Refinement of the Source Parameters ofthe 1948 M8.1 Panay Island Earthquake
By
Bartolome C. Bautista, Maria Leonila P. Bautista, Ishmael NaragNarag,, Angelito G
Lanuza Janila B. Ocampo,Ocampo, Ramil Atando,Atando, Renato U. Solidum Jr.
PHILIPPINE INSTITUTE OF VOLCANOLOGY AND SEISMOLOGY(PHIVOLCS)

10th International Workshop on Seismic Microzoning and Risk Reduction
September 24-25, 2013
GRIPS,, Roppongi,
pp g , Tokyo,y , Japanp

PANAY
ISLAND
Iloilo City

GUIMARASNEGROSISLAND

1948 Panayy Earthquakeq

The earthquake ischaracterized byy intense
ground shaking, extensiveliquefaction and landslides
and a report of small tsunami
Date: January 25, 1948
Time: 12:45 AM
Magnitude: 8.1 (Secondbiggest in the Philippines
Depth: fixed to 33
Location may not be
accurate due to very few
local seismic stations

Whyy is this earthquake interesting?
gThe 1948 M8.2 Panay Island Earthquake is one of the biggest
but least studied earthquake in the Philippines
Philippines.
 There are no scientific studies made and written accounts arealso veryy meager
g since it happenedppbarelyy two yyears after WorldWar II when the Philippines was still trying to recover from theravages of the war.
 The location of the epicenter which is at the southern tip of
Panay island is doubted since the simulated intensities do notg distribution.
match the actual damage
 There were accounts of tsunamis which need to be verified.
 It is important to ascertain the source fault for future seismic
hazards and risk assessment Purposes.
 The people who witnessed the earthquake are now 75 years oldand above and are now disappearing one by oneone.
Methodologygy Accurately determine the actual intensity

distribution
 CompareC
thisthi withith simulated
i l t d intensityi t itdistribution computed using the adjusted fault
plane location and orientation.
 The fault plane parameters that give intensity
distribution that closely match the actuali t it distribution
intensity
di t ib ti isi considered
id d as theth more
acceptable source parameters of the event

Methodology for Generating Historical
Intensity Distribution Map
 Gathered written accounts of earthquake impacts

from old newspapers which are archived in theNational Libraryy and from historical reports
pfrom historians of local government units
 Conducted actual interviews of residents whoare 75 years old and above (12 years old andabove in 1948)
 Onsite observations of existing damaged
structures such as old churches which are theonly remaining masonry structures at that time

Official Documents that tell of the effects of the earthquake: Bugasong, Antique

Town Historian

Minutes of Meeting,February 16, 1948
Municipality of Bugasong

Intensity Survey thru Eyewitness Accounts
We searched for eyewitnesses
y75 yrsy old and above (Antique,Iloilo(qand Guimaras))Interview Proper
Part I: Personal information and setting the time recollection of events
Part II: Earthquake and Tsunami recollections

Evaluation of Intensity Based on
Damage to Churches

Description of Construction Type Pre 1787 Construction Materials

 Pre 1948 Construction Materials

PHIVOLCS
Earthquake Intensity
Scale (PEIS)

Criteria for Evaluating Intensities Based on
Damage to Old Churches
Facade√√√√Cracks(wall)

Partial
Total Damage
Damage(Wall) (Wall)

√√
√√√√√

RoofBelltowercollapse Collapse

Intensity(PEIS)√√Int IX√√√√√√Int VIIIInt VIIIInt VIIIInt VIIIInt VIIInt VI

In the epicentral area in Anini-y, Antique

Near the epicentral area in San Joaquin, Iloilo

Photos of earthquake damage courtesy of Jaro Archbishop Archives OfficeEffects in Duenas, Iloilo

Photo of earthquake damage courtesy ofDuenas Church

Effects in Miagao, Iloilo

Present-day
MiagaoChurch:
UNESCO
heritage site
(Photo of damaged Courtesy of Jaro Archdiocese Archives)Effects to Guimbal Church in Guimbal, Iloilo

Photo Courtesy of Jaro Archdiocese Archives

Effects to Lambunao, Iloilo

Photo of damage Courtesy of Lambunao Fiesta ProgramEffects to Alimodian, Iloilo

Present-dayyAlimodian
ChurchUpper Photos Courtesy of Alimodian Historical Society (upper left)
and Jaro Archdiocese Archives

Effects to Igbaras, Iloilo

Present- day Igbaras Church
Photo of damage Courtesy of Jaro ArchdioceseArchives)

Coronet Tower, Arevalo District, Iloilo City

Eff t iin Oton,
Effects
Ot
IloiloIl il

Observed
Intensity
Distribution
MapFactors that determine the severityy of groundgshakingLocation
 Proximity to seismic sourceLocal soil condition Effect
Eff t off llocall soils
il tto groundd shakingh ki
 Amplification/deAmplification/de--amplification

