Caltech's Seismological Laboratory investigates, researches, and informs the public about earthquakes that occur in Southern California and worldwide. Caltech works in collaboration with the scientific community to provide information that enhances our knowledge and understanding of earthquakes.
FREQUENTLY ASKED QUESTIONS
- What causes earthquakes to happen?
- Can human activity cause earthquakes?
- What is a fault?
[more]
LINKS
[ SCEC: TriNet Digital Broadband Instrument Responses ]
[ USGS: Recent Earthquakes in California and Nevada ]
[ SCEC: Recent Earthquakes in California and Nevada ]
[ USGS: Did you feel it? ]
[ SCEC: General Earthquake
Information ]
What do these movies show? | ||
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These movies illustrate the up-and-down velocity of the Earth's surface. Strong blue waves indicate the surface is moving rapidly downward. Strong red waves indicate rapid upward motion. When the waves pass through soft soils (sediments) they slow down and amplify. Waves speed up when they pass through hard rock. The color of the waves oscillates between red and blue indicating alternating up and down motion. These are synthetic movies, created from a simulation based on actual seismographic data for each event. They were created via the following procedure: | ||
[1] |
Every time an earthquake occurs in Southern California, thousands of seismograms are recorded at hundreds of seismic stations [EXTERNAL]. From these seismograms, information about the earthquake can be quickly inferred, including its epicenter, depth, as well as its intensity [FAQ]. |
 + zoom in SCEC Stations in Southern California |
[2] |
We collect these seismic recordings automatically from the Southern California Seismic Network (SCSN) through the internet. We subsequently simulate the seismic waves generated by the earthquake in a three-dimensional (3-D) southern California seismic velocity model [PDF] using the source parameters determined by the seismic network. At periods of about 4 seconds and longer, the resulting wave motion simulated at most stations closely match the gathered data recorded at these stations. |
(a) (b)  + zoom in |
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(a) Topographic map with shaded relief of southern California showing
the extent of our 3-D Southern California model. The major late
Quaternary faults are shown. (b) Compressional-wave speed variations
along two cross sections AA' and BB'. Cross section AA' runs from the
Coast Ranges through the Ventura and Los Angeles basins to the
Peninsular Ranges. Notice the significant 3-D wave-speed variations, and
in particular the very low wave-speed sedimentary basins. Also notice
the shallow Moho (top of the dark blue region) underneath the Los
Angeles basin. Cross section BB' runs from Death Valley through the
Mojave and San Gabriel Mountains to the Los Angeles basin. Note again
the low wave-speeds underneath Los Angeles and the shallow Moho under
the continental borderland. This cross section also highlights the
substantial topography and bathymetry (exaggerated 5 times) that is
incorporated in the 3-D model.
[ download Simulations of Ground Motion in the LA Basin paper (PDF, 5.6MB) ] | ||
[3] |
Finally, after the full 3-D wave simulation, we collect the surface
motion data
(displacement, velocity, or acceleration [FAQ]),
and map them on top of the topography of Southern California. The data is thus rendered and bundled into movies, or filtered and processed into synthetic maps (e.g. Synthetic ShakeMap [FAQ]). |
 + zoom in |
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Transverse component data (black) and 3D SEM synthetic seismograms
(red) for the 22/February/2003 Big Bear event are plotted ona map of southern
California. Stations are denoted by blue triangles and labeled by their station codes.
The instrument response was deconvolved from the data to obtain ground displacement.
Both the data and the synthetic seismograms were subsequently band pass filtered
between 6 and 35 sec with a four-pole two-pass Butterworth filter.
[ download Simulations of Ground Motion in the LA Basin paper (PDF, 5.6MB) ] | ||
[4] |
The resulting earthquake movies are then approved by a geophysicist at Caltech's
Seismo Lab. The movies are published via this portal and automatic
emails are sent to our Email Notification List.
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| The color scale of the wave motion movies changes from event to event in order to accommondate the events maximum intensity. | Notice that since intensity grows exponentially, if we used one static color scale, then either small earthquakes would not be seen, or large earthquakes would oversaturate the map. | |
Scientific Synthetics DataBase
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About ShakeMovie© ]