Abstract: The Caribbean-South American (CAR-SA) plate boundary is a complex transform fault system connecting oppositely vergent subduction zones, the Antilles in the east, and a currently locked CAR-SA flat slab subduction zone in the west. Teleseismic P-wave tomography shows both the Atlantic (ATL) and the Caribbean (CAR) plates subducting in opposite directions to transition zone depths under northern South America. Receiver functions (RF) show a depressed 660 discontinuity and thickened transition zone associated with each subducting plate. In the east, the ATL part of the SA plate subducts westward beneath the CAR. The eastern end of the El Pilar-San Sebastian strike-slip system, a subduction-transform edge propagator (STEP) fault, lies above the point where the ATL tears away from SA as it descends into the mantle. The Paria cluster seismicity is the mechanical expression of the plate tear. Body wave tomography and LAB depth determined from RFs and Rayleigh waves suggest that the descending plate also viscously removes the bottom third to half of the SA continental margin lithosphere. This has left thinned continental lithosphere under northern SA as the subduction zone has migrated along its northern coast. The thinned lithosphere extends almost the entire length of the El Pilar-San Sebastian fault system, from ~65o to 69oW, and inland more than 100 km. In northwestern SA the CAR plate subducts at < 30o to the ESE under northern Colombia to about Lake Maracaibo, Vn, and extends laterally from northernmost Colombia to perhaps as far south as the Bucaramanga nest seismicity. The flat slab is associated with three Neogene, basement cored, Laramide-style uplifts: the Santa Marta block, the Perija Range, and the Merida Andes. To the SE under Lake Maracaibo and the Merida Andes the CAR descends steeply to the transition zone. The steep descent suggests that the CAR plate is internally torn, separating the subducting CAR from CAR forming the seafloor to the north.
Slidecast:
https://vimeo.com/276984598
Abstract: Treasure Island is located in the central San Francisco Bay, immediately north of Yerba Buena Island, between the active San Andreas and Hayward faults. Treasure Island was constructed by placing hydraulic sand fill over natural shoal deposits within perimeter rock dikes. The natural shoal deposit consists of layers of clean sand, silty sand, and lenses of highly plastic clay. Full-scale and high-energy in-situ dynamic ground improvement test results indicated that, unlike the fill material, no appreciable ground improvement (i.e. densification) was observed within the shoal deposits. From a thorough geologic characterization of the shoal deposit and the results of laboratory cyclic direct simple shear tests on high-quality samples, it was concluded that the dynamic behavior of the natural shoal deposit could not be adequately captured by simplified conventional analytical methods, as the shoal deposit was found to be more resistant to seismically induced lateral deformation than could be predicted by simplified methods. Therefore, this study was undertaken to evaluate the seismic deformation of the existing shoreline at Treasure Island through a nonlinear dynamic deformation analysis. The scope of the study included seismic site response analyses, lateral deformation analyses using two-dimensional finite-element models in PLAXIS, pseudo-static hybrid deformation analyses, and comparisons with observed seismic performance of similar sites during past earthquakes. The shoal deposit was modeled using the UBC Sand model, with input parameters carefully selected to capture material behavior obtained through cyclic simple shear tests. Examination of PLAXIS analysis results indicates that the magnitude of lateral deformations at the location of the proposed development was negligible. A simplified method was also developed to be used as a screening tool for estimating the potential for lateral movement at other sites along the Treasure Island shoreline.
Slidecast:
https://vimeo.com/277159601
Abstract: Large historical mega-thrust subduction earthquakes, such as the 2004 Aceh-Andaman, 2010 Maule, and 2011 Tohoku earthquakes, have triggered numerous aftershocks in subduction plate interfaces and continental crusts. The crustal seismicity occurs much closer to the population and buildings than the subduction earthquake which is likely to occur with a larger magnitude and at a greater distance. Therefore, the crustal earthquake can have a greater potential impact on seismic damage and loss than the subduction earthquake. Generally, times between major events may be too short to inspect and repair damaged buildings; in such situations, damage accumulation of buildings can be major issues. A new method for assessing spatiotemporal seismic hazard and risk due to a mega-thrust subduction earthquake that triggers both subduction and crustal aftershocks is developed. The Epidemic Type Aftershock Sequences (ETAS) model is used to generate synthetic earthquake catalogs and capture spatiotemporal earthquake clustering. The conventional isotropic ETAS simulation is extended to account for spatial anisotropic distribution of aftershocks by applying the scaling law of the rupture model, and implementing a 2D uniform distribution and a power law decay inside and outside of the rupture area, respectively. Moreover, to evaluate seismic hazard and risk, the ETAS model is convolved with ground motion prediction equations (GMPEs) and seismic fragility curves. A case study is set up for the 2011 Mw9 Tohoku event in Japan. By incorporating more realistic spatial anisotropy of aftershocks, we quantify how the spatiotemporal seismic hazard rate is changed by the triggered crustal and subduction aftershocks in comparison with long-term time-independent hazard rate. Furthermore, we propose to evaluate the impact of increased crustal and subduction aftershocks to seismic hazard and risk assessments for making various risk management decisions more effectively in the post-mainshock period.
