Seismicity and structure of the Orozco transform fault from ocean bottom seismic observations

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy and the Woods Hole Oceanographic Institution February 1982In this thesis, seismic waves generated by sources ranging from 2.7 kg shots of TNT to magnitude 5 earthquakes are studied in order to determine the seismic activity and crustal structure of the Orozco transform fault. Most of the data were collected by a network of 29 ocean bottom seismometers (OBS) and hydrophones (OBH) which were deployed as part of project ROSE (Rivera Ocean Seismic Experiment). Additional information is provided by magnetic anomaly and bathymetric data collected during and prior to ROSE and by teleseismic earthquakes recorded by the WWSSN (Worldwide Seismic Station Network). In Chapter II, the tectonic setting, bathymetry and teleseismic history of the Orozco Fracture Zone are summarized. Covering an area of 90 x 90 km which includes ridges and troughs trending both parallel and perpendicular to the present spreading direction (approximately east-west), the bathymetry of the transform portion of the fracture zone does not resemble that of other transform faults which have been studied in detail. A detailed study of one of the largest teleseismic earthquakes (mb=5.1) indicates right lateral strike-slip faulting with a strike parallel to the present spreading direction and a focal depth of less than 5 km. The moment sum from teleseismic earthquakes suggests an average fault width of at most a few kilometers. Because the teleseismic earthquake locations are too imprecise to define the present plate boundary and the magnetic anomaly data are too sparse to resolve the recent tectonic history, more questions are raised than are answered by the results in this chapter. These questions provide the focus for the study of the ROSE data. Chapter III contains an examination of the transfer function between seafloor motion and data recorded by the MIT OBS. The response of the recording system is determined and the coupling of the OBS to the seafloor during tests at two nearshore sites is analysed. Applying these results to the ROSE data, we conclude that the ground motion in the absence of the instrument can be adequately determined for at least one of the MIT OBS deployed during ROSE. Hypocentral parameters for 70 earthquakes, calculated for an assumed laterally homogeneous velocity structure which was adapted from the results of several refraction surveys in the area, are presented in Chapter IV. Because of the large number of stations in the ROSE network, the epicentral locations, focal depths and source mechanisms are determined with a precision unprecedented in marine microseismic work. Relative to the assumed model, most horizontal errors are less than ±1 km; vertical errors are somewhat larger. All epicenters are within the transform region of the Orozco Fracture Zone. About half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough which forms the northern boundary of the transform zone (region 1). Most well determined depths are very shallow (<4km) and no shallowing of activity is observed as the rise-transform intersection is approached. In fact, the deepest depths (4-10km) are for earthquakes within 10 km of the intersection; these apparent depth differences are supported by the waveforms recorded a t the MIT OBS. First motion polarities for all but two of the earthquakes in region 1 are compatible with right lateral strike-slip faulting along a nearly vertical plane striking parallel to the spreading direct ion. Another zone of activity is observed in the central part of the transform (region 2). The apparent horizontal and vertical distribution of activity is more scattered than for the first group and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism. No difference can be resolved between the stress drops or b values in the two regions. In Chapter V, lateral variations in the crustal structure within the transform region are determined and the effect of these structures on the results of the previous chapter is evaluated. Several data sources provide information on different aspects of the crustal structure. Incident angles and azimuths of body waves from shots and earthquakes measured at one of the MIT OSS show systematic deflections from the angles expected for a laterally homogeneous structure. The effect of various factors on the observed angles and azimuths is discussed and it is concluded that at least some of the deflection reflects regional lateral velocity heterogeneity. Structures which can explain the observations are found by tracing rays through three dimensional velocity grids. High velocities are inferred at upper mantle depths beneath a shallow, north-south trending ridge to the west of the OBS, suggesting that the crust under the ridge is no thicker, and perhaps thinner, than the surrounding crust. Observations from sources in region 2 suggest the presence of a low velocity zone in the central transform between the sources and the receiver. That the presence of such a body provides answers to several of the questions raised in Chapter IV about the hypocenters and mechanisms of earthquakes in region 2 is circumstantial evidence supporting this model. These proposed structures do not significantly affect the hypocenters and fault plane solutions for sources in region 1. The crustal velocity structure beneath the north-south trending ridges in the central transform and outside of the transform zone is determined by travel time and amplitude modeling of the data from several lines of small shots recorded at WHOI OBH. Outside of the transform zone, a velocity-depth structure typical of oceanic crust throughout the world oceans is found from three unreversed profiles: a 1 to 2 km thick layer in which the velocity increases from about 3 to 6.7 km/sec overlies a 4 to 4.5 km thick layer with a nearly constant velocity of 6.8 km/sec. A reversed profile over one of the north-south trending ridges, on the other hand, indicates an anomalous velocity structure with a gradient of 0.5 sec-1 throughout most of the crust ( from 5.25 km/sec to 7.15 km/sec over 3.5 km). A decrease in the gradient at the base of the crust to about 0.1 sec-1 and a thin, higher gradient layer in the upper few hundred meters are also required to fit the travel time and amplitude data. A total crustal thickness of about 5.4 km is obtained. An upper mantle velocity of 8.0 to 8.13 km/sec throughout much of the transform zone is determined from travel times of large shots of TNT recorded at MIT and WHOI instruments. "Relocations" of the large shots relative to the velocity model assumed in Chapter IV support the conclusion from the ray tracing that results from region 2 may be systematically biased because of lateral velocity heterogeneity whereas results from region 1 are not affected. In the last chapter, the results on crustal structure and seismicity are combined in order to define the present plate boundary and to speculate on the history of the present configuration.This research was supported by the Office of Naval Research, under contracts N00014-75-C-0291 and N00014-80-C-027

