Abstract: Lately, there have been newly identified, potentially seismogenic crustal faults (PSCFs) in Chile, many of them are now published by the South America Risk Assessment Project of the Global Earthquake Model. These faults pose a great risk to the community and infrastructure; therefore, it is necessary to estimate ground motion that can be produced by these seismic sources. Unfortunately, there are very few recorded ground motion data from these active faults, not sufficient to build ground motion prediction equations—a necessary element to estimate earthquake hazard. Because of the lack of recorded data, it has become necessary to estimate ground motion with physics-based models. With recorded ground motion, we have estimated path and site attenuation parameters, namely kappa, for different sites in Central Chile. Preliminary results show that the regional value of kappa is approximately 40 ms. The PSCFs database contain variable information depending on the different sources used to compile the fault datum. Nevertheless, if absent, the necessary parameters to simulate seismic scenarios such as the dimension, sense of slip and dip angle and direction can be extracted from the fault traces themselves and inferred from the seismotectonic setting of each individual fault. We used the 2010 Leonard scaling relationships to determine a magnitude based on the length of each active fault. For each fault, we computed several kinematic earthquake rupture scenarios, and propagated seismic waves using Green’s functions with the UCSB method (Crempien and Archuleta, 2015). The Green’s functions incorporate the attenuation measured previously and are calculated up to 25 Hz. For all earthquake scenarios, we estimate peak ground acceleration intensities of over 1g within 10km of the closest distance to the fault. These preliminary results show that these active crustal faults need to be studied in depth in order to better characterize the earthquake hazard in Central Chile.
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Abstract: Probabilistic seismic hazard analysis (PSHA) is typically performed by combining an earthquake rupture forecast (ERF) with a set of empirical ground motion prediction equations (GMPEs). ERFs have typically relied on observed fault slip rates, scaling relationships, and regional magnitude-frequency distributions to estimate the rate of large earthquakes on pre-defined fault segments. GMPEs, which regress against recorded ground motions, often lack data at short site-rupture distances and for large, complex ruptures. The CyberShake platform (Graves et al., 2011) replaces GMPEs with deterministic three-dimensional ground motion simulations, characterizing the effects of basin response and other path effects which are parameterized or treated as aleatory variability in GMPEs. We replace traditional ERFs with a multi-cycle physics-based earthquake simulator, the Rate-State Earthquake Simulator (RSQSim), developed by Dieterich & Richards-Dinger (2010). RSQSim simulations on the Uniform California Earthquake Rupture Forecast, Version 3 (UCERF3) fault system produce seismicity catalogs that match long term rates on major faults and yield remarkable agreement with UCERF3 when carried through to GMPE-based PSHA calculations. Averaged over a representative set of sites, the RSQSim-UCERF3 hazard-curve differences are comparable to, or even less than, the differences between UCERF3 and its predecessor, UCERF2, used in all CyberShake studies to date. Unlike traditional ERFs, RSQSim produces full slip-time histories for all simulated ruptures which can be used directly as input to deterministic wave propagation simulations. We couple the RSQSim model with CyberShake and the SCEC Broadband Platform to create the first fully physics-based PSHA model. Resultant ground motions match GMPE estimates of mean and variability of shaking well over magnitudes and distances for which GMPEs are well constrained. We will present these comparisons and preliminary CyberShake results using RSQSim.
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Abstract: On August 8 2017, an Ms 7.0 earthquake struck the Jiuzhaigou County, Sichuan Province, China. 67 stations of China Strong Motion Network Center (CSMNC) were triggered and recorded ground motions from the event. There were eight stations with epicentral distance less than 100 km of the earthquake. Two temporary stations were installed near station Jiuzhai Zhangzha (JZZ) after the main event. 18 aftershocks with magnitude between 1.2 and 3.9 were recorded from August 19 to August 23, 2017. The largest PGA, greater than 1.0g, was recorded at station JZZ. However, this large ground motion recording is under investigation for possible instrumental or other contaminations. The horizontal peak ground accelerations (PGA) and peak ground velocities (PGV) were obtained and compared to ground motion prediction equation (GMPE) used in the 5th generation of seismic ground motion parameters zonation map of China. The comparisons show that PGAs and PGVs at all stations, except station JZZ are below the GMPE. The S-wave horizontal to vertical spectral ratios (HVSR) were calculated for the main event and aftershocks at station JZZ and the two temporary stations. The results show that the HVSRs at the three sites are quite similar. The HVSRs at station JZZ also show nonlinear site response: the peak frequency shifting from ~10Hz to ~2Hz with amplitude decreasing. Stochastic finite fault source model with dynamic corner frequency was used to simulate ground motions at the eight stations for the main event. The simulated ground motions were compared to the recordings at the eight stations.
