In an effort to contribute towards the understanding of the Marikina Valley fault system which has been reported active based on geologic and geomorphic evidences, a preliminary radon survey was launched. The survey was undertaken primarily to establish a baseline data for radon and to relate its significance to the possibility of an earthquake along this major structural line. The field coverage included the two northeast trending faults that bound the Marikina Valley, here referred to as the West Marikina Valley Fault (WMVF) and the East Marikina Valley Fault (EMVF). The survey involved mainly soil-gas measurement across the different segments of the Marikina Valley faults and in some selected stations outside the fault zones to differentiate the radon concentration for the various lithologic units. The results obtained for the different rock types indicate the small disparity in the radon distribution between the East Marikina Valley Fault and the West Marikina Valley Fault. The traverse results, on the other hand, disclosed the apparent increasing radon gas readings towards the fault. This implies that more radon gas are coming out along the fault line than in other areas even though the fault is covered with colluvial deposits.

           In general, it can be inferred that the overall relatively low radon concentration indicates the absence of any significant subsurface activity that may initiate ground movement.

           The applicability of the radon technique in the monitoring of earthquake zones, active volcanoes, and others which are considered potential hazards has been demonstrated through the initiative of Dr. Philip D. Roberts of the British Geological Survey (BGS) during his short stint in the country under the auspices of the Mines and Geosciences Bureau. Part of his many field activities included on-site radon measurements at Taal Volcano in Batangas which had shown signs of restiveness since March 1991 and in the well known Digdig Fault in Nueva Ecija which had been the epicenter of the July 16, 1990 strong earthquake that ravaged the Central-Northern Luzon region. The outcome of his preliminary works was very encouraging and disclosed interesting results which he hoped would be pursued and utilized in other areas.

           The rationale behind this radon monitoring technique lies in the fact that an earthquake or ground rupture is usually accompanied by regular changes in certain components of the subsurface, radon gas in particular. Likewise, an earthquake in the making is usually preceded by attendant changes in the elastic properties of the rock resulting in the development of fracture systems which serve as avenues or passages for the escape of radon gas to the surface, thus the increase in its concentration. Surface detection, therefore indicates the level of ground radon activity useful in the prediction of a possible earthquake. Radon is a radioactive gaseous nuclide, formed by the emission of an alpha particle from the disintegration of radium. It does not combine with other elements which permits its free migration or dispersion through pore spaces in rock and soil. In nature, there are three isotopes of radon, namely Rn S0222 T (Radon), Rn S0220 T (Thoron) and Rn S0219 T (Actinon) all derived from the decay series of U S0238 T, Th S0232 T and U S0235 T respectively. Their corresponding half lives are 3.82 days for radon, 54.5 seconds for thoron and 3.92 seconds for actinon. It is interesting to note that actinon has the least half life and the isotopic occurrence of its parent source accounts only for 0.72% of the total uranium in the natural environments. Thus, radon and thoron are the two isotopes commonly measured.

           This report summarizes the preliminary radon survey carried out at the well-known Marikina Valley Fault which has been reported to be active based on geologic and geomorphic evidences. The survey was undertaken primarily to establish a baseline data for radon and to study, monitor and possibly forecast any possible subsurface movements that may be related to the Marikina structures. This undertaking was conducted during the periods 9-25 September and 23-24 October 1991 through a joint effort of the Philippine Nuclear Research Institute (PNRI), Philippine Institute of Volcanology and Seismology (PHIVOLCS) and the Mines and Geosciences Bureau (MGB). The field coverage was composed of the two northeasterly trending faults that bound the Marikina Valley (Fig. 1). These faults encompass the Marikina Valley and the neighboring areas of San Mateo, Montalban, Antipolo and the eastern section of Metro Manila.

           The survey involved mainly radon gas measurements across the different segments of the Marikina Valley faults and in selected points at some distance from the fault structures.

           The geology of Metro Manila area was reported by Gervacio (1968) and NEPC (1987). Shown in Fig. 2 is the geology of Metro Manila reproduced from the 1:50,000m map published by the Metropolitan Waterworks and Sewerage System (1981).

           In general, recent alluvial deposits probably derived from the Guadalupe Formation cover a considerable portion of Metro Manila, particularly the plain areas. They are composed of silts, sands and unconsolidated or very poorly consolidated and unsorted pebbles, cobbles and small boulders of the underlying rocks. Gervacio (1968) disclosed two separate depositional environments for these alluvial strata; the Manila Deltaic Plain and the Marikina Valley Alluvial Plain. The thickness of these alluvial deposits vary accordingly from place to place. In the deltaic area, the deposits measure over 50 meters thick near the coast and thin out toward the east. In the Marikina plain, it ranges from 30 meters to as much as 130 meters. However, alluvial succession up to 200 meters in thickness was noted in Cainta and Pasig areas (NEPC, 1987).

