16 research outputs found
Tectonic framework and relative ages of structures within the Ottawa-Bonnechere graben.
No full textThe Ottawa-Bonnechere graben forms a 55 km wide topographic low extending from near Montreal through Ottawa. It is part of the St. Lawrence rift system, which also includes the Saguenay graben in Quebec. Cambrian to Ordovician carbonate and fine clastic sedimentary rocks of the St. Lawrence Platform overlie Grenvillian basement rocks and, in the Ottawa-Hull area, are cut by several generations of brittle faulting. Relative ages of faults and associated structures are determined or inferred from field studies of key outcrops. Three periods of faulting are defined, and the orientations of the paleostress fields associated with each period are modelled using fault and fault surface lineations orientation data. The oldest generation of fault structures, referred to as D\sb1, formed in response to a stress field in which the greatest principal horizontal stress was oriented northwest. The second and third periods of faulting (D\sb2, D\sb3) occurred when the greatest principal horizontal stress was oriented west-northwest and southwest, respectively. Both D\sb2 and D\sb3 involved the reactivation of the existing faults and the development of new faults. (Abstract shortened by UMI.
Tectonic framework and relative ages of structures within the Ottawa-Bonnechere graben.
No full textThe Ottawa-Bonnechere graben forms a 55 km wide topographic low extending from near Montreal through Ottawa. It is part of the St. Lawrence rift system, which also includes the Saguenay graben in Quebec. Cambrian to Ordovician carbonate and fine clastic sedimentary rocks of the St. Lawrence Platform overlie Grenvillian basement rocks and, in the Ottawa-Hull area, are cut by several generations of brittle faulting. Relative ages of faults and associated structures are determined or inferred from field studies of key outcrops. Three periods of faulting are defined, and the orientations of the paleostress fields associated with each period are modelled using fault and fault surface lineations orientation data. The oldest generation of fault structures, referred to as D\sb1, formed in response to a stress field in which the greatest principal horizontal stress was oriented northwest. The second and third periods of faulting (D\sb2, D\sb3) occurred when the greatest principal horizontal stress was oriented west-northwest and southwest, respectively. Both D\sb2 and D\sb3 involved the reactivation of the existing faults and the development of new faults. (Abstract shortened by UMI.
Morphotectonic Kinematic Indicators along the Vigan-Aggao Fault: The Western Deformation Front of the Philippine Fault Zone in Northern Luzon, the Philippines
No full textThe Vigan-Aggao Fault is a 140-km-long complex active fault system consisting of multiple traces in the westernmost part of the Philippine Fault Zone (PFZ) in northern Luzon, the Philippines. In this paper, its traces, segmentation, and oblique left-lateral strike-slip motion are determined from horizontal and vertical displacements measured from over a thousand piercing points pricked from displaced spurs and streams observed from Google Earth Pro satellite images. This work marks the first instance of the extensive use of Google Earth as a tool in mapping and determining the kinematics of active faults. Complete 3D image coverage of a major thoroughgoing active fault system is freely and easily accessible on the Google Earth Pro platform. It provides a great advantage to researchers collecting morphotectonic displacement data, especially where access to aerial photos covering the entire fault system is next to impossible. This tool has not been applied in the past due to apprehensions on the positional measurement accuracy (mainly of the vertical component). The new method outlined in this paper demonstrates the applicability of this tool in the detailed mapping of active fault traces through a neotectonic analysis of fault-zone features. From the sense of motion of the active faults in northern Luzon and of the major bounding faults in central Luzon, the nature of deformation in these regions can be inferred. An understanding of the kinematics is critical in appreciating the distribution and the preferred mode of accommodation of deformation by faulting in central and northern Luzon resulting from oblique convergence of the Sunda Plate and the Philippine Sea Plate. The location, extent, segmentation patterns, and sense of motion of active faults are critical in coming up with reasonable estimates of the hazards involved and identifying areas prone to these hazards. The magnitude of earthquakes is also partly dependent on the type and nature of fault movement. With a proper evaluation of these parameters, earthquake hazards and their effects in different tectonic settings worldwide can be estimated more accurately
Complex Shear Partitioning Involving the 6 February 2012 MW 6.7 Negros Earthquake Ground Rupture in Central Philippines
