Addressing Geohazards Through Ocean Drilling
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Science Reports Addressing Geohazards Through Ocean Drilling by Julia K. Morgan, Eli Silver, Angelo Camerlenghi, Brandon Dugan, Stephen Kirby, Craig Shipp, and Kiyoshi Suyehiro doi:10.2204/iodp.sd.7.01.2009 Introduction societal effects requires a diverse array of observational techniques. Ocean drilling can be a key element in under- Natural geohazards, such as earthquakes, volcanic erup- standing oceanic geohazards, given that the submarine tions, landslides, and volcanic collapse, are of immediate geologic record preserves structures and past evidence for societal concern. In an oceanic setting (Fig. 1), all are capable earthquakes, landslides, volcanic collapse, and even bolide of generating tsunami that threaten coastal zones at distances impacts. This record can be read and interpreted through of many thousands of kilometers. This power and its effects drilling, coring, in situ characterization, observatory studies, were forcefully shown by ����������������������the giant������������������ earthquake �(��w 9.2�����) ���and monitoring, and laboratory studies to provide insight into tsunami of 26 December 2004 off the coast of northern future hazards and associated risks to society. Sumatra. Smaller magnitude submarine earthquakes and landslides occur with shorter recurrence intervals and ���the With these concerns and opportunities in mind, an capability�������������������������������������������������� of tsunami generation, creat���������������������ing������������������ hazards for local Integrated Ocean Drilling Program (IODP) workshop on coastal communities as well as for offshore industry and oceanic geohazards was held at �c�enamins �dgefield, infrastructure. At the other end of the scale, the geologic outside Portland, Oregon (U�����������.����������S���������.��������A�������.������) on���� 2���7–30����������������������� ����������August 200�. record suggests that less common, large-volume volcanic A primary objective of the workshop was to document how collapses and extraterrestrial meteorite and comet impacts scientific oceanic drilling could provide fundamental infor- in ocean basins have the potential to initiate tsunami of mation on the frequency and magnitudes of these destructive extraordinary power that can threaten huge sections of events, a������������������������������������������������s well as��������������������������������������� ��������������������������������������provide ������������������������������scientific insights into their coastlines with growing populations. These events also variability and underlying physics. The workshop was disperse enormous volumes of ash, steam, and ejecta into the attended by �����������������������������������������������eighty-nine������������������������������������ scientists from �������������������eighteen����������� countries, atmosphere, with short- and long-term consequences, who were charged with the following goals: (1) establish the including climate change. All of these processes������������, which����� have state of knowledge regarding conditions and distribution of operated throughout ���the �����arth��’������������������������������������������������������������s����������������������������� history,����������������������������������������� are instrumental in catastrophic geohazards; (2) define key unresolved scientific shaping the �arth system today. However, they are charac- questions relating to geohazards; (�) formulate realistic teristically difficult to predict, and viable risk assessment science plans to answer them; (4) evaluate the tools and and hazard mitigation depend on a clearer understanding of technologies available for geohazards study; (5) identify the causes, distributions, and consequences of such natural potential drilling targets for specific hazardous phenomena; events. and (6) enhance international collaborations and stimulate proponent teams to develop competitive IODP proposals. Understanding the spatial and temporal variability of Participants contributed to the workshop through oral and submarine geohazards, their physical controls, and their poster presentations, white paper preparation, proposal and Figure 1. Geologic settings in which oceanic geohazards may Bolide Impact be generated. - Major tsunami! - Earthquakes - Landslides - Ejecta of rock, melt, Glacier dust, vapor Delta Passive Margins - Submarine landslides Rift & Transform Margins Subduction Margins Oceanic Volcanoes - Debris flows, turbidity currents - Moderate earthquakes - Large earthquakes - Submarine/subaerial landslides - Shoreline subsidence, retreat - Submarine/subaerial landslides - Submarine/subaerial landslides - Debris flows, turbidity currents - Modest earthquakes - Volcanic