Getting Started with Visible Earthquakes
Choose an event
First, pick an earthquake to model from those displayed on the front page. Clicking on the earthquake will bring you to an information page with a Model button; clicking that button will bring you to the modeling page. In order to use the modeling page, you must first Sign in with Google.
In Visible Earthquakes, draggable sliders control the modeled fault parameters (strike, dip, rake, etc). Values for these parameters are displayed in the bottom panel and update as you move the corresponding sliders. A full revision history is saved so you may undo changes.
Data and residuals
Measured InSAR data is displayed in the left panel. You may toggle between the wrapped phase interferogram and unwrapped total displacement. The modeled data is displayed in the right panel; this updates in real time as you adjust your earthquake model.
In the right panel, you may also display the residual, the difference between the real and modeled data. If your earthquake model fits well, you should have a fairly featureless residual. Note that the residual colorbar is different than the data colorbar.
Submitting your model
When you are happy with the fit of your model to the data, you may submit the model to our database. Clicking the Submit Model button in the lower right brings up a window where you can answer a couple questions and finalize your submission. After you do this, you will be able to see histograms of all the modeling results submitted to Visible Earthquakes for this earthquake.
See how it's done
The following movie shows the process of modeling an earthquake using the Visible Earthquakes tool:
Learn About Earthquakes, Faults, and InSAR
What is an earthquake?
Earthquakes are caused by the sudden release of stored elastic energy within the Earth. As the tectonic plates that comprise Earth’s lithosphere (its rigid outer layer, which includes the continents and the ocean floor) move around, they can pull, push or drag on each other at their edges, the plate boundaries. This causes the lithosphere to start to bend near the plate boundaries. As the plates continue to move, the lithosphere continues to bend. As the lithosphere is an elastic solid, the more it is bent, the more elastic potential energy it accumulates. And, like a ruler that you bend over the end of the table, the more bending and stored elastic energy there is, the more violent it is when that energy is released. In the lithosphere, the stored elastic energy is released by slip on faults. We call that sudden energy release an earthquake.
What is a fault?
Faults are weaknesses in the lithosphere. They are usually represented as lines on a map, but they are in fact surfaces within the Earth. In many cases, these surfaces are planar, or close to planar. Most of the time, the rocks on either side of a fault are held in place by friction. However, if the stress due to the pull of the bent lithosphere on a fault is stronger than the frictional stress holding it in place, the rocks on either side of the fault will move past each other. This movement is not smooth or gentle – the two sides of the fault will grind against each other, causing the generation of seismic waves that we feel as shaking of the surface.
After an earthquake, the previously bent rocks of the lithosphere will have become unbent again. This unbending causes displacement of the Earth’s surface that can be measured, and that we can use to understand what happened in the earthquake.
How do geologists describe faults and earthquakes?
Geologists classify faults into several different categories, based on how the fault moves:
These are vertical or near-vertical surfaces where the rocks on either side move in opposite, horizontal directions. There are two kinds – left-lateral strike slip faults, where, if you are looking across the fault, the rocks on the opposite side move to the left; and right-lateral strike-slip faults where, in the same situation, the rocks move to the right.
These are inclined (a geologist would say ‘dipping’) surfaces where the rocks on either side move in a mixture of horizontal and vertical motion. The rocks above the fault move down and away from the rocks below the fault.
These are dipping surfaces where the rocks on either side move with a mixture of horizontal and vertical motions. Here, the rocks above the fault move up and towards the rocks below the fault (i.e. the reverse situation to a normal fault). A commonly-used alternative name for these faults is ‘thrust faults’.
These are faults that show a mixture of strike-slip and normal or reverse motion. To be more descriptive, it is common to add the types of motion involved, e.g. ‘oblique left-lateral-normal slip’.
Earthquake source parameters
Geologists use a set of standard terms, that we call ‘source parameters’ to describe the orientation, position and movement of faults and the earthquakes that occur on them. One of the aims of Visible Earthquakes is to find these source parameters by modeling the surface deformation.
Strike and dip
‘Strike’ is the name geologists give to the orientation or direction of a fault on the map – it takes the form of a compass bearing. ‘Dip’ is the name given to the maximum slope of a fault, from 0° (horizontal) to 90° (vertical). Since strike, and indeed any line on a map, could have two potential compass bearings that are 180° apart (e.g. a fault that runs from east to west could have a compass bearing of 90° or 270°) we use a convention to pick which value to use. We choose the direction where, if you are facing in that direction, the fault would be sloping (dipping) away from you to the right (e.g. in the case of an E-W-striking fault that dips to the north, we would use 270° for strike).
