The Earth Story

Watch how the clouds sometimes punch through to a higher layer in the atmosphere - that’s the part where it becomes a “pyrocumulus”, the cloud is going so high under its own energy that it begins condensing water and forming a normal water cloud.

Folded and warped These sedimentary rocks in the southern Australian outback date from the latest Precambrian, and were deposited some 540 million years ago in a basin, before being uplifted into a mountain range and eroded into soft hills. The region is known as the Adelaide Rift Complex (or to use an older, now mostly abandoned, vocabulary geosyncline) and flows from the flinders ranges, through the Fleurieu Peninsula (of wine growing terroir fame) southwards to Kangaroo Island.

I don’t know that I’m a fan of the camera motion on this one, careful if you’re sensitive to too rapid fire of motion. But, it’s a landscape video that seems to have won a ton of awards for the filmmaking, so see what you think. Original caption:

Original caption:

The hills of East L.A.

In 1987, a fault east of Los Angeles let loose a magnitude 5.9 earthquake that caused over $350 million in damage and 8 deaths. In addition, in 2008 there was a magnitude 5.4 earthquake nearby, centered in a location known as the Chino Hills (shown in this photo).

These earthquakes illustrate some fundamental properties of the Los Angeles Basin and thus are worth a bit of extra discussion. By analyzing the properties of the earthquake in detail, geologists can determine the kind of fault it occurred on. All of these earthquakes take place on faults that are mixed – they’re mostly thrust faults, with a bit of strike-slip (side to side) motion on them. The strike-slip component makes sense because these faults are a short drive from an enormous strike-slip fault; the San Andreas. The stress from giant fault permeates the area, sometimes adding a small strike-slip component when faults do move.

The thrust fault component tells the other part of the story of the L.A. Basin. The mountains surrounding L.A., like the San Gabriel Mountains and the Santa Monica Mountains, started their lives far to the south, somewhere near San Diego. As the San Andreas Fault began to move, these rocks were ripped from the coast to the south and pushed north. Eventually, the rocks ran back into the continent, causing them to be squeezed upwards.

This compression forced mountains like the San Gabriels to shoot upwards rapidly, rising from sea level to over a thousand meters in only a couple million years. The forces that built these mountains impact the entire area; creating faults throughout the L.A. basin.

Almost every set of hills in the basin is created by a thrust fault resulting from this collision. The force that built the mountains north of L.A. is propagating into the basin, creating faults everywhere, beneath everyone’s feet. 10 million people live on top of rocks penetrated by these faults, causing the ground to, on occasion, rock and roll.

These faults are capable of producing much larger events as well, including the Northridge earthquake in 1994. Small quakes like this should serve as a reminder of the necessity of preparation for the L.A. basin; bigger ones are coming.

-JBB

Image credit: http://www.flickr.com/photos/modern_artifacts/4719307111

http://earthquake.usgs.gov/earthquakes/eventpage/ci15481673#summary http://earthquake.usgs.gov/earthquakes/eqinthenews/2008/ci14383980/#summary