关键词: 电火花震源, GI枪震源, 小多道地震, 活动断裂, 台湾海峡

Abstract:

The neotectonic activity is intense in the Taiwan Straits and the coastal area of South China. This region is one of the earthquake-prone areas of the world. In history, earthquakes of magnitude 6-7 occurred repeatedly in this region with a high recurrence rate. Therefore, this area has always been the focus of seismicity research and coastal earthquake prevention and disaster reduction. The exploration of active faults is the basis for seismic zoning, but the detection and identification of active faults in sea area are more difficult because of the coverage of sea water, which leads to a large number of “blind areas” in marine fault exploration for a long time. Seismic exploration methods are economical, suitable and efficient in detecting active faults in the sea area. This study compares the detection effect of different seismic sources.
In this study, geophysical exploration of active faults was carried out in the southeast Fujian uplift zone in the Taiwan Straits. A mini-multichannel seismic profile of GI gun source and sparker source at the same location was selected for comparative analysis and illustration. Five reflection interfaces(T₁—T₄, Tg)were interpreted on the GI gun profile, and five sets of seismic sequences(A—E)were classified. Six reflection interfaces(T'₁, T₁—T₄, Tg)were interpreted on the sparker source profile, and six sets of seismic sequences(A—D and E₁—E₂)were classified. Three basement faults and two shallow faults with small vertical extension were found, which are active since the late Pleistocene. Among them, the scale of fault F₁ is large, the displacement of the basement fault F₁ is 51ms, and the overall displacement of (T₁—T₄) in the sediments is 35ms. Faults F₂—F₅ are located on the continental side of fault F₁ and can be combined into grabens and horsts in forms, which are inferred to be the associated faults of Fault F₁. It’s found that basement faults can be identified by both GI gun profile and sparker source profile, while the small faults can only be identified by the sparker profile. At the same time, the depth of upper breakpoint on the sparker profile is shallower, and the latest fault activity can be traced back to the Holocene. The locations and geometrical shapes of the three basement faults are similar on the two profiles, but there are imaging differences in the formation shapes around the faults and the distribution patterns of the secondary faults due to the influence of resolution. The similarity of fault detection results shows the effectiveness of the two methods, while the difference of profile imaging shows the necessity of combined detection in practical work.
According to the comparison of the two kinds of data, the sparker profile reveals a finer shallow structure than the GI gun profile does, and the GI gun profile can obtain a clearer basement structure. Based on the fusion results of the two kinds of data, the structural attributes of fault F₁ are further analyzed and explained in detail in this paper, and the Fault F₁ is the result of the reactivation of a basement pre-existing fault in the late Pleistocene and is a depression-boundary fault with an activity pattern of extensional normal faulting, and it is considered in this paper to be part of the South China Binhai fault zone. Therefore, it is necessary to attach importance to the combination of multiple detection methods in marine seismic zoning and marine seismic hazard assessment in order to obtain more detailed fault information.

Key words: sparker source, GI gun source, mini-multichannel seismic, active fault, Taiwan Straits