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Spatio-temporal analysis of main seismic hazard parameters in the Ibero-Maghreb region using an Mw homogenized catalog

Mobarki Mourad, Talbi Abdelhak

Acta Geophysica

2022

Vol. 70, no 3

979--1001

As a step toward probabilistic seismic hazard assessment in the Ibero–Maghreb region, this study focuses on the estimation and the analysis of main seismic hazard parameters, namely the magnitude of completeness mc, the occurrence rate λ, the Gutenberg–Richter b value, and the maximum expected magnitude Mmax. The most recent earthquake catalog database has been compiled using different earthquake sources, including historical and instrumental earthquake records covering the time period 1045−2019. International Seismological Centre catalog database has been used to get the preliminary instrumental catalog covering the study region. Then, a magnitude inter-scale conversion analysis has been carried out to obtain magnitude conversion empirical laws. These later have been used to compile a homogenized catalog with all magnitudes unified to the moment magnitude Mw scale, which is quite reliably linked to fault physical parameters. The completeness magnitude has been estimated for different time periods using the maximum curvature and the entire magnitude range methods. Subsequently, the spatio-temporal variation of the completeness magnitude has been studied to better appreciate regional data quality. This analysis resulted in three complete sub-catalogs corresponding to different magnitude of completeness, namely mc=3.5, 4.5, and 5.5, starting from 1997, 1967 and 1959. The remaining seismic hazard parameters (λ, b and Mmax) were analyzed taking into account former magnitudes of completeness. Indeed, the use of incomplete data may add significant bias to seismic hazard parameters estimates. Main results are presented as spatial maps showing variation of seismic hazard parameters for different mc values. In particular, for the first time, a combined maximum magnitude–intensity map is elaborated and analyzed. The region corresponding to the largest maximum possible magnitude Mmax has been delimited as including Chelif basin and its surrounding areas west of the capital city Algiers. Mmax hotspots with values exceeding 7 have been delimitated close to Lakhdaria and Boumerdes cities east of the capital Algiers. The corresponding hotspots include the epicenter of the 1910 Mw7 and the 2003 Mw6.8 Aumale and Zemmouri earthquakes, respectively. The magnitude of completeness mc has been found to decrease significantly during the last two decades, especially in the western part. The results obtained in this study can serve as a road map for future seismic hazard studies performed on the Ibero–Maghreb region.

One cause of pulse-like anomalies observed at Guza before the Wenchuan earthquake

Zhou C., Wang Q., Zhu L., Wang C.

Acta Geophysica

2018

Vol. 66, no. 5

833--842

Many precursor-like anomaly observations prior to the Wenchuan earthquake were reported and analyzed, especially the abnormal strain pulses observed at the Guza station, but there are few discussions of the causes. Stick–slip motion is the basis for description of a great variety of phenomena characterized by the presence of sliding friction. In this article, perturbed Sine–Gordon (SG) equation is established from Bykov’s unsteady-state slip model. Stable solitary solutions of displacement and strain dimension are obtained and nonlinear pulse propagation is simulated using finite-difference modeling, while numerical stability is obtained by the flux-corrected transport method. Considering the solution of SG equation as initial source, a comparison between the modeling results and actual data at the Guza station gives one possible interpretation for this anomaly. During the seismogenic process of the Wenchuan earthquake, faults may likely occur as stick–slip tectonic movements which might be described by SG equation and would generate solitary wave signal. This kind of pulse experiences a forward tilting distortion due to nonlinear effect of the Earth and is received by the borehole strainmeter. Two kinds of nonlinear effects could lead to these special pulses. One is the nonlinear effects in the wave propagation process, and the other is effects of friction and stress in the process of unsteady-state slip. Because of the convergence effect of pulse, the wave would be collapsed at a certain time due to an excessive increase in the tilt angle. Hence, this kind of pulse cannot propagate for a long distance.