Ground-motion simulations within ShakeMap methodology: application to the 2008 Iwate-Miyagi Nairiku (Japan) and 1980 Irpinia (Italy) earthquakes (original) (raw)
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Journal of Seismology, 2012
The ShakeMap software automatically generates maps of the peak ground motion parameters (shakemaps) and of instrumental intensity soon after an earthquake. Recorded data are fundamental to obtaining accurate results. In case observations are not available, ShakeMap relies on ground motion predictive equations, but due to unmodelled site conditions or finite fault effects, large uncertainties may appear, mainly in the near-source area where damage is relevant. In this paper, we aim to account for source effects in ShakeMap by computing synthetics to be used for integrating observations and ground motion predictive equations when nearsource data are not available. To be effective, the computation of synthetics, as well as of the finite fault, should be done in near real time. Therefore, we computed rapid synthetic seismograms, by a stochastic approach, including the main fault features that were obtained through inversion of regional and teleseismic data. The rapidity of calculation is linked to a number of assumptions, and simplifications that need testing before the procedure can run in automatic mode. To assess the performance of our procedure, we performed a retrospective validation analysis considered as case study of the M w =6.3 earthquake, which occurred in central Italy on April 6, 2009. In that case, the first shakemaps, generated a few minutes after the earthquake, suffered large uncertainties on ground motion estimates in an area closer to the epicenter due to the lack of near-field data. To verify our approach, we recomputed shakemaps for the L'Aquila earthquake, integrating data available soon after the earthquake at different elapse times with synthetics, and we compared our shaking map with the final shakemap, obtained when all the data were available. Our analysis evidences that (1) when near-source data are missing, the integration of real data with synthetics reduces discrepancies between computed and actual ground shaking maps, mainly in the near-field zone where the damage is relevant and (2) the approach that we adopted is promising in trying to reduce such discrepancies and could be easily implemented in ShakeMap, but some a priori calibration is necessary before running in an automatic mode.
Two procedures are described to generate hybrid synthetic seismograms for ground motion estimation: the Compex developed by the authors and the CERS downloaded from the eQuake-RC website. Both methods are tested on the earthquakes occurred at Iwate-Miyagi (Japan) and L'Aquila (Italy); the synthetic seismograms are computed utilizing the parameters published in previous studies and the results are compared with the recordings through a qualitative analysis and the calculation of the goodness of fit.
The 2011 October 23 Van earthquake occurred at 13:41 local time in Eastern Turkey with an epicentre at 43.36 o E, 38.76 o N (Kandilli Observatory Earthquake Research Institute (KOERI)), 16 km north-northeast of the city of Van, killing around 604 people and leaving thousands homeless. This work presents an overview of the main features of the seismic ground shaking during the Van earthquake. We analyse the ground motion characteristics of the mainshock in terms of peak ground acceleration (PGA), peak ground velocity (PGV) and spectral accelerations (SA, 5 per cent of critical damping). In order to understand the characteristics of the ground motions induced by the mainshock, we also study the site response of the strong motion stations that recorded the seismic sequence. The lack of seismic recordings in this area imposes major constraints on the computation of reliable seismic hazard estimates for sites in this part of the country. Towards this aim, we have used a stochastic method to generate high frequency ground motion synthetics for the M w 7.1 Van 2011 earthquake. The source mechanism of the Van event and regional wave propagation parameters are constrained from the available and previous studies. The selected model parameters are then validated against recordings. We also computed the residuals for the ground motion parameters in terms of PGA and PGV at each station and the model parameter bias by averaging the residuals over all the stations. The attenuation of the simulated ground motion parameters is compared with recent global and regional ground motion prediction equations. Finally, since it has been debated whether the earthquake of November 9 was an aftershock of the October 23 earthquake, we examine whether static variation of Coulomb stress could contribute to the observed aftershock triggering during the 2011 Van Lake sequence.
