C. Milkereit

New Relationships between Vs, Thickness of Sediments, and Resonance Frequency Calculated by the H/V Ratio of Seismic Noise for the Cologne Area (Germany)

Noise measurements were carried out in the Cologne area (Germany), and the resonance frequency of each site was estimated from the main peak in the spectral ratio between the horizontal and vertical component. For 32 of these sites, the thickness of the sedimentary cover was known from boreholes, and a clear correlation between resonance frequency and sediment thickness was observed. A formula that correlates cover thickness with frequency of the main peak in the horizontal-to-vertical spectral ratio was derived. In addition, a best-fitting shear-wave-velocity distribution with depth, vs ( z ), as well as a relationship between average shear-wave velocity Vs and thickness of the sedimentary cover, was calculated. By using all of the noise measurements and applying the derived relationships, we obtained a subsoil classification for the Cologne area.

ASSESSMENT OF THE NATURAL FREQUENCY OF THE SEDIMENTARY COVER IN THE COLOGNE AREA (GERMANY) USING NOISE MEASUREMENTS

Noise measurements were carried out at 381 sites in the Cologne area (Germany) using both short period and broad band sensors. The large number of data allowed both assesment of the influence of different sensors in the site response estimation and to compare the widely used H/V technique with the recently proposed Fourier Phase Spectral Method (FPSM). The results show that short period sensors are able to reliably retrieve site effects at frequencies well below their corner frequencies. Moreover, the H/V method should be preferred to the FPSM in determining the fundamental resonance frequency of soils. Finally, a map showing the resonance frequency distribution in the studied area was drawn using the results obtained applying the H/V technique.

Comparison of Different Site Response Estimation Techniques Using Aftershocks of the 1999 Izmit Earthquake

On 17 August 1999, the M W 7.4 Izmit earthquake occurred in northwestern Turkey. A temporary seismic network was installed to improve the geometry of a previously installed network. In this study, 262 aftershocks of the Izmit earthquake with magnitude M L ranging between 0.4 and 4.5 were analyzed using digital recordings from 17 stations of these networks. S -wave and P -wave spectral records, corrected for path attenuation function, were inverted by means of the generalized inversion technique (GIT). The GIT site responses were compared with those calculated by horizontal-to-vertical (H/V) ratios applied to both earthquake data and to pre-event noise. In most cases, the H/V of the earthquake data provided site responses with shapes consistent with those obtained using GIT. However, the level of amplification was occasionally found to be different. In general, these discrepancies can be explained either by the amplification affecting the vertical component of the ground motion that we detected by GIT or by the waves having a propagation in almost the vertical direction. Using the H/V ratio applied to noise, for most cases, we obtained not only the peaks for the fundamental resonance frequencies, but site responses with shapes similar to those obtained by GIT. The detected level of amplifications using the H/V ratio showed a tendency not to exceed those calculated by the inversion. Finally, amplifications up to a factor 5 are found for stations placed over complex topography. For these sites, amplifications are not always correctly identified by the H/V technique, using either earthquakes or noise.

An Improved Automatic Scheme for Empirical Baseline Correction of Digital Strong-Motion Records

Abstract Compared with seismic waves, near-field static deformation can provide more robust constraints on earthquake size and slip distribution because it is less sensitive to the rupture process and Earth structure. The static deformation data are now obtained using space-geodetic measurements. For early warning and rapid hazard assessment, such geodetic measurements are less useful because they are usually available only with a time delay of days to weeks or even longer. Recent studies have shown that coseismic static displacements can be estimated from modern seismometer records after an appropriate correction for baseline errors that may be caused by rotational motion, tilt, instrument, and other effects. For this purpose, several empirical baseline correction methods have been proposed. Algorithms, in which an acceleration or other acceleration record derived threshold is used to determine the timing of baseline shift, can be easily implemented. In practice, however, the baseline shift is not necessarily accompanying the strongest ground shaking; methods based on the threshold approach tend to lead to an over- or underestimation of the true baseline shift. Other correction schemes, which can be performed by manual calibration, rely on subjective decisions for the choice of correction parameters. In this paper, an automatic scheme is presented, in which the method used to determine the baseline shift has a stronger physical basis. Case studies on the 1999 Chi-Chi, 2007 Tocopilla, 2008 Wenchuan, and 2010 Maule earthquakes, where the strong-motion data were obtained from different types of accelerometers, show that this automatic scheme is more robust than the previously suggested ones. In all test cases, the coseismic displacements recovered from the strong-motion records agree within 10%–20% with direct GPS measurements or indirect model predictions. In addition, the automatic scheme was also successfully used to correct the strong-motion records of a low-cost sensor obtained from a laboratory test.