Basement and Regional Structure Along Strike of the Queen Charlotte Fault in the Context of Modern and Historical Earthquake Ruptures

The Queen Charlotte fault (QCF) is a dextral transform system located offshore of southeastern Alaska and western Canada, accommodating similar to 4.4 cm/yr of relative motion between the Pacific and North American plates. Oblique convergence along the fault increases southward, and how this convergence is accommodated is still debated. Using seismic reflection data, we interpret offshore basement structure, faulting, and stratigraphy to provide a geological context for two recent earthquakes, an M-w 7.5 strike-slip event near Craig, Alaska, and an M-w 7.8 thrust event near Haida Gwaii, Canada. We map downwarped Pacific oceanic crust near 54 degrees N, between the two rupture zones. Observed downwarping decreases north and south of 54 degrees N, parallel to the strike of the QCF. Bending of the Pacific plate here may have initiated with increased convergence rates due to a plate motion change at similar to 6 Ma. Tectonic reconstruction implies convergence-driven Pacific plate flexure, beginning at 6 Ma south of a 10 degrees bend the QCF (which is currently at 53.2 degrees N) and lasting until the plate translated past the bend by similar to 2 Ma. Normal-faulted approximately late Miocene sediment above the deep flexural depression at 54 degrees N, topped by relatively undeformed Pleistocene and younger sediment, supports this model. Aftershocks of the Haida Gwaii event indicate a normal-faulting stress regime, suggesting present-day plate flexure and underthrusting, which is also consistent with reconstruction of past conditions. We thus favor a Pacific plate underthrusting model to initiate flexure and accommodation space for sediment loading. In addition, mapped structures indicate two possible fault segment boundaries along the QCF at 53.2 degrees N and at 56 degrees N.USGS Earthquake Hazards External Grants ProgramNational Earthquake Hazards Reduction ProgramUTIG Ewing/Worzel FellowshipInstitute for Geophysic

Integrating Science Needs with Advanced Seafloor Sensor Engineering to Provide Early Warning of Geohazards: Visioning Workshop and Roadmap for the Future

The workshop was funded by the National Science Foundation (NSF), OCE Division of Ocean Sciences (Award # 1817257). This report summarizes the key findings, outcomes, and recommendations of the workshop and serves as a draft of the comprehensive roadmap

The Cascadia Initiative : a sea change In seismological studies of subduction zones

Author Posting. © The Oceanography Society, 2014. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 27, no. 2 (2014): 138-150, doi:10.5670/oceanog.2014.49.Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.The Cascadia Initiative is supported by the National Science Foundation; the CIET is supported under grants OCE- 1139701, OCE-1238023, OCE‐1342503, OCE-1407821, and OCE-1427663 to the University of Oregon

