(Article published in the
issue of Manila Standard Today)
The apparently unceasing high-intensity earthquakes that have been visiting Japan since the middle of this month inevitably raise the question of the Philippine’s own vulnerability to and preparedness for (or lack thereof) the Big One. I do not know if there is any study covering the entire archipelago but I do know that one exists (or, more precisely, had existed) for Metro Manila.
It was done from October 1976 to February 1977 under the auspices of the Office of the United Nations Disaster Relief Co-ordinator (the UNDRO) and was a project headed by Jean Paul Chardin, with the objective of generating (a) a systematic vulnerability analysis and, as its most significant output, (b) a risk map for consideration in the crafting of the urban master plan of Metro Manila.
Two UNDRO consultants,
Messrs. Michel Couillaud and Jacques Didon conducted the necessary survey
and collected the data needed to produce the report; J. P. Chardin put the
report in layman’s terms and submitted it to the then Governor of Metro
Manila. But, unfortunately, like many other studies, usually
foreign-funded, the “Composite Vulnerability Analysis” with the subtitle “A
methodology and case study of the Metro Manila Area” (“UNDRO Report”) was
not given the attention it deserved. Perhaps because it sounded too
technical (despite the efforts of J.P. Chardin, who years later also became
special adviser to the UN International Decade for Disaster Reduction, to
put it in understandable language) and/or because it sounded very much like
a party-pooper and was therefore not in tune with then much touted vision of
a beautiful Metro Manila, the report was simply ignored.
The UNDRO, as an office, is no more; it has been absorbed since then by the UN Department of Humanitarian Affairs but the report’s observations remain accurate and its recommendations still worth considering. Besides, last week’s 5.7 intensity quake, felt in Metro Manila, give the report both urgency and currency.
The ideas that follow I owe exclusively from the aforementioned UNDRO Report. I thus dispense herein with the pesky quotation marks to indicate that the ideas are not mine and the text, for the most part, my own words.
Destructive earthquakes, based on existing lists of earthquakes that have affected Metro Manila in historical times, have occurred many times in the last 300 or so years. On an average, Metro Manila has been shaken by damaging earthquakes once in every 15 years. The strongest of said earthquakes have been assessed at Intensity 10 in the Rossi-Forel scale which, reflects the grades of earthquake intensities as they are felt by the affected populace. Intensity 10 earthquakes, according to historical records, have had an average return period of 130 years.
Metro Manila’s vulnerability to earthquakes is made understandable in a large measure, by its geomorphological structure. Metro Manila is composed of a low elevation relief extending from the foot of the Sierra Madre range at the north to the slopes of Taal Volcano in the south. It forms an arm of land which separates Laguna Lake from Manila Bay and is bordered by sendentary plains sloping gently eastward to the lake and westward to the bay and ultimately to the sea.
The bedrock of this land arm is a volcanic formation called the Guadalupe formation (locally named Adobe) which is a thick sequence of well-bedded volcanic tuff and tuffaceous elastics which date from the earliest Pleistocene Ice Age, about a million years ago.
On both sides of this bedrock are the sedimentary plains, dating from the Second Glacial Age about 7 hundred thousand years ago, which were formed essentially from alluvial sediments transported by the Marikina River as well as the sediments in the lake water impounded by the Pasig River. This alluvium is made up of an unconsolidated mixture of sand, some gravel, and considerable silt and clay derived cliefly from the weathering of pyroclastic and volcanic rocks.
A third category of current subsoil in Metro Manila need be added to adobe and alluvial deposits and that is the “fill”, or “tambak” in the vernacular. Probably the most recent and largest expanse of “tambak” in Metro Manila the area recently reclaimed from the Manila Bay starting with the term of President Ferdinand E. Marcos.
Neither bedrock adobe nor soft alluvium behave like school children in a straight line nor do they observe uniform layering of subsoil throughout their varying length and witdth. Hence, the combination of bedrock and alluvium in one area is not necessarily exactly the same as the next adjacent unit of measure. For an accurate picture, one needs to consult detailed geologic maps (two of which are part of the UNDRO report) in order to know whether he is standing on bedrock or on ground liable to shifting.
It may not be amiss to state at this point that, based on actual drilling and core analyses, generally, the commercial district of Sta. Cruz, Sampaloc, Quiapo, Escolta, Intramuros, Port Area, Ermita, Paco and Malate, all in the City of Manila, are underlain by plastic clays, silts, sand and gravels with an intricate admixture of marine shells, corals and decayed plants. In other words, said sites are not on bedrock.
Everytime the earth heaves and hos, those areas atop bedrock will necessarily be affected differently from those on softer ground. Generally, less impact will be felt by those on bedrock like Novaliches, Quezon City, Guadalupe and Parañaque Hills; more will be felt by those near the Pasig River mouth and in the souther part of Marikina plain. That is Geology 101.
But geology alone is not determinative of the total impact of earthquakes on Metro Manila. In my next pieces, I will deal with other factors that shake Metro Manila guaranteed at the very least to rattle our nerves.