Coseismic Ground Rupture of the 15 October 2013 Magnitude (M_W) 7.2 Bohol Earthquake, Bohol Island, Central Philippines

Abstract

The 15 October 2013 magnitude (MW) 7.2 Bohol earthquake produced an ~50‐km‐long, ~12‐km‐wide northeast trending zone of uplift with an ~8‐km‐long discontinuous ground rupture indicating predominantly reverse‐slip movement on a southeast dipping fault. Documentation of the nearly continuous northern terminus of the 2013 Bohol earthquake ground rupture revealed its association to preexisting scarps of the previously unmapped, Quaternary‐active North Bohol Fault. Trenching across the rupture at four sites not only reveals the geometry and kinematics of the fault but also shows at least one or two pre‐2013 surface rupturing events. Onshore geologic mapping and offshore seismic reflection profiles demonstrate the presence of an island‐wide, northeast‐southwest trending fold‐and‐thrust belt through which deformation related to the regional shortening across the Visayan Sea Basin in the central Philippines is likely distributed.

Complex Shear Partitioning Involving the 6 February 2012 MW 6.7 Negros Earthquake Ground Rupture in Central Philippines

“…There has been no specific mention in previous models of the participation and magnitude of involvement of reverse/thrust faults in Central Philippines and the Negros Trench in accommodating deformation in the region through shear partitioning. The recognition of the ground rupture affirms the critical role of reverse/thrust faults that previous studies on the 2013 and 1990 Bohol earthquakes [3], which occurred along the NBF and EBF(? ), respectively (Figures 9 and 10), should have revealed earlier.…”

“…The entire area is covered by an up to 3-km-thick Pliocene-Pleistocene sedimentary sequence, as shown by offshore seismic profiles and onshore geologic cross-sections across the Tañon strait and Negros Island, respectively [8]. However, our experience from mapping the 1990 M W 7.7 Luzon [18], 1994 M W 7.1 Mindoro [49], and 2013 M W 7.2 Bohol [3] earthquake ruptures has exposed us to different examples of scarps which cut through both exposed bedrock and soft materials. Surveying these ruptures and many other active faults in the Philippines has shown us that the key to determining if scarps are fault-related is to recognize their morphotectonic nature, regardless of the type of surface materials.…”

“…The Negros Oriental Fault's trace complexity and scarp morphologies resemble the ground rupture characteristics of large magnitude reverse/thrust fault-generated earthquakes in the past few decades, such as the 2013 M W 7.2 Bohol earthquake, also in the Visayas region, Philippines [3], and the M W 7.9 2008 Wenchuan (Sichuan) earthquake in China [31][32][33][34][35][36][37], M W 7.6 2005 Kashmir earthquake in Pakistan [38], and the M W 7.6 1999 Chi-Chi earthquake in Taiwan [39,40]. The Negros Oriental Fault's trace complexity and scarp morphologies resemble the ground rupture characteristics of large magnitude reverse/thrust fault-generated earthquakes in the past few…”

“…In some areas where LiDAR data were available, the displacement from the 2012 earthquake was either too small (<0.25 m) to confidently measure displacement by constructing scarp profiles or indistinguishable from previous displacement on pre-existing scarps. However, a study of the ground rupture associated with the 2013 M W 7.2 Bohol earthquake by Rimando et al (2019) [3], which also utilized Phil-LiDAR data, demonstrates that a good correspondence exists between scarp heights measured in the field using a tape measure and those measured from scarp topographic profiles derived from LiDAR DEM. Indeed, field-measured scarp heights from Poblacion, Mckinley, and Martilo of 0.6,~0.4, and~1.7 m (observation points 25, 44, and 52 in Table S1, respectively), are very similar to the scarp heights of~0.85,~0.62, and~1.67 m calculated from select LiDAR-derived scarp topographic profiles ( Figure 5G-I).…”

“…Locally, its recognition allows for a better assessment of seismic hazards within the island. Lastly, recognition of the ground rupture of the 2012 Negros earthquake (which we propose to call the 'Negros Oriental Fault'), along with other recently recognized active structures in the Visayas region [3], allows us to explore the role that these structures possibly play in the accommodation of deformation in the complex tectonic setting of the Philippines. Previous models of shear partitioning in or near the area suggest distribution of relative plate motion of the Philippine Sea Plate (PSP) only on the Philippine Trench and the Philippine Fault Zone (PFZ) [4][5][6], or mainly on the Philippine Trench, the PFZ, and other strike-slip faults [7].…”