Ground-Motion Simulations for the 1980 M 6.9 Irpinia Earthquake (Southern Italy) and Scenario Events
Bulletin of the Seismological Society of America, 2011
In this paper, we adopt three ground-motion simulation techniques (the stochastic finite-fault simulation code from ; the hybrid deterministic-stochastic approach with approximated Green's functions from and the broadband hybrid integral-composite technique with full-wavefield Green's functions from , with the aim of investigating the different performances in near-fault strong-motion modeling and prediction from past and future events. The test case is the 1980 M 6.9 Irpinia earthquake, the strongest event recorded in Italy in the last 30 years. First, we simulate the recorded strong-motion data and validate the model parameters by computing spectral acceleration and peak amplitude residual distributions. The validated model is then used to investigate the influence of site effects and to compute synthetic ground motions around the fault. Afterward, we simulate the expected ground motions from scenario events on the Irpinia fault, varying the hypocenters, the rupture velocities, and the slip distributions. We compare the median ground motions and related standard deviations from all scenario events with empirical ground-motion prediction equations (GMPEs). The synthetic median values are included in the median 1 standard deviation of the considered GMPEs. Synthetic peak ground accelerations show median values smaller and with a faster decay with distance than the empirical ones. The synthetics total standard deviation is of the same order or smaller than the empirical one, and it shows considerable differences from one simulation technique to another. We decomposed the total standard deviation into its between-scenario and within-scenario components. The larger contribution to the total sigma comes from the latter, while the former is found to be smaller and in good agreement with empirical interevent variability.
Peak ground velocity ShakeMaps derived from geodetic slip models
Geophysical Journal …, 2009
We develop a methodology to derive fast and reliable peak ground velocity (PGV) ShakeMaps from kinematic finite-source models of earthquake rupture inferred from geodetic static displacements. The temporal variations in slip on the fault are based on the simple assumption that larger slip takes longer time to accumulate. Assuming constant rupture and slip velocities, slip is distributed in time to produce a variable rise-time model. Sensitivity tests on finite-source models of the 1994 M w 6.7 Northridge, California earthquake show that distribution of peak ground velocities derived using 3.0 km s −1 rupture velocity and 76.8 cm s −1 slip velocity matches, to a large extent, observed PGV, suggesting that rapid assessment of strong ground motions derived from geodetic data can aid emergency response, particularly in areas with sparse seismic station coverage.
S U M M A R Y The devastating 2010 M w 7.0 Haiti earthquake demonstrated the need to improve mitigation and preparedness for future seismic events in the region. Previous studies have shown that the earthquake did not occur on the Enriquillo Fault, the main plate boundary fault running through the heavily populated Port-au-Prince region, but on the nearby and previously unknown trans-pressional Léogâne Fault. Slip on that fault has increased stresses on the segment of Enriquillo Fault to the east of Léogâne, which terminates in the ∼3-million-inhabitant capital city of Port-au-Prince. In this study, we investigate ground shaking in the vicinity of Port-au-Prince, if a hypothetical rupture similar to the 2010 Haiti earthquake occurred on that segment of the Enriquillo Fault. We use a finite element method and assumptions on regional tectonic stress to simulate the low-frequency ground motion components using dynamic rupture propagation for a 52-km-long segment. We consider eight scenarios by varying parameters such as hypocentre location, initial shear stress and fault dip. The high-frequency ground motion components are simulated using the specific barrier model in the context of the stochastic modeling approach. The broad-band ground motion synthetics are subsequently obtained by combining the low-frequency components from the dynamic rupture simulation with the high-frequency components from the stochastic simulation using matched filtering at a crossover frequency of 1 Hz. Results show that rupture on a vertical Enriquillo Fault generates larger horizontal permanent displacements in Léogâne and Port-au-Prince than rupture on a south-dipping En-riquillo Fault. The mean horizontal peak ground acceleration (PGA), computed at several sites of interest throughout Port-au-Prince, has a value of ∼0.45 g, whereas the maximum horizontal PGA in Port-au-Prince is ∼0.60 g. Even though we only consider a limited number of rupture scenarios, our results suggest more intense ground shaking for the city of Port-au-Prince than during the already very damaging 2010 Haiti earthquake.