No full textA 75 km-long, generally NE-striking ground rupture associated with the 6 February 2012 MW 6.7 (Mb 6.9) Negros earthquake was mapped on the eastern side of Negros Island, Philippines. It closely follows a previously unmapped, pre-existing fault trace along the coast which is marked mostly by terrace-forming scarps. The dominance of vertical separation (west side up) is consistent with a west-dipping reverse fault, as indicated by focal mechanism solutions. The ground rupture map eliminates the ambiguity in the focal mechanism solution regarding the orientation, sense of motion, and location of the seismogenic fault plane, which are indispensable in the assessment of seismic hazards and the nature and distribution of deformation. This study uses the ground rupture map of the 2012 Negros earthquake in sorting out the mechanism of deformation in the Visayas Islands region. The ground rupture’s length is well within the aftershock area while its scarp heights are consistent with an earthquake of its magnitude and nature of movement. The 2012 Negros earthquake rupture’s pattern, scarp types, and offset of man-made structures are similar to those of recent reverse/thrust ground ruptures mapped globally and are distinct from those associated with erosion, landslide, and liquefaction. The onshore coseismic reverse fault of the Negros earthquake, which contradicts a model of coseismic slip on an offshore blind thrust fault by previous workers, represents the first thoroughly mapped ground rupture of its kind in the Philippines. The ground ruptures of the 2012 Negros and 2013 Bohol earthquakes, along with the Philippine Trench and the Philippine Fault Zone (PFZ), represent a complex shear partitioning mechanism in the Visayas Islands region. This departs from the current simple shear partitioning model for the region and is distinct from those for other regions along the PFZ and adjacent subduction zones. This study shows how an appreciation of morphotectonic features can lead to a better understanding of the distribution of deformation and the nature of earthquake hazards
Earthquake History and Rupture Extents from Morphology of Fault Scarps Along the Valley Fault System (Philippines)
No full textThe morphologic dating of single-event fault scarps along the dextral strike-slip Valley Fault System (VFS) yielded distinct clusters of relative ages (kt), which we interpret as evidence of independent surface ruptures. The boundaries between structural and geometric segments of the East Valley Fault (EVF) appear to have been nonpersistent during the recent rupture cycle. We associate the youngest cluster with the largest historical earthquake (M > 7 in 1863) felt in Manila, which is believed to have come from three segments of the EVF. Thus, future multiple-segment events, M > 7, could occur on the EVF. Our results do not support rupturing of the entire length of the West Valley Fault (WVF), but its northern segment (segment I) is capable of generating an M > 7 earthquake. This is the first time that diffusivity and relative ages of fault scarps are determined from this part of the world and is one of the few studies applying analysis of recent fault scarps to rupture segmentation studies. The recent scarps along the WVF’s segment II are due to aseismic creep and occur along pre-existing tectonic structures. Continued groundwater overextraction within the creeping zone could induce seismicity and modulate the natural timing of future earthquakes along the WVF
Tectonic Control of Aseismic Creep and Potential for Induced Seismicity Along the West Valley Fault in Southeastern Metro Manila, Philippines
No full textVertical creep along 15 ground ruptures within a 15 km long and 1.5 km wide zone has been occurring along the southeastern part of Metro Manila. Though the unusually high rates of vertical slip point to excessive groundwater withdrawal as the trigger, the evidence presented herein indicates that these may not be simple irregular subsidence fissures. Tectonic control of creep along these traces is suggested by the following: the occurrence of some of these ground ruptures along pre-existing scarps that coincide with topographic and lithologic boundaries, the left-stepping en echelon pattern of surface rupturing, and the distribution of the creeping zone within the dilational gap of the dextral strike-slip West Valley Fault (WVF). Furthermore, interpretation of an exposure across one of the creeping faults indicates reactivation by creep of a pre-existing tectonic fault zone. The paleoseismic evidence also suggests that the pre-creep slips are coseismic and dominantly strike-slip. Recognizing the occurrence of coseismic slip preceding aseismic creep is a primary consideration in assessing the potential of the WVF’s creeping segment and its adjacent segments in generating earthquakes. Tighter groundwater extraction regulations may be necessary to avoid exacerbating the effects of vertical ground deformation and the occurrence of induced seismicity
DataSheet1_Spatial and temporal variation of aseismic creep along the dilational jog of the West Valley Fault, Philippines: Hazard implications.pdf
