ML20196B402
ML20196B402 | |
Person / Time | |
---|---|
Site: | Perry ![]() |
Issue date: | 09/30/1988 |
From: | WESTON GEOPHYSICAL CORP. |
To: | CLEVELAND ELECTRIC ILLUMINATING CO. |
Shared Package | |
ML20196B393 | List: |
References | |
NUDOCS 8812060251 | |
Download: ML20196B402 (19) | |
Text
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EIGHTH QUARTERLY REPORT CEI SEISMIC MONITORING NETWORK JULY l THROUGH SEPTEMBER 30,1988 l
Prepared for CLEVELAND ELECTRIC ILLUMINATING COMPANY NOVEMBER 1988
([Qh Weston GeophyCORPORATION sical 8812060251 DR estggg ADOCK 05000440 PDC
1 TABLE OF CONTENTS EAAA LIST OF FIGURES i LIST OF TABLES 11
1.0 INTRODUCTION
1 2.0 SEISMIC NETWORK 1 3.0 OBSERVED SEISMICITY 1 3.1 Epicentral Area of January 31, 1986 1 3.2 ThG corridor Between the January 31, 1986 Epicenter and the Injection Wells 2 3.3 Other Events Recorded by AUT0 STAR 2 4.0 DISCUSSION 2
5.0 CONCLUSION
4 6.0. REFERENCES 4 TABLES FIGURES APPENDIX A FIGURE Al 2354J Weston Geophysie:ol
LIST OF FIGURES FIGURE 1 Telemetered Network Station Configuration FIGURE 2 Cumulative Epitentral Distribution of Aftershocks -
January 31, 19'64 SegJence FIGURE 3 Seismicity: Ju11 1 - September 30, 1988 FIGURE 4 Cumulative Seismicity - Up to September 30, 1988 I
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l LIST OF TABLES l TABLE 1 Microcarthquakes in Epicentral Area of January 31, 1986 Event.
TABLE 2 Microcarthquakes in the Corridor Between the Injection Wells and the Main Shock Area.
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1.0 INTEODUCIlDE In accordance with its agreement with the U.S. Nuclear Regulatory Commission, Cleveland Electric Illuminating [CEI) continues to monitor the seismic adivity in a restricted region of Northeastern Ohio, encompassing the local
- of the Perry Nuclear Power Plant, two deep injection wells operated by CALHIO, and the epicentral area of the January 31, 1986 earthquake. This eighth Quarterly Report covers the period from July 1 to September 30, 1988. An update of the CALHIO volumetric injection data is also provided in Appendix A.
2.0 SEISMIC NETWORK During this period, the Automated Seismic Telemetering and Recording System (AUTOSTAR), fully described in the third Quarterly Report, has performed in a reliable manner, except for one digitizing unit, which had to be sent back to the REFTEK Corporation for repairs. Thus station ANT, located on Antioch Road, was not operative during most of August and September. Nonetheless, because of the network configuration, shown en Figure 1, the detection threshold for the corridor between the Calhio wells and the January 31, 1986 epicentral area was not reduced. The total percentage of uptime for the network operation was 87%, the same as for the last quarter.
3.0 OBSERVED SEISMICITY 3.1 Eoicentral Area of the_ unuary 31. 1986 Earthauake During the quarter, only one small microcarthquake (Mc = 0.1) was detected on August 5. Table 1 gives an update of all the aftershocks recorded in that area and Figure 2 presents both the locations and the sequential order of occurrence. The August 5 epicenter is also shown on Figure 3.
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3.2 The Corridor Between the January 31. 1986 Epicenter.aad the Iniection Wells During this quarter, two small events (Mc = -0.2 and 0.1) were detected in the area of interest. The first one, on August 20, is located in the cluster defined by previous low level activity; the second one is located to the north of the injection wells. Because of h - U.T.), it cannot be ruled its size and the time of occurrence (15 out with certainty that the event could be man-made, i.e. related to construction or to gas well fracting. The two epicenters are shown on Figure 3. Table 2 lista the location parameters of all microevents detected since the beginning of the seismic monitoring. Figure 4 illustrates all corresponding locations.
