SAGE Facility MUSTANG metrics Web Service Documentation

This metric computes the short-term average / long-term average (STA/LTA) ratio at half second intervals over a 24-hour period, reporting the day’s maximum value and the time of the sample at which it was calculated. The metric uses an STA window length of 3 seconds and an LTA window of 30 seconds. Because the measurement represents the daily maximum value, no triggering threshold is set.

The maximum STA/LTA ratio can help identify smaller events with good signal to noise ratio. The sample_snr metric is calculated for events of magnitude 5.5 or larger, but for local networks, smaller, closer events often result in strong signal that wouldn’t be flagged otherwise. Other activities that can cause large STA/LTA ratios at these window lengths include impulsive noise (cultural, wind and tilt), pings from sensor components that aren’t pivoting freely and mass recentering. In contrast to these effects, STA/LTA maxima caused by earthquakes are more likely to appear on all channels of an instrument and multiple stations in a network.

Traces – one N.S.L.C (Network.Station.Location.Channel) per measurement
Window – 24 hours starting at 00:00:00 UTC
Data Source – IRIS miniSEED archive or IRIS PH5 archive

SEED Channel Types – ?H?, ?L?, ?N?, ?G?, ?P? | High Gain, Low Gain, Accelerometer, Gravimeter, Geophone | excluding very long period channels

• Request 24 hours of time series data for a single N.S.L.C.
• Remove the mean and trend.
• Average the amplitudes over a 3-second window that begins at the current sample x[n]:
  

  STA(n) = mean(x[n+i]) 0 <= i < 3*sample_rate
  

• Average the amplitudes over a 30 second window that ends at the current sample x[n]:
  

  LTA(n) = mean(x[n-i]) 0 <= i < 30*sample_rate 
  

• Calculate and store the STA/LTA ratio in a vector:
  

  StaLtaRatio(n) = STA / LTA
  

• Advance the STA and LTA windows by [ceiling(sample_rate/2)] samples (a half second) for next calculation.
• Find the maximum value and index sample time from the vector of all STA/LTA ratios for the day:
  

  maxSTALTA = maximum(StaLtaRatio(n))
  

value – maximum daily STA/LTA ratio (unitless)
time – index sample time at which the maximum STA/LTA ratio was found (start time of STA window and end time of LTA window)
target – the trace analyzed, labeled as N.S.L.C.Q (Network.Station.Location.Channel.Quality)
start – beginning of the data day requested (00:00:00 UTC)
end – end of the data day requested (truncated as 23:59:59 UTC)
lddate – date/time the measurement was made and loaded into the MUSTANG database (UTC)

Because the STA/LTA ratio is calculated at half-second increments, the reported time of the maximum value will have up to a half-second uncertainty compared to an algorithm that calculates the ratio at each sample. The increment of a half second was chosen because it identifies the same signal features as an algorithm with a one-sample increment, but increases the speed of the metric significantly (although the value of the maximum ratio may be slightly less).