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

Lead Research Organisation: Imperial College London
Department Name: Earth Science and Engineering

Abstract

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.

Planned Impact

This research will have four major classes of non-academic beneficiaries:

(1) At an international level, the results from this and the wider Hikurangi projects will improve our understanding of the slow slip process. In turn this will help us understand the fault slip process in general and related hazards (earthquake, tsunami), and the role slow slip might play in potentially leading to earthquakes or tsunami earthquakes. This will significantly impact our ability to assess geohazard potential worldwide with implications for government policy makers and authorities in areas of seismic hazard globally;.
(2) The hydrocarbon and mining industry continue to be keen sponsors of the development of full-waveform inversion (FWI) methods because of the improved imaging of fine-scale structure that this technique allows. Modifications in the application of the technique developed in this project are expected to have specific applicability to hydrocarbon exploration in frontier areas as well as future academic surveys. Increased academic use of FWI will ultimately result in large numbers of highly-trained specialists who, like many previous graduates, are being recruited by industry and boosting their expertise;.
(3) Local authorities in New Zealand will benefit from improved knowledge of the seismic hazard associated with the Hikurangi subduction margin;.
(4) The UK public will benefit from and be informed about subduction zone and fault slip processes from research-led outreach activities aimed at a broad range of age groups and backgrounds. The international public will benefit from coverage surrounding the results of this study in the media, and the advances made in terms of earthquake and tsunami hazard at subduction zones and potential for the resources industry.

We will interact with these beneficiaries by:
(1) Fully engaging with our project partner, GNS Science New Zealand, who have links with local authorities in New Zealand. We will work together with social scientists at GNS to translate our results into information that will be of use to regional policy makers and geohazard management teams. In particular, new rock property information from the megathrust that we will recover from riser drilling and integration with geophysical datasets will directly be used to update seismic and tsunami hazard models for New Zealand. We will work closely with hazards teams at GNS to make sure they are using the most realistic parameters in their models based on the results of this project. This material will also be fed, via our international partners (e.g., US, Japan), into other regional hazard assessment communities where subduction zone geohazards are significant.
(2) Contributing to the maintenance of a dedicated FWI website at Imperial and through a series of ongoing workshops. We will host academics at Imperial for longer periods of training and release software to specifically target other academic users, which will increase the number of trained specialists in this new field.
(3) Maintaining a dedicated project website which will provide public education material related to the research. We will also produce an exhibition for the Imperial College Festival which attracts visitor numbers of c. 10,000 including government policy makers, industry professionals and the general public of all backgrounds and age groups. In addition, we will develop materials suitable for delivery in Imperial, University of Southampton and University of Cardiff outreach activities. At the end of the project we will submit a Royal Society Summer Exhibition proposal, to develop an exhibit for this prestigious event. We will engage with the general public internationally through releasing press releases to the world's media.

Publications

10 25 50
 
Description Revealing the geophysical environment of slow slip using core-log-seismic integration
Amount £24,068 (GBP)
Funding ID NE/S00291X/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 06/2018 
End 12/2018
 
Title New seismic data 
Description We have collected seismic data from 50 6TD, 200 Cube and 25 GSX's from 14th Dec to 16th February during a 3D active-source seismic reflection survey partially funded by our award. These data have only just been collected and they will be uploaded to SeisUK and IRIS datacentres in the coming months. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? No  
Impact The data has only just been collected and will be analysed by my research team over the coming years before it is publically released. 
 
Description GFZ Potsdam 
Organisation Helmholtz Association of German Research Centres
Department German Research Centre for Geosciences
Country Germany 
Sector Private 
PI Contribution I appplied for an equipment loan from the GFZ Potsdam Geophysical Instruement Pool and was succesful in being loaned 120 Cube seismometers to contribute to the NERC funded experiment
Collaborator Contribution GFZ Potsdam loaned use (free of charge) 120 Cube seismometers which allowed us to greatly uplift the originally funded NERC project which involved the use of only 60 seismometers
Impact Collection of seismic data on 120 Cube seismometers from 14th Dec to 16th Feb
Start Year 2017
 
