ALBERT

Description: On 6 April 2009, an earthquake of M w  6.13 ( Herrmann et al. , 2011 ) occurred in central Italy, close to the town of L’Aquila. Although the earthquake is considered to be a moderate-size event, it caused extensive damage to the surrounding area. The earthquake is identified with significant directivity effects: high-amplitude, short-duration motions are observed at the stations that are oriented along the rupture direction, whereas low-amplitude, long-duration motions are observed at the stations oriented in the direction opposite to the rupture. The complex nature of the earthquake combined with its damage potential brings the need for studies that assess the seismological characteristics of the 2009 L’Aquila mainshock. In this study, we present the strong-ground-motion simulation of this particular earthquake using a stochastic finite-fault model with a dynamic corner frequency approach. For modeling the resulting ground motions, we choose two finite-fault source models that take into account the source complexity of the L’Aquila mainshock. In order to test the sensitivity of ground-motion parameters to the seismic wave attenuation parameters, we use two different attenuation models obtained in the study region using weak-motion and strong-motion databases. Comparisons are made between the attenuation of synthetics and ground-motion prediction equations (GMPEs). Synthetic ground motions are further compared with the observed ones in terms of Fourier amplitude and response spectra at 21 strong-ground-motion stations that recorded the mainshock within an epicentral distance of 100 km. The spatial distribution of shaking intensity obtained from the "Did You Feel It?" project and site survey results are compared with the spatial distributions of simulated peak ground-motion intensity parameters. Our results show that despite the limitations of the method in simulating the directivity effects, the stochastic finite-fault model seems an effective and fast tool to simulate the high-frequency portion of ground motions.

Description: On 23 October 2011 an M w  7.1 earthquake occurred in eastern Turkey, close to the towns of Van and Ercis, causing more than 600 casualties and widespread structural damage. The earthquake ruptured a 60–70 km long northeast–southwest fault with a thrust mechanism, in agreement with regional tectonic stress regime. We studied the fault process of the event and the recorded ground motions using different sets of data. Regional records (0.005–0.010 Hz) are used to constrain the centroid moment tensor solution. Near-regional data, 100–200 km from the fault, are used for relocation of the hypocenter and, in the frequency range 0.05–0.15 Hz, for inversion of the rupture propagation by two methods: multiple point-source model (ISOLA) and multiple finite-extent (MuFEx) source model. MuFEx also provides an estimate of the model uncertainty, which is quite large due to unfavorable station distribution. We arrive at several plausible scenarios (equally well fitting the observed data including Global Positioning System coseismic displacements) with different styles of the rupture propagation. A few alternative source models are used for broadband (0.1–10 Hz) ground-motion simulations by means of the hybrid integral-composite source model. Only models comprising source complexities, such as a delayed rupture of shallow asperities, enable explanation of the acceleration record at the only available near-fault station, which exhibits a long duration and two prominent wave groups. These complex rupture models are used to simulate the ground motion in the near-fault area, specifically, at Van and Ercis, where records of the mainshock were missing, providing reasonable agreement with the observed spatial distribution of damage.

Description: In this paper, empirical relationships between modified Mercalli intensity (MMI) and recorded peak ground-motion parameters are developed for Turkey. Strong ground motion data from moderate-to-large earthquakes are employed along with the corresponding MMI information inferred from isoseismal maps and earthquake damage reports. Linear least-squares regression technique is used to derive the following simple relationships between MMI and peak ground acceleration (PGA), peak ground velocity (PGV), and pseudospectral acceleration (PSA): MMI=0.132+3.884 x log(PGA), MMI=2.673+4.340 x log(PGV), MMI=–0.247+3.404 x log[PSA(0.3 s)], MMI=–0.934+4.119 x log[PSA(1.0 s)], and MMI=–0.313+4.453 x log[PSA(2.0 s)]. Despite weak dependencies of the residuals on magnitude or distance terms, we also developed refined predictive relationships that include M w and epicentral distance as independent variables. The simple predictive equations are then compared with similar relationships developed with data from other regions in the world. These comparisons confirm that such relationships should be derived from regional datasets because both the ground-motion content and damage types exhibit local properties. Alternatively, refined relationships, which do not show any regional dependencies, can be employed. Finally, an application is presented in terms of a comparison between the estimated (computed) and observed intensity map of the 17 August 1999 Kocaeli ( M w  7.4) earthquake. The estimated maps from both MMI–PGA and MMI–PGV relationships are found to be in close agreement with the observed intensity map.

Description: A major thrust-fault earthquake of MW = 7.0 occurred on 23 October 2011 at 10:41:21 UTC in the eastern Anatolian region of Turkey, severely affecting the nearby towns of Van and Erciş. In this study, a few strong-motion records from the epicentral area are analyzed in order to investigate the characteristics of the ground motions. Also reported are the post-earthquake field observations for various types of structures, such as buildings, bridges, historical structures, tunnels, and dams within the vicinity of the fault plane. The spatial distribution of damage indicates a noticeable hanging-wall effect. The special-type structures are observed to experience far less damage, as opposed to the building structures in the region pointing to the need for strict compliance to seismic building code and the corresponding construction requirements.

