# Presentation: 180475

## Earth’s Interior

- Visual 2.1

**Notes:**
Visual 2.1: Depiction of structure of the interior portion of the earth. The primary focus for this lesson is on the lithosphere and asthenosphere.

## Oceanic and Crustal Plates

- Visual 2.2

**Notes:**
Visual 2.2: Depiction of typical relationship between the lithosphere and asthenosphere, as well as difference between heavy oceanic crust and lighter continental crust. Lighter continental crust tends to “float,” and heavy oceanic crust sinks or subducts below lighter continental crust when they collide. Continental crust is typically composed of silicic or granitic rocks with lighter minerals such as quartz and feldspar, whereas oceanic crust is colder and denser, typically consisting of mafic or basaltic rock rich in heavy mineral such as pyroxene or olivine. 

## Convection Drives the Plates  

- Visual 2.3

## Locations of Earthquakes

- Visual 2.4

## Earth’s Major  Plates

- Visual 2.5

## Plate Tectonics Animation

- Visual 2.6

## San Andreas Fault – Well-Known Plate Boundary

- Visual 2.7

**Notes:**
Visual 2.7: Aerial photograph looking northward along the San Andreas Fault, the plate  boundary between the North American and Pacific Plates. This fault system is responsible for most of the seismicity in California. Note that there are at least two prominent continuous fractures that can be seen in the photo. There are many smaller faults associated with the San Andreas Fault System, as would be expected where two major plates meet.  The San Andreas Fault involves mostly right-lateral strike-slip type faulting movement. Photo credit: USGS

## Map of Plates In the Western US

- Visual 2.8

## Subduction Zone

- Visual 2.9

## Elastic Rebound Theory

**Elastic Rebound Theory**

- Visual 2.10

**Notes:**
Visual 2.10: Visual one of three consecutive figures depicting the build-up of strain due to  tectonic forces and sudden slippage that releases strain energy in  an earthquake. Tectonic forces (red arrows) are typically associated with plate boundaries. What produces seismic waves? The rocks that generate earthquakes have elastic properties that cause them to deform when subjected to tectonic forces (red arrows) and to “snap back” and vibrate when energy is suddenly released. During the rupture, the rough sides of the fault rub against each other. Energy is used up by crushing of rock and by sliding friction. Earthquake waves are generated by both the rubbing and crushing of rock as well as the elastic rebounding of the rocks along adjacent sides of the ruptured fault.

## Visual 2.11

**Notes:**
Visual 2.11: Visual two of three consecutive figures depicting the build-up of strain due to  tectonic forces. The rocks that generate earthquakes have elastic properties and will deform elastically, building up strain energy, in response to the steady tectonic forces (red arrows).  The rocks will continue to build up strain energy to a point... 

## Visual 2.12

**Notes:**
Visual 2.12: Visual three of three consecutive figures depicting the sudden slippage that releases strain energy in an earthquake. When the interface resistance along the fault is exceed, sudden slippage occurs and the rocks  “snap back” and vibrate when energy is suddenly released– we feel the effects of this motion as an earthquake. Earthquake waves are generated by both the rubbing and crushing of rock, as well as the elastic rebounding of the rocks along adjacent sides of the ruptured fault. The relative movement along the ruptured portion of the fault results in permanent ground displacement (see offset fence line). 

## San Andreas Fault Offset- 1906 S.F. EQ

**fault trace**

**fence offset from **
**fault movement**

- Visual 2.13

**Notes:**
Visual 2.13: Fence shown above was offset during fault movement associated with the 1906 San Francisco Earthquake. Photograph shows evidence of fault movement along the San Andreas fault during the 1906 San Francisco Earthquake. Photo credit: USGS. 

## Magnitude vs. Ruptured Fault Length

- Visual 2.14

**Notes:**
Visual 2.14: Data source: USGS

## Types of Faults

- Visual 2.15

**Notes:**
Visual 2.15: Active faults: faults that have moved in the "recent" geologic past (i.e., during last 11,000 years), or that have a certain average slip rate per year (i.e., 1 mm/year). This definition, in terms of age or slip rate can vary, depending upon the specific project and/or the governing agency.  Fault movements can be determined from modern observations or prehistoric geologic evidence from paleoseismic studies. 
Inactive or dead faults: faults that have not moved in the recent geologic past and are judged to no longer be capable of producing earthquakes. 
Capable faults: faults that can produce felt earthquakes (slippage along faults in hard rock). Evidence of faulting may or may not be visible at surface, depending on the sediments overlying the fault.  In many areas, hard, competent rock capable of generating earthquakes is typically encountered at a depth of about 5 km.  
 - Large, capable faults usually require a certain, critical level of stress to be exceeded before relative slip occurs (based on strength of rock, frictional characteristics, etc.). Therefore, these faults tend to produce large, recurring earthquakes of about the same size. These earthquakes are referred to as characteristic earthquakes.
 - Faults can move aseismically. That is, no earthquake will occur if there is slow and steady creep such that the "lock up" of the fault does not allow stresses to build up to the critical slipping point. Figure Credit: USGS