Journal of Seismology

We analyzed the available instrumental data on Italian earthquakes from1960 to 1996 to compute the parameters of the time-magnitudedistribution model proposed by Reasenberg and Jones (1989) andcurrently used to make aftershock forecasting in California. From 1981 to1996 we used the recently released Catalogo Strumentale deiTerremoti `Italiani' (CSTI) (Instrumental Catalog Working Group, 2001)joining the data of the Istituto Nazionale di Geofisica e Vulcanologia(INGV) and of the Italian major local seismic network, with magnituderevalued according to Gasperini (2001). From 1960 to 1980 we usedinstead the Progetto Finalizzato Geodinamica (PFG) catalog(Postpischl, 1985) with magnitude corrected to be homogeneous with thefollowing period. About 40 sequences are detected using two differentalgorithms and the results of the modeling for the corresponding ones arecompared. The average values of distribution parameters (p= 0.93±0.21, Log10(c) = –1.53±0.54, b = 0.96±0.18 and a = –1.66±0.72) are in fair agreementwith similar computations performed in other regions of the World. We alsoanalyzed the spatial variation of model parameters that can be used topredict the sequence behavior in the first days of future Italian seismic crisis,before a reliable modeling of the ongoing sequence is available. Moreoversome nomograms to expeditiously estimate probabilities and rates ofaftershock in Italy are also computed.

Assessing the detection threshold of seismic networks becomes of increased importance namely in the context of monitoring induced seismicity due to underground operations. Achieving the maximum possible sensitivity of industrial seismic monitoring is a precondition for successful control of technological procedures. Similarly, the lowest detection threshold is desirable when monitoring the natural seismic activity aimed to imaging the fault structures in 3D and to understanding the ongoing processes in the crust. We compare the application of two different methods to the data of the seismic network WEBNET that monitors the earthquake swarm activity of the West-Bohemia/Vogtland region. First, we evaluate the absolute noise level and its possible non-stationary character that results in hampering the detectability of the seismic network by producing false alarms. This is realized by the statistical analysis of the noise amplitudes using the ratio of 99 and 95 percentiles. Second, the magnitude of completeness is determined for each of the nine stations by analysing the automatic detections of an intensive swarm period from August 2011. The magnitude–frequency distributions of all detected events and events detected at individual stations are compared to determine the magnitude of completeness at a selected completeness level. The resulting magnitude of completeness M c of most of the stations varies between −0.9 and −0.5; an anomalous high M c of 0.0 is found at the most distant station, which is probably due to inadequate correction for attenuation. We find that while the absolute noise level has no significant influence to the station sensitivity, the noise stationarity correlates with station sensitivity expressed in low magnitude of completeness and vice versa. This qualifies the method of analysing the stationary character of seismic noise as an effective tool for site surveying during the seismic station deployment.

Stochastic finite-fault modeling is an important tool for simulating moderate to large earthquakes. It has proven to be useful in applications that require a reliable estimation of ground motions, mostly in the spectral frequency range of 1 to 10 Hz, which is the range of most interest to engineers. However, since there can be little resemblance between the low-frequency spectra of large and small earthquakes, this portion can be difficult to simulate using stochastic finite-fault techniques. This paper introduces two different methods to scale low-frequency spectra for stochastic finite-fault modeling. One method multiplies the subfault source spectrum by an empirical function. This function has three parameters to scale the low-frequency spectra: the level of scaling and the start and end frequencies of the taper. This empirical function adjusts the earthquake spectra only between the desired frequencies, conserving seismic moment in the simulated spectra. The other method is an empirical low-frequency coefficient that is added to the subfault corner frequency. This new parameter changes the ratio between high and low frequencies. For each simulation, the entire earthquake spectra is adjusted, which may result in the seismic moment not being conserved for a simulated earthquake. These low-frequency scaling methods were used to reproduce recorded earthquake spectra from several earthquakes recorded in the Pacific Earthquake Engineering Research Center (PEER) Next Generation Attenuation Models (NGA) database. There were two methods of determining the stochastic parameters of best fit for each earthquake: a general residual analysis and an earthquake-specific residual analysis. Both methods resulted in comparable values for stress drop and the low-frequency scaling parameters; however, the earthquake-specific residual analysis obtained a more accurate distribution of the averaged residuals.

