the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
The quest for reference stations at the National Observatory of Athens, Greece
Abstract. The assumption of reference station conditions is investigated for the first time across 60 rock stations belonging to the broadband and accelerometric networks of the National Observatory of Athens. We select the stations based on the established belief that they lie on rock, and provided their data are publicly available through EIDA/NOA and have been for long enough to yield a substantial number of recordings. No site effects studies have been conducted before for the ensemble of the stations under study. Furthermore, no ad hoc field campaigns have been performed to characterise them, save in 2 cases. The first step is to compile all existing information for these stations from all publicly available sources and past studies, including geology, topography, station installation, Vs30 estimates and any other known metadata. The second step is to compile ad-hoc information from maps combined with the operator’s first-hand experience of the sites, to better describe the geological unit and age, along with other characteristics such as station installation and morphology. The third and largest step is to compile the first Greek ground-motion dataset on rock and perform a detailed analysis of the recordings to estimate site-specific transfer functions and hence assess local site response characteristics for each station. A strong-motion dataset of 6840 recordings is developed and curated for this purpose, visually inspected and processed in the time and frequency domains. Single-station amplification functions (horizontal-to-vertical spectral ratios, HVSR) are estimated from the seismic data, and the site resonance characteristics are assessed, not only in the conventional way of combining components, but also assessing directional sensitivity. ‘True’ reference site behaviour implying low, flat amplification with no directional dependence, these elements these transfer function characteristics are combined with the compiled existing and new metadata, to evaluated the stations’ capacity as reference sites. The stations are grouped in terms of behaviour and the preferred ones are recommended, hoping to facilitate the better use of data in future hazard applications.
- Preprint
(32679 KB) - Metadata XML
-
Supplement
(17752 KB) - BibTeX
- EndNote
Status: final response (author comments only)
-
CC1: 'Comment on nhess-2023-233', Alexandra Moshou, 01 Feb 2024
Although this study makes a significant contribution to the understanding of reference station conditions for rock sites in the broadband and accelerometric networks of the National Observatory of Athens, it is important to acknowledge a key limitation. The study relies heavily on existing data from publicly available sources and past studies, and the selection of stations is based on the belief that they are situated on rocks.
This report exhibits several notable shortcomings that may impact the validity and reliability of the findings. First, the reliance on an established belief that the selected stations are situated on the rock without conducting prior site effect studies raises concerns about the accuracy of the assumed geological conditions. Additionally, the absence of ad hoc field campaigns for characterizing the stations, except in two cases, introduces a significant limitation in understanding geological units and age, as well as other critical characteristics. The use of ad-hoc information from maps and operator experience while attempting to enhance site descriptions may introduce subjective biases and lack the rigor of systematic field studies. Furthermore, the report acknowledges the absence of previous site effect studies for the ensemble of stations under investigation, suggesting a potential gap in foundational understanding. The reliance on publicly available data and compilation of existing information may lead to incomplete or outdated datasets, compromising the overall robustness of the analysis. The report's recommendation of preferred reference sites is contingent on the assumptions and methodologies employed, raising questions regarding the generalizability and applicability of the findings to broader hazard applications. Overall, these limitations underscore the need for a more comprehensive and rigorous approach to ensure the credibility of a report's conclusions.
While the article compensates by combining available information, including operator experience and ad-hoc data, it highlights a potential gap in the comprehensive understanding of the geological and site-specific features of these stations. Future research could benefit from targeted field campaigns to fill this gap, enhance the robustness of the findings, and provide a more accurate assessment of the suitability of the stations as reference sites.
It should not be accepted as a research article than a report in such a highly acknowledged journal such as Natural Hazards and Earth System Sciences.
Citation: https://doi.org/10.5194/nhess-2023-233-CC1 - AC1: 'Reply on CC1', Olga-Joan Ktenidou, 03 Jul 2024
-
RC1: 'Comment on nhess-2023-233', Chuanbin Zhu, 17 Mar 2024
- AC2: 'Reply on RC1', Olga-Joan Ktenidou, 03 Jul 2024
-
RC2: 'Comment on nhess-2023-233', Giovanni Lanzano, 05 Apr 2024
The manuscript studies for the first time the conditions of reference stations across 60 rock stations belonging to the broadband and accelerometric networks of the National Observatory of Athens. The analysis is based on selecting stations situated on rock and whose data availability is sufficient for a meaningful collection of recordings.