Description: We present a least-squares optimization method for solving the nonlinear full waveform inverse problem of determining the crustal velocity and intrinsic attenuation properties of sedimentary valleys in earthquake-prone regions. Given a known earthquake source and a set of seismograms generated by the source, the inverse problem is to reconstruct the anelastic properties of a heterogeneous medium with possibly discontinuous wave velocities. The inverse problem is formulated as a constrained optimization problem, where the constraints are the partial and ordinary differential equations governing the anelastic wave propagation from the source to the receivers in the time domain. This leads to a variational formulation in terms of the material model plus the state variables and their adjoints. We employ a wave propagation model in which the intrinsic energy-dissipating nature of the soil medium is modeled by a set of standard linear solids. The least-squares optimization approach to inverse wave propagation presents the well-known difficulties of ill posedness and multiple minima. To overcome ill posedness, we include a total variation regularization functional in the objective function, which annihilates highly oscillatory material property components while preserving discontinuities in the medium. To treat multiple minima, we use a multilevel algorithm that solves a sequence of subproblems on increasingly finer grids with increasingly higher frequency source components to remain within the basin of attraction of the global minimum. We illustrate the methodology with high-resolution inversions for two-dimensional sedimentary models of the San Fernando Valley, under SH-wave excitation. We perform inversions for both the seismic velocity and the intrinsic attenuation using synthetic waveforms at the observer locations as pseudoobserved data.

Description: This article is concerned with the problem of seismic inversion in the presence of model uncertainty. In a recent article (Askan et al., 2007), we described an inverse adjoint anelastic wave propagation algorithm for determining the crustal velocity and attenuation properties of basins in earthquake-prone regions. We formulated the tomography problem as a constrained optimization problem where the constraints are the partial and the ordinary differential equations that govern the anelastic wave propagation from the source to the receivers. We employed a wave propagation model in which the intrinsic energy-dissipating nature of the soil medium was modeled by a set of standard linear solids. Assuming no information was initially available on the target shear-wave velocity distribution, we employed a homogeneous shear-wave velocity profile as the initial guess. In practice, some information is usually available. The purpose of the present article is to modify our nonlinear inversion method to start from an initial velocity model, and include a priori information regarding the initial model parameters in the misfit (objective) function. To represent model uncertainties, given an initial velocity model, in addition to the data misfit term in our objective function, we include an L (super 2) -normed weighting term, which quantifies the model estimation errors, independently of the measured data. We use total variation (TV) regularization to overcome ill posedness. We illustrate the methodology with pseudo-observed data from two-dimensional sedimentary models of the San Fernando Valley, using a source model with an antiplane slip function.

Description: On 6 April 2009, an earthquake of M (sub w) 6.13 (Herrmann et al., 2011) occurred in central Italy, close to the town of L'Aquila. Although the earthquake is considered to be a moderate-size event, it caused extensive damage to the surrounding area. The earthquake is identified with significant directivity effects: high-amplitude, short-duration motions are observed at the stations that are oriented along the rupture direction, whereas low-amplitude, long-duration motions are observed at the stations oriented in the direction opposite to the rupture. The complex nature of the earthquake combined with its damage potential brings the need for studies that assess the seismological characteristics of the 2009 L'Aquila mainshock. In this study, we present the strong-ground-motion simulation of this particular earthquake using a stochastic finite-fault model with a dynamic corner frequency approach. For modeling the resulting ground motions, we choose two finite-fault source models that take into account the source complexity of the L'Aquila mainshock. In order to test the sensitivity of ground-motion parameters to the seismic wave attenuation parameters, we use two different attenuation models obtained in the study region using weak-motion and strong-motion databases. Comparisons are made between the attenuation of synthetics and ground-motion prediction equations (GMPEs). Synthetic ground motions are further compared with the observed ones in terms of Fourier amplitude and response spectra at 21 strong-ground-motion stations that recorded the mainshock within an epicentral distance of 100 km. The spatial distribution of shaking intensity obtained from the "Did You Feel It?" project and site survey results are compared with the spatial distributions of simulated peak ground-motion intensity parameters. Our results show that despite the limitations of the method in simulating the directivity effects, the stochastic finite-fault model seems an effective and fast tool to simulate the high-frequency portion of ground motions.

Description: In this paper, empirical relationships between modified Mercalli intensity (MMI) and recorded peak ground-motion parameters are developed for Turkey. Strong ground motion data from moderate-to-large earthquakes are employed along with the corresponding MMI information inferred from isoseismal maps and earthquake damage reports. Linear least-squares regression technique is used to derive the following simple relationships between MMI and peak ground acceleration (PGA), peak ground velocity (PGV), and pseudospectral acceleration (PSA): MMI=0.132+3.884Xlog(PGA), MMI=2.673+4.340Xlog(PGV), MMI=-0.247+3.404Xlog[PSA(0.3 s)], MMI=-0.934+4.119Xlog[PSA(1.0 s)], and MMI=-0.313+4.453Xlog[PSA(2.0 s)]. Despite weak dependencies of the residuals on magnitude or distance terms, we also developed refined predictive relationships that include M (sub w) and epicentral distance as independent variables. The simple predictive equations are then compared with similar relationships developed with data from other regions in the world. These comparisons confirm that such relationships should be derived from regional datasets because both the ground-motion content and damage types exhibit local properties. Alternatively, refined relationships, which do not show any regional dependencies, can be employed. Finally, an application is presented in terms of a comparison between the estimated (computed) and observed intensity map of the 17 August 1999 Kocaeli (M (sub w) 7.4) earthquake. The estimated maps from both MMI-PGA and MMI-PGV relationships are found to be in close agreement with the observed intensity map.