THESIS PROPOSAL. DEGREE PROGRAMME: Ph. D. FIELD OF SPECIALIZATIOB: Geophysics

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1 THESIS PROPOSAL DEGREE PROGRAMME: Ph. D. FIELD OF SPECIALIZATIOB: Geophysics SUPERVISOR and COMMITTEE: Supervisors - Mladen Nedimovic Committee Member - Keith Louden (Dalhousie Oceanography) / Donna Shillington (Lamont- Doherty Earth Observatory) TITLE OF PROPOSAL: Seismic megathrust hazards by reflection mapping KEY WORDS Megathrust earthquake, 2D reflection seismics, subduction zone, Alaska-Aleutian arc, interplate boundary, seismogenic zone, wide-angle refraction, seismic velocity distribution LIST INNOVATIONS or EXPECTED SIGNIFICANT OUTCOMES: 1. Validation of the hypothesis on using seismic reflection imaging to determine locked zones on subduction thrusts. 2. Localization of potential sites for great earthquakes and slow slip zones at the Semidi Segment and the Shumagin Gap of the Aleutian arc. 3. Determination of arc structure composition along two wide-angle refraction profiles. SUMMARY OF PROPOSED RESEARCH: Effective megathrust earthquake damage prevention depends on a few major quantities - accurate estimation of the occurrence time, spatial location, and expected strength of large future events in the regions of interest. Whereas the recurrence rates and magnitudes of megathrust earthquakes can be estimated with long-term earthquake observation, accurate determinations of megathrust earthquake occurrence times and rupture areas are more difficult to obtain. A new approach to determining megathrust earthquake rupture areas using seismic reflection imaging was introduced by Nedimovic et al and will be tested at the Aleutian subduction arc. This subduction zone was chosen because of its accessibility for marine surveying across the full width of the seismogenic zone. For optimal imaging over geometrically complex subsurfaces, we acquired six trench-normal multichannel seismic (MCS) profiles using two 8-km streamers of the R/V Langseth. The streamers had 640 channels each and were used to collect state of the art data with an effective penetration depth >30 km. Additionally, on two of these profiles we acquired wide-angle refraction data using 21 ocean bottom seismometers (OBS) to obtain seismic velocity models for interpretation and to support the MCS processing. Our goal is to produce clear images

2 of the interplate boundary, the contact zone between the subducting oceanic plate and the overriding continental plate. This is the area where megathrust earthquakes originate. The reflection character of the locked (seismogenic) and slow slipping zones can then be obtained by correlating reflection signature of the interplate interface with the locations of historic megathrust earthquakes and dislocation models based on GPS data. Statement of Problem The Alaska-Aleutian subduction zone is one of the most seismically active regions on Earth. Megathrust earthquakes with magnitudes greater than 8-9 generated in this area are a threat to coastal communities in Alaska and around the Pacific. The quantification of future megathrust earthquake damage depends largely upon the location and size estimation of its rupture area, or seismogenic zone. We will use high quality seismic reflection and refraction data from offshore Alaska (Fig. 1) in an attempt to map the down-dip extent of the seismogenic zone within the Semidi Segment. We will also compare these results with the megathrust reflection signature of the near by Shumagin Gap, an area assumed to be driven by aseismic slip as indicated by recent GPS measurements and the absence of earthquake activity in the last 100 years. Comparison of the megathrust reflection signatures in these two areas will test the hypothesis that it is possible to detect seismically locked areas of the megathrust using deep seismic reflection imaging.

3 Figure 1 Investigation area of this study. Red and black lines indicate position of seismic profiles which are processed by either the Lamont-Doherty Earth Observatory (red) or the Dalhousie University (black). Two additional profiles provide wide-angle reflection/refraction information acquired by 21 Ocean bottom seismometers (yellow dots). Note that seismic line 3 is crossing the Semidi Segment, which is assumed to be seismically locked, whereas line 5 crosses the Shumagin Gap - a region where no large earthquake activity occurred since at least 100 years (Davies et al. 1981). Figure 2 Shematic diagram of the great megathrust earthquake cycle. (a) The downgoing and overiding plates become seismically coupled along what is referred to as the locked zone. This seismogenic behavior changes to slow slip further downdip. (b) Slip along the locked and transition zones generates earthquakes (From Hyndman and Wang 1995).