Distribution of Seismic Velocities and Attenuation in the Crust beneath the North Anatolian Fault (Turkey) from Local Earthquake Tomography

We investigate the crustal structure beneath the western part of the North Anatolian fault zone (NAFZ), an area where at least five damaging earthquakes occurred during the twentieth century. This study is based on local earthquake tomography using the data from aftershocks of the Izmit event (17 August 1999, M 7.4) recorded by stations of permanent and temporary networks. We derive the distribution of V P , V S , and the V P / V S ratio based on the iterative inversion for both V P - V S and V P - V P / V S using the LOTOS code. Innovatively, in this study we perform an inversion for frequency-dependent S -wave attenuation (1/ Q S ). The reliability of the results is assessed through synthetic tests. The distributions of the resulting seismic parameters ( V P , V S , V P / V S , and Q S ) highlight important geodynamical features in the study area. The low-velocity and high-attenuation patterns mostly correlate with the fracturing zones of the NAFZ. Low velocities are also observed beneath the main sedimentary basins (e.g., Adapazari, Duzce, and Kuzuluk). High-velocity and low-attenuation patterns correlate with blocks presumed to be rigid (Kocaeli, Armutlu, and Almacik blocks). The rupture traces of the largest earthquakes in this area pass generally in the transition areas between high and low velocities, while moderate and weak seismicity is mostly concentrated in low-velocity areas. Based on these results we propose and discuss the role that the Almacik block could have played in producing the largest earthquakes in the study area in the twentieth century.

Crustal attenuation characteristics in northwestern Turkey in the range from 1 to 10 Hz

We have analyzed the aftershocks ( M L <4.5) following the 1999 Izmit earthquake ( M w 7.4) to infer the frequency-dependent attenuation characteristics of both P and S waves, in the frequency range from 1 to 10 Hz and in the distance range from 10 to 140 km. A linear-predictive model is assumed to describe the spectral amplitudes in terms of attenuation and source contributions. The results show that both P and S waves undergo a strong attenuation along ray paths shorter than 40 km, while the secondary arrivals significantly contribute to the spectral amplitudes over the distance range from 40 to 60 km, as also confirmed by the computation of synthetic seismograms. For longer ray paths, the decrease in attenuation suggests an increase in the propagation efficiency with depth. Finally, the spectral attenuation curves are flattened, or sloped upward at low frequencies in the range from 100 to 140 km, due to the contemporary arrivals of direct waves and postcritical reflections from the Moho. In terms of geometrical spreading and anelastic attenuation, the attenuation in the range from 10 to 40 km is well described by a spreading coefficient n = 1 for both P and S waves, and the quality factors can be approximated by QS ( f ) = 17 f 0.80 for 1 ≤ f ≤ 10 Hz and QP ( f ) = 56 f 0.25 for 2.5 ≤ f ≤ 10 Hz. For ray paths in the range from 60 to 80 km, the attenuation weakens but the interaction between seismic waves and propagation medium is more complex. The multilapse time window analysis (mltwa) is applied to quantify the amount of scattering loss and intrinsic absorption for S waves. The seismic albedo B decreases from 0.5 at 1 Hz to 0.3 at 10 Hz, while the total quality factor QT increases from about 56 to 408. The multiple lapse time-window analysis (mltwa) results provide only an average estimate of the attenuation properties in the range from 10 to 80 km. In fact, by neglecting the variation of attenuation with depth, the mltwa results underestimate attenuation for distances less than 40 km, and do not capture the significant features caused by the integrated energy of the secondary arrivals observed in the range from 40 to 60 km.

Calibration of an ML Scale in Northwestern Turkey from 1999 Izmit Aftershocks

A local magnitude scale is derived for northwestern Turkey, using data collected by a temporary network installed by the German Task Force for Earthquakes after the 1999 Izmit earthquake ( M w 7.4) and the permanent Sapanca-Bolu network. We computed Wood–Anderson seismograms for over 5353 arrivals at 27 three-component stations, from 530 earthquakes. The hypocentral distances considered range from 5 to 140 km, with the best represented range being from 5 to 70 km. We inverted the measured amplitudes following both the nonparametric Richters (1958) and parametric Bakun and Joyner (1984) approaches. These methods yield consistent magnitude values and station corrections. However, the calculated nonparametric distance correction, log A , implies that ground-motion attenuation is higher than what is accounted for by the equation calibrated in central California. In the range 5–62 km, the best fit is provided by -log A = log( R /17) + 0.00960( R - 17) + 2. This equation is obtained constraining the geometrical spreading parameter n to 1. The calculated value of k = 0.00960 confirms that in this range of distance seismic waves could propagate through a low- Q volume. Station corrections, which allow for a significant reduction of M L residuals, range between ±0.5 magnitude units, suggesting a strong influence of local site effects on the amplitude of ground motion. In accord with the obtained attenuation and station corrections, the magnitudes of the considered events range from 0.4 to 4.8.

