An aftershock study of the El Asnam (Algeria) earthquake of 1980 October 10

Summary. An array of 28 portable seismic stations was operated in the region of El Asnam following the magnitude 7.3 ( M _s) thrust earthquake of 1980 October 10. Locations of 494 events are presented in this paper and provide an indication of the overall form of the aftershock distribution. Tests to establish location accuracy (particularly depth) reduce this set to 277 events which, it is argued, are well constrained. P -waves alone are used in this study as a consequence of a debate about the reliability of reading S -phases. From the reduced set of 277 events, 81 events provide well-constrained focal mechanisms.
The locations are presented in the form of maps and cross-sections, and discussed in relation to information already derived from field mapping of surface breaks and teleseismic studies of the waveforms of the main event. The zone of surface faulting (including secondary normal faulting) extended for 35 km but the aftershock distribution extends for twice this distance. Along the part of the fault which experienced substantial displacement in the main shock, the fault plane itself appears to be devoid of aftershocks, although many lie in the footwall beneath the fault. At junctions between segments of thrust faulting, strike-slip motion occurs. This is apparent in the aftershock focal mechanisms, and in the surface ruptures in one place.
The large number of aftershocks in the north-east area appears to be due to the reactivation of a fan-like system of smaller reverse faults associated with surface folding. Activity at the south-west end is considerably less than that in the north-east, and is not obviously associated with recognizable geological or morphological features.     

Postseismic deformation at El Asnam (Algeria) in the seismotectonic context of northwestern Algeria

We use a combination of seismicity. tectonic features, focal mechanisms, seismic strain and postseismic movement to study the western part of North Algeria, the El Asnam region and its surrounding area in particular. A seismotectonic map of this part of Algeria, delimited by the Mediterranean Sea in the north and the Tellian mountains in the south, was built from available geological and seismological data. An examination of this map shows that the most significant earthquakes are concentrated along tectonic features and quaternary basins elongated in an east-west direction, suggesting NNW-SSE compressional movements. During the large El Asnam earthquake of 1980 October 10, M _w= 7.1, vertical movement was measured along a 40 km northeast-southwest thrust fault. These movements were determined geodetically in 1981 with reference to a basic network previously measured in 1976. In order to control postseismic movement and to ensure the monitoring of the seismic area, a dense geodetic network has been regularly measured since 1986, both in planemetry and altimetry. The results of the altimetric remeasurements show significant vertical movements. The elevation changes of the benchmarks have been deduced from precise levelling measurements: a remarkable uplift (5.1 ± 1.9 mm yr⁻¹) of the northwestern block, during the 1986-91 period has been observed, whereas the southeastern block is seen to be relatively stable. The Sar El Marouf anticline, situated along the central segment of the El Asnam surface breaks, appears to be growing with a maximum postseismic slip rate of (9.6 ± 1.4 mm yr⁻¹). The mean uplift rates computed for the northwestern block support the view that the 1954 earthquake did not occur on the same reverse fault as the 1980 event.     

Reply to comment by A. Douglas on 'Systematic determination of earthquake rupture directivity and fault planes from analysis of long-period P-wave spectra'

We respond to the comments by Douglas regarding our earlier paper by emphasizing that our automated method was intended to distinguish between the primary and auxiliary fault planes in earthquake focal mechanisms and does not always produce reliable results for rupture velocity and rupture length.     

Progressive failure on the North Anatolian fault since 1939 by earthquake stress triggering

10 M ≥ 6.7 earthquakes ruptured 1000 km of the North Anatolian fault (Turkey) during 1939–1992, providing an unsurpassed opportunity to study how one large shock sets up the next. We use the mapped surface slip and fault geometry to infer the transfer of stress throughout the sequence. Calculations of the change in Coulomb failure stress reveal that nine out of 10 ruptures were brought closer to failure by the preceding shocks, typically by 1–10 bar, equivalent to 3–30 years of secular stressing. We translate the calculated stress changes into earthquake probability gains using an earthquake-nucleation constitutive relation, which includes both permanent and transient effects of the sudden stress changes. The transient effects of the stress changes dominate during the mean 10 yr period between triggering and subsequent rupturing shocks in the Anatolia sequence. The stress changes result in an average three-fold gain in the net earthquake probability during the decade after each event. Stress is calculated to be high today at several isolated sites along the fault. During the next 30 years, we estimate a 15 per cent probability of a M ≥ 6.7 earthquake east of the major eastern centre of Ercinzan, and a 12 per cent probability for a large event south of the major western port city of Izmit. Such stress-based probability calculations may thus be useful to assess and update earthquake hazards elsewhere.     

Excitation of the normal modes of a rotating earth model by an earthquake fault

Summary. The motion excited in a rotating earth model by a kinematically prescribed earthquake fault is solved for in closed form. In addition, expressions for the total energy released and the energy dissipated by bodily friction subsequent to faulting are obtained in terms of the normal-mode excitation amplitudes.