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接近加州州弗朗西斯科湾区的地震危险潜力。 在这两个地方，一场7级地震威胁着一个海洋大城市。伊斯坦布尔目前的人口接近1500万，每年的旅游人口为300万，未来20年发生7.1级地震的概率估计为35- 70%。 1766年，一场与预期相似的地震摧毁了人口不足50万的伊斯坦布尔。 一些人认为，这场地震即将重演。 GeoGONAF是一个国际合作项目的缩写，该项目旨在监测地震前地下和海底的应变变化。它的科学团队并不期望预测未来地震的准确时间，但他们乐观地认为，现在密切关注即将发生的破裂的密集监测仪器阵列将捕获丰富的信号，这些信号将表明北安纳托利亚断层的几个分支中的哪一个最有可能破裂。 在城市周围和近海岛屿上安装这些仪器花了近十年的时间，但现在数据流流入土耳其，德国和美国的实验室，原则上允许这些合作国家的科学团队识别接近破裂的信号特征。在土耳其吸取的经验教训将直接用于解释发生在圣弗朗西斯科湾的海沃德断层下的类似地震过程。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）的最后阶段，在钻孔中安装了六个应变计。 应变仪可以检测到10 e-12（1皮应变）的应变变化，相当于地球直径变化人类头发直径的五分之一。在地下30米至100米深处插入这些超灵敏仪器后的稳定阶段几乎已经结束（1-3年），六个100 Hz采样率的应变数据流（截至2016年6月）现在补充了GeoGONAF德国，美国和土耳其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利用潮汐和入境地震波校准应变仪，然后监测马尔马拉地区出现的偶发和连续信号。 具体来说，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，但在GPS数据上被视为增量，在应变仪上被视为持续时间从几分钟到几天的独特的时间演变信号。 2016年3月18日，六台应变仪中的第一台上的一个大的缓慢滑动事件被非常详细地捕捉到，其重要性在于，这些事件逐渐推动已经准备好即将破裂的岩石走向未来的地震。 虽然每增加一次地震都会使地震更近，但要知道这将发生在伊斯坦布尔以南的三个断层中的哪一个还为时过早。 拟议的项目将每天检查收到的应变和全球定位系统数据，以寻找传感器阵列上同步出现的相干信号，以便确定断层的滑动区域及其滑动的面积和程度。 初步的解释表明，大型蠕变事件不能发生超过一年一次，在任何给定的补丁的故障，但蠕变事件可能是相当频繁的整个互联网络的故障之下和附近的马尔马拉海。 作为断层系统更大研究的一部分，将专门安装额外的表面蠕变仪，以确定马尔马拉以东沿着北安纳托利亚断层的2-16 mm振幅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.