           The Pleistocene Guadalupe formation consisting of massive and thick sequence of volcanic and sedimentary rocks constitutes a vast portion of Metro Manila, extending from Quezon City and Novaliches in the north to as far as Cavite in the south. This formation is differentiated into three members, namely, a) the Alat member, b) a pyroclastic member and c) a sedimentary member. The pyroclastic member and sedimentary member are referred to as the Diliman Tuff. The sedimentary Alat series, well exposed in the northeastern section of La Mesa-Novaliches district, is composed of a sequence of massive conglomerate, deeply weathered silty mudstone and tuffaceous sandstone. The thickness of the Alat formation is about 100 meters (BMG, 1982, NEPC, 1987).

           The Diliman tuff as the name implies is well defined in the locality of Diliman, Quezon City. This unit is made up mainly of fine grained vitric tuffs and welded volcanic breccias with subordinate amounts of tuffaceous, fine to medium grained sandstone. This member is also characterized by a relatively high clay content and presence of abundant organic materials.

           The thickness of the Diliman tuff ranges from 1300 to 2000 meters (BMG, 1982; NEPC, 1987).

           Towards the Antipolo, Rizal area, is an exposure of a small body of Tertiary sedimentary rocks composed of shale, sandstone, conglomerate wacke and limestone (MWSS, 1981). Confined in the eastern section of Metro Manila are igneous bodies of basalt and minor diorite.

           Tectonic development and vulcanism accompanied by wide fluctuations of sea level during the Late Tertiary to Quaternary periods greatly influenced not merely the geology and geomorphic aspects but also the structural elements. The most significant structures which have important bearing to the Metro Manila area are the existence of the two parallel northeast trending Marikina Valley faults. Several concepts have been formulated by various workers with regards to its development. Alvir, A. D. (1929) described the fault as graben in connection with his geological study of the Angat-Novaliches Region. Irving, E. M. (1947) proposed that it developed from the normal block uplift of Quezon City and Guadalupe Ridge whose southern extension is traceable up to Tagaytay. This is in relation to his study on the geomorphological implications of the Marikina drainage pattern in Rizal province. Gervacio, F. C. (1968) in his study of the geology, structures and landscape development of Manila and suburbs suggested that Marikina Valley and its neighboring areas are integral parts of the geomorphic development of the southern extension of Central Valley and the southeastern extension of the Sierra Madre Range.

           The Marikina structures which have long been recognized consist of two northeasterly trending faults, here referred to as the East Marikina Valley Fault (EMVF) and the West Marikina Valley Fault (WMVF). These two faults enclosed the Marikina Valley and the adjacent municipalities of Montalban, San Mateo, Antipolo and the eastern section of Metro Manila.

           The EMVF forms the eastern side of the graben and is well exemplified in the vicinity of San Mateo by prominent scarps and truncated ridges. The extent of this fault, however, is poorly defined because of the extensive modification. The PHIVOLCS Team traced this fault for about 8 km.

           The WMVF which constitutes the western margin of the valley is topographically expressed as steep scarps. Gervacio (1968) traced this fault for about 80 km from the northern portion of Quezon City to the western side of the Laguna de Bay extending south to Tagaytay Ridge in Cavite. The PHIVOLCS Team, however, mapped only the Montalban area in the north to Pasig in the south for a distance of about 23 km. The major disparity between these two faults is the amount of vertical displacement which is greater along WMVF than the EMVF. In Montalban over 180 meters of displacement was measured, about 80 meters near Rosario and around 150 meters in Pasig. Shown in Fig. 3 is the structural map of Metro Manila and vicinity adapted after Gervacio (1968).

           The field survey involved mainly soil-gas measurement utilizing a portable EDA RD-200 radon detector, an adapter with a hand pump and a BGS soil-gas spike consisting of a hollow stainless steel tube and a solid inner rod. The radon detector measures alpha particle emissions from the decay of isotopic radon. Field determinations were carried out along established traverse lines across the fault. Measurements were taken at convenient intervals from 5 to 20 meters depending on the proximity to the fault line. At every established point, the soil gas spike is driven at least 50 cm into the ground and the solid inner rod is then slowly withdrawn. One end of the adapter (with hand pump) is connected to the hollow stainless tube of the spike and the other end to the scintillation cell in the radon detector. At least 750 ml of soil gas (equivalent to about 15 hand pumps) is then pumped manually into the cell. The counting is made in three sequential one-minute counts. All data are recorded including site location and other pertinent information. A total of 113 measurements were made in this field survey.