The study is relevant since no systematic previous site effects studies were conducted for the ensemble of stations under examination. Similar studies have been conducted in other contexts and at different scales by Lanzano et al. (2020, 2022) for Italy and by Pilz et al. (2020) for Europe.
I believe that this work is a very useful contribution to the scientific community and to seismological studies in Greece. I suggest accepting the publication with minor revisions.
Some comments below:
Station and data selection
- Please explain further, on the basis of which criterion you consider the 60 stations to be installed on rock. Did you use geological and/or topographical proxies? Do any of the stations also have a geophysical survey?
- Letters a and b are not present in Figure 1 and 2
Creation of a new strong motion dataset
The analysis of the signals for the creation of the dataset is a very important step. I realised that the authors did a very thorough job, taking advantage of already available codes. However, I suggest that this section be reorganised schematically by indicating the data processing work in steps.
For example:
- Identification of clipped records (which criteria?)
- Waveform picking
- Calculation of signal-to-noise ratio
- Identification of corner frequencies
- Etc.
I am not saying to repeat things that have been explained in other works, but to list the actions in a schematic and sequential manner. I think the work would benefit in terms of clarity. I think it would also be helpful to understand which signal analyses are done by the NGA-East code and which are not.
Do you check for double events recordings? How do you treat?
I also suggest improving figure 3 to make it more self-explaining. A legend is missing.
Transfer function
- For the calculation of transfer functions with the horizontal-to-vertical spectral ratio method, I would pay attention to the recordings of co-located stations.
From what I understand, all recordings for stations equipped with accelerometer and velocimeter were considered for the estimation of HVSR. My experience with INGV's Italian seismic network, which has a large number of collocated stations, is that this should be done carefully. I suggest conducting a preliminary analysis by keeping the instruments separate (consider them as two different stations) to verify that the transfer function is equal. First of all, the sensors could suffer from fixed scaling (a comparison of intensity measurements of recordings of the same event is also recommended) caused by incorrect conversion constants in the station xml. In addition, Hollander et al. (2020) and Castellaro et al. showed that the behaviour could be different at specific frequency ranges (especially in high frequency) due to different station installations.
- I think it is also useful to explain how the groupings in Figure 7 were made. Was a clustering analysis conducted? Or is it based on a visual analysis of the curves? I have the impression that the transition from one group to another may not be clearly defined and some stations may be in one group rather than another arbitrarily. Wouldn't it be useful to set a quantitative criterion to isolate stations that have a flat response?
Discussion and conclusions
I noticed that the authors never considered the high-frequency near-site attenuation parameter k0 among the parameters for identifying reference sites. Considering the great experience of the authors, do you think this parameter could have any weight in the future? For example, in the work of Morasca et al. (2023) we started from the 36 stations of the study by Lanzano et al. (2020) and restricted to 6, as reference sites for a GIT in central Italy. The selection was made on the basis of k0, identifying those with k0<0.015s.
Additional reference
Castellaro, S., Alessandrini, G., & Musinu, G. (2022). Seismic station installations and their impact on the recorded signals and derived quantities. Seismological Society of America, 93(6), 3348-3362.
Hollender, F., Roumelioti, Z., Maufroy, E., Traversa, P., & Mariscal, A. (2020). Can we trust high‐frequency content in strong‐motion database signals? Impact of housing, coupling, and installation depth of seismic sensors. Seismological Research Letters, 91(4), 2192-2205.
Morasca, P., D'Amico, M., Sgobba, S., Lanzano, G., Colavitti, L., Pacor, F., & Spallarossa, D. (2023). Empirical correlations between an FAS non-ergodic ground motion model and a GIT derived model for Central Italy. Geophysical Journal International, 233(1), 51-68.
Citation: https://doi.org/10.5194/nhess-2023-233-RC2 - AC3: 'Reply on RC2', Olga-Joan Ktenidou, 03 Jul 2024
Viewed
HTML | XML | Total | Supplement | BibTeX | EndNote | |
---|---|---|---|---|---|---|
572 | 188 | 48 | 808 | 68 | 37 | 31 |
- HTML: 572
- PDF: 188
- XML: 48
- Total: 808
- Supplement: 68
- BibTeX: 37
- EndNote: 31
Viewed (geographical distribution)
Country | # | Views | % |
---|
Total: | 0 |
HTML: | 0 |
PDF: | 0 |
XML: | 0 |
- 1