triggerOnset {IRISSeismic}

Event onset triggering

Description

The triggerOnset method of Trace objects uses the numeric vector returned by the STALTA "first break picking" method and a user selected threshold to determine the arrival time of a seismic event.

Usage

triggerOnset(x, picker, threshold, index)

Arguments

x

a Trace object

picker

results from applying the STALTA method to this trace

threshold

optional numeric value of the threshold at which triggering should occur

index

optional logical to return the index (rather than the time) of event onset (default=FALSE)

Details

This method simply identifies the point at which the picker first rises above the threshold.

When no threshold is supplied, an appropriate value is calculated from the picker with:

threshold <- quantile(picker,0.999,na.rm=TRUE).

Value

A single value is returned identifying the onset of the seismic event or NA if none is detected. The returned value wil be a POSIXct time by defult or a numeric index if index=TRUE.

Note

The appropriate value for the threshold will depend upon the exact STA/LTA algorithm used and the noise level in the signal.

Author(s)

Jonathan Callahan [e-mail]

See Also

STALTA

Examples

## Not run: 
# Open a connection to IRIS DMC webservices
iris <- new("IrisClient")

starttime <- as.POSIXct("2010-02-27 06:00:00",tz="GMT")
endtime <- as.POSIXct("2010-02-27 09:00:00",tz="GMT")
  
# Get the waveform
st <- getDataselect(iris,"IU","ANMO","00","BHZ",starttime,endtime)
tr <- st@traces[[1]]
picker <- STALTA(tr,3,30)

# Identify the onset of the event
to <- triggerOnset(tr,picker)

plot(tr)
abline(v=to, col='red', lwd=2)

## End(Not run)