Array Analysis
Prominent Event
Example of a single array image spanning 2-hours of data. Time is on the x-axis and the yaxis is a list of stations sorted by distance from the corner of the array.
Within the array image, the color white (black) is mapped to large (small) amplitudes on the seismic waveforms.
Large earthquakes can 'light up' the entire array, producing a verticle whte stripe on the array iamge such as shown here. This is an example from an off-array magnitude 4.2 earthquake on June 07, 2011 near Festus Missouri, which is ~30 miles south of St. Louis.
Hail Storm
In this array image the sprinkling of white 'specs' along the top portion of the array image are from a hail strom on May 23, 2011. We can easily determine that these are not earthquakes because they do not produce the 'white verticle streak' that is a signature of earthquakes.
There are, however, earthquakes on this array image, the largest of which is on the right side of the array image and has a verticle white 'streak' that spans ~1/2 of the image from the top to half way down.
Array Image with no Events
Some array images have no discernable events within the two hour window. As seen in this example, this can be seen as a virtually fully-light blue image with no significant 'white vertical streaks.'
Cataloging events using Google Spreadsheet
All of the auto-detections and Array Network Facility (ANF) catalogued events are compiled onto a Google Spreadsheet. During analyst review, each of the auto-detections are first checked for detection on events already indexed by the ANF. Those events that match an ANF event are combined into one listing. The new events listed on the spreadsheet are then viewed on the arrays (such as those seen above). A ranking on a scale of 1-5 (5 being absolute certainty) is provided for each event. These rankings are based on a number of factors such as the intensity of the whiteness of the event, the number of stations the event was detected on, and the shape of the event cluster.
Examining Results
For Basin #2 we are typically finding about ~25 events per month that were not listed in the ANF catalog.
Below is a histogram of the LTX detecitons that were deemed earthquakes that were not listed in the ANF catalog (pink) and the LTX detections that were in the ANF catalog (orange), for events in Basin #2 that have lattitudes about 38.5°. The reason for the restriction on lattitude is to avoid categorizing already known events and portions of events outside the basin.