4 Figure 3 Two details from seismic reflection sections of the Cascadia megathrust subduction zone (Nedimovic et al. 2003). (a) shows a thin and sharp reflection sequence (< 2 km, probably even < 0.5 km as Nedimovic et al suggest) of the assumed locked part of the interplate boundary, whereas (b) was acquired in an area further downdip, showing a broad reflection band (>4 km width) of the presumably slow slipping (aseismic) interplate interface between the plates (Dragert et al. 2001). Between (a) and (b) is a transitional area of conditional stability characterized by a transitional megathrust reflection package km thick. Background Large subduction megathrust or great earthquakes frequently lead to human casualties and major material damage due to tsunamis and strong ground shaking that collapse buildings and destroy infrastructure (e.g. Fujii et al. 2011, Kiser and Ishii 2011, Lay et al. 2005). Therefore, megathrust earthquakes are a constant threat for many coastal communities and urban centers around the world, including those in North America, such as Vancouver, Anchorage, Seattle and Portland. Predictions of future earthquake magnitudes are challenging but can be estimated by determining the potential seismic moment which depends upon the rupture area, friction and slip magnitude (Hanks and Kanamori 1979). Much of what we know today about seismogenic zones at the

5 subducting interplate boundaries comes from long-term seismological studies (e.g. Atwater and Hemphill-Haley 1997, Goldfinger et al. 2003), deformation studies of post-seismic uplift or GPS movements from geodetic data (Wang et al. 2001, Fournier and Freymueller 2007) and numerical modeling of thermal behavior (Hyndman and Wang 1995). Despite the useful insights gained, earthquake, geodetic and numerical modeling studies have limitations. Earthquake seismology and geodesy require extremely long observation periods but modern seismological instruments began to emerge in the first half of the last century and GPS measurements exists for only the last couple of decades. Furthermore, even after long periods of observation earthquake seismology only yields data about the most recent rupture and GPS studies only provide deformation information about the present state of coupling behavior. Numerical modeling, which relies on the earthquake and geodesy information is therefore not well constrained. These limitations of the current methology motivate us to test an alternative approach that has the potential to provide higher detail short-term characterization of long-term slip behavior of megathrusts. From a margin normal perspective, seismic and aseismic slip occur on different parts of a subduction thrust (Fig. 2). They are the result of different temperatures, pressures and likely fundamentally different types of rock deformation. Brittle failure is dominant where seismic slip is observed (along fault planes) and plastic deformation is likely where slip is aseismic and appears to include a larger rock volume. Because subduction processes appear to be stable thermally and compositionally for extended periods of time, brittle and plastic deformation tend to occur along the same downdip ranges of the megathrust. Therefore, it seems reasonable to think that the structural signature (and its reflection expression) for parts of the megathrust that have experienced brittle deformation should differ from those that have experienced plastic deformation. A recent study of the northern Cascadia megathrust (Nedimovic et al. 2003) appears to confirm this idea. The seismic reflection signature of the presumably locked seismogenic part of the interplate boundary is focused and distinct, whereas in the slow slipping part a broad band of reflections is observed (Fig. 3). However, further testing of the hypothesis on using seismic reflection imaging to determine the locked and slow-slipping zones of subduction thrusts is needed. To achieve this, we focus on the eastern Alaska-Aleutian arc (Fig. 1). The targeted area appears to be the best place globally to test the reflection imaging hypothesis because: 1. Wide continental shelf allows for an investigation of the locked, transition and slow slip zones using relatively inexpensive marine surveying; 2. Changes in megathrust coupling can be investigated in both downdip and along strike direction (Semidi Segment is believed to be locked and Shumagin Gap is believed to be freely slipping); 3. Earthquake and dislocation models are available from previous studies for this region (Freymueller and Beavan 1999, Fournier and Freymueller 2007). These models provide a good base for further investigations but have certain limitations, which have to be considered and are described below.

