(last revised 10 May 2010)
GMEW is an Earthworm module for computing ground-motion parameters: peak ground acceleration, velocity and displayment, plus spectral response values.
When in starts up, gmew connects to a transport ring and reads HYP2000_ARC messages, produced by hypoinverse. Gmew also uses the transport ring for sending strong-motion messages, as well as heartbeat and status messages. Gmew reads the summary line of the HYP2000_ARC message, starts collecting trace data from one or more wave_servers, and processes the data for that event. Once the processing (described below) is completed, the gmew module becomes dormant, waiting for the next event to arrive and beating its heart at the specified interval.
(5/10/11) If the <watchForAlarm> command is included in the config file, gmew will now also read ACTIVATE_MODULE messages that have been tagged for gmew, but instead of computing the start of the window, uses the time provided in the message (using the duration specified in the <watchForAlarm> command.
Gmew prepares the new data for processing by checking for gaps and removing the mean. Then gmew transforms the trace data into the frequency domain using an FFT (fast fourier tranform). A cosine taper is applied to remove the lowest and highest frequency components. The default values for the cosine taper ranges are 0.05 to 0.1 Hz at the low end and 45% to 50% of samplerate at the high end. Individual SCNLs may be configured to use other filter values if necessary. The low frequency cutoff is essential to stabilize the displacement trace.
The instrument response is removed, and acceleration, velocity and displacement traces are computed. In addition, spectra response data is computed for the three periods of 0.3 seconds, 1.0 seconds, and 3.0 seconds, with 0.5% damping. All six of these traces are computed in the frequency domain, since integration or differentiation are easily computed there. The spectral response traces are computed by generating the response function for a one degree of freedom harmonic oscillator with the specified free period and damping. All six of these synthetic traces are then transformed back to the time domain by an FFT that handles multiple arrays simultaneously.
(5/10/11) If one or more <AddSpectraResponseAt> commands are included in the config file, then those periods will be computed instead of the standard ones listed above. This does not affect what gets written to the ShakeMap XML.
When the synthetic traces have been returned to the time domain, the peak amplitudes are located for each trace. The search for peak amplitude is done within a time window related to the estimated S-phase arrival time and specified in the configuratiion file.
Gmew writes a TYPE_STRONGMOTIONII message for each SCNL that it processes. This message contains the PGA, PGV, PGD and spectral response values for the SCNL. In addition, the user can configure gmew to write an XML file for the entire event. This file is in the format used by ShakeMap (version 2) for station readings.
Some warnings about this program: Currently gmew does not know to wait for trace data to appear in the wave_servers. So when run as an Earthworm module, it may not be able to obtain enough trace data for adequate processing. You may want to run gmew manually after all the desired trace data has accumulated in the wave_server.
Gmew uses a quite a bit of CPU time, depending on the number and duration of traces requested. If this and other CPU-intensive modules such as localmag are running on the same machine, they will have a significant impact on the machine load after one or more large events. Until Earthworm has some sort of scheduling system, you may want to run these programs at reduced priority.
Gmew will gradually use increasing amounts of memory. This is normal. It is keeping information about FFT factors and trig functions, so that these do not need to be recalculated for each trace calculation.