Goal: The purpose of Reference Strong Motion Stations is to provide recorded motion for post-earthquake damage analysis of buildings, and to provide reliable information for the generation of accurate characterization of the shaking by ShakeMap and other means.

Location Strategy: Reference stations shall be located to record representative ground motion in the vicinity of all significant business districts or groupings of structures. The stations shall be located on representative site geology and within 1 mile (1.6 km) of the most significant business district or structure grouping.

There should be at least one reference station in each zip code. An existing strong motion station meeting the technical specifications below can serve as a reference station, unless the station is too far from significant structures or is on significantly different site geology. More than one reference station shall be required in zip code areas in which the significant building inventories are located on different site geologies, are too distant from a reference station (>1 mile), or are expected to experience significant variation in ground shaking due to other factors (such as topography, distance to fault, soil-structure interaction, etc.).

1. Accelerograph system with acceleration range of 1 mg to 2 g (.001 g - 2.0 g) meeting industry-accepted accelerograph specifications.

 

2. Recorder dynamic range: 12 bits or more, with 1 LSB ~1 mg. Fixed gain (no gain ranging).

 

3. Noise floor: Average signal-to-noise ratio (SNR) > 1 at 1mg over 0 < f < 40 Hz.

 

4. Communication: Either auto-dial out or continuous reliable communication.

 

5. Station housing: Small building without significant basement (1-2 story light structure, wood frame preferable but reinforced masonry acceptable); building generally less than ~ 4000 sq.ft. in plan.

 

6. Station housing foundation: Concrete pad (6" or less), on grade, without piles or piers.

 

7. Environment: No nearby structures likely to cause significant soil-structure interaction effects that may be recorded in the data. At least one major structure dimension away from large/massive structures in the area.

 

8. Three-axis acceleration recording with vertical and two orthogonal horizontal sensors mounted with a fixed and accurate orientation relative to the sensor housing enclosure.

 

9. Orientation of sensor enclosure: Well documented initially; subsequently controlled by an effective process.

 

10. Station Location accuracy: One thousandth of a degree in latitude and longitude; an accuracy of one ten-thousandth of a degree (GPS) is preferable.

 

11. Attachment of sensors and recorder to pad: Bolted securely, with concrete anchors that remain tight over time.

 

12. Timing accuracy: Generally 10 ms or less of absolute (UTC).

 

13. Sample rate: 200 samples per second (5 ms resolution) preferable; 100 samples per second generally acceptable; adequate anti-alias filtering by ½ the sampling rate frequency required.

 

14. Triggering: Triggered instruments shall trigger at ~5 mg (½ %g horizontal acceleration. The recorder shall stay triggered for at least 30 seconds after the last occurrence of horizontal acceleration over 5 mg. The recorders at a selected groups of stations likely to show significant basin effects should stay triggered for 60 seconds or longer after the last 5 mg acceleration.

 

15. Recorder shall include pre-event memory of 5 seconds or more and be able to record, on-site, 20 minutes or more of ground motion data.

 

16. Autonomy: On-site recorder shall operate for approximately 4 days without power in case of loss of AC power. AC power connection shall be hardwired or securely connected to its source.

 

17. TriNet strong-motion stations located within the built environment shall satisfy the Reference Station specifications described below.

Revised/Approved at 4/2/98 SMIAC Meeting

Second Revision at 4/17/98 TriNet TAC Meeting

c:\text\RefStds4.doc Third Revision after 4/22/98 TriNet TSC Meeting