ML20245A679

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Second Quarterly Rept Cleveland Electric Illuminating Co, Seismic Monitoring Network,870115-0415
ML20245A679
Person / Time
Site: Perry  FirstEnergy icon.png
Issue date: 04/15/1987
From:
WESTON GEOPHYSICAL CORP.
To:
Shared Package
ML20209H271 List:
References
CON-#189-8101 2.206, NUDOCS 8705010184
Download: ML20245A679 (13)
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'l-Il SECOND QUARTERLY REPORT CEI SEISMIC MONITORING NETWORK

0 January 15 - April 15,1987 0

o Prepared for CLEVEL AND ELECTRIC ILLUMINATING COMPANY APRIL 1987

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1.0 INTRODUCTION

Cleveland Electric Illuminating [CEI), at the request of the U.S.

Nuclear Regulatory Commission, has been operating a seismic network to monitor the activity in a small area encompassing the Calhio injection wells and the January 31, 1986 epicenter. This second Quarterly Report. l

.1 prepared by its consultant , Weston Geophysical, covers the time period

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extending from January 15, 1987 to April 15, 1987, and contains an update of the recorded seismicity.

2.0 SEISMIC NETWORK i l

During the last three months, eight portable MEQ-800 vertical seismographs have been operated in the configuration shown on Figure 1.

Compared to last quarter 's configurat. ion, the portable network has two additional temporary stations, TUR2 and NARC. The purpose of this change was to improve the locationing ability of the portable array until the new digital network takes over. Figure 1 shows the locations of the eight portable seismometers; it includes also the location of the PNPP, the January 31, 1986 epicentral area, and the two Cals.io well locations.

i The installation of the five three-component stations of the CEI micronet began on April 10, 1987, and operational testing is expected to be completed by the end of April. It is anticipated that both the portable network and the digital micronet will operate concurrently for  !

a period of time to permit the tuning of the new equipment. When this is achieved, the portable equipment will be removed from the injection j wells /1986 main shock corridor.

3.0 OBSERVED SEISMICITY BETWEEN JANUARY 15 AND APRIL 15, 198'1 observed mic,c> arm. quakes will be reported by areas. First, in the  !

epicentral area of the January 31, 1986 event, then in the injection j wells - main shock corridor, and finally in the area east of Cleveland.

0922J *1* Weston Geophysical

3.1 Ericer.ttcl Area of the January 31, 1986 Earthquake one tr.icroear thquake [Mc = 1.7) was detected on February 12, 1987 by the I portable CEI network, as well as by the John Carroll University i network.

The calculated epicenter is within the cluster formed by the 15 previous aftershocks.

The NRC staff was immediately informed by CEI after a preliminary location was obtained.

Table 1 updates the list of all aftershock locations: Figure 2 shows the

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aftershock cluster with the recent event identified. The previous aftershock had occurred on July 17, 1986. The February 12, 1987 reh roawthrph ves apperent1; net fellowed by any other smallet e vem ta .

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k 3.2 Iniection Wells - Main Epicenter Corridor During the last quarter, the CEI portable network has detected and located three small events, all of them in the same area, about four kilometers east-southeast of the two injection wells. 1

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Table 2 presents an update of the recorded microearthquakes, and j includes slight revisions of locations reported in the last quarterly report.

These revisions are based on additional sensitivity analysis.

One of the two detected but unlocated events previously reported, the December 24. 1986 event, has now been located by using one additional arrival time provided by JCU. Figure 3 shows an updated distribution of the recorded activity in the area of interest. Besides the two Calhio  ;

i wells, four other injection wells - one of them now closed - near Painesville have been included on this figure.

3.3 Other Microearthquakes in the Region on January 21 and February 28, 1987, two microearthquakes [Mc = 1.5 and 1 i

1.4 respectively) were detected by the CEI network. Both are clearly

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outside the CEI network, too far to be accurately located by the CEI c.

network alone. Both microearthquakes were recorded by some stations of '

the JCU network, including the station on JCU campus.

0922J *2*

Weston GeoptWd

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i The first event occurred before the Mentor station of the JCU network was on line, leaving a large azimuthal gap. Because of that, a stable l

solution is not yet available. The second event, on February 28, 1987, f was located in the Willoughby area by pooling data from JCU and CEI nets (41.62N, 81.44W).

The occurrences of these two events, surely tectonic in nature and not induced, are in agreement with the accepted historical activity in the Cleveland area [FSAR: Appendix 2D-D].

4.0 DISCUSSION

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( Compared to the last quarter, the recorded seismicity within the CEI network aperture has slightly decreased. The data set from network operations to date is insufficient to establish any relationship between i 1

deep injection and event occurrences; similarly, a causal relationship between oil / gas wells and microearthquakes has not gained any further

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support. The possibility of some seasonal correlation has not been  !

ruled out. l Attempts to improve the confidence in calculated focal depths have not yet given any significant results. A higher average velocity for the first two kilometers, based in part on a sonic log [down to 3,000 feet) and in part on some synthetic estimates, does not seem to improve the mean residuals. On the other hand, recent borings for installation of borehole seismometers of the new digital array confirm that rock is very shallow. 10 to 20 feet, at four of the five sites; at site ANT. rock was reached at 62 feet. On this basis, a thin [0.05 km.] surficial layer of very low velocity does not seem appropriate, given the smal.1 aperture of the micronet.

An interesting observation from the last quarter is the occurrence of two small earthquakes with coda magnitude of about 1.5 near Willoughby.

This is consistent with the historical seismicity.

0922J e 3*

Weston Geophysical

It is expected that with John Cartoll University's recent acquisition of j magnetic -tape. playback facility, and the installation of the five {

three-component digital stations of- the CEI network, future phase  !

identification and arrival time readings will'. be more reliable, I

.especially those of the S-phases. For the last two quarters, sensitivity tests on readings and phase identification have shown that true location uncertainties are larger than the calculated errors.

5.0 CONCLUSION

S During the January 15 - April 15, 1987 period, the CEI network has detected and located three small events, about tnree to four Kilometers

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east of the Calhio injection wells. In addition, a microcarthquake was located in the epicentral area of the January 31, 1986 earthquake.

Finally, two microearthquakes occurred to the northeast of JCU campus, tens of kilometers west of the injection wells and the location of the January 1986 main shock. Thus, there appears to be some tectonic activity present in the region at some low' magnitude level and totally unrelated to fluid injection. Closer to the Calhio wells, the current seismicity observed has not increased during the last quarter.

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0922J *4*

Weston Geophysical 1

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1 0 5 KM 1986-1987 FIGURE 3 Weston Geophysical

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