Revealing the geophysical environment of slow slip using core-log-seismic integration

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发生在海床以下不到2公里的深度，每1 - 2年定期发生一次。这个SSE区域是世界上唯一一个具有现代钻探能力的区域，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。钻探为我们提供了有关岩石性质和流体含量的直接信息，但这些数据是非常一维的，只能提供地下的“粗略”一瞥。另一方面，地震数据对地下的几何形状进行成像，并使我们能够推断出更广泛区域内岩石的声速，但是，它并不能提供流体含量或其他岩石性质的任何直接证据。在钻井数据和地震数据同时存在的地方，可以建立地震属性和岩石属性之间的关系。然后，这些关系可以应用于整个地震数据体，以估计尚未钻探的区域中的岩石性质。在Hikurangi的情况下，这将包括俯冲板块边界断层的较深部分，该部分经历缓慢滑动。

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 less than 2 km below the sea bed, and they occur regularly every 1-2 years. This SSE zone is the only such zone worldwide within 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. Drilling provides us with direct information about rock properties and fluid content, however the data is very one-dimensional, and only provides a "pot-shot" glimpse at the subsurface. Seismic data on the other hand images the geometry of the subsurface and allows us to deduce the speed of sound through the rocks over wider areas, however, it does not provide any direct evidence of fluid content or other rock properties. In places where drilling data and seismic data exist together relationships between seismic properties and rock properties can be developed. These relationships can then be applied throughout seismic data volumes to estimate rock properties in areas that have not been drilled. In the case of Hikurangi this would include the deeper part of the subduction plate boundary fault which undergoes slow slip.