6 During the last century almost the entire Alaska-Aleutian subduction thrust has ruptured in large to great earthquakes (Davies et al. 1981). Seismological observations of these earthquakes were used to gain a basic understanding about the distribution of subduction earthquake rupture areas along this subduction zone. Additionally, recent deformation studies (e.g. Freymueller and Beavan 1999, Fournier and Freymueller 2007) provide information on the current spatial distribution of the subduction thrust coupling. Unfortunately, seismological instruments along the Alaska-Aleutian subduction zone have been sparsely distributed and data from the global network have not been sufficient to allow for a detailed determination of rupture zones. Furthermore, most parts of the currently locked zones are located offshore and are inaccessible to contemporary GPS measurements. The GPS data points for the region of interest in this work are therefore reduced to the Alaska Peninsula, Kodiak Island and smaller islands offshore. Nevertheless, available seismological and geodetic results on the Alaska-Aleutian megathrust rupture zones and interplate coupling provide a good initial estimate needed for comparison with seismic reflection images. Objectives The long-term objective of this research is focused on using seismic reflection imaging to map coupling on subduction thrust to improve probabilistic megathrust seismic hazards maps globally. The short-term objectives, i.e. those that can be fully addressed within this thesis work are: 1. Test the hypothesis on using seismic reflection imaging to map the locked and slowslipping zones of subduction thrusts; 2. Determine the location of locked asperities along the eastern section of the Alaska- Aleutian subduction thrust; 3. Derive geological properties from seismic velocity distribution to help infer composition and evolution of the Semidi Segment and Shumagin Gap sections of the Aleutian arc. Approach and Methods We have collected MCS data on six profiles (~300 km) perpendicular to the trench, one profile (~400 km) along the continental shelf in trench-parallel direction and several smaller strike profiles both at the continental shelf and the deep sea (Fig. 1). Coincident with profile line 3 and 5, we have additional wide-angle refraction data from 21 OBS's. The dataset is divided between the Lamont-Doherty Earth Observatory (LDEO) in New York and Dalhousie University. LDEO will be working with the data from the assumed aseismic western part of the research area near the Shumagin Gap. Dalhousie University will be analyzing data from the eastern part at the Semidi Segment, which is assumed to be seismically locked. The seismic data were collected with the R/V Marcus G. Langseth at a combined MCS and wide-angle refraction survey in the Gulf of Alaska near Kodiak Island (Fig. 1). We used two 8-km long streamers with 640 receivers (hydrophones) each for the MCS survey. The streamers were towed at different depths (9 m and 12 m) behind the vessel to enhance both shallow and deep imaging of the crust, respectively. The seismic source was a 6600 cu. in. tuned airgun array. The shot interval was 62.5 m and recording

7 time was 22.5 s at a sampling rate of 2 ms. Wide-angle recordings were conducted with 21 fourcomponent OBS's with average spacing interval of approximately 15 km. The aim of seismic data processing is to produce reflection or velocity images of subsurface geology, i.e. structures of different rock properties, layer geometry and faults. But the seismic signal of interest is always combined with unwanted signals such as noise, blurry wavelets and artifacts. This can result in wrongly appearing dips in seismic sections or in disguising the wanted signal. Also the unknown and sometimes highly heterogeneous propagation velocity (seismic velocity) in the subsurface is affecting travel-time arrivals. If not corrected by an accurate velocity model it decreases the reliability of the final results. The following paragraphs describe the main processing methods for reflection and refraction data to address these shortcomings and produce a more reliable description of the geologic situation. Filter and Gain - to improve the signal-to-noise (s/n) ratio Raw seismic data is always disguised by unwanted signals (noise). Sources of noise can be manifold and may include: surface water waves (swell noise), damaged hydrophones, ringing noise from the seismic source and other diverse sources. The first step in data processing is to enhance the reflection signal and decrease the energy (amplitude) of the noise. The signal strength of the reflecting signal decreases with depth and towards large offsets due to amplitude attenuation and spherical spreading. To get a higher s/n ratio, it is necessary to apply a gain to increase amplitudes of the desired reflected signal and use filters to attenuate noise energy (Yilmaz 1987). Deconvolution - increasing vertical resolution of seismic data An ideal seismic source signal would be a spike (or delta function) - a signal with high amplitude over a very small time interval. In practice, the generating such signals is not possible. Generated signals always have a certain duration associated with decaying amplitude and are called wavelets. The result is a blurry seismogram without sharp boundary separation. The deconvolution process aims to sharpen the input signal by compressing the seismic wavelet. Therefore, it improves the temporal (vertical) resolution of the seismogram (Yilmaz 1987). Migration - processing method for "accurate" imaging of earth structures While deconvolution improves the temporal resolution of a seismogram, migration increases the spatial resolution. Detailed subsurface structures can be delineated with this process by moving wrongly appearing dipping reflectors into their true subsurface position and by collapsing diffractions resulting from obstacles or sharp edges in the subsurface. There are different approaches to the migration process, but one of the most widely used migration algorithms is the

