Unlocking the secrets of slow slip with IODP drilling and next-generation seismic experiments

俯冲带位于地球的一个构造板块在另一个板块之下滑动的地方-这种运动由板块边界断层控制。这些板块边界断层能够产生地球上最大的地震和海啸，例如2011年日本的Tohuku-oki和2004年苏门答腊-安达曼地震，共造成约25万人死亡。虽然一些板块边界断层在灾难性地震中失效，但在一些俯冲边缘，板块毫不费力地相互蠕动，没有应力沿断层沿着积聚，因此不会产生大地震。确定是什么控制了断层在大地震中是蠕动还是滑动，对于评估生活在板块边界断层附近的社区面临的地震危险以及我们对地震过程本身的理解至关重要。在过去的15年里，在俯冲带发现了一种全新的地震现象：无声地震或慢滑动事件（SSEs）。这些事件释放的能量与大地震一样多，但持续了几周甚至几个月，根本没有地面震动。SSE可能有可能引发高度破坏性的地震和海啸，但这是否可能以及为什么SSE会发生是当今地震学中最重要的两个问题。我们之所以知道SSE的存在，是因为它们导致了地球的运动，而这些运动可以用GPS技术来测量。现在，在几乎所有有良好、连续GPS网络的俯冲带都发现了慢滑事件，包括日本、哥斯达黎加、美国西北部和新西兰。重要的是，最近有证据表明，SSE发生在本十年来最大的两次地震之前，并可能引发了这两次地震，即2011年Tohuki-oki和2014年智利伊基克地震。因此，社会迫切需要更好地了解SSE及其与破坏性地震的关系。我们对SSE知之甚少，因为它们大多发生在25-40公里的深度：太深了，无法使用地震数据进行钻探和清晰成像，这是一种使用高能声波探测地壳的远程方法。新西兰北方的希库朗吉边缘是一个重要的例外。非常浅的SSE发生在c的深度处。在海床下5公里处，它们每1-2年定期发生一次。这个SSE区域是世界上唯一一个在现代钻井能力范围内的此类区域，我们可以通过地震技术清晰地对断层进行成像-这个位置为我们提供了对缓慢滑动的断层带进行采样和成像的机会。这将允许测试一些不同的假设提出来解释SSE。我们还可以将这些岩石的性质与来自日本等其他地区的钻井和地震数据进行比较，这些地区的断层表现不同，并产生非常大的地震。通过这种比较，我们可以更进一步地了解为什么一些俯冲边缘断层在大地震中失败，而另一些则没有，以及什么断层属性控制着不同的滑动过程。在钻探之前，我们需要三维地震数据来模拟钻探现场，以突出任何潜在的风险，并使我们能够更多地了解远离钻探现场的岩石属性在三维空间中的变化。即使在钻探之前或没有钻探，地震图像也将提供缓慢滑动过程和断层性质的重要细节。我们将使用一种新技术，称为全波形反演（FWI），可以产生高分辨率的声波通过地壳的速度模型。声波在含有大量流体的岩石中传播速度较慢，因此我们将寻找表明流体存在的低速异常，模型表明这可能会产生SSE。新西兰俯冲带的开创性FWI成像将是同类中的第一个，以前所未有的分辨率提供有关断层带属性的信息。

Subduction zones are located where one of the Earth's tectonic plates slides beneath another - this motion is controlled by the plate boundary fault. These plate boundary faults are capable of generating the largest earthquakes and tsunami on Earth, such as the 2011 Tohuku-oki, Japan and the 2004 Sumatra-Andaman earthquakes, together responsible for ~250,000 fatalities. Although some plate boundary faults fail in catastrophic earthquakes, at some subduction margins the plates creep past each other effortlessly with no stress build-up along the fault, and therefore large earthquakes are not generated. Determining what controls whether a fault creeps or slips in large earthquakes is fundamental to assessing the seismic hazard communities living in the vicinity of plate boundary faults face and to our understanding of the earthquake process itself. In the last 15 years a completely new type of seismic phenomena has been discovered at subduction zones: silent earthquakes or slow slip events (SSEs). These are events that release as much energy as a large earthquake, but do so over several weeks or even months and there is no ground-shaking at all. SSEs may have the potential to trigger highly destructive earthquakes and tsunami, but whether this is possible and why SSEs occur at all are two of the most important questions in earthquake seismology today. We only know SSEs exist because they cause movements of the Earth that can be measured with GPS technology. Slow slip events have now been discovered at almost all subduction zones where there is a good, continuous GPS network, including Japan, Costa Rica, NW America and New Zealand. Importantly, there is recent evidence that SSEs preceded and may have triggered two of the largest earthquakes this decade, the 2011 Tohuki-oki and 2014 Iquique, Chile earthquakes. Therefore, there is an urgent societal need to better understand SSEs and their relationship to destructive earthquakes.We know little about SSEs because most of them occur at depths of 25-40 km: too deep to drill and to image clearly using seismic data, a remote method that uses high-energy sound waves to probe the Earth's crust. The Hikurangi margin of northern New Zealand is an important exception. Very shallow SSEs occur here at depths of c. 5 km below the sea bed, and they occur regularly every 1-2 years. This SSE zone is the only such zone worldwide within likely range of modern drilling capabilities and where we can image the fault clearly with seismic techniques - this location provides us with an opportunity to sample and image the fault zone that slowly slips. This will allow testing of a number of different hypotheses proposed to explain SSEs. We can also compare the properties of these rocks with drilling and seismic data from other locations such as Japan, where the faults behave differently and generate very large earthquakes. Through this comparison we can get closer to understanding why some subduction margin faults fail in large earthquakes and others do not and what fault properties control the different slip processes.Before the drilling can take place we need 3D seismic data to characterise the drill site to highlight any potential risks and to allow us to learn more about how rock properties vary in three dimensions away from the drill sites. Even before or without drilling the seismic images will provide important details of the slow slip process and fault properties. We will use a new technique, called full-waveform inversion (FWI) that can produce high resolution models of the speed of sound waves through the Earth's crust. Sound waves travel slower through rocks that contain a lot of fluids so we will look for low velocity anomalies signifying the presence of fluids, which models have suggested could allow generation of SSEs. The groundbreaking FWI imaging of the New Zealand subduction zone will be the first of its kind, providing information on fault zone properties at unprecedented resolution.