Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

indicative of strike‐slip faulting with Mo=5.3x 10 17 Nm
equivalent to a seismic moment magnitude Mw 5.8
(PHIVOLCS, 2003).
Fig. 2: The plot of 1‐month aftershock distribution of the 2003
Masbate earthquake
The Ms 6.2 mainshock struck early in the evening of 15
February 2003 with extensive effect including the
nearby provinces. The focal mechanism solution
released by PHIVOLCS (2003) suggests a dominantly
left‐lateral slip along a vertical NW‐SE fault that
correlated with PFZ in the area. Several days following
the mainshock, numerous aftershocks were recorded
by the Masbate Seismic Station of PHIVOLCS.
Aftershock plots in Fig. 2 were drawn using GMT by
Wessel and Smith (1998). The aftershock area
indicated at least a 60 km long rupture and distributed
about 20 km wide along the Masbate Fault of PFZ.
GROUND RUPTURE
Field mapping in the municipalities of Dimasalang,
Palanas and Cataingan showed varying surface effects
on the ground and manmade structures caused by the
horizontal displacements of the Masbate Fault of the
PFZ. Surface manifestations varied from hairline cracks,
soil mounds and en echelon fractures along a left‐
lateral fault. Fig. 3 shows the extent of the ground
rupture mapped along the Masbate Fault in Masbate
Island which extends to about 23 km. Following are
some examples of the mapped ground rupture.
Ground rupture related to the 1:47pm foreshock
Field investigation found that the 1:47pm foreshock
caused damages in the epicentral areas. Residents of
the affected structures straddling the fault in the
municipality of Palanas reported that their houses
were damaged by this foreshock with visible cracks on
walls and floors as well as on rice fields and gardens.
Most of the observed damages caused by fissuring
along the fault were mainly on concrete floors and
asphalt roads with hairline or ~2‐4 mm‐wide cracks. In
Sta. Cruz, Palanas, the road was fissured during this
foreshock and booming sound was heard preceding the
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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quake. Within the epicentral area such as in Nabangig
and Matugnaw site, sulfur‐like and/or gun powder‐like
smell was reported that possibly indicate ground
fissuring during the foreshock event.
Fig. 3: Map of the ground rupture produced during the 2003
earthquake along the Masbate Fault. Shaded area and stars in
the index map are the location of the ground rupture map and
the epicenters of historical events in the area, respectively.
Small letters in the left table indicate the location of the
measured horizontal and vertical displacements along the
surface rupture as indicated in the map
Ground rupture related to the 7:0lpm Mainshock
‐ Dimasalang area
In the northernmost portion of Dimasalang, the ground
rupture produced during the main shock transected
the coastline with apparent left‐lateral displacement
(Fig. 4). The fault trace is manifested by alternating
smooth sand mounds and depressions that eventually
were obliterated by waves during high tides and
human activities. During low tide, the ground rupture
in the shallow portion of Naro Bay was discerned as en
echelon faults cutting through the thinly mud‐covered
dead corals. The en echelon faults are at least a meter
in length trending N76 o W with a general trend of
N62 o W. Horizontal displacement measured at this