Tabletop Earthquakes: A Slider Block Experiment

Tabletop Earthquakes:
A Slider Block Experiment

We will set up a slider-block model to demonstrate how fluid injection can cause a fault to slip more easily.  Specifically, we will show how a heavier block has a longer time delay after applying the weight to it before it will slip.  A lighter block will represent the fault supported by fluid and slips more easily under the same weight.  We will also show that different springs (with different spring constants) cause the system to change behavior.  Specifically, a stiffer spring will lead to more frequent and smaller events.  The students will have the opportunity to test these observations by using a timer to record the times between slip events for all situations and a ruler to measure the distance (magnitude) of all slip events.

Materials:

Long piece of wood for the base

-wider than the slider block so you don’t have many derailments

Slider blocks

-blocks of wood or brick with different weights or just one block and multiple weights to put on top

String to attach slider block to spring and spring to weight

Springs with two different spring constants

Weight heavy enough to get the system moving but not too heavy to prevent the sticks

Method:

1.     Hook system up as shown in diagram with weight hanging over the table

2.     Start the block as far away from the edge of the table as allowed by the length of the base wood and let the weight go

3.     Have one student use a timer to measure the time between slip events

4.     Have another student record these times

5.     Have another student mark in pencil on the wood the position of the block at the end of each slip event (choose the front or back of the block)

6.     After several slip events, begin to fill in the following table (preferably on a board where the students can all see the results) with the mean time between slip events and the mean magnitude of each slip event :

	Heavier Block		Lighter Block
Stiffer spring	time	distance	time	distance
Less stiff spring	time	distance	time	distance

7.     Repeat the procedure with each combination of block weight/spring stiffness

Summary:

We discuss the observations and how they relate to earthquake generation.  The spring stiffness represents different rock types.  If done in conjunction with a fracking example, make sure to draw parallels between the reduced normal stress cases in the slider block example (lighter block) and fluid injection.  This offers an easily accessible explanation for induced seismicity in wastewater injection sites.  For questions, contact hannahr@ldeo.columbia.edu

Budget

Material	Use	Cost
Slider Block Model
Long piece of wood	Base of model	Borrowed from lab/professor
2 blocks of wood	Of different weights to represent the initial and reduced normal stress	Borrowed from lab/professor
Sandpaper	To have a rough fault so that the block actually sticks before slipping	Borrowed from lab/professor
Weight	To apply shear stress to the fault	Borrowed from lab/professor
String	To attach the block to the weight	Borrowed from lab/professor
Springs	To store the energy while the block is sticking	~$10