Fault location and source process of the Boumerdes, Algeria, earthquake inferred from geodetic and strong motion data.

International audienceThe Boumerdes earthquake occurred on a fault whose precise location, offshore the Algerian coast, was unknown. Geodetic data are used to determine the absolute position of the fault. The fault might emerge at about 15 km offshore. Accelerograms are used to infer the space-time history of the rupture using a two-step inversion in the spectral domain. The observed strong motion records agree with the synthetics for the fault location inferred from geodetic data. The fault plane ruptured for about 18 seconds. The slip distribution on the fault indicates one asperity northwest of the hypocenter with maximum slip amplitude about 3 m. This asperity is probably responsible for most of the damage. Another asperity with slightly smaller slip amplitude is located southeast of the hypocenter. The rupture stops its westward propagation close to the Thenia fault, a structure almost perpendicular to the main fault

The 2000 Tottori earthquake: a shallow aerthquake with no surface rupture and slip properties controlled by depth.

International audienceThe M w 6.8 Tottori earthquake, Japan, does not exhibit any surface trace but was particularly well instrumented. Strong motion displacement records and GPS coseismic data are used to constrain the evolution of the slip on the fault plane in time and space. We adopt in this study a two-plane fault geometry based on aftershock distributions and analysis of close station records. In a first step, our inversion allowed surface slip. The model obtained has a significant surface slip, which contradicts the absence of clear surface slip reported by geologists. In a second step, models with no slip at the surface (buried faults), compatible with geological observations, have been tested. The tests with different fault depths show that when slip is allowed to occur close to the surface, the fit to seismological and geodetic data is increased. These tests confirm that slip actually occurred at shallow depth. Despite the nonuniqueness of the solution, all the inverted source models show (1) a large slip amplitude patch at a depth of about 4–5 km and (2) relatively small slip in the hypocentral area. The rupture velocity is about 2750 m/s in the asperity region. The total rupture duration is about 8 s. The slip distribution seems to be controlled by the variation of fault properties with depth. Another feature that could control the rupture of this earthquake is a fault plane almost perpendicular to the main fault NW of the epicenter, which apparently inhibits further rupture propagation

The new Algerian Digital Seismic Network (ADSN): towards an earthquake early-warning system

Seismic monitoring in Algeria has seen great changes since the Boumerdes earthquake of 21 May 2003. Indeed, the installation of a new digital seismic network has resulted in a significant upgrade of the previous analog telemetry network. <br><br> During the last four years, the number of stations in operation has increased substantially from 25 to 69, and 20 of these are broadband, 2 are very broadband, 47 are short period. 21 are equipped with accelerometers. They are all managed by Antelope software from Kinemetrics (US Cie), and they are all connected in real time and use various modes of transmission (e.g., satellite, internet, mobile phone). The spatial repartition of the stations now cover most of northern Algeria. In addition, 70 GPS stations have recently been added to this seismological network, most of them collocated with the seismological stations. <br><br> Since the installation of the network, the records of local or distant events have improved significantly. The automatic processing of the data in a few minutes allows alert messages to be distributed to Civil Defense and other national authorities to react promptly to any emergency. The current strategy is to improve the data quality, to increase the density of the network by adding about 50 new stations, to reduce the processing time, and to reduce the time needed to send out an alert message. The result should be greatly improved network performance, which will lead to an effective early-warning system

Zemmouri earthquake rupture zone (Mw 6.8, Algeria) : aftershocks sequence relocation and 3D velocity model

International audienceWe analyze the aftershocks sequence of the Zemmouri thrust faulting earthquake (21 May 2003, M w 6.8) located east of Algiers in the Tell Atlas. The seismic sequence located during ~2 months following the mainshock is made of more than 1500 earthquakes and extends NE-SW along a ~60-km fault rupture zone crossing the coastline. The earthquake relocation was performed using handpicked P and S phases located with the tomoDD in a detailed 3D velocity structure of the epicentral area. Contrasts between velocity patches seem to correlate with contacts between granitic-volcanic basement rocks and the sedimentary formation of the eastern Mitidja basin. The aftershock sequence exhibits at least three seismic clouds and a well-defined SE-dipping main fault geometry that reflects the complex rupture. The distribution of seismic events presents a clear contrast between a dense SW zone and a NE zone with scattered aftershocks. We observe that the mainshock locates between the SW and NE seismic zones; it also lies at the NNS-SSE contact that separates a basement block to the east and sedimentary formations to the west. The aftershock distribution also suggests fault bifurcation at the SW end of the fault rupture, with a 20-km-long ~N 100° trending seismic cluster, with a vertical fault geometry parallel to the coastline juxtaposed. Another aftershock cloud may correspond to 75° SE dipping fault. The fault geometry and related SW branches may illustrate the interference between pre-existing fault structures and the SW rupture propagation. The rupture zone, related kinematics, and velocity contrasts obtained from the aftershocks distribution are in agreement with the coastal uplift and reflect the characteristics of an active zone controlled by convergent movements at a plate boundary