earthquake prompts me to remind everyone: newspapers are not themselves authoritative sources regarding scientific interpretation. They generally can be deemed reliable in the reporting of observed effects, but for anything prospective they, at best, quote authoritative sources ("researchers"). Which might be only one researcher's off-hand, and possible wp:fringe, opinion. Agencies like the USGS, CGS, and SCEC do compile secondary reports for the public (and public officials), which are not only soundly authoritative, but often quite interesting, and with a lot of detail most popular media skim over. ~ J. Johnson (JJ) (talk) 00:24, 25 February 2014 (UTC)

Geography of the three zones[edit]

The listing of the locations is confusing. The locations within each of the three zones - Northern, Central, Southern, in that order - are listed in a South-North direction, so there is no continuity. Either the zones should be featured in the reverse order, or the locations within each of them should be listed in the reverse order. Valetude (talk) 11:53, 6 May 2014 (UTC)