Collaborative Research: GeoGONAF: Analysis of active deformation and strain transfer along the Izmit Bay-Marmara Sea segment of the North Anatolian Fault

世界上很少有地方比土耳其的马尔马拉海地区更符合加利福尼亚州旧金山湾区的地震危险潜力。在这两个地方，一场7级地震都有可能撼动一个海上特大城市。伊斯坦布尔目前人口接近1500万，每年旅游人数为300万，未来20年发生7.1级地震的可能性估计为35%-70%。1766年，一场与预期类似的地震摧毁了伊斯坦布尔，当时伊斯坦布尔的人口不到50万。一些人认为，这场地震即将重演。GeoGONAF是一个国际合作项目的首字母缩写，该项目旨在监测地震前的地下和海底应变变化。其科学团队预计不会预测未来地震的确切时间，但他们乐观地认为，目前密切监听即将到来的破裂的密集监测仪器将捕捉到丰富的信号，这些信号将表明北安纳托利亚断层的几个分支中最有可能破裂的是哪一个。在城市周围和离岸岛屿上安装这些仪器花了十年的大部分时间，但现在有一股数据流流入土耳其、德国和美国的实验室，这些实验室原则上允许这些合作国家的科学团队识别出接近破裂的信号。在土耳其学到的经验将直接用于解释位于旧金山湾海沃德断层之下发生的类似前兆过程。在GeoGONAF(研究伊斯坦布尔附近北安纳托利亚断层的地球物理观测站的大地测量部分)最后阶段，在钻孔中安装了6个应变仪。应变计可以检测到10E-12(1皮应变)的应变变化，相当于地球直径的变化是人类头发直径的五分之一。在地下30米至100米深处安装这些超灵敏仪器后的稳定阶段几乎已经结束(1-3年)，目前有6个100赫兹采样率的应变数据流(截至2016年6月)补充了由德国、美国和土耳其的GeoGONAF合作者安装的钻孔地震仪和GPS仪器的数据。目前的项目计划首先使用潮汐和入射地震波校准应变仪，然后监测马尔马拉地区发生的间歇性和连续信号。具体地说，计划在伊斯坦布尔南部和东南部的海底断层上搜索慢滑事件。这些缓慢的蠕变事件对地震仪来说是不可见的，但被视为GPS数据的增量，以及应变仪上独特的时间演变信号，持续时间从几分钟到几天。2016年3月18日，在即将投入使用的六台应变仪中的第一台上，捕捉到了一次重大的慢滑事件，其重要性在于，这种事件逐渐将已经准备好即将破裂的岩石推向未来的地震。尽管每增加一次地震就会使一次地震更近，但现在就知道伊斯坦布尔以南三条断层中的哪一条会发生地震还为时过早。拟议的项目将每天检查传入的应变和全球定位系统数据，以搜索传感器阵列上同步出现的相干信号，以确定断层的滑动斑块及其滑动的区域和程度。初步解释表明，在任何给定的断层地块上，大型蠕变事件每年不会超过一次，但在马尔马拉海下方和附近相互关联的断层网络中，蠕变事件可能相当频繁。作为对断层系统的更大研究的一部分，将专门安装额外的地表蠕变测量仪，以确定马尔马拉以东的北安纳托利亚断层上2-16毫米幅度蠕变事件的传播特征。

There are few places in the world that more closely match the seismic hazard potential of the San Francisco Bay Area of California than the Marmara Sea area of Turkey. In both places a magnitude 7 earthquake threatens to shake a maritime megacity. The current population of Istanbul is approaching 15 million, with an annual tourist population of 3 million, and the probability of an M7.1 earthquake in the next 20 years estimated as 35-70%. In 1766 a previous earthquake, similar to the one anticipated, destroyed Istanbul when its population was less than 0.5M. Some believe that a repeat of this earthquake is imminent. GeoGONAF is an acronym for an international collaborative project to monitor the subterranean and submarine strain changes that will precede the earthquake. Its scientific team does not expect to predict the precise time of this future earthquake, but they are optimistic that the dense array of monitoring instruments now listening closely to the approaching rupture will capture a rich harvest of signals that will indicate which of the several branches of the North Anatolian fault is most likely to rupture. It has taken the better part of decade to install these instruments around the city and on offshore islands, but now a stream of data flows into laboratories in Turkey, Germany and the US, that in principle permit the scientific teams of these collaborating nations to identify signals characteristic of approaching rupture. Lessons learned in Turkey will have direct application in the interpretation of similar precursory processes occurring beneath the Hayward fault located in the San Francisco Bay.Six strain-meters were installed in boreholes during the final phase of GeoGONAF (the Geodetic component of the Geophysical Observatory to study the North Anatolian Fault near Istanbul). The strain-meters can detect strain changes of 10e-12 (1 picostrain), equivalent to a change in the diameter of the Earth of one fifth of the diameter of a human hair. The stabilization phase following inserting these ultra-sensitive instruments at depths of 30 m to 100 m underground is almost over (1-3 years) and six streams of strain data at 100 Hz sampling rate (as of June 2016) now complement data from the borehole seismometers and GPS instruments installed by the German, US and Turkish collaborators of GeoGONAF. The current project plans first to calibrate the strain-meters using tides and inbound seismic waves, and then to monitor episodic and continuous signals occurring in the Marmara region. Specifically a search for slow slip events on the submarine faults south and southeast of Istanbul is planned. These slow creep events are invisible to seismometers but are seen as increments on GPS data and as distinctive time-evolving signals on strain-meters with durations of minutes to days. On 18 March 2016 a large slow slip events on the first of the six strain-meters to be commissioned was captured in remarkable detail, its importance being that such events incrementally advance rocks already poised for imminent rupture towards a future earthquake. Although each increment brings an earthquake closer, it is too early to know on which of three faults south of Istanbul this will occur. The proposed project will examine incoming strain and GPS data on a daily basis to search for coherent signals occurring synchronously on the array of sensors, in order to identify slipping patches of faults and the area and extent of their slip. Preliminary interpretations indicate that large creep events cannot occur more than once per year on any given patch of a fault but that creep events may be quite frequent throughout the interconnected network of faults beneath and near the Sea of Marmara. As part of a larger study of the fault system additional surface creep-meters will be installed specifically to identify the propagation characteristics of 2-16 mm amplitude creep-events along the North Anatolian fault to the east of Marmara.