Slip, slip vector and rake
‘Slip’ is the term given to the distance that the rocks on one side of the fault move with respect to the other. The direction in which the slip takes place depends on the type of the fault you have; we define a ‘slip vector’ – a vector within the fault surface that describes both the size and the direction of fault slip. For a strike-slip fault, this vector is parallel to the strike direction (hence the name ‘strike-slip’). For a normal fault, the direction will be parallel to the dip direction. The direction of fault slip, and thus the type of fault you have, is described by an angle that we call the rake. In detail, this is the angle between the strike direction and the slip vector, but for our purposes, it is enough to know which values correspond to which kinds of faults:
-180° – right-lateral strike-slip
-90° – normal slip
0° – left-lateral strike-slip
90° – reverse slip
180° – right-lateral strike-slip
Moment, length, width and depth
The overall size of an earthquake is described by a quantity called ‘moment’, that essentially measures how much of the lithosphere was unbent by an earthquake, and by how much (and also, how hard it is to bend the lithosphere in the first place).
moment = rigidity x slip x fault area
Rigidity is a material property of rocks that measures a rock’s resistance to shearing (bending). For the continents, values of around 30 GPa are commonly assumed. The greater the rigidity of the lithosphere, the more stress it requires it is to bend, and so the larger an earthquake (and its moment) will be.
Slip is defined above. The more slip there is in an earthquake, the larger it (and its moment) will be.
The larger the area of fault involved in an earthquake, the larger it will be. Assuming that the fault is rectangular, then its area can be estimated from its length (in the along-strike direction) multiplied by its width (in the along-dip direction). The width can be calculated from the depths to the top and the bottom of the fault and the dip of the fault:
width = (bottom depth – top depth) / sin (dip)
The InSAR method
Side-looking radar antennas, such as those mounted on remote sensing satellites, transmit microwave-band radar signals towards the ground, and record the backscattered echoes. The resolution of such a radar is inversely proportional to its ‘aperture’ (antenna length); by making use of the concept of a ‘synthetic aperture’, i.e. the idea that echoes measured at different positions along the orbital track of the satellite can be combined to produce a much longer virtual (‘synthetic’) aperture, resolutions of a few to tens of meters can be achieved in processing.
Each full resolution synthetic aperture radar (SAR) image processed in this way contains a measurement of two quantities at every pixel: the amplitude and phase of the returned radar. The amplitude is a measure of the intensity of the radar returns, controlled by the roughness and slope of each pixel on the ground. The phase, on the other hand, is a function of the distance between the satellite and the ground target. If a pixel on the ground were to move between successive passes of the satellite, then in principle the phase measurement associated with that pixel should change; by differencing the phase measurements made before and after the movement, the phase change associated with the movement can be isolated and the displacement of the surface quantified.
In practice, each SAR image is composed of hundreds of thousands or millions of such pixels. Differencing the phase components of two SAR images of an area on the ground (a technique called ‘Interferometric SAR’, or ‘InSAR’) will generate an interference pattern image, or ‘interferogram’, which is a map of the displacement of the surface that occurred between the two SAR image acquisitions. The spatial pattern, amplitude and sense of displacement recorded in the interferogram can then be used to determine the nature of the source of the deformation.
Interpreting InSAR data
A key concept when interpreting InSAR data is the idea of satellite line-of-sight (LOS). The displacement measurement made by InSAR is the difference in the path length taken by the radar between the two SAR image acquisitions, at each pixel on the ground. This is a one-dimensional measurement in the LOS direction, i.e. the direction linking the satellite and the ground pixel.
InSAR measurements are thus to some degree ambiguous, since a range of potential movements of the ground could result in the same displacement in the LOS direction.
Where significant displacement of the ground surface has occurred, e.g. at the epicenter of a shallow earthquake, typically a cluster of concentric interference ‘fringes’ is observed in the interferogram. Here, each fringe represents a contour of ground displacement, towards or away from the satellite, equal to half of the radar wavelength (a movement of a few centimeters, typically). The total surface displacement due to the earthquake can be estimated by counting the concentric fringes from the outside to the center of the ‘deformation pattern’. This process is known as ‘phase unwrapping’, and is typically automated during interferogram processing.
Which satellites are used for InSAR?
Over the last two decades, there have been over a dozen satellites that have collected radar imagery suitable for InSAR, from several different space agencies:
ERS-1 (1991-2000) and ERS-2 (1995-2011), European Space Agency.
ERS-1 was the first imaging radar satellite to be used consistently for InSAR, including the iconic interferogram of the 1992 Landers, CA earthquake that made the cover of Nature magazine that heralded a new method for studying earthquakes. It was followed a few years later by the near-identical ERS-2, which followed its sibling one day behind in orbit. Both satellites had identical 'C-band' radar antennas (wavelength 5.6 cm), which are moderately sensitive to displacement of the ground, but affected by vegetation for periods for periods of more than a few months.