           Shown in Table I are the results of soil-gas measurements obtained across the different sections of the two northeast trending Marikina faults. The radon and thoron values drawn for each traverse are presented in bar graphs to depict distribution pattern as well as the radon-thoron ratios. Moreover, in-situ measurements were also conducted outside the fault zones to determine the radon and thoron levels for the various lithologic units traversed by the faults. The results are summarized in Table II.

TRAVERSE EMVF-1

           This line is situated at the northeastern side of Gloria Subdivision, San Mateo, Rizal. It is 80 meters long and surveyed towards the east at 20m interval along the gently rolling slope. Five readings were made with radon values ranging from 17 to 122 cpm. It is clearly shown in the graph that radon is at its maximum as it approaches the fault and drops after intersecting it. The thoron distribution, on the other hand, is quite variable and does not conform with the radon pattern. The thoron counts vary from 2 to 8 cpm. The radon and thoron ratios also do not show any pattern. The ground in this area is moderately soft and covered with colluvial materials of basalt derivatives.


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TRAVERSE EMVF-2

           This traverse was conducted at Sitio Marang, Bgy. Maly, San Mateo, Rizal. The line is 80 meters long and measured along the flat to gently rolling fault scarp characterized by moderately soft ground. A total of nine measurements were made at 10m intervals with radon values ranging from about 13 to 97 cpm. As shown in the graph the radon rises at the fault and decreases after crossing it. The thoron values do not display any defined pattern with values ranging from 0 to 20 cpm. The thoron value is zero at the fault which indicates that radon is the prevailing gas. The radon and thoron ratios also do not show any pattern.


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TRAVERSE EMVF-3

           This line is 100 meters long and measured from east to west at 10m and 20m intervals. It is located in a vacant residential area at Bgy. Abuab, San Mateo, Rizal. The ground surface is flat and terraced covered with alluvial deposits. Eleven readings were recorded. As shown in the graph there are several radon peaks which were observed along every terrace divide and dropped when the surface becomes smoothly flat. These observations may indicate the occurrence of step faults where the terrace boundaries represent the fracture lines. The radon counts range from 33 to 162 cpm. At 10m west interval a radon value of 115 cpm marked the main fault as it forms the base of the escarpment as indicated by its general direction. Thoron value at this point is zero indicating radon as the principal gas. The thoron counts vary from 0 to 44 cpm and do not follow the radon pattern. The radon and thoron ratios exhibit no defined pattern.

TRAVERSE WMVF-1

           This line is situated at Bgy. Haranga, Montalban, Rizal. Here the northern part of the fault bifurcates in two structural lines where the traverse measurement was conducted. The line is 120 meters long and surveyed westerly along the moderately flat ground at 10m interval. The area is underlain by massive tuff with thin soil cover in some portions. Twelve readings were obtained. The radon counts range from about 8 to as high as 310 cpm. The radon distribution has three prominent peaks as shown in the graph. At point 10m west the radon peaked to maximum as this line intersected the main fault. The two other peaks at intervals 50m W and 100m W represent the branching faults. The thoron distribution, on the other hand, is quite inconstant and does not follow the radon pattern. The thoron readings range from about 2 to 32 cpm. The radon and thoron ratios do not show any pattern.


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TRAVERSE WMVF-2

           This traverse was conducted in Bgy. San Jose, Montalban. It is 120 meters long and surveyed from east to west along the flat to gently rolling slope at 10m and 20m intervals. The ground is covered by tuff with thin overburden. A total of nine readings were obtained. Both radon and thoron do not exhibit any trend. The radon values range from 18 to 214 cpm while thoron vary from 0 to 64 cpm. At point 40m west where the line traverse crossed the fault, the highest radon count was recorded while the thoron value was about 36 cpm. The ratios for radon and thoron are likewise variable on either side of the fault.


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TRAVERSE WMVF-3

           This line is situated in Northview I Subdivision, Quezon City. It is 90 meters long and surveyed towards the west starting at the base of the escarpment upward along the gently rolling surface at 10m intervals. The ground is masked with massive tuff and covered with thin soil. Ten readings were made with radon values ranging from 33 to 197 cpm. Right at the base, at point 10m E where the traverse crosses the fault, a significant rise in the radon concentration was evident. Highest thoron value is recorded at this point. The thoron values as has been measured in other WMVF areas are quite inconstant and do not agree with the radon trend. The thoron readings vary from 14 to 66 cpm. The radon and thoron ratios are similarly variable.


