Citation:Oestar, J., T. Hayashida (2021), Estimation of source, propagation, and site amplification
factors using broadband seismic data in the Philippines, Synopsis of IISEE-GRIPS Master’s Thesis.

ESTIMATION OF SOURCE, PROPAGATION, AND
SITE AMPLIFICATION FACTORS USING
BROADBAND SEISMIC DATA IN THE PHILIPPINES
Joel Oestar1,2

Supervisor: Takumi HAYASHIDA3
ABSTRACTThis study estimates the source, propagation path, and site amplification factors of the two regions inthe Philippines: the Luzon and Mindanao. Using the generalized spectral inversion technique, weseparated the acceleration Fourier spectra of the ground motions recorded by the Philippine Seismic
Network (PSN) from 2016-2020. The source, site amplification, and anelastic attenuation (Q) factors inthe two regions were estimated using the "reference event" and the "reference site" constraints. We
obtained almost similar results in the Q factor using the two constraints. The "reference site" approachshows stable results in the Mindanao region. However, when using the "reference event" approach, eachsite's estimated site amplification factors show spectral decay in the high frequencies for the Mindanao
region. The same spectral decay in the Luzon region using the "reference site" constraint. The obtained
source spectra also differ among the two constraints, especially in the Mindanao region. We found thatthe spectral amplitudes in the higher frequencies possibly depend on the earthquake's source depth, nottheir source type. In the Luzon region, spectral amplitudes in the higher frequencies are observed forcrustal earthquakes when we adopt the "reference site" constraint. The Q-values in the Luzon region
using the reference site approach shows a weaker frequency-dependent than those estimated in theMindanao region.
Keywords: Seismic wave attenuation, Site amplification factor, Source factor, Spectral inversion.
1. INTRODUCTION
Earthquake is one of the natural hazards that occur in a seismically active region such as the Philippines.
In general, the earthquake ground motions depend on the seismic source, path, and site amplification
factors. Characterization of these factors, especially the site amplification effects that may amplify theseismic motion, is important for estimating earthquake damage for future earthquakes. On the other hand,understanding the attenuation parameter such as regional Q-values is indispensable for strong groundmotion prediction from the viewpoint of engineering seismology. One of the examples of site
amplification effect is the 1968 Casiguran, Aurora earthquake (7.3Mw), which caused the collapse ofRuby Tower in Manila, and several major buildings near the area sustained varying levels of structural
damages. The severely damaged area was about 225 km away from the epicenter. This event iscomparable to the 1985 Michoacan, Mexico Earthquake (8.0 Mw), which caused severe damage in areas
400 km away from the epicenter.

1Department of Science and Technology-Philippine Institute of Volcanology and Seismology (DOSTPHIVOLCS), Philippines.
2
IISEE-GRIPS Master’s course student.
3International Institute of Seismology and Earthquake Engineering, Building Research Institute, Japan.

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In the last half-century, different studies attempted to estimate the site amplification effects.
Numerous empirical and numerical methods have been developed. The widely used method was theStandard Spectral Ratio (SSR) method (Borcherdt, 1970). Borcherdt (1970) used the spectral ratio
between the sediment site and a reference site (usually on bedrock) from the same earthquake to remove
the path effect. Andrews (1982, 1986) proposed the generalized spectral inversion technique andconsidered the geometrical spreading factor for the path term.
On the other hand, Boore (1983) added the geometrical spreading factor and anelastic
attenuation of ground motion to the path term. Iwata and Irikura (1986, 1988) used reference site
constraints to estimate stably the source, site, and Q factors. However, Boatwright et al. (1991) andFletcher and Boatwright (1991) constrained the shape of the source spectrum to perform the spectral
inversion. This study estimates the strong ground motions factors observed by the Philippine Seismic
Network. We follow the method of Moya and Irikura (2003) in data processing, which contained theshape of the source spectrum before performing a spectral inversion. We also used an alternative
approach—the reference site constraint proposed by Iwata and Irikura (1988), especially in regions
where no event pairs are located to each other.
2. DATAThis study used earthquake records at 49 Satellite-Telemetered Seismic Stations (STSS) of thePhilippine Seismic Network (PSN), operated by the Philippine Institute of Volcanology and Seismology(PHIVOLCS). The seismic stations were divided into three regions to facilitate more convenient spectral
inversion for Luzon, Visayas, and Mindanao. Based on the PHIVOLCS SWIFT CMT catalog, weselected more than 300 earthquakes for the minimum magnitude of 4.5 and a maximum source depth of60 km. The earthquake records are in velocity waveform in SEED data format and manually extracted
from the continuous data recording based on the event catalog. The velocity waveforms were trimmed
1 minute before the P-wave onset with a 20-minute duration and converted SEED velocity waveform
into SAC acceleration data format.
3. METHODOLOGY
In this study, we first automatically extract the S-wave part of the waveforms using the Husid plot (Husid,1969) by accumulating the acceleration waveform amplitudes and find when the accumulated amplitude
becomes 5% and 95% of the total energy. Generally, the 5%-95% range corresponds to the S-wave partof the waveform. Next, we remove the DC and linear trends and apply a cosine taper of approximately
5% of the 20.48-seconds duration from the head and tail of the waveform data. Then, we conduct theFast Fourier Transform (FFT) to obtain the acceleration Fourier amplitude spectra and horizontal-tovertical (H/V) spectral ratio of the S-wave part.
Naturally, in the frequency domain, the observed S-wave Fourier amplitude spectrum 𝑂 𝑓of an earthquake can be expressed as the product of these three factors— 𝑆 𝑓 is the source of the ithearthquake, 𝑃 𝑓 is the propagation path effect, and 𝐺 𝑓 is the site effect at the jth station as
expressed in equation (1):(1)
𝑂 𝑓
𝑆 𝑓 ∙𝑃 𝑓 ∙𝐺 𝑓
This study mainly follows Moya and Irikura's (2003) method in data processing to separate
these three factors and estimate each contribution to the strong ground motion, then used the Fortranprogram package developed by Dr. Hayashida (IISEE-BRI) for spectral inversion. For the reference
event constraint, spectral shape of the source effect 𝑆 𝑓 is given by the 𝜔 model (Brune, 1970):