8 diffraction summation method. It sums up amplitudes along a diffraction hyperbola and collapses them in its apex. Assuming a horizontally layered velocity model of the earth, the velocity function used to compute the travel-time trajectory is the root mean square (rms) velocity at the apex of the hyperbola. For a successful migration it is essential to use the correct velocity. Other migration techniques use the wavefield downward continuation (Finite-Difference Migration, e.g. Ristow and Rühl 1994) or the Fourier Transforms to perform migration in the frequency and wavenumber domain (f-k migration, introduced by Chun and Jacewitz 1981). These techniques are usually used when lateral variations in seismic velocities are light to moderate and are carried out in the time domain. Depth migration, which will produce a depth section, is a more advanced migration technique and is applied when strong lateral changes in velocity are predominant. This would be most likely the case in our data set, where we will encounter dipping subducting crust, conflicting dips and a highly irregular sea bottom surface. Imperative is the use of an accurate velocity-depth model of the subsurface, which has to be determined before depth migration. Velocity analysis Seismic surveys are in most cases done by recording acoustic waves propagating between seismic sources, reflectors (or refractors) in the subsurface, and the receivers. The recorded data and processed seismic sections are usually shown in two-way-travel (TWT) time. However, we are not interested in the time the signals need to reach a certain layer, but would rather like to know at which depth the layer is located. For this, we need to know the propagation velocity of the medium the signal is traversing. In most cases, wave velocity is analyzed by examining the reflection moveout curves. Seismic travel-time inversion and forward modeling - FAST and RAYINVR Unlike the seismic reflection method, which is used to locate rock boundaries, the refraction method can be used to accurately determine seismic velocity distribution of the subsurface. Rays of refracted waves travel through the layered subsurface according to Snell's law. Their traveltimes are determined by the different velocity zones they pass through. Inversion of these traveltimes enables us to derive an accurate velocity-depth model of the subsurface. The greater the raypath coverage of an area (or volume) is, the more accurate the resulting velocity model is. The process of inversion uses differences in observed and theoretical calculated travel-times (of an initial velocity model) and aims to minimize the residuals. This will be done by the inversion program FAST (First Arrival Seismic Tomography) developed by Zelt and Barton (1998). Another method to derive seismic velocities from refracted data is the "layer stripping" strategy used by the program RAYINVR (Zelt and Smith 1992). This program uses an initial velocitydepth model for forward modeling the first arrivals in OBS data. The synthetic arrivals are then compared with the observed arrival times and in case of a mismatch, the velocity-depth model will be modified, and synthetic arrivals will be calculated again. The method is designed to