3.3 Other Events Recorded by AUT0 STAR As in the past, AUTOSTAR recorded several seismic events originating outside its limited aperture. Two of these, on July 22 at 16:09 U.T.
and on September 30 at 17:25 U.T., come from the Fairport Harbor area, about 10 km. west of the CEI micronet. Both of these events are very small, (Mc near 0.0) and cannot be located with the same precision and confidence as those occurring within the net. In addition the John Carroll University network cannot provide any reliable input in these two cases. For this reason, both events are given the same average coordinates: 41.75N and 81.25W. Due to the times of occurrence, relatively close to noon, there is some doubt on the true nature of these events. Other more distant events were also recorded, including the September 7, 1988 Kentucky earthquake (Mb = 4.6).
4.0 DISCUSSION The occurrence of the August 5, 1988 event in the epicentral area of the January 31, 1986 earthquake is not surprising. The last af tershock had been observed on February 12, 1987, more than eighteen months ago. Af ter such a long gap, one could possibly question if the 2354J *2*
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August 5, 1988 event should be called an aftershock. More interesting is the fact that it occurred within the original zone of hypocenters, very close to the last aftershock (Figure 2).
In view of the small magnitude (Mc = -0.2) of the August 20, 1988 event and the fact that the last small event occurred more than six months before, nothing abnormal or significant is inferred from this occurrence. The small event of September 27, 1988 northeast of the wells, cannot be correlated to the injection process with any confidence, considering the time of the day, the distance, and the novelty of this epicentral location.
As reported at the Eastern Section of the Seismological Society meeting last October by Leblanc et al. (1988), the detection threshold of the CEI micronet is very low due to the small aperture and the sensitive instrumentation. As a result, some of the observed events are extremely small and at times below the average cultural noise level. These small microcarthquakes, (Me < 0.0) represent energy releases that are negligible compared to those of earthquakes contributing to the seismic hazard. Although these small events are certainly not correlated noise, their true tectonic nature is often questione1, particularly for daytime occurrences. Realistically, CEI cannot attempt to track down all man-made sources of seismic noise in or around its network.
Processing all small events presents a problemt how far down into the seismic noise should one go without risking to obscure with lesser quality data the snore important information conveyed by the more reliable epicenters of the larger events, (Mc > 0.5). As events get smaller, the reading accuracy of first arrival times and the correctness of phase identification decreases; in addition, the computed errors frequently lose their significance. This problem is not new; the introduction of quality estimates in different hypocentral determination codes constitutes a caution sign for the users.
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I In view of these difficulties of interpretation, guiding criteria are now being prepared on the basis of the experience acquired during the last two years. These guidelines will specify a minimum numbe.- of stations and phases, certain noise conditions, and a minimum duration (Mc) required for a solution to be accepted. Once finalized, these guidelines will be presented in a future Quarterly Report.
The injection data presented on Figure Al of Appendix A continue to show no visible correlation with the microactivity. It is becoming apparent that this type of data may not provide absolute confirmation that induced seismicity is occurring. On a short term scale, no correlation has been observed for more than two years and a half. On a long term scale, any suggested correlation (Ahmad and Smith, 1988, Armbuster et al.,1987, Nicholson et al., 1988) remains a speculative possibility.
5.0 CONCLUSION
During the last quarter, the seismic activity observed by the CEI micronet consisted of one small event in the January 31, 1986 epicentral area, and two other ones in the corridor / area near the Calhio wells. No correlation of the observed microseismicity with the deep injection has yet been established.
6.0 REFERENCES
Ahmad, M.U. and Smith, J.A, 1988: Earthquake injection wells, and the Perry Nuclear Power Plant, Cleveland, Ohio, Geology, Vol. 16,
- p. 739-742.
Armbuster, J.G. , Seeber, L. , and Evans, K.F. ,1987: The July 1987 Ashtabula earthquake (Mb = 3.6) sequence in northeastern Ohio and a deep fluid injection well: Abstracts of the 59th Meeting of the Eastern Section of the Seismological Society of America, St. Louis.
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' Leblanc, G., Klimkiewicz, G.K., Filipkowski, F. and Rice, J.: Thirty months of seismic monitoring in Northeastern Ohio: Program and Abstracts of the 60th annual meeting of the Eastern Section of the Seismological Society of /,merica, p. 53, October 12, 1988, PennsyIvania State University.
Nicholson, C.,, Roeloffs, E. and Wesson, R.L., 1988: the Northeastern Ohio Earthquke of January 31, 1986: was it induced?, Bulletin of the Seismological Society of America, Vol. 78, p. 188-217.
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