Intensity Simulation Using Empirical Method

We used the Fukushima and Tanaka ground attenuation
model (1990) to compute for PGA at rock site
 Fault
F lt rupturet
was calculated
l l t d fromftheth Fault
F lt lengthlengthl thMagnitude relation of Wells and Coppersmith (1994)
 Vs30 Site amplification model determined from SRTM
slope map
 This amplification model was verified using H/Vmicrotremor observation and REMI microtremor
measurements.
Generation of Site Amplification Map
Method of Allen and Wald, 2009

Topo MApSlope
Sope MAppVS30 Map

Basis for Estimating Site Amplification
Rock site

Using Topographic and Slope data
Assumptions
1) Thinner residual soils due tohigh erosion rate in steep slopes
2) Thicker residual soils due tolow erosion rate in gentle slopes
Steep Slope

3) Deposition and accumulationof loose water saturated alluvial
materials in low-lying flat areas
Gentle Slope
Flat

GEOLOGICAL MAPQuaternary alluvial, lacustrine, beach and residual
deposits

Late Oligocene-Middle Miocene sediments andvolcanics; marine ss, shale, ls
Upper Miocene sediments and volcanics; largely
marine clastics, reef ls & andesitic-basaltic pf and lv

Pliocene to Pleistocene sediments (marine &terrestrial); includes extensive reef limestone & waterlaid PF; alsolocalized terrace gravel deposits
Undifferentiated Cretaceous to Paleogene strata;
metavolcanics and metasediments
Basic & ultrabasic; mainly peridotite, dunite (generallyserpentinized) & layered gabbro; troctolite, norite,trondjhemite
Carboniferous to Middle Jurassic radiolarite, sandstone,
shale, limestone and conglomerateIntermediate to acid; mainly diorite, granodiorite, quartz
diorite and monzonite

Source: Mines and Geosciences Bureau-DENR
Liquefaction Hazard Mapping. Antique Province– 11.18 to 11.28.2009

Generation of Site Amplification Map (Method 2)
PAGER Vs30 Map

Amplification Map

Amp = -0.0014 *(vs30) + 1.6357

From Allenll andd
Wald, 2009

Microtremor H/V measurement

Microtremor Period VS
Sediment Thickness

From Malte Ibs-von Seht and Jtirgen
Wohlenberg
Bulletin of the Seismological Society ofAmerica, Vol. 89, No. 1, pp. 250-259,February 1999

Nakamura Microtremor method

SR=H / V

SR=Site response Spectra
H= Average spectra of the two horizontal
components
V = Spectra
pof the vertical componentp

Fo = Cs / 4D

Fo=predominant Frequency which is obtained for SR
Cs=shear-wave velocity of surface sediments
D=depth of basement rock

Ao=Cb / Cs

Ao=amplification factor related to impedeance
Cb=shear-wave velocity of basement rockRefraction Microtremor (REMI) Sample Results

Vs Model for Lambunao
0200400600

800

10000-25Depth, m-5075-75-100-125-150-175

Shear-Wave Velocity, m/s@Vs30=375m/s

Determistic Method of Estimating GroundShaking Hazard

Amplified PGA

Attenuation
Equations
B. Applying effects of Site response

Peak Groundp
Acceleration ((PGA)) map

X

Amplification Map
Vs30 Map

Intensity Map

Scenario 1

Scenario 3

Scenario 2

Scenario 3 explains
well the distribution ofdamages incurredduringg the 1948 Panayy
Island Earthquke

Conclusion and Recommendation
The results of the study showed that the most appropriate source isalong an active thrust fault located 30 km west of Iloilo City (Scenario3).
 Not much information was obtained about tsunami which means that
it is probably too small and did not cause any serious damage.
 More eyewitness
yinterviews and intensity
y surveys
y in Northern Panayyand Negros Island need to be done to improve damage distribution.
 More microtremor measurements in Northern Panay and Negros
Island to enhance site amplification data.
 Consider directivity in intensity simulations
 Paleoseismic study of the fault is recommended to verify its latest
movement and estimate the magnitude and recurrence time ofearthquakes
th k

Thank YouForListening

Eyey Witness Interview
Miag-ao, Iloilo:•Miagao
MiCh
Churchhwas damaged•Tsunami
Ti killedkill d fish
fi hwhen water receded•Numerous
Nllong anddwide fissures(“litik”)
Earthquake Eyewitnesses fromMi
Miag-ao:
Lili Gecobe
Lilia
G b anddSalvacion Tuale

Before and After
pictures of the Jaro
Belfry in Iloilo City
photos courtesy of the Museo IloiloJaro Belfry Today