ESTIMATION OF PEAK HORIZONTAL GROUND ACCELERATIONS

COVERING THE MANOLO FORTICH 1 HEPP & 2 HEPP

PROJECT SITES

ESTIMATION OF PEAK HORIZONTAL GROUND ACCELERATIONS

COVERING THE MANOLO FORTICH 1 HEPP & 2 HEPP PROJECT SITES

INTRODUCTION

The RSJ & Associates requested the again the undersigned for his professional

comments on the estimation of peak ground acceleration (PGA) values for the Manolo

Fortich 1 & 2 Hydro Electric Power Plant (HEPP) structures. Said estimation was

sought by the HEDCOR Manolo Fortich Project Engineering Team to the former for the

latter’s consideration on the design of the different structures to be constructed in the

area. Taking consideration on the request, and with reference to the works of several

authors (e.g. Thenhaus, et al., 1994; Zhang, et al., 1999) on estimation of ground-

motion hazards, presented hereunder the estimated values for peak ground horizontal

accelerations for the project site.

THENHAUS ET AL.’S PROBABILISTIC GROUND-MOTION HAZARD MODEL

The probabilistic ground-motion hazard model developed by Algermissen et al. (1982)

was further improved by Thenhaus, et al. (1994) with the addition of a computer

algorithm that calculates ground-motion from planar seismic sources that dip. Such

consideration provides a more accurate representation of the geographic distribution of

strong ground-motion from earthquakes that rupture the shallow plate interface of

subduction zones (Thenhaus, et a., 1994)

Inasmuch as there is a lack of adequate strong ground-motion in the Philippines, the

recently developed attenuation relation for Japan (Fukushima and Tanaka, 1990),

described below, was applied by Thenhaus, et al. (1994).

log 10A = 0.41M – log 10(R + 0.032 x 100.41M) – 0.0034R + 1.30

where A is the mean peak acceleration (in cm/sec2), R is the shortest distance between

the site and the fault rupture (km), and M is the surface-wave magnitude. As described

by Thenhaus, et al., (1994), the standard deviation of log A for the relation is 0.21with a

multiple correlation coefficient of 0.89.

The results of such improvement are presented on the following maps (Figures 1 and

2) covering the entire Philippines.

(1) (2)

Fig. 1 – Map showing peak horizontal acceleration amplitudes in rock for the Philippines. Acceleration

values have a 10% probability of exceedance in 50 years. Fig. 2 – Map showing peak horizontal

acceleration amplitudes in medium soil for the Philippines. Acceleration values have a 10 percent

probability of exceedance in 50 years. All contours are in terms of the acceleration of gravity (g).

ZHANG, ET AL.’S GLOBAL SEISMIC HAZARD ASSESSMENT PROGRAM

Zhang, and others in 1999 developed the Global Seismic Hazard Assessment Program

(GSHAP) in Continental Asia, for which the Philippines cooperated in the program

implementation. Result of the program is presented on a map on Figure 3. The seismic

hazard map depicts PGA with a 10% chance of exceedance in 50 years and the site

classification is rock.

Zhang, et al. (1999) applied the relationship determined by Huo and Hu (1992) for

strong ground attenuation relation, where the intensity and strong ground motion data

from both China and western USA were utilized. Zhang used the relationship in their

seismic hazard computation in Asia and it is expressed as:

lnY = 0.1497 + 1.9088M - 2.049M2 - 2.049ln[R+0.1818exp(0.7072M)]

where Y is the peak ground acceleration at the site. R is the shortest distance (in km)

from the site to the vertical projection of the earthquake fault rupture to the surface. M is

the surface wave magnitude.

The result of their computations is presented in a map shown in Figure 3.

Fig. 3 – Seismic hazard map of Asia depicting Peak Ground Acceleration given in units of m/s

2 with a

chance of exceedance in 50 years. The site classification is rock (adapted from Zhang, et al., 1999).

This seismic hazard map of Asia depicts the shaking hazard that will have the largest

effect on one to two story structures (the largest class of structures in Asia). In addition,

the seismic hazard (PGA) values in Asia reflect the complicated regional

seismotectonics. The highest hazard values are along the subduction zones that border

eastern and southwestern Asia.

