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This poster was presented by N. Germenis at AGU Fall Meeting 2019, San Francisco, CA.

Abstract

As local earthquakes don’t produce very low frequencies, the use of expensive broadband seismometers, for local and regional seismicity monitoring, may be avoided. Low frequency geophones have been used in the past for this type of monitoring, however their response is typically very limited below 2Hz, and thus a magnitude overestimation may occur. In most cases, wide band seismic sensors are typically used in local and regional seismicity monitoring experiments, but it is not always clear what are the achievable measurement quality levels and how they compare to solutions that are more expensive.

This work tries to shed light into this problem, by presenting a comparative study among three widely available sensors, representing the state of the art in the market of both wideband and broadband sensors. Two different earthquake experiments have been used, with magnitudes ML 4.9 and 4.7 and epicenter distances of 200km and 100km, respectively. For each experiment, the waveform and spectrum plots of each vertical channel have been analyzed.

It is shown that, despite the complexity and cost differences, wideband sensors provide superior performance compared to broadband sensors in the range of their recording spectrum, with no loss of any low period information. Coupled with the cost differences, that allow more stations to be installed, the use of wideband sensors is the recommended way to go, for local and regional seismicity as well as micro-seismicity monitoring.

Figure 1. Seismometers in the seismic vault.

1. Experiment Setup

The scope of the experiment was the investigation of the performance of a wide band sensor (WB), in regional  and  local  earthquake  recording.  The  WB sensor was tested against a typical short period (SP) BB sensor while a Broad Band sensor (BB) was used as a reference. The seismic sensors were installed in a seismic vault, for a period of six months, and common WB recordings of local and regional events were extracted SP for analysis. The scope of the experiment was the investigation of the performance of a wide band sensor (WB), in regional  and  local  earthquake  recording.  The  WB sensor was tested against a typical short period (SP) BB sensor while a Broad Band sensor (BB) was used as a reference. The seismic sensors were installed in a seismic vault, for a period of six months, and common WB recordings of local and regional events were extracted SP for analysis.

Figure 2. Event A (left) and event B (right) records from the three instruments.

2. Waveform data

We have used waveform data from two regional events. The first one (Event A), was an ML 4.9 earthquake recorded at an epicentral distance of 200km, the second one (Event B) had a magnitude of ML 4.7 and was recorded at an epicentral distance of 100km. Instrumentally corrected records are plotted in Fig.2. Wide Band (WB) and Broad Band (BB) records are perfectly matched while the Short Period (SP) sensor clearly lacks longer periods, as expected.

Figure 3. Event A (left) and event B (right) records from the three instruments.

2. Results

In Fig.3 the Power Spectral Density plots of the two events, are presented. In both cases the WB and BB records have equal spectral characteristics, even in the low frequency band. As expected, the SP sensor cannot record accurately in the low frequency part.

References 

[1] Sokos, E. N., Zahradnik, J. ISOLA a Fortran code and a MatlabGUI to perform multiple-point source inversion of seismic data, Computers & Geosciences, Volume 34, Issue 8, August 2008, Pages 967-977.

[2] Sokos, E. and Zahradník, J. Evaluating Centroid‐Moment‐Tensor Uncertainty in the New Version of ISOLA Software, Seismological Research Letters, July/August 2013, v. 84, p. 656-665

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