9 determine the correct velocities from the upper layers first before proceeding to the deeper parts in the subsurface. An initial velocity-depth model or structural build-up of the upper layers can be derived from MCS seismic reflection data. The applicability of this method to subduction zones has to be tested, since the dipping of the downgoing plates does not fit into the geometry of a horizontally layered subsurface geology for which this method is intended. Anticipated Results and Significance On-board preliminary processing showed excellent seismic data imaging potential, suggesting that we will be able to obtain detailed structural information about this part of the Alaska- Aleutian subduction zone. A seismically locked zone, Semidi Segment, and a zone of assumed aseismic slow slip, Shumagin Gap, will be analyzed in greater detail. The gathered results will then be compared to validate the hypothesis that it is possible to use seismic reflection mapping to detect locked parts of subduction thrusts. Successful detection of seismogenic zones could help to improve probabilistic seismic hazard maps in this region. The Semidi Segment is assumed to rupture in a great earthquake every years, and the last great earthquake occurred roughly 74 years ago in 1938 (Sykes et al. 1981). Hence, this area is of great topical interest to both the general public and the scientific community. Our study will also make an important contribution to the GeoPRISMS Program. GeoPRISMS is an US scientific initiative of interdisciplinary researchers, who chose the Aleutian arc as a discovery corridor to understand the origin and evolution of the Aleutian Island chain and the continental crust in general. Reflection data from this study will illuminate crustal structures in great detail and wide-angle data will result in subsurface velocity information, both of which might help to determine crustal rock composition of the Aleutian volcanic arc (Shillington et al. 2007). REFRENCES Atwater, B.F., and Hemphill-Haley, E Recurrence intervals for great earthquakes of the past 3,500 years at northeastern Willapa Bay, Washington: US Geological Survey Professional Paper 1576: 108 p. Chun, J.H. and Jacewitz, C Fundamentals of frequency-domain migration, Geophysics 46, Davies, J., Sykes, L., House, L. and Jacob, K Shumagin seismic gap, Alaska peninsula: History of great earthquakes, tectonic setting, and evidence for high seismic potential, Journal of Geophysical Research 86: Fournier, T.J. and Freymueller J.T Transition from locked to creeping subduction in the Shumagin region, Alaska, Geophysical Research Letter 34: 5pp. Fujii, Y., Satake, K., Sakai, S., Shinohara, M. and Kanazawa, T Tsunami source of the

10 2011 off the Pacific coast of Tohoku, Japan earthquake, Earth, Planets and Space 63, no7: Goldfinger, C., Nelson, C.H., Johnson, J.E., and the Shipboard Scientific Party Holoscene earthquake records from Cascadia subduction zone and northern San Andreas fault based on precise dating of offshore turbodites, Annual Review Earth Planetary Science 31: Hyndman, R.D. and Wang, K The rupture of Cascadia great earthquakes from current deformation and thermal regime, Journal of Geophysical Research 100: Kiser, E. and Ishii, M The 2010 Mw 8.8 Chile earthquake: Triggering on multiple segments and frequency-dependent rupture behavior, Geophysical Research Letters 38: L Lay, T., Kanamori, H., Ammon, C.J., Nettles, M., Ward, S.N., Aster, R.C., Beck, S.L., Bilek, S.L., Brudzinski, M.R., Butler, R., DeShon, H.R., Ekstroem, G., Satake, K. and Sipkin, S The Great Sumatra-Andaman Earthquake of 26 December 2004, Science 308: Nedimovic, M.R., Hyndman, R.D., Ramachandran, K. and Spence, G.D Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface, Nature 424: Ristow, D. and Rühl, T Three dimensional finite-difference migration by multiwaysplitting, Society of Exploration Geophysicists Annual meeting 1995, Shillington, D.J., VanAvendonk, H.J.A., Holbrook, W.S., Kelemen, P.B. And Hornbach, M.J Compostion and structure of the central Aleutian island arc from arc-parallel wideangle seismic data, Geochemistry Geophysics Geosystems 5, no 10: 32pp. Sykes, L.R., Kisslinger, J.B., House, L., Davies, J.N. and Jacob, K.H Rupture zones and repeat times for great earthquakes along Alaska-Aleutian arc, , Maurice Ewing Series 4: Wang, K., Jiangheng, H., Dragert, H. and James, T.S Three-dimensional viscoelastic interseismic deformation model for the Cascadia subduction zone, Earth Planets Space 53: Yilmaz, O Seismic Data Processing. Society of Exploration Geophysicist.

11 Zelt, C. and Barton P.J Three-dimensional seismic refraction tomography: a comparison of two methods applied to data from the Faeroe Basin, Journal of Geophysical Research 103: Zelt, C. and Smith R.B Seismic traveltime inversion for 2D crustal velocity structure, Geophysical Journal International 108,