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6.1: Earthquake Geometry and Process

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  • Generally speaking, it is the movement of faults that leads to earthquakes. A fault is a plane with localized displacement separating two blocks of rock. As you might already know from your intro to geology class, there are three basic types of faults; strike slip, normal, and reverse. The figures below show the three types of faults.

    FIGURE three faults

    Another type of fault is called a thrust fault, which is a low angle reverse fault. 

    FIGURE thrust fault

    On a fault plane, there are several main locations that help us qualify earthquakes. 

    FIGURE fault plane

    The focus is the location where the earthquake initiates. The epicenter is the projection of the focus to the surface. On the surface, the rupture above the focus is visible. 

    FIGURE fault scarp

    Looking at the area from a different view, we can see that the fault scarp is separated from the epicenter. If we look at the area from yet another different angle (map view), we can develop a model of stick-slip faulting. 

    FIGURE stick slip

    Tectonic forces are moving in opposite directions and cause applied shear stress, \(\sigma_s\).

    FIGURE stuck together and frictional surface

    The equation for frictional strength is 


    If \(\sigma_s>\sigma_f\), then failure (an earthquake) occurs. 

    FIGURE stress drop

    The top part of the graph, above the \(\sigma_f\) line, shows where the force overshoots 'average \(\sigma_s\)' because some places will be stronger/weaker and \(\sigma_s\) is heterogeneous. There needs to be a 'large enough' area above \(\sigma_f\) to trigger a rupture. 

    FIGURE high stress region

    The above figure shows that if there is a large enough region with high enough stress, this will trigger an earthquake. Now that we know a bit about where earthquakes occur, let's go through how they actually happen. 

    In a fault rupture, the earthquake initiates at the focus. 

    FIGURE step by step rupture from handout

    At 5 seconds, the rupture continues to expand as a crack along the fault plane. When the rupture front reaches the surface, displacements occur along the surface trace, and rocks at the surface begin to rebound from their deformed state.

    At 10 seconds, the rupture front progresses down the fault plane, reducing the stress and allowing the rocks on either side to rebound. Seismic waves continue to be emitted in all directions as the fault propagates. 

    At 20 seconds, the rupture has progressed along the entire length of the fault. The fault has reached its maximum displacement, and the earthquake stops. 

    In general, backtracking seismic energy allows us to see its source on the fault. We can also get total slip this way.

    FIGURE step by step rupture with t

    Only a finite width behind the rupture front is slipping. So slipping stops at the focus while regions further away are still slipping. We can infer the distribution of slip on a fault surface from seismograms. Contours indicate location of the rupture front. Stars indicate where the earthquake started. 

    FIGURE seismograph handout

    We can get lots of information from seismograms. They are used for determining:

    1. Time of an earthquake and distance to epicenter
    2. Earthquake location 
    3. Magnitude (energy release) of the quake
    4. Type of fault (normal, reverse, strike slip)
    5. Distribution of slip on the fault surface
    6. Structure of the interior of the Earth

    FIGURE seismograph paper moves handout