eruptions, ash, gases - Explosive eruptions, lahars - Volcanic eruptions, ash, gases - Tsunami - Debris flows, turbidity currents - Debris flows, turbidity currents - Earthquakes - Tsunami - Tsunami - Tsunami Scientific Drilling, No.7, March 2009 15 Science Reports “concept” presentations, focused breakout discussions, and they������������������������������������������������������� can produce tsunami much larger than predicted for the open plenary discussions. earthquake. As a dramatic reminder, more than 1600 people died in 1998, when the � �.0 Sissano earthquake in Papua Submarine Landslides New Guinea triggered a massive submarine landslide, gener- ating a tsunami that inundated a small stretch of coastline Submarine landslides occur at a wide range of scales and (Synolakis et al., 2002). In North America, a large earth- settings. They often comprise distinctive mass transport quake in eastern Canada in 1929 triggered the Grand Banks deposits recognized on the seafloor or in seismic reflection landslide, turbidity flow, and tsunami that resulted in�������twenty- profiles (Fig. 2). Small�������������������������������-������������������������������scale submarine landslides are nine������������������������������������������������������ deaths and substantial coastal damage (Whelan, 1994). relatively frequent.����������������������������������������������������������������������������� �������������������������������������They��������������������������������� have displaced oil rigs, damaged The possible role of co-seismic landsliding in generating a pipelines, broken deep-sea communication cables (Piper et local tsunami in Hawaii in 19�5 is still debated (Lander and al., 1999), and, in a few cases, damaged segments of coastline Lockridge, 1989; �a et al., 1999). Some of the largest (Longva et al., 200�; Sultan et al., 2004). Large and small submarine landslides, however, have occurred on relatively events along coastal zones also create local, destructive aseismic passive margins. The best known example is the tsunamis (Lee et al., 2003�����������������������; ���������������������Tappin et al., 2001). Storegga slide on the mid�����������������������-����������������������Norwegian margin (Fig. ���������), which disrupted 90,000 km2 of the continental slope about 8100 A range of conditions and triggers has been implicated in years ago (Solheim et al., 2005). Although the cause of this the initiation of submarine landslides; these depend on slide is still debated, it is thought to have produced tsunami geologic setting, slope evolution, and tectonic and volcanic inundations in Norway, Iceland�����������������������,���������������������� and the British Isles activity. �arthquake triggering of landslides is well-known; (Bondevik et al., 199�). Hypothesized triggers include local fluid overpressures, groundwater A seepage forces, and storm-induced Head wave-action. Sea level or sea tempera- Scarp MASS TRANSPORT tures may also cause slope failure Rotated Slide DEPOSIT Blocks through gas hydrate dissociation or Slump Blocks dissolution, which can release free gas to the atmosphere (Bünz et al., 2005; Debris 1 km 1.4 U1324 Flows Increasing Internal Disruption �ienert et al., 2005). This process fits B th) into the more general “Clathrate Gun Debris Apron & Turbdity Flows Hypothesis”, relating methane release 1.6 . ~ 1300 m nor (proj U1323 and global climate change (Kennett et 899-1 U1322 al., 2000). 1.8 T (s) To date, there are no known exam- TW 2.0 ples of medium- to large-sized subma- rine mass movements where the geo- metry, in situ stresses and pressures 2.2 have been characterized prior to, during, and after the failure. Thus, it is 2.2 SW NE still unclear how and why failures U1324 1 km 1.4 initiate where and when they do, and S10 C what governs their subsequent flow S20 Distal Le Hemipelagic Drape vee behaviors. For example, some land- 1.6 S30 U1323 slides disintegrate rapidly, transitioning Southwest Pass Canyon S10 Seaoor U1322 into debris flows, avalanches�����,���� and East Levee 1.8 Slope Failure 2 Slump Blocks S20 S10 turbidity currents, whereas others S40-1 S20 T (s) S30 remain cohesive, undergoing incre- TW S30 S40-2 S50-1 S40-3 2.0 S50-3 mental down-slope creep and deforma- Slope Failure 1B S50-2 S60-3 Slope Failure 1A S70-3 tion, with impacts on their tsunami- S80 S60-1 West Levee Core S80 Top Blue S80 Top Blue genic behavior. The Storegga landslide 2.2 Ursa Canyon East Levee Blue Unit is one of the best-characterized Base Blue examples (Solheim et al., 2005); borings Base Blue 2.2 SW NE and in situ measurements have been Figure 2. [A] Schematic diagram of a mass transport deposit. [B] Seismic cross-section showing col-lected inside and outside of the land- stacked submarine slides/slumps within the Ursa region of the northern Gulf of Mexico. Some