No full textAccelerated creep, primarily through vertical displacement, has been occurring along 15 en-echelon faults belonging to a 15 km-long and 1.5 km-wide, N-S-trending dilational jog of the active West Valley Fault (or West Marikina Valley Fault), on the southeastern part of Metro Manila, Philippines. The much-larger-than-known tectonic slip rates had been the only reliable evidence in support of excessive groundwater withdrawal as the trigger of creep in the 1990s. Recently available groundwater extraction data (1977–2019) could more directly and consistently link groundwater withdrawal to accelerated creep in the 1990s. Twenty years (1999–2019) of precise displacement measurements could also reveal significant spatial and temporal links between slip rate changes and patterns of groundwater extraction. Our analysis shows that greater rates of vertical displacement are related to the timing of faster extraction rates. Variations in slip rates between the northern and southern measurement sites are primarily due to regional differences in groundwater extraction, which are influenced by differences in the implementation of water extraction reduction regulations. Proximity to the key source of groundwater recharge (Laguna Lake) is also an influencing factor. Although there are many unknowns inherent to this type of study, continued depressurization could induce static stress changes that could modulate the timescale of earthquake occurrence due to the natural course of stress loading driven by regional tectonics. The current and potential effects of continued depressurization in the areas with high slip rates and surrounding regions are paramount considerations in crafting and implementing tighter and extended groundwater extraction regulations.</p
Reappraisal of the 2012 magnitude (MW) 6.7 Negros Oriental (Philippines) earthquake intensity and ShakeMap generation by using ESI-2007 environmental effects
No full textThe macroseismic intensity of the February 6, 2012, Negros Oriental earthquake (MW 6.7), which affected the islands of Negros and Cebu, central Philippines, has been reassessed in this study using the Environmental Seismic Intensity Scale (ESI-2007). This earthquake caused a ∼75-km-long surface rupture along a previously unmapped fault and resulted in extensive landslides, localized liquefaction, lateral spreading, a tsunami, and widespread damage to infrastructure near the epicentral area. Considering the widespread earthquake environmental effects (EEEs), ESI-2007 intensities were evaluated for 324 locations covering an area of approximately 1000 km2 within the Negros and Cebu Islands. A systematic comparison was conducted between the ESI-2007 scale and the traditional intensity scales (PHIVOLCS earthquake intensity scale (PEIS) and Modified Mercalli Intensity scale (MM) along with the generation of an ESI-2007 shake map, which is solely based on site-specific ESI-2007 intensity values. According to the ESI-2007 scale, the epicentral intensity I0=X is assessed. This is two degrees higher than the intensity of the PEIS, and three degrees higher than the modified MM intensity provided by the United States Geological Survey (USGS). The intensity difference may also be due to the lack of suitable observations of building damage data in this sparsely populated region of the Philippines. Comparison of the ShakeMap that was constructed using the ESI-2007 intensities with the PHIVOLCS and USGS ShakeMap suggests that the instrumental or structural damage-based intensity maps underestimate the seismic intensity for the 2012 Negros Oriental earthquake. The ESI-2007 ShakeMap presented in this work is pertinent for the assessment of future seismic risk associated with other earthquake generators in the vicinity of the islands of Negros and Cebu. It can be integrated with the PEIS or MM intensity scale to improve disaster management and planning, post-earthquake recovery efforts, and damage estimation
Seasonal and Episodic Variation of Aseismic Creep Displacement Along the West Valley Fault, Philippines
No full textCreep through mainly vertical displacement along en echelon ground ruptures within the creeping segment of the West Valley Fault (WVF) in the Luzon Island, Philippines, has been occurring since first documented in the 90s. It is believed to have been triggered by excessive groundwater withdrawal, mainly because of the high rates of slip recorded in the 90s. Near-field displacements measured by locally fabricated linear variable differential transformer (LVDT) and ultrasonic creepmeters are compared with near-field long-term displacements as measured by precise leveling surveys. Though the ultrasonic creepmeter is less accurate in measuring short-term displacement than the LVDT creepmeter, both are reliable in measuring longer-term displacements. Data from creepmeters can reveal association of displacement with seasonal precipitation and correlation between short-term displacement and episodic rainfall. In the case of the WVF’s creeping segment, rainfall episodes and wet seasons do not always result in immediate abrupt displacement changes. Nevertheless, the results of our monitoring with creepmeters underscores the contribution of precipitation in triggering creep, through its effect on the ground and by releasing stored tectonic strain, in the southern region of the WVF’s creeping zone where groundwater withdrawal remains largely unregulated. Continuous monitoring and periodic leveling surveys should continue as creep continues to cause damage and the potential for induced seismicity remains
