When an earthquake occurs, what does it look like on our instruments?

Example of a realtime display

Figure 1: Example waveforms recorded by ANZA network stations. Those stations closest to the hypocenter are at the top, with the further away stations increasing down.

Clicking on the hyperlink below will download a movie of the waveforms our Anza broadband instrument array recorded for the October 30th 2001 magnitude 5.1 earthquake that occurred right in the heart of our Southern California array (click here for details on this particular event).

The image to the right (Figure 1) will help you understand what you are seeing in the movie.

The movie is a screenshot of the waveforms our instruments recorded. Each station (station ID on the y-axis) has three motion sensors that correspond to x, y and z components - north-south (x), east-west (y) and vertical (z). The movie on this page only has the vertical (z) traces shown (Channel HHZ), which are where the P wave arrivals are recorded. The yellow lines that make up most of the display are the motion waveforms (x, y or z) recorded at each station. The x-axis (horizontal axis) is time, measured in the format HH:MM:SS YYYYJJJ where JJJ is the Julian day (0-366). The red 'P' flag is the time that the event's primary wave was recorded. This location is picked by one of our analysts.

The movie shows what the array was recording at each station prior to, and then during, the earthquake. Note that the earthquake does not occur at exactly the same time at each station.

Frequently Asked Question

Q: Why would different stations record the arrival of the P wave at different times?

A: Earthquake waves radiate from the hypocenter of an earthquake in all directions. Stations closest to the hypocenter will therefore record arrival times of the P waves first. For a local earthquake (within 2° of the network stations) these arrivals are only milliseconds apart, but for large teleseismic events, the arrivals can be several seconds apart.

MPEG movie of the Anza 2001-10-31 earthquake

If you have the Quicktime plugin installed the movie should load below. Click the triangle 'play' button on the left-side to start playing the movie.

URL: http://eqinfo.ucsd.edu/faq/display.php [Last updated: 2015-10-22 (295) 23:07:44 UTC]