Slidecast:
https://vimeo.com/277156673
Abstract: In order to obtain sound risk assessments, it is important to quantify, among others, building characteristics and socioeconomic and environmental, attributes of the potential impacted area. Risk assessment studies conducted at detailed scale (individual dwellings) are not common because of difficulties in getting the appropriate data both, at the intensity of shaking and at the vulnerability levels. Here we present the main results of the risk study carried out in Portoviejo after the 7.8 Mw Jama-Pedernales earthquake. Damage is concentrated in tall buildings (about 31.7% for 4 or more story buildings versus 8.4% for 1-2 story), mostly located downtown Portoviejo, although soil properties there are not different from others around the city. The damage ratio persists in new buildings (built after 2001), even for those built under the first national Construction Building Code that includes locally calculated seismic zonation (2001). Damage to official buildings was also high (hospitals, schools, police departments, local and national government). Old buildings were heavily damaged too, so there are no evidences of vulnerability reduction in recent years. Most of fatalities, 133 in Portoviejo, were caused by tall buildings collapses, but it could have been worse if the earthquake would have occurred during business ours. Economic losses assessment considered the exposure (based on the cadastral value of the buildings) and the replacement cost. Such losses were estimated in 450-700 million US$. The first ever empirical fragility and vulnerability curves in Ecuador were also calculated after assigning to buildings a typology based on cadastral, morphometric and site information. Portoviejo is a standard city in the Ecuadorian context, so our results could be extrapolated to others in the country for the same type of earthquakes. In Portoviejo the paradigm that directly correlates social vulnerability with high earthquake risk has been challenged so new approaches must be considered.
Slidecast:
https://vimeo.com/277178348
Abstract: The main shock of the 2016 Kumamoto earthquakes occurred on April 16, 2016, at 01:25 (JST), with Mw7.0. The rupture of the crustal earthquake reached to the surface and more than 30 km of surface ruptures appeared after the earthquake. Strong motions around the surface ruptures showed large peak velocities and permanent displacements, which could affect the damage around the source region. In general, permanent displacements around surface ruptures are generated from the shallow part of the fault because near and intermediate field terms attenuate with distance. Thus, strong motion records around surface ruptures can be used to investigate the behavior of the shallow part of a fault. In this study, in order to reveal the dynamic behavior of the near surface fault and its effects on strong motions, we analyzed near fault strong motion records of the 2016 Kumamoto earthquake. Velocities and displacements were estimated by numerical integration of accelerations in time domain and baseline correction (c.f. Iwan 1985, Boore 2001) to avoid distorting the waveforms. The result showed that large velocity pulses of around 0.5 Hz occurred simultaneously with the onset of permanent displacements at many stations. This could imply that these large velocities were generated near the surface. In addition to well-known fault parallel fling pulses, velocity pulses exceeding 50 cm/s in the radial direction were significant at some stations around the Futagawa fault such as KMM006 and KAWAYO. These pulses could be due to the radial component of near and intermediate-P terms. These results show that strong motions from the shallow part of a fault have to be considered in the strong ground motion simulations especially for near fault region although they have often been ignored.
Slidecast:
https://vimeo.com/277154148
Abstract: Portoviejo is located in a sedimentary basin characterized by soft sedimentary deposits of fluvial-marine origin that gradually filled the estuary, surrounded by outcrops of heavily weathered shales. The seismic hazard is dominated by interface earthquakes of the subduction of the Nazca plate beneath the North Andean Sliver, but local faults with potential Q activity were identified during the study. The depth of the quaternary sediments and their properties has been studied with geotechnical and geophysical methods. Depth of the sedimentary basin is 50 m in the center of Portoviejo increasing towards NE to 160 m. The depth of the seismic basement, characterized by Vs > 1400 m/s (consolidated layers of shale or sandstone within the shales) ranges between 100 and 200 m in the urban area. Seismic response is mainly conditioned by Vs30, and geotechnical characteristics, and only to a lesser extent to sediment thickness. Vs30 varies between 155 and 360 m/s in the basin, corresponding to silty – clayey materials. The shales show Vs between 360 and 600 m/s. Five seismic microzones of similar seismic response are defined by the distribution of Vs30 ranging from rigid soils with Vs30 > 360 m/s to very soft soils with Vs30 < 180 m/s; microzone 6 is located in the floodplain of the Portoviejo River with very high liquefaction potential, and lateral spread due to the lack of lateral confinement. Downtown Portoviejo, strongly damaged in 2016, does not present any special subsoil configuration, nor in the observations from the aftershock recordings (greater amplifications are observed to the north), with the exception of liquefaction phenomena near the cemetery. Our results, which include PSH assessment, allow a greater detail with respect to the building code with spectra with values of the plateau 10-50% higher than the respective code spectra. Landslide hazard due to earthquakes is small in Portoviejo, with the exception for extreme events (2475 years return period).
Slidecast:
https://vimeo.com/276984990