Interferometric Analysis of Strong Ground Motion for Structural Health Monitoring: The Example of the L’Aquila, Italy, Seismic Sequence of 2009

Abstract Structural health monitoring (SHM) aims to improve knowledge of the safety and maintainability of civil structures. The usage of recording systems exploiting wireless communication technology is particularly suitable for SHM, especially for rapid response following earthquakes. In this study, both of these issues are combined, and we report on the application of seismic interferometry to SHM using a dataset of seven earthquakes collected using a novel wireless system of accelerometers during the L’Aquila, Italy, seismic sequence in 2009. We show that interferometric analysis allows the estimation of the shear-wave velocity of seismic phases propagating throughout a structure, and, most important for SHM purposes, allows the monitoring of the velocity variations during the aftershock sequence. Moreover, innovatively we apply the S transform to the building response functions retrieved by interferometry to estimate the fundamental resonance frequency and the quality factor Q .

Rayleigh wave dispersion curves from seismological and engineering-geotechnical methods: a comparison at the Bornheim test site (Germany)

Active and passive procedures for estimating the local seismic response from surface-wave measurements are compared for a test site in the Bornheim area (Germany), where independent geophysical and geological information is available. Recording was done using geophones, as well as seismometers, in various configurations. Five popular and standardized techniques were used for analysing the data: multichannel analysis of surface waves (MASW), the refraction microtremor technique (ReMi), the extended spatial autocorrelation technique (ESAC) and frequency–wavenumber analysis (beam-forming and maximum likelihood methods). The resulting surface wave dispersion curves are largely consistent, but differ in their respective low-frequency ranges due to the resolving capabilities of the respective acquisition geometries. Two joint inversions of dispersion and H/V curves, one for the lower frequency range (2.3–9.2 Hz) and the other for the complete range (2.3–45 Hz) of the dispersion curves resulted in fairly similar S-wave profiles, but increasing the frequency range allowed better estimates for the lower velocities at shallow depths. The results also compare well with borehole information. The site responses obtained from the two S-wave profiles are very similar, even at higher frequencies. The use of combined procedures (geotechnical-engineering and seismological) allows a high quality estimation of the S-wave velocity structure to be obtained, both at shallow and large depth. However, if a combined approach is not possible, for site response estimation at sites with sedimentary cover thicker than 30 to 50 m and where knowledge of the average S-wave velocity is more important than higher resolution estimates at shallower depths, the use of passive seismological 2D arrays is strongly recommended.

GFZ Wireless Seismic Array (GFZ-WISE), a Wireless Mesh Network of Seismic Sensors: New Perspectives for Seismic Noise Array Investigations and Site Monitoring

Over the last few years, the analysis of seismic noise recorded by two dimensional arrays has been confirmed to be capable of deriving the subsoil shear-wave velocity structure down to several hundred meters depth. In fact, using just a few minutes of seismic noise recordings and combining this with the well known horizontal-to-vertical method, it has also been shown that it is possible to investigate the average one dimensional velocity structure below an array of stations in urban areas with a sufficient resolution to depths that would be prohibitive with active source array surveys, while in addition reducing the number of boreholes required to be drilled for site-effect analysis. However, the high cost of standard seismological instrumentation limits the number of sensors generally available for two-dimensional array measurements (i.e., of the order of 10), limiting the resolution in the estimated shear-wave velocity profiles. Therefore, new themes in site-effect estimation research by two-dimensional arrays involve the development and application of low-cost instrumentation, which potentially allows the performance of dense-array measurements, and the development of dedicated signal-analysis procedures for rapid and robust estimation of shear-wave velocity profiles. In this work, we present novel low-cost wireless instrumentation for dense two-dimensional ambient seismic noise array measurements that allows the real–time analysis of the surface-wavefield and the rapid estimation of the local shear-wave velocity structure for site response studies. We first introduce the general philosophy of the new system, as well as the hardware and software that forms the novel instrument, which we have tested in laboratory and field studies.