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TRAVERSE WMVF-4

           This line is situated in a vacant residential area inside the Capitol Hills Subdivision, Quezon City. It is 180 meters long and measured towards the east at 10m and 20m intervals along the moderately rolling ground to flat surface with shallow overburden. Measurement commenced on the upper slope downward where a total of 14 readings were obtained. At interval 130m east where the line intersects the fault, a maximum radon value of 147 cpm was recorded. This observation is clearly shown in the graph. The thoron values vary from 24 to 82 cpm and do not follow the radon distribution. The radon and thoron ratios are also variable.


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TRAVERSE WMVF-5

           This traverse was conducted inside the Ateneo Compound in Quezon City. Here the line is measured from west to east along the flat and steep to gently rolling fault scarp at 10m, 15m and 20m intervals. The ground is covered by massive tuff with thin soil cover. Eleven measurements were made with radon values ranging from 17 to as high as 544 cpm. A cursory look at the graph indicates two prominent peaks that lie in opposite directions, one on the upper slope and the other on the base of the cliff. The peak on the downslope at interval 80m W with a radon count of 482 cpm (thoron count is 51 cpm) rested on the edge of the cliff where the fault line crossed. The peak on the upper slope at point 30m W with a radon count of 544 cpm (thoron value is 99 cpm) could be one of the accompanying secondary fractures defined on the western part of the WMVF in Quezon City district (Fig. 2). The thoron distribution is quite variable and does not conform with the radon pattern. The thoron values range from 3 to 159 cpm. The radon and thoron ratios are likewise inconstant.


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TRAVERSE WMVF-6

           This traverse was conducted within a vacant residential area in the Valle Verde Subdivision, Pasig, Metro Manila. The length is 80 meters and surveyed toward the east direction at 10m interval. The ground is flat and covered with shallow overburden. A total of nine readings were made. The radon value as shown in the graph is at its highest as it passes the fault. The radon count obtained is 399 cpm with a thoron value of 49 cpm. The thoron distribution is quite variable and does not agree with the radon trend. The thoron counts range from 19 to 114 cpm. The ratios for radon and thoron showed no defined pattern.


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TRAVERSE WMVF-7

           This traverse is located in an opened residential area inside the Kawilihan Subdivision, Pasig, Metro Manila. Here the line is 40 meters long and surveyed toward the east along the gently rolling tuff formation with thin soil cover. Measurement started at the base of the cliff upward at 5m and 10m intervals. The radon counts range from 52 to 347 cpm. As shown in the graph at point 25m east, a conspicuous peak in radon reading marked the main fault line. The thoron distribution, on the other hand, does not conform with the radon pattern. The thoron values range from 0 to 165 cpm. The radon and thoron ratios also do not show any trend.

           A total of 16 measurements were undertaken in several areas away from the fault structures to determine the radon and thoron levels for the various lithologic units. The tuff formation which covered a considerable portion of the fault area showed low to moderate radon and thoron values. The radon levels range from 17 to as high as 179 cpm while the thoron values vary from 7 to 126 cpm. Most of these measurements were carried out on the higher elevation located on the western part of the WMVF.            Few readings obtained within the Marikina alluvial plain disclosed very low radon and thoron concentrations. However, one station in Montalban exhibited high radon and thoron with values of 139 cpm and 51 cpm respectively. This area is possibly characterized by alluvial deposits of tuff derivatives.            Measurement undertaken on the upper section of the EMVF which is underlain by basalt generally exemplified very low radon and thoron concentrations. The radon values range from 29 to 50 cpm while the thoron equivalents vary from 13 to 47 cpm.            The above observations explain the slight disparity in the distribution of radon between the EMVF and the WMVF which could be attributed to the differences in lithology.

           The foregoing discussion demonstrates the applicability and usefulness of radon technique in the study and monitoring of the Marikina valley fault system which has been reported active. This technique could serve as a supplementary tool for a more integrated earthquake prediction program.

           The important highlights of this study are the baseline radon activity disclosed along the fault zones and on the various rock units found in the Marikina Valley fault system. The soil-gas measurements revealed the radon distribution for the different rock types in the area. The traverse results likewise showed the apparent high radon gas concentration along and near the fault lines. The radon gas concentration, in general, rises as the line crosses the main fault. This indicates the existence of gas migration. Furthermore, the controlling fractures which serve as channelways for the escape of gas were covered by colluvial materials. Moreover, the appreciable thoron concentrations indicate that the gas coming out is of shallow origin.

           Based on the radon analysis, it can be seen that the traverse results showed no large disparities with the values obtained outside the fault. This could be attributed to the underlying lithology (degassing of radon in rocks) and not to the seismic activities or strain changes. It can be inferred that the overall relatively low radon concentration manifests the absence of any significant subsurface activity that may initiate ground movement.