ML19260C935
| ML19260C935 | |
| Person / Time | |
|---|---|
| Site: | 05000574 |
| Issue date: | 01/11/1980 |
| From: | Kelleher J WOODWARD-CLYDE CONSULTANTS, INC. |
| To: | |
| Shared Package | |
| ML19260C924 | List: |
| References | |
| CON-NRC-19-80-470 NUDOCS 8002070151 | |
| Download: ML19260C935 (4) | |
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Woo'dward Clyde Consultants REVIEW OF REPORTS TO PUNO COMMISSION r
by J. Step and C. lomnit:
This report comments briefly on the assessments of seismic design - for the Napot Point site presented to the Puno Commissi by J. Stepp and C. Lomnit on and dated 30 October 1979.
Some comparisons are made with my own earlier review of several documents relating to the same facility and dated July 18 Finally, an attempt is made to evaluate the consequences of
, 1979.
forward with the construction and operation of the facility at going the currently proposed level of seismic design.
Overview of the Review Process - Stepp and Lomnitz are experienced and competent and their conclusions warrant careful consideration The report by Stepp reveals reasonable profensional procedur reasonable thought processes.
es and Stepp had the considerable advantage of visiting the site, examination of regional structures and tecto i setting,and direct e.scess to geologi. cal and geophysical data and to nc the methods used to obtain these data.
He was able to discuss the nuances of the data with local scientista and other profess-ionals most familiar with these investiga1. ions.
in particular concerning the absence c#Stepp makes some strong points of offshore faulting, any indication the indications of long term tectonic stability of the Napot Point site and the su; rounding region, and the qualit of the data and the investigations which support y
these points.
Stepp concludes that.4g as a reference for Reg Guide 1.60 is a reasonable design value and presents his arguments for this conclusion in a forthright and professional manner.
Adecuacy of Seismic Data Base - I have strong reservations about the statement.by Stepp, that
"...the investigations are extensive and form an adequate basis for defining the relevant earthquake 1.936 M 2 8002070 M
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=ones."
To me this statement is unjustified until the observations are available from an adequate local network even though the conclusien by Stepp may eventually be borne out by such a procedure.
In my judgment, the available seismic data base is inadequate to assess hazards to the level which would be required by good professional practices.
This deficiency will remain until the installation and operation of a local seismic network of good qua_lity for a duration of two years or more.
At present, the available seismic data include observations from a global seismic network, from a local network with limited detection capability and. from a portable microearthquake survey of several months duration.
In my view, the quality of available data is not sufficient (1) to fully interpret the location and level of activity of the boundary of subduction between the two interacting plates of lithosphere and (2) to provide full assurance that there exists no features near the site which have not yet developed strong surface expression and which wculd be revealed only by seismic instrumentation and not by surface investigation.
Stepp states that state-of-the-art geophysical procedures were used and, these kinds of investigations certainly reduce the likelihood that hazardous structures exist undiscovered near the cite region.
Nevertheless, normal professional practice would require data from a local seismic network particularly in this location because of the high level of seismicity throughout the Philippine Islands and because of the indications of active and complex tectonics.
A network installed specifically to monitor volcanic hazard would not necessarily be adequate to assess seismic hazard but modifica-tion of such a network would probably encompass this goal.
The region near the island of Luzon appears to be a zone of tectonics in trancition whereby one subduction zone may be terminating and.another incipient zone may be forming.
Such areas of tectonic transition require, in my judgment, a particularly careful search for potential younger structures at a depth that have not developed a strong surface expression.
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Seismic Hazard Appraisal Based on Current Data -
In my report of July 18, 1979, I discussed the option of proceeding at the Proposed design level of.4g and stated "The likelihood of exceeding the high value of seismic design during the lifetime of the facility is definitely. low but not necessarily remote.
By way of comparison the staff of the NRC would require for a domestic site that the likelihood of exceedance of the design level be demonstrably remote."
I wish to emphasize and expand these comments and develop more clearly the reasoning process.
A design level of.4g when coupled with the response spectrum of Reg Guide 1.60 is a relatively high and conservative value.
With such a design value the facility would not suffer significant damage except from a large earthquake occurring relatively close to the site.
Extensive geologic and geophysical investigations have, in fact, not developed evidence for the presence of a structure appropriate for such an event.
It is not unreasonable that if an adequate network were installed, the resulting observations might justify at some later time a design level of.4g.
In my earlier report I stated "Certain kinds of technical information for example, suggest that major subduction activity may be operating at a reduced level along that segment of the platz boundary in question.
If the appropriate kinds of information were fully investigated and if data were available from a local network, it is possible that a strong case could be made against the future occurrence of an earthquake of magnitude 8 or larger immediately beneath the plant site."
Following are the reasons for these statements:
1.
The absence of large historic earthquakes along western Luzon is significant.
Despite evidence elsewhere for recurrence times longer than 200 years, many active subduction segments appear to have at least one significant shock somewhere along their length during a 200 year interval.
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The presence of irreg21ar bathymetric features in the ocean floor seaward (westward) of the Manila Trench may be in-dicative of a reduced level of subduction activity.
The presence of a possible zone of modified subduction along precisely this segment of the plate boundary was pointed out by Rowlett and Kelleher (1976; Fig.
7, reprint attached).
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The virtual absence along western Luzon of low angle thrust-type focal mechanisms drawn from world-wide seismic data is a possible indication that the zone is not being loaded tectonically in preparation for a great earthquake.
This criteria (i.e.,
the presence of low-angle thrust-type focal mechanisms) is not well established and is the focus of a current research project.
The foregoing suggest that were the seismic zone near western Luzon evaluated by a local network, the indications of tectonic stability described by Stepp may well be found to be justified.
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