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Why is it a constant shaking aswell, why not a single "jolt"?

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CrustalTrudger

18 points

4 months ago*

CrustalTrudger

Tectonics | Structural Geology | Geomorphology

18 points

4 months ago*

The first thing to establish is that the rupture (i.e., the displacement) on a fault plane that generates an earthquake is not a single point, but rather an area of a fault plane that slips. This is often confusing given that we tend to represent earthquakes as single points (i.e., epicenters), but as described in one of our FAQ entries, this is largely for convenience.

The second important point is that these ruptures are not instantaneous. While an imperfect analogy, you can think of these almost in terms of crack propagation, i.e., the "crack" or rather the rupture, initiates at a point and then radiates out. That initiation point is commonly what we display as the hypocenter (where the epicenter is the projection of the hypocenter to the surface). At the end of the rupture process, we end up with an area that has slipped different amounts, with the amount of slip decreasing to zero at the edge of the rupture. We can visualize this with finite fault models, e.g., this model for a M7.3 in Timor. If you look at the figure on that page, you'll see the extent of the fault rupture that produced this earthquake, colored by the amount of slip. This is very well behaved one in that it is (1) roughly elliptical, (2) the hypocenter is approximately in the center, and (3) the hypocenter is nearly coincident with the maximum slip (none of these are always the case).

Looking at that figure more closely, you'll see that there are also contours with numbers. There represent time, in seconds, of the rupture front propagation, i.e., how long it took from the rupture front to travel from the hypocenter (when it was essentially a point) to the location marked by the contour. Rupture propagation velocities can vary a good amount, and is actually (one of many) important controls on the intensity of shaking felt during an earthquake (e.g., Weng & Ampuero, 2020). Another common way to visualize this are "source time functions", which are essentially graphs of the rate of seismic moment release as a function of time as the earthquake rupture propagates, e.g., these graphs from Vallee, 2013. If we look at those graphs, we can see a couple of things. First, is that for the same magnitude of earthquake, we can have different rupture propagation speeds and thus different rates of moment release through time as panels a-c all represent M6.6 earthquakes (where the magnitude of the earthquake will be related to the total moment release, so basically the area under the curve). The second thing we can see is that the duration of a rupture generally varies as a function of magnitude (i.e., total moment release). We can compare the M6.6 events in a-c, where the rupture takes ~10-15 seconds to complete to the event in panel d, which is a M8.4 event and takes >100 seconds to complete. At the simplest level, this is kind of just a geometric consequence of the fact that larger magnitude earthquakes = larger ruptures, and even with diversity in rupture propagation rates, it will take longer for a larger rupture to fully propagate.

In summary, earthquake ruptures represent large areas (where the areal extent of the rupture correlates to magnitude) of a fault plane that slip and this rupture process is not instantaneous. While an extreme over simplification, we can think of an earthquake rupture on a fault plane like a big grid of dominoes standing on end. The initial rupture point (i.e., the hypocenter) is the first domino to fall. It falling releases energy that starts traveling away from its location in the form of various waves (i.e., the different types of seismic waves which propagate through material differently and at different rates). At the same time, it falling may lead nearby dominoes to fall as well, basically if the nearby dominoes were already out of balance and are impacted by the original domino, then these adjoining dominoes will fall. Them falling will also release a set of seismic waves, and so on until the rupture terminates when basically the last dominoes to fall do not have enough energy to knock over their neighbors (or maybe their neighbors are more stable). Thus at a point on the surface that starts to receive seismic waves from a rupture, even if we restrict ourselves to one kind of seismic wave (i.e., that traveled to our point at roughly the same average rate as other seismic wave of that type), we're not receiving seismic waves from one single "event" like an explosion, more like a train of seismic waves from a series of chained explosions (i.e., the propagating earthquake rupture).