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Abstract: At least 75 tsunamis have impacted the region over the last 500 years causing over 3000 deaths. Tsunamis are rapid onset potentially high impact hazards. Due to increasing urbanisation in coastal locales and the low frequency of tsunamis, preparedness and response must be in the psyche of our vulnerable populations to save lives and property. The Tsunami Public Awareness and Education (PAE) Strategy emphasises a mitigation approach in building long-term tsunami awareness and education within the 48 Member States and territories of the Intergovernmental Coordination Group for the Tsunami and Other Coastal Hazards Warning System for the Caribbean and Adjacent Regions (ICG/CARIBE EWS). The strategy is placed within the context of broader international, regional and national disaster risk reduction initiatives, the need for institutional capacity and support, partnerships and shared responsibility across all levels of society to secure resilience. Critical guidance includes audience selection, the effectiveness of multi-media tools and redundant warning dissemination mechanisms, message standardization and increased information flow for successful PAE programming. In recognising the fundamental role of PAE for effective evacuation and warning dissemination to minimise hazard impacts; the Strategy also addresses key synergies between tsunamis and other hazards – earthquakes, landslides, hurricanes and other coastal hazards e.g. coastal flooding and storm surges, which frequently impact the region. These realities of linkages were vividly displayed during the 2017 Atlantic Hurricane season. Regional-level Strategy implementation has largely focused on developing tools for broad audiences, trainings and executing community programmes. Proposed initiatives advance specialised education and sectoral application. The Strategy provides a pioneering, harmonized, regional-level framework for tsunami risk reduction through PAE.
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Abstract: Tall buildings, increasing in number throughout the world, including the Middle East, are greatly affected by long trains of seismic waves from regional earthquakes. Seismic motions, measurable with accurate GPS, are greatly amplified with the increasing height of buildings. Here we show the first successful GPS measurements of the motions of the 414 m tall, Al-Hamra Tower in Kuwait, due to the 11/12/2017 magnitude 7.3 earthquake, 642 km north of Kuwait City on the Iran-Iraq border. The GPS direct measurements of the movements near the top of this building show oscillations of the building, lasting more than ten minutes, with maximum peak-to-peak displacements of 30 cm. The period of the oscillations corresponds to the fundamental mode of the building, and the scalloping nature of the oscillations is produced by the interference of two lowest modes with 7.1 and 5.7 second periods. The large oscillations, occurring in two broad pulses – one and three minutes – after the initial shaking, correlate with GPS and seismic measured ground displacements in the resonance band of the building. GPS measurements show that there was no permanent deformation despite the large swaying of the building. The response of other tall buildings to seismic wave trains can be determined from the seismic power in the resonance band of these buildings and these methods can be applied widely around the world. Measurements of this type with low-cost GPS receivers will soon be possible.
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Abstract: Observations of the wave field at the surface of the Earth are fully characterized at a point by 3 orthogonal vector displacements, 3 horizontal strains, and 3 rigid body rotations. These 9C seismic observables can further be interpreted with a wave propagation model to infer basic attributes of the seismic waves including wave type (from the curl and divergence), azimuth of propagation, and horizontal phase velocity, among other parameters. If these point measurements are then distributed over a surface grid, then images of the wave field and its attributes can be empirically mapped forming tomographic images of the Earth structure effect on the seismic waves. Both the point measurements and expansion into a grid can be made by using dense arrays of seismometers where the seismic observations are incorporated into a finite difference scheme to compute strains and rotations. These kinds of arrays, sometimes called “geodetic” arrays, are becoming feasible to efficiently and cheaply deploy using high frequency (5 or 10Hz dominant frequency) nodal instruments. Accurate computations of strains and rotations from seismic array data depend on precise knowledge of instrument orientations in the field. Estimates of relative amplitude statics and orientations can be made empirically by using teleseismic arrivals simultaneously recorded by the nodal instruments. Two nodal arrays are used to demonstrate the use of gradiometry for inferring wave attributes at a point and over an area. The IRIS Community Wavefields Experiment deployed during June-July 2016 affords a detailed look at instrument orientation and stability for computing wave gradients at a point and for comparison with standard array beamforming techniques. A Large-N experiment consisting of 384 vertical nodal seismometers deployed during an industrial 3D seismic experiment near Utica, Ohio, in 2013 demonstrates the variation of the high frequency wave field over a relatively small area of ~200x300m.
Slidecast:
https://vimeo.com/278571176
