Marine geophysical study of the Solomon Arc convergent zone

所罗门岛弧地区是一个复杂的板块会聚带。北侧是北所罗门海沟，太平洋板块在那里被吃掉。南侧的另一个海沟是圣克里斯托瓦尔海沟，在那里，印度-澳大利亚板块和所罗门海板块似乎正在俯冲到所罗门弧下。圣克里斯托瓦尔海沟在瓜达尔卡纳尔以西变得不那么明显。板块俯冲带通常最好地用地震数据来描述，也就是通过确定定义俯冲板块几何形状的震源来描述。然而，在所罗门地区这样的复杂地区，除非假设该地区的构造活动，否则全球地震台网数据集无法解决复杂性问题。在瓜达尔卡纳尔以西，每年只有不到10次地震被确定，在确定震源时获得高空间分辨率的唯一方法是进行非常高灵敏度的现场地震观测。我们于1993年改进了我们的数字海底地震仪(OBS)，并于1994年8月28日至9月7日在瓜达尔卡纳尔岛以西用5台OBS进行了临时微震活动观测。初步结果表明：(1)地震可探测性比国际地震中心的测定高约200倍；(2)印澳板块俯冲至约50公里，倾角约为50度；(3)在双弧之间的小盆地几乎没有地震活动。(4)可能与太平洋板块俯冲有关的60-90公里深的事件。我们正在规划一条弧形断面，以获得弧系的整个地壳结构，这将有助于将上述发现与结构和构造联系起来。

The Solomon island arc area is a complex plate convergence zone. On the northern side is the North Solomon Trench at which the Pacific plate is being consumed. Another trench on the southern side is the San Cristobal Trench, where the Indo-Australian plate and the Solomon Sea plate seem to be subducting beneath the Solomon arc. The San Cristobal Trench becomes less conspicuous west of Guadalcanal.A plate subduction zone is usually best described by the seismic data, that is by the determination of hypocenters that define the geometry of the subducting slab. However, in a complex region like the Solomon area, the worldwide seismic network dataset is unable to resolve the complexity unless one assumes the tectonics operating in the region. Only less than 10 events are determined each year west of Guadalcanal.The only way to obtain high spatial resolution in hypocenter determination is to conduct an on-site seismic observation with very high sensitivity. We have improved our digital ocean bottom seismograph (OBS) in 1993 and made a temporary microseismicity observation west of Guadalcanal using 5 OBS's for the period of Aug.28 through Sep.7 in 1994.Our preliminary results indicate (1) about 200 times higher seismic detectability compared to the International Seismological Center determinations, (2) subduction of the Indo-Australian plate to about 50 km with a surprisingly large dip angle of about 50 degrees, (3) almost no seismicity in the small basin between the double arc, (4) 60 to 90 km deep events probably associated with the Pacific plate subduction.We are planning an arc transect to obtain the whole crustal structure of the arc system, which will help to link the above findings with the structure and tectonics.