A Seasonally Modulated Earthquake Swarm near Maupin, Oregon

From December 2006 to November 2011, the Pacific Northwest Seismic Network (PNSN) reported 467 earthquakes in a swarm 60 km east of Mt Hood near the town of Maupin, Oregon. The swarm included 20 MD ≥ 3.0 events, which account for over 80 per cent of the cumulative seismic moment release of the sequence. Relocation of 45 MD≥ 2.5 earthquakes and moment tensor analysis of nine 3.3 ≤ Mw ≤ 3.9 earthquakes reveals right-lateral strike-slip motion on a north-northwest trending, 70° west dipping, 1 km2 active fault patch at about 17 km depth. The swarm started at the southern end of the patch and migrated to the northwest at an average rate of 1–2 m d−1 during the first 18 months. Event migration was interrupted briefly in late 2007 when the swarm encountered a 10° fault bend acting as geometrical barrier. The slow migration rate suggests a pore pressure diffusion process. We speculate that the swarm was triggered by flow into the fault zone from upwards-migrating, subduction-derived fluids. Superimposed on the swarm is seasonal modulation of seismicity, with the highest rates in spring, which coincides with the maximum snow load in the nearby Cascade Mountains. The resulting surface load variation of about 4 × 1011 N km−1 arc length causes 1 cm annual vertical displacements at GPS sites in the Cascades and appears sufficient to modulate seismicity by varying normal stresses at the fault and fluid flow rates into the fault zone

Hydrate Ridge—A Gas Hydrate System in a Subduction Zone Setting

Hydrate Ridge is a 6–10 km wide, 22 km long N–S striking thrust ridge within the Cascadia accretionary prism offshore of Oregon in the NE Pacific Ocean. Over the past four decades it has been a primary focus site for studies of gas hydrate/free gas systems within a convergent margin setting. A local peak called the North Hydrate Ridge (NHR), located at a depth of 590 m, hosts the first documented cold seep system driven by convergent margin processes and supports chemosynthetic communities sustained by the anaerobic oxidation of methane. A southern peak at 780 m depth, known as the South Hydrate Ridge (SHR), is actively venting gas around an area of seafloor bacterial mats and a 40 m high carbonate chimney within a long-lived vent system separate from NHR. Bottom simulating reflections (BSRs) observed in seismic profiles indicate these vents are part of a broad gas hydrate province that extends across all of Hydrate Ridge. Hydrate Ridge has been the focus of extensive geophysical surveys, water column acoustic and sampling surveys, high-resolution seafloor mapping using remotely operated, autonomous and deep-towed vehicles, seafloor fluid flow monitoring, and a site for the Ocean Observatories Initiative (OOI). All of these are in support or complementing Ocean Drilling Program (ODP) drilling efforts during Legs 146 and 204 to quantify and characterize the gas hydrate/free gas system. Hydrate concentrations are up to 45% of pore space (30% of total volume), but typically 2–20%, and are strongly coupled with the structure and stratigraphy within the thrust ridge

Seismic Sequence Stratigraphy and Tectonic Evolution of Southern Hydrate Ridge

This paper presents a seismic sequence and structural analysis of a high-resolution three-dimensional seismic reflection survey that was acquired in June 2000 in preparation for Ocean Drilling Program (ODP) Leg 204. The seismic data were correlated with coring and logging results from nine sites drilled in 2002 during Leg 204. The stratigraphic and structural evolution of this complex accretionary ridge through time, as inferred from seismic-stratigraphic units and depositional sequences imaged by the seismic data, is presented as a series of interpreted seismic cross sections and horizon time or isopach maps across southern Hydrate Ridge. Our reconstruction starts at ~1.2 Ma with a shift of the frontal thrust from seaward to landward vergent and thrusting of abyssal plain sediments over the older deformed and accreted units that form the core of Hydrate Ridge. From ~1.0 to 0.3 Ma, a series of overlapping slope basins with shifting depocenters was deposited as the main locus of uplift shifted northeastward. This enigmatic landward migration of uplift may be related to topography on the subducted plate, which is now deeply buried beneath the upper slope and shelf. The main locus of uplift shifted west to its present position at ~0.3 Ma, probably in response to a change to a seaward-vergent frontal thrust and related sediment underplating and duplexing. This structural and stratigraphic history has influenced the distribution of gas hydrate and free gas by causing variable age and permeability of sediments beneath and within the gas hydrate stability zone, preferential pathways for fluid migration, and varying amounts of decompression and gas dissolution