From the recorded ground motion available for 1999 Chi-Chi earthquake, few pairs of stations are located at the same place (i.e., same lat-long, and soil type) but drastically different PGA values. For example, CHY109 & CHY110. If source, path and site conditions are same, then what are the reasons for observing different PGA values at stations.
Intressting question.
As I understood from your words, the record is different, but source and site seems to be the same. So, in my attention is - how much close could be conditions for these two records ?
First - the identic seismometer type for Earthquake records should be at these seismic stations. Is this condition valid for your case? If no, this could be the answer. The sensitivity for recording signal could be different. If yes, we go forward to source, and next element on the way is the basement of seismic station. Here the isolation from the ground is important, because, the station building usually affect the records when the signs arrive. So, if the seiamometers are located in different seismic buildings - could be a reason. If the seiamometers are placed in same building but one is in middle and one close to building corner, could be a reason. If their in same condition for record - close one to other (2-10 meters) than you should consider near geometry of the environment. If exist some geologic conditions which could affect the arriving waves. Some soil density gradients which can reflect some arrived energy, to one of seiamometers, and none for second. For me these looks essential. Good luck!
You should make your question more specific:
1) What strong motion database do you refer to?
2) Where are those stations - at what distance from the source and also are they on the same depth? If they are in an vertical array they shall have for sure different spectra. Are they in neighbouring buildings with very different sizes?
3) What is the difference in the spectra/PGA is it huge or not?
Thank you Dr. Ilie Sandu
1. I am not sure of the type of seismometer installed. What is the sensitivity of PGA to type of seismometer? At some stations, it is observed that the difference is equal to and sometimes more than double.
2. I do not have any details about the basement of seismic stations. I could know that all the stations recorded free-field accelerations, so they may not get influenced by building vibrations.
Thank you Mr. Stoyan Andreev
1. Horizontal Accelerogram of 1999 Chi-Chi earthquake
2. Yes, I will find out whether they are in a vertical array or not.
3. Yes, though not in all cases, the difference is huge sometimes.
Neelima
Technically, there cannot be any difference. A large difference may occur, if there is a big boulder or large loose pockets, just below one of the foundation of Seismometer. This is my wild guess. Such discontinuity may alter the recorded ground motion. During installation, no one might have done detailed investigation of the founding medium below the free field, as it may not be needed. Such surprises are quite common, in soil sites. In such a case, the same observation could be there for other recorder time histories, at the same site with the same channels, i.e., CHY109 & CHY110. Without these details, it is difficult to conclude.
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