## 2 The Study Area  
Metropolitan Manila (Metro Manila) is the capital region of the Philippines (Figure 1), which is also the center of the economy and the most populated areas. The region is composed of several cities and municipalities, sprawled over an area of around 620 square kilometers. It lies in the southern extension of the Luzon Central Valley. In the early stagesof the development of the Central Valley, rapid sedimentation from highlands persisted until the Late Cretaceous, intercalated with tuff and other volcanic products [37]. During the Late Tertiary and Quaternary periods, complex tectonic events led to the basic geologic structure of Metro Manila at present. This geologic evolution has produced three distinct terranes in the metropolis: the Coastal Lowland, Central Plateau, and the Marikina Valley. East of the metropolis, the uplands of Rizal province with basement-ophiolite complexes can be found. To the west, this region is bounded by Manila Bay.  
The Coastal Lowland is the flat and low plain section of Metro Manila, facing the Manila Bay. This section can be further subdivided into deposits from sand bars, back-marshes, tidal flats, the Pasig River delta, and some reclaimed land. This Quaternary Alluvium deposit has a thickness that can reach up to 40 m [40]. Sixty percent of the Metro Manila area is underlain by the Central Plateau, which is the geomorphic manifestation of the bedrock of the region, namely the Diliman Tuff member of the Pleistocene Guadalupe Formation. This lithologic member is made of flat-lying sequences of medium to thin-bedded, fine-grained vitric tuff and welded pyroclastic brecias. Minor fine to medium-grained tufaceous sandstone was also reported to be part of this unit [41]. The Marikina Valley is the flat section on the eastern bounds of the metropolis, with lower elevations than the Central Plateau. This is produced by a pull-apart basin associated with the West Valley Fault (WVF) and East Valley Fault (EVF), collectively called the Marikina Valley Fault System [42]. Quaternary alluvium deposits in this section are associated with the floodplain of the Marikina River and the delta along the Laguna de Bay. Surface deposits on this sector are reported to reach up to 50 m in thickness [40].  
Geologic field mapping and different subsurface investigations have revealed that most of the Metro Manila soils and sediments are composed of thick sequences of pyroclastic and epiclastic deposits. Pyroclastic deposits, primarily tuff materials, are derived from eruptions of nearby volcanoes in the region. The epiclastic deposits come from pyroclastic materials that were reworked by fluvial and lacustrine processes. [PERSON] (1983) [43] reported that these epiclastic fluvial deposits are generally made of a sequence of loose to firm silty fine sand at the surface, underlain by very soft clayy silt or silty clay, further underlain by stiff to very stiff clay, and finally hard clay and silt, sometimes intercalated with very dense sand and/or gravel that grade into bedrock. The thickness and lateral continuity of these  
Figure 1: (**A**) Geological and (**B**) geomorphologic map of Metro Manila, Philippines, and the locations of the refraction microtremor surveys. Geologic map modified from [PERSON] (1983) [38] and geomorphologic map obtained from [PERSON] (2004) [39]. Locations of the testing sites are also depicted in black squares.  
layers is highly variable, such that during borehole investigations on a particular site, it is normal to encounter different thicknesses and sequences of soil and sediment deposits.