The seismicity of Algeria since the nineteenth century is relatively well documented. However, compared with the numerous damaging earthquakes that are documented since 1850, fewer than a dozen reports of earthquakes are listed for the pre-1850 ad period, suggesting that the historical record is missing a substantial number of earthquakes. This paper examines the use of literary and epigraphic sources relevant to the investigation of seismicity in Algeria during Roman times. We provide examples where the meager written literary record may be supplemented with appropriate archaeological and epigraphic data describing damage to ancient Roman sites. The examples show that collaboration between earth scientists and archeologists is of utility in improving the seismic record and highlights the need for further study of data sources and repositories located both inside and outside of Algeria.

1-D models for the quality factor along a few profiles crossing the Vrancea region and the adjacent extra-Carpathian zone have been recently developed, to reconstruct the ground motion generated by weak-to-moderate crustal earthquakes of Vrancea. The retrieved Q-structures reveal a consistent pattern of high attenuation at the bend of the Southeastern Carpathians. We discuss this result in relation with the outcomes of several studies that analyze wave attenuation along travel paths crossing both crustal and upper mantle structures of the study region, to emphasize the contribution of the shallow structure to the observed attenuation pattern.

The propagation of seismic waves is influenced by changes in crustal structure as for example the transition from continental to oceanic crust along the Norwegian margin. We analyzed Lg wave propagation to map lateral crustal changes in Norway and adjacent areas. We used 1369 observations from 279 earthquakes recorded mostly by the Norwegian National Seismic Network between 1990 and 2017. First, we classified Lg wave propagation in terms of efficiency through Lg/Pn ratios and found significant changes between ray paths crossing offshore and onshore areas. Then we derived an average QLg(f) = 529 f0.42 model for Norway, which is in the expected range for a stable tectonic environment. This was used as starting model for a tomographic inversion. We present tomographic models of Lg wave attenuation at frequencies 2 Hz, 4 Hz, and 6 Hz, respectively. We observed the most significant variation between offshore and onshore regions. This can be explained by changes in crustal structure and the occurrence of unconsolidated sediments in the offshore areas.

In 1991, a digital seismic monitoring network was installed in Iceland with a digital seismic system and automatic operation. After 20 years of operation, we explore for the first time its nationwide performance by analysing the spatiotemporal variations of the completeness magnitude. We use the Bayesian magnitude of completeness (BMC) method that combines local completeness magnitude observations with prior information based on the density of seismic stations. Additionally, we test the impact of earthquake location uncertainties on the BMC results, by filtering the catalogue using a multivariate analysis that identifies outliers in the hypocentre error distribution. We find that the entire North-to-South active rift zone shows a relatively low magnitude of completeness Mc in the range 0.5–1.0, highlighting the ability of the Icelandic network to detect small earthquakes. This work also demonstrates the influence of earthquake location uncertainties on the spatiotemporal magnitude of completeness analysis.

In this study, non-principal waves propagating in an isotropic elastic half-space covered by an orthotropic layer are examined. The main objective is to establish a formula for the SH transfer function induced by an vertically incident SH wave and a formula for the H/V ratio of surface waves. The peak frequencies of both the SH transfer function and the H/V ratio curve are examined for models with low to high impedance contrasts to verify the applicability of the quarter wave-length rule for both SH body waves and surface waves. It is numerically shown that the quarter wave-length rule applies well for non-principal SH body wave. Non principal surface waves are shown to be a composition of Love and Rayleigh waves, and their peaks follow the quarter wave-length rule only in the case of high impedance contrast. For medium or low impedance contrasts, the peak frequencies of surface waves could differ from the peak frequencies of SH body wave with relative differences up to $$50\%$$ .