Description GNS 
Organisation GNS Science
Country New Zealand 
Sector Public 
PI Contribution GNS were conducting a seismometer deployment in New Zealand at the same time as the NERC funded experiment so we could assist by paying half the costs of tools and garage hire etc. We also collaborated on seeking permissions from landowners and forestry owners to access land as we were working in the same area. We will work with GNS as collaborators in the analysis of the data.
Collaborator Contribution A team from GNS Science assisted us with the deployment of almost 200 seismometers in New Zealand as part of this research grant. We benefitted from their time in the field (an estimated total of over 500 man hours in the field), which we were not charged for. GNS also donated the use of their 4WD vehicles (5 vehicles for a period of 3 weeks and 1 vehicle for a period of 10 weeks). GNS were also doing their own seismometer deployment at the same time so we could share costs of materials and garage hire with them. GNS alsohelped us network with the local council and civil defence as well as Maori communities in Gisborne.
Impact Collection of seismic data from 50 6TD seismometers, 120 Cube seismometers and 25 GSX seismometers from 14th Dec to 16th February. The 50 6TD seismometers will continue recording until October 2018
Start Year 2017
 
Description NSF 
Organisation National Science Foundation (NSF)
Country United States 
Sector Public 
PI Contribution Our funded NERC proposal provided 10 days of ship time on the Marcus Langseth to collect 3D seismic data. After the success of our proposal an NSF proposal was funded to provide 25 days of ship time. Together these proposals provided the necessary 35 days of ship time for us to acquire the 3D seismic data.
Collaborator Contribution NSF funded 25 days of the total 35 day offshore expedition.
Impact Collection of a 3D seismic reflection volume offshore Gisborne, North Island, New Zealand
Start Year 2017
 
Description Geological society outreach talks 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Other audiences
Results and Impact I have given three outreach talks at geological societies since the start of the NERC grant. These include at the Keele Geography Society ball, Lapworth Museum lecture series, University of Birmingham, the London branch of the Open University and the Hertfordshire geological society. My talk was aimed at introducing the mixed audience of industry professionals, academics, students, the general public to the phenomenon of slow slip and discussing the aims of our new experiment. This talk sparked many questions and discussion afterwards and I have been invited back to present the results of the experiment in the coming years.
Year(s) Of Engagement Activity 2017,2018
 
Description Gisborne Herald 
Form Of Engagement Activity A press release, press conference or response to a media enquiry/interview
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact In Jan 2018 I gave an interview to the Gisborne Herald (the most-read local paper in the Gisborne area where our fieldwork was focused) to talk about our seismometer deployment. After the publication of this article we were approached by people in supermarkets and in the study area who had seen our photos in the paper and wanted to learn more about the experiment.
Year(s) Of Engagement Activity 2018
URL http://gisborneherald.co.nz/environment/3166768-135/earthquake-studies-converge-on-gisborne
 
Description Newspaper article on Gisborne mud volcanoes 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact On 15th Dec 2018 there was a mud volcano eruption in the area where we deployed seismometers funded by this grant. I was asked for a brief interview and the project led by Imperial College is mentioned in the article.
Year(s) Of Engagement Activity 2019
URL http://gisborneherald.co.nz/localnews/3897921-135/mud-volcano-erupts-at-waimata-valley
 
Description Public meeting Gisborne council 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact East Coast Lab organised a "ship to shore" public event where members of the public could speak to members of IODP expedition 372 who were drilling offshore New Zealand in Dec 2017. Some members of the public are extremely interested in the science being done by the drilling and our NERC funded seismic experiment, whereas others are cautious and somewhat suspicious that we are working to find oil and gas. The event was aimed at providing a venue for the public to meet the scientists involved and answer any questions and alleviate concerns. I was involved as an expert panel member due to my involvement in the NERC funded project.
Year(s) Of Engagement Activity 2017
URL http://www.eastcoastlab.org.nz/news/ship-to-shore-opportunity-for-east-coast-communities/
 
Description Talk to school pupils during Imperial Summer School 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Schools
Results and Impact I gave a talk to sixth form students on the rsearch funded by this grant. These students came from around the UK and were considering taking a degree in geoscience. Formal feedback was requested from the students after the event and 56% of the attendees said that after attending the event they would definitely consider a degree in geoscience with 31% possibly considering a degree in geoscience.
Year(s) Of Engagement Activity 2018