Ω𝑆 𝑓12𝑓𝑓(2)where Ω is the low-frequency flat level, and 𝑓 is the corner frequency of the i-th earthquake. Toestimate the source model in equation (2), we need to find the low-frequency flat level Ω and the cornerfrequency 𝑓 of the target event. We used the obtained seismic moment in the catalog of PHIVOLCSSWIFT CMT of the target event, and we found the Ω from,𝑅𝑀(3)Ω4𝜋𝜌𝑉
where R = 2/5 is the root mean square radiation pattern (Andrews, 1986), 𝜌 = 2700 kg/m3 is the density,
and 𝑉 = 3.2 km/s is the shear-wave velocity of the medium around the source. 𝑀 is the seismicmoment (Nm) obtained from the PHIVOLCS-SWIFT CMT catalog.
For the estimation of corner frequency 𝑓 , we took the spectral ratios from the two events,
which are relatively close to each other and have different magnitudes. The spectral ratio of the bigger
magnitude event over the smaller magnitude event will remove the site and path effects and follows the𝜔 model. The 𝑓 can be found by comparing the average of the target events and the best-fitting
function,𝑓𝑓(4)
1𝐹 𝑓Ω ∙ 1
𝑓𝑓
where Ω is the ratio of the low-frequency flat levels between the bigger and the smaller events, 𝑓
is the corner frequency of the small event, and 𝑓 is the corner frequency of the big event.
In regions with no events relatively close to each other, we used the method of Iwata andIrikura (1988)— the reference site constraints. We selected a reference site with no amplification for all
frequencies using the S-wave H/V spectral ratio and assumed the site amplification at this site (i.e., rocksite) to be G(f)=2.0 due to the free surface effect.
4. RESULTS AND DISCUSSION
4.1 Source Effect
In the Mindanao region, we used two alternative constraints—the reference event and reference siteconstraints. We evaluate and compare the estimated source factors between the two constraints (seeFigure 1) and found that some of the source spectra using the reference event approach do not follow
the 𝜔 - model (Aki, 1967; Brune, 1970). However, using the reference site approach, all of the source
spectra follow the 𝜔 - model. Moya and Irikura (2003) suggest that this is caused by the contaminationof the path and site effects that distort the spectral decay from the observed spectra. We checked eachspectrum and event location and found that all estimated source spectra having larger spectral amplitudes
in the high frequencies are subduction zone earthquakes with depths between 20-60 km. This trend
confirms the investigation of the spectral shape differences for intraslab/interplate earthquakes innortheast Japan conducted by Kasatani and Kakehi (2014). For the Luzon region, constraining thereference event was not possible due to the lack of event pairs located near each other. Instead, we onlyused the reference site constraint for inversion. However, the estimated source factor in the Luzon region
shows similar spectral decay with those in the Mindanao region using a different approach, the reference
event constraint. Both of the estimated source spectra do not follow the 𝜔 - model.
We failed to estimate the source, site amplification, and Q factor for the Visayas region due tolittle earthquakes accepted by the set criteria for inversion for the two approaches. Estimating the siteand Q factor in the Visayas region will take more time until we meet the necessary event or try othercriteria for selecting earthquake

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Figure 1. Estimated source factors in Mindanao
Region. The left and right panel shows the source
spectra of all the events used for inversion using
the reference event and reference site constraints,respectively.