ESTIMATION OF PGA VALUES FOR MANOLO FORTICH 1 & 2 HEPP

Presented below in Table 1 are the classifications of the lithology underlying where the

component structures for the two HEPP project sites are to be erected.

PROJECT NAME/COMPONENT

GEOGRAPHIC COORDINATES CLASSIFICATION OF UNDERLYING

LITHOLOGY LATITUDE LONGITUDE

Manolo Fortich 1 HEPP

Weir 1 (Tanaon R.) 8° 27’ 43.27” N 125° 02’ 07.75” E Rock

Weir 2 (Guihean R.) 8° 24’ 48.60” N 125° 01’ 18.52” E Rock

Weir 3 (Amusig R.) 8° 27’ 18.22” N 125° 00’ 00.06” E Rock

Surge Tank 8° 25’ 55.07” N 124° 59’ 11.83” E Rock

Head Pond 8° 25’ 36.02” N 125° 00’ 03.54” E Rock

Power Station 8° 25’ 48.30” N 124° 58’ 56.65” E Rock

Manolo Fortich 2 HEPP

Weir 4 (Lower Amusig R.) 8° 25’ 49.13” N 124° 58’ 53.05” E Rock

Surge Tank 8° 25’ 04.43” N 124° 55’ 26.19” E Medium Soil

Power Station 8° 25’ 01.82” N 124° 55’ 12.44” E Medium Soil

Table 1 – Comparison of underlying lithology for each project component for the two HEPP sites

Taking the above information/data and plotting each respective project component on

the corresponding map showing respective peak ground acceleration values, the

following plots were obtained (Figures 4, 5 & 6).

Fig. 4 – Map showing the locations of project components using the Thenhaus, et al. Probabilistic

Ground-Motion Hazard model for rock as site conditions

Fig. 5 – Map showing the locations of project components using the Thenhaus et al. Probabilistic Ground-

Motion Hazard model for medium soil as site conditions

Fig. 6 – Map showing the locations of project components using the Zhang et al. Global Seismic Hazard

Assessment Program for rock as site conditions.

Shown on Table 2 are the estimated peak ground acceleration values for each project

component when the two previously described relationships are applied.

PROJECT NAME/COMPONENT

CLASSIFICATION OF UNDERLYING

LITHOLOGY

ESTIMATED PEAK GROUND ACCELERATION

USING THENHAUS, ET AL. MODEL

(g)

USING ZHANG, ET AL. GSHAP

(m/ s2)

Manolo Fortich 1 HEPP

Weir 1 (Tanaon R.) Rock 0.21 3.4

Weir 2 (Guihean R.) Rock 0.21 3.4

Weir 3 (Amusig R.) Rock 0.21 3.4

Surge Tank Rock 0.21 3.4

Head Pond Rock 0.21 3.4

Power Station Rock 0.21 3.4

Manolo Fortich 2 HEPP

Weir 4 (Lower Amusig R.) Rock 0.21 3.4

Surge Tank Medium Soil 0.30

Power Station Medium Soil 0.30

Table 2 – Comparison of estimated peak ground acceleration (PGA) values for each project component

for the two HEPP sites using the Thenhaus model and Zhang GSHAP program.

Based from the above values described in Table 2, the estimated PGA values when

using the Thenhaus model are comparatively lower than the PGAs if the Zhang’s

GSHAP approach is utilized. In the same manner the former is of a lesser cost in terms

of economics. For the above, this writer believes that the Zhang’s approach has a

greater emphasis which takes into consideration the global seismotectonic aspects but

of a higher cost in terms of structural design implementation.

It is thus suggested that the user of this information generated possibly check on the

values obtained by using the mathematical relationships applied both by Thenhaus

(1994), Zhang (1999) to have a more appropriate estimates of PGA values for each of

the project component planned for the Manolo Fortich 1 & 2 HEPP.

Prepared by :

JUANITO A. MANZANO, JR., MSc. EnGeo

Consulting Geologist

PRC License No. 0000655

PTR No. 9187534

Issued on: 01/30/2014

Issued at: Quezon City