Data Requirements from Low–Frequency Seismology and the Dearth of STS-1 Sensors

Data Requirements from Low–Frequency Seismology and the Dearth of STS-1 Sensors a) Grading of available GSN and equivalent vertical recordings. A grade E record has some surface wave trains but no obvious spectral lines. An ‘F’ grade record has no seismic signal or large data gaps. b) Scoring for co-located sensors. Number of records (dark blue) and instances when the sensor provided the best seismogram (light blue).
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Free-oscillation seismology has exceptionally high demands on data quality. We need long, gap-free records, with consistently high signal-to-noise performance. The data harvest for a typical very large earthquake (scalar seismic moment M0 ≥ 5.0 x 1020 Nm is rather sobering. Only 75% of the records downloaded from the IRIS DMC can be used for study.
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For the 2004 IRIS Broadband Instrumentation workshop I summarized the experience I gathered from analyzing over 20 large earthquakes in the last 10 years. I inspected each vertical component spectrum visually and assigned grades A-F. A little less than 50% of the records of Wielandt-Streckeisen STS-1 vault sensors (132) meet the highest quality requirements, while the 46 Teledyne-Geotech KS5400 borehole installations (and its predecessor KS36000) yield less than 10% high-quality records. Surprisingly, the fraction of such records at 85 STS-2 and Guralp CMG3 (CMG-3t and CMG-3b) installations is almost as large. Overall, I find that 70% STS-1 records are ‘’acceptable’’ for analysis, 50% KS54000 records, but less than 30% STS-2/CMG3 records which clearly stresses the importance of observatory-quality very-broadband installations. It is often argued that KS54000s are typically deployed in noisy environments so that my comparison should not be used directly to judge the value of a KS54000. I have also inspected the data from co-located sensors for the 3 largest earthquakes in 2003. Typically, the primary sensor is either an STS-1 or KS54000 and the secondary sensor a STS-2 or CMG-3. In 92% of the cases when an STS-1 is involved, it provides the best records. The same is true for only 46% of the KS54000 records, which implies that in more than half of the cases the secondary sensor (STS-2 or CMG3) provides the better record. The new station at the South Pole, QSPA, hosts all four sensors. For two of the three largest earthquakes in 2003, the STS-1 delivered the best record, while the STS-2 delivered the best for one event. The quality of the CMG-3 is not far behind and, occasionally better, while the KS54000 delivers grade C data. For the Sumatra-Andaman earthquake, the CMG-3 delivered the best data at very low frequencies, while the STS-1 performed relatively poorly, the reason for which is not understood.
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Clearly, the STS-1 is the ultimate workhorse of low-frequency seismology. After the 2004 Sumatra earthquake, low-frequency modes could be seen on STS-2 records of many networks, not just the GEOFON network that has consistently been delivering good STS-2 records. The Sumatra earthquake and its March 2005 aftershock were extremely large and it is probably not a good idea to include its data to adjust GSN design goals. Rather, we need to find an adequate successor for the STS-1. This perhaps necessitates the co-deployment of several sensors rather than one very-broadband sensor that can do it all.
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Laske, G.. The Needs for/of Low-Frequency Seismology, IRIS Broadband Instrumentation Workshop, 2004.


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