I'm trying to understand why in the Loma Prieta quake the fault didn't crack through the earth, yet the earth cracked. It is very confusing. I've read several sites, have watched one video, and consistently it's stated that one of the surprises of this quake was that the fault didn't crack through the earth. Yet there are many pictures of the earth with many cracks in it, but they don't count. I guess they don't count because the cracks went from the crust downward due to various effects of the quake.

Anyway, I'm going to paste an especially confusing paragraph from some of my reading to see whether anyone here can crack it open in elementary terms:

"A somewhat surprising aspect of the October 17, 1989 Loma Prieta earthquake was the lack of recognizable surface faulting or rupture along the trace of the San Andreas Fault. Aftershock patterns (see the first page) demonstrated that actual fault rupture teminated approximately 5 km below the ground surface. Although no throughgoing fault rupture occurred at the surface, a 5 km wide zone of complex ground cracking formed along the summit and seaward flank of the Santa Cruz Mountains in the Summit/Skyland Ridge area between Highway 17 and the forest of Nisene Marks. These ground cracks varied widely in dimension and lateral extent, often occurring in complex interconnected patterns both along the upper slopes of ridges as well as along ridgetops themselves. "


OK. If I understand this, the fault cracked well below the earth's surface. And the ground cracking was simply consequential cracking, but not the fault.

Here's the url from the quote:

http://www.es.ucsc.edu/~es10/fieldtripEarthQ/Damage3.html


Thanks for any elucidating comments.
WW

I'm guessing here, as I'm no seismologist or geologist or any kind of gist....
imagine a bowl full of flour: the bowl breaks and the surface of the flour will show the effects of that break, but it wasn't really the flour that broke(cracked).
maybe...?

Aftershock patterns ... demonstrated that actual fault rupture teminated approximately 5 km below the ground surface.
Apparently, this earthquake was "unusually deep" - 11 miles deep compared to "typical California earthquake focal depths" of 4 to 6 miles, as this passage explains:

Surface Effects

Surface displacements with offsets of up to 3 or 4 feet along a zone about 20 miles long would normally be expected to accompany an earthquake of this magnitude. Instead many cracks have been found over several discontinuous and indistinct zones. There are several possible explanations for this lack of clear surface expression. The earthquake was unusually deep, making it difficult for the bedrock rupture to propagate to the ground surface. The combination of rugged topography, thick soil, and forest cover could also make surface breaks less distinct. The State Commission report of the 1906 earthquake described very similar surface rupture characteristics along the Santa Cruz Mountains portion of the San Andreas Fault.

Thanks, troubadour.

This was also from the same site:

The earthquake hypocenter occurred at a depth of 18km below the surface.

So the quake generated at this great depth? I've never heard of a hypocenter till reading the info. on the site.

I would like to have just a little understanding of why one plate moves north and the other south here. Clueless so far from my reading.

I would like to have just a little understanding of why one plate moves north and the other south here.
Does this help, Wordwind:
Three types of movement are recognized at the boundaries between plates: convergent, divergent and transform-fault.

At transform-fault boundaries, plates move horizontally past each other. The San Andreas Fault zone is an example of this type of boundary where the Pacific Plate on which Los Angeles sits is moving slowly northwestward relative to the North American Plate on which San Francisco sits.
Or this from another source:
The San Andreas fault in California is a nearby example of a transform, separating the Pacific from the North American plate. At transforms the plates mostly slide past each other laterally, producing less sinking or lifing of the ground than extensional or compressional environments.

etaoin-- <GRIN>

some of earth quake geologoly can be understood as blankets on a bed...
think of a nicely layered bed, with some sheets, and comforters, and some small afghans... they all move differently, the sheets slide better.. the small afghans might tend to lump more, and the comforter, which is several layers thick, has all the layers moving as one...

every time something moves in the bed, the different layers move (earthquakes.) the sheets can get wrinkled and lumpy, but the comfortor might not be as wrinkled.

and you can go under the covers (subductions,) that causes the sheets and all the other layers to rise up, (the coastal mountains of california, in all there different names)

you can lie on top of the covers, (and they get crushed, and wrinkled all the same... or you can push them diagonally... (when you get over heated, and twist the covers so you arms and sholders have a covers, but you legs are exposed..)

you can push the covers aside and bunch them up where india meet the main land of asia, the two continents are just bunching up and making the himalyas.. where as in california, one layer (say your body) is sliding under other layers (the covers) and at the same time raising the upper layer --

and finally, if say the bedframe or box spring cracked or broke ( a deep earth quake) there might be some wrinkles on the top of the bed, but not the same sort as if someone had slept in the bed... so deep earth quakes look different.

can you visualize the movements better now?

Can you visualize movements better now, asks of troy...

Yep. That worked well, especially the bedframe's cracking.

But I do find the lateral movement most interesting of all the movements: one tectonic plate moving north against another moving south.

Subduction makes a lot of sense; and so does its near opposite that causes rifts and even eventual formation of continents that have been pushed apart. But lateral movement--one plate crunching against another moving north and the other plate crunching against the former moving south....well, that just blows my mind.

Magma pushing huge chuncks of land apart through rifts just makes mechanical sense. And Subduction makes a tremendous amount of sense, particularly when an ocean plate moves against a continental plate and is subducted because ocean earth is so much heavier and denser.

But back to lateral movement: To consider two enormous plates comprised of all kinds of soil and bedrock layers moving very, very hard against each other--one moving north and the other south. Why? Why would one tectonic plate move north and the other tectonic plate move south? Some huge force underneath the earth causing one plate to move north and the other south...but what's really going on there? Although in the case of the Loma Prieta quake the Pacific plate did rise upward...

Fortunately, there are hundreds of sites that offer the kind of elementary explanations I'm seeking. But I still haven't found a single site that really explains why tectonic plates should move laterally against each other--you know, the cause and effect kind of thing. I'll keep plowing away on google...

Thanks for the bed cover metaphor, of troy. That's something good to pull into the maelstrom of images I have going on to try to get some working visual image that works.

WW


PS: The thought just occurred: I wonder whether tectonic plates might move laterally when they're composed of equally dense material? For subduction to occur, I think one plate has to be denser than the other and is therefore subducted under the other. Maybe with laterally moving plates both plates are equally dense...and so neither really gives into the other and their push against each other causes opposite lateral movement...? Just a thought. Oh, well. On to the tectonic sites for children...