Tectonophysics, 2011
We test a recently developed ShakeMap model for the Friuli Venezia Giulia (NE Italy) region ) by comparing the macroseismic observations, related to three past earthquakes (Cansiglio 1936, Friuli 1976 and Bovec-Krn 1998 which occurred in this region, with the instrumental intensities obtained from the ShakeMaps, and based on synthetic seismograms computed from available source models of these events. Because a comparison requires that the simulation resembles as close as possible the real situation, ShakeMaps are computed from synthetics calculated at receivers placed at the sites where the recording instruments are already operative and at the sites where the instruments are going to be installed in the near future. Since the available instrumental recordings are very few, the resulting instrumental intensity maps are compared with the macroseismic observations wherever the latter are available. We make use of various relationships between the ground motion parameters and the macroseismic intensity during the comparison. The best results are obtained when the macroseismic intensities (I ≤ VII) are estimated using the relationship proposed by .
Real-time Finite Fault Rupture Detector (FinDer) for large earthquakes
Geophysical Journal International, 2012
To provide rapid estimates of fault rupture extent during large earthquakes, we have developed the Finite Fault Rupture Detector algorithm, 'FinDer'. FinDer uses image recognition techniques to detect automatically surface-projected fault ruptures in real-time (assuming a line source) by estimating their current centroid position, length L, and strike θ. The approach is based on a rapid high-frequency near/far-source classification of ground motion amplitudes in a dense seismic network (station spacing <50 km), and comparison with a set of pre-calculated templates using 'Matching by Correlation'. To increase computational efficiency, we perform the correlation in the wavenumber domain. FinDer keeps track of the current dimensions of a rupture in progress. Errors in L are typically on the same order as station spacing in the network. The continuously updated estimates of source geometries as provided by FinDer make predicted shaking intensities more accurate and thus more useful for earthquake early warning, ShakeMaps, and related products. The applicability of the algorithm is demonstrated for several recorded and simulated earthquakes with different focal mechanisms, including the 2009 M w 6.3 L'Aquila (Italy), the 1999 M w 7.6 ChiChi (Taiwan) and the M w 7.8 ShakeOut scenario earthquake on the southern San Andreas Fault (California).
A threshold-based earthquake early warning using dense accelerometer networks
Geophysical Journal International, 2010
Most earthquake early warning systems (EEWS) developed so far are conceived as either `regional' (network-based) or `on-site' (stand-alone) systems. The recent implementation of nationwide, high dynamic range, dense accelerometer arrays makes now available, potentially in real time, unsaturated waveforms of moderate-to-large magnitude earthquakes recorded at very short epicentral distances (<10-20 km). This would allow for a drastic increase of the early warning lead-time, for example, the time between the alert notification and the arrival time of potentially destructive waves at a given target site. By analysing strong motion data from modern accelerograph networks in Japan, Taiwan and Italy, we propose an integrated regional/on-site early warning method, which can be used in the very first seconds after a moderate-to-large earthquake to map the most probable damaged zones. The method is based on the real-time measurement of the period (τc) and peak displacement (Pd) parameters at stations located at increasing distances from the earthquake epicentre. The recorded values of early warning parameters are compared to threshold values, which are set for a minimum magnitude 6 and instrumental intensity VII, according to the empirical regression analyses of strong motion data. At each recording site the alert level is assigned based on a decisional table with four alert levels defined upon critical values of the parameters Pd and τc, which are set according to the error bounds estimated on the derived prediction equations. Given a real time, evolutionary estimation of earthquake location from first P arrivals, the method furnishes an estimation of the extent of potential damage zone as inferred from continuously updated averages of the period parameter and from mapping of the alert levels determined at the near-source accelerometer stations. The off-line application of the method to strong motion records of the Mw 6.3, 2009 Central Italy earthquake shows a very consistent match between the rapidly predicted (within a few seconds from the first recorded P wave) and observed damage zone, the latter being mapped from detailed macroseismic surveys a few days after the event. The proposed approach is suitable for Italy, where, during the last two decades, a dense network of wide dynamic-range accelerometer arrays has been deployed by the Department of Civil Protection (DPC), the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and other regional research agencies.