Figure 2. The estimated source factor (in theleft panel) and 1/Q (in the right panel) factor
results in Luzon Region using reference sitet i t

4.2 Site EffectFigure 3 shows relations of estimated site amplification factors (thick red line) and H/V spectral ratio(thin gray line) at station LCV in the Mindanao region using two different approaches, i.e., reference
event and reference site constraints. The reference site approach in the Mindanao region shows the same
spectral shape between the estimated site amplification effect and the S-wave H/V spectral ratio.
However, when the reference event constraint is adopted, the shape of the estimated site amplification
effects are different from those of the S-wave H/V spectral ratio—showing rapid spectral decay in thehigher frequencies. It only indicates that in the Mindanao region, inversion constraining a reference site
has more stable results than a reference event constraint. Figure 4 shows relations of estimated site
amplification factors (thick red line) and S-wave H/V spectral ratio (thin gray line) at stations RTB andTIR in the Luzon region using the reference site constraint. Here, we failed to compare the estimatedsite factor results between the two constraints. Instead, we compared the estimated site factor results
between the Luzon and the Mindanao region. The estimated site factor at station TIR in the Luzon region
using the reference site constraint (Figure 4, right panel) shows similar spectral decay as station LCV inthe Mindanao region using the reference event (Figure 3, left panel). The rapid spectral decay in thehigher frequencies does not depend on the constraints used for inversion but instead can be explainedby fmax [e.g., Hanks, (1982), Boore, (1983)] and kappa [Anderson and Hough (1984)]. On the other hand,a slight deviation between the estimated site factor (thick red line) and the assumed free surface effect(G(f) = 2.0) at the reference site in Figures 3 and 4 was due to the uncertainties in the inverse problem,

Reference site

Reference siteFigure 3. Comparison of estimated site factorresults (thick red line) and the S-wave H/Vspectral ratio (thin gray line) of LCV station inthe Mindanao region using the reference site andreference event constraints.

Figure 4. Estimated site factor results (thick redline) and the S-wave H/V spectral ratio (thingray line) in the Mindanao region using thereference site constriant.

44.3 Q EffectFigure 5 shows the comparison of the estimated Q values between the Luzon and Mindanao regions.
The Q-values in the Luzon region show smaller values in comparison with those in the Mindanao region.
One possible reason is that the Q-values are dependent on the traveling path or subsurface soil medium.
All of the earthquakes used in the inversion in the Luzon region were crustal-type earthquakes. For
crustal-type earthquakes, the seismic wave propagates in the shallow part where Q is low. On the otherhand, most of the earthquakes used in the Mindanao region were subduction zone earthquakes. The
attenuation model results for these two regions will be useful for various scientific studies in seismichazard assessment and engineering seismology.

Figure 5. Comparison of estimated Q factor
results between the Luzon (orange) andMindanao (blue) region using reference site

Figure 6. Comparison of estimated Q factor
results in the Mindanao region using thereference event and reference site constraint.

5. CONCLUSIONSThis study applied the spectral inversion method to strong ground motion data recorded by the Philippine
Seismic Network (PSN) from 2016 to 2020. Fourier spectra of the observed S-wave portion were used
to estimate the source, site amplification, and Q-factor of the three regions in the Philippines— theLuzon, Visayas, and Mindanao in a frequency range from 0.1 to 10 Hz.
The Q-values obtained in the Mindanao region were almost similar for both approaches: thereference event and the reference site. We estimated the low-frequency flat level and the cornerfrequency of one event to model source characteristics for the reference event. We utilized the seismic
moment from the catalog and the obtained corner frequency of the reference event to make the inversion
process more stable. For the reference site approach, we used S-wave H/V spectral ratio information ateach site and selected the station that showed a flat value entire the target frequency range. We obtained
the Q-values in the Luzon region using only the reference site constraint due to the difficulty of cornerfrequency estimation.
In this study, the estimated site amplification factor using the reference site approach shows
more stable results than the reference event. We used the reference site approach for the Luzon andMindanao regions and estimated site factors assuming the site, which shows flat H/V spectral ratios as
the reference site. It is an indication that the generalized spectral inversion technique well captures theaverage site amplification characteristics. However, there is a comparison between the spectral decay inthe site amplification factor of the Luzon region, which follows the reference site constraint, and thosewith the Mindanao region that used the "reference event" constraint.
We compared the best method to follow for each region between the two approaches; thereference event and the reference site. We analyzed the source spectra obtained in both methods. Wealso found that the obtained source spectra in the subduction zone earthquakes using the reference event
have higher high-frequency spectral levels. We also validated the relationships between higher highfrequency spectral levels and source depth ranging from 20-60 km. Estimating the site and Q factor in5

the Visayas region will take more time until we meet the necessary event or try other criteria for selectingearthquakes.
ACKNOWLEDGEMENTS
This research was conducted as the individual study of the training course “Seismology, Earthquake
Engineering and Tsunami Disaster Mitigation” by IISEE/BRI, JICA, and GRIPS. I would first like toexpress my utmost gratitude to Dr. Takumi Hayashida for supervising me and giving me very usefultools, advice and suggestions during my study. The completion of this study could not have beenpossible without his expertise and to Dr. Yokoi for the constructive comments and ideas regarding thisstudy during every rehearsal presentation.
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