McGuire Jeffrey J.

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Jeffrey J.

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  • Article
    A long-term geothermal observatory across subseafloor gas hydrates, IODP Hole U1364A, Cascadia accretionary prism
    (Frontiers Media, 2020-12-21) Becker, Keir ; Davis, Earl E. ; Heesemann, Martin ; Collins, John A. ; McGuire, Jeffrey J.
    We report 4 years of temperature profiles collected from May 2014 to May 2018 in Integrated Ocean Drilling Program Hole U1364A in the frontal accretionary prism of the Cascadia subduction zone. The temperature data extend to depths of nearly 300 m below seafloor (mbsf), spanning the gas hydrate stability zone at the location and a clear bottom-simulating reflector (BSR) at ∼230 mbsf. When the hole was drilled in 2010, a pressure-monitoring Advanced CORK (ACORK) observatory was installed, sealed at the bottom by a bridge plug and cement below 302 mbsf. In May 2014, a temperature profile was collected by lowering a probe down the hole from the ROV ROPOS. From July 2016 through May 2018, temperature data were collected during a nearly two-year deployment of a 24-thermistor cable installed to 268 m below seafloor (mbsf). The cable and a seismic-tilt instrument package also deployed in 2016 were connected to the Ocean Networks Canada (ONC) NEPTUNE cabled observatory in June of 2017, after which the thermistor temperatures were logged by Ocean Networks Canada at one-minute intervals until failure of the main ethernet switch in the integrated seafloor control unit in May 2018. The thermistor array had been designed with concentrated vertical spacing around the bottom-simulating reflector and two pressure-monitoring screens at 203 and 244 mbsf, with wider thermistor spacing elsewhere to document the geothermal state up to seafloor. The 4 years of data show a generally linear temperature gradient of 0.055°C/m consistent with a heat flux of 61–64 mW/m2. The data show no indications of thermal transients. A slight departure from a linear gradient provides an approximate limit of ∼10−10 m/s for any possible slow upward advection of pore fluids. In-situ temperatures are ∼15.8°C at the BSR position, consistent with methane hydrate stability at that depth and pressure.
  • Article
    Imaging the deep structure of the San Andreas Fault south of Hollister with joint analysis of fault zone head and direct P arrivals
    (Blackwell Publishing, 2007-04-08) Lewis, M. A. ; Ben-Zion, Yehuda ; McGuire, Jeffrey J.
    We perform a joint inversion of arrival time data generated by direct P and fault zone (FZ) head waves in the San Andreas Fault south of Hollister, CA, to obtain a high-resolution local velocity structure. The incorporation of head waves allows us to obtain a sharp image of the overall velocity contrast across the fault as a function of depth, while the use of near-fault data allows us to resolve internal variations in the FZ structure. The data consist of over 9800 direct P and over 2700 head wave arrival times from 450 events at up to 54 stations of a dense temporary seismic array and the permanent northern California seismic network in the area. One set of inversions is performed upon the whole data set, and five inversion sets are performed on various data subsets in an effort to resolve details of the FZ structure. The results imply a strong contrast of P-wave velocities across the fault of ~50 per cent in the shallow section, and lower contrasts of 10–20 per cent below 3 km, with the southwest being the side with faster velocities. The presence of a shallow low velocity zone around the fault, which could corresponds to the damage structures imaged in trapped wave studies, is detected by inversions using subsets of the data made up of only stations close to the fault. The faster southwest side of the fault shows the development of a shallow low velocity FZ layer in inversions using instruments closer and closer to the fault (<5 and <2 km). Such a feature is not present in results of inversions using only stations at greater distances from the fault. On the slower northeast side of the fault, the presence of a low velocity shallow layer is only detected in the inversions using the stations within 2 km of the fault. We interpret this asymmetry across the fault as a possible indication of a preferred propagation direction of earthquake ruptures in the region. Using events from different portions of the fault, the head wave inversions also resolve small-scale features of the fault visible in the surface geology and relocated seismicity.
  • Article
    Imaging along-strike variations in mechanical properties of the Gofar transform fault, East Pacific Rise
    (John Wiley & Sons, 2014-09-23) Froment, B. ; McGuire, Jeffrey J. ; van der Hilst, R. D. ; Gouedard, P. ; Roland, Emily C. ; Zhang, H. ; Collins, John A.
    A large part of global plate motion on mid-ocean ridge transform faults (RTFs) is not accommodated as major earthquakes. When large earthquakes do occur, they often repeat quasiperiodically. We focus here on the high slip rate (∼14 cm/yr) Gofar transform fault on the equatorial East Pacific Rise. This fault is subdivided into patches that slip during Mw 5.5–6 earthquakes every 5 to 6 years. These patches are separated by rupture barriers that accommodate slip through swarms of smaller events and/or aseismic creep. We performed an imaging study to investigate which spatiotemporal variations of the fault zone properties control this segmentation in mechanical behavior and could explain the specific behavior of RTFs at the global scale. We adopt a double-difference approach in a joint inversion of active air gun shots and microseismicity recorded for 1 year. This data set includes the 2008 Mw 6 Gofar earthquake. The along-strike P wave velocity structure reveals an abrupt transition between the barrier area, characterized by a damaged fault zone of 10–20% reduced Vp and a nearly intact fault zone in the asperity area. The importance of the strength of the damage zone on the mechanical behavior is supported by the temporal S wave velocity changes which suggest increased damage within the barrier area, during the week preceding the Mw 6 earthquake. Our results support the conclusion that extended highly damaged zones are the key factor in limiting the role of major earthquakes to accommodate plate motion along RTFs.
  • Article
    Earthquake swarms driven by aseismic creep in the Salton Trough, California
    (American Geophysical Union, 2007-04-10) Lohman, Rowena B. ; McGuire, Jeffrey J.
    In late August 2005, a swarm of more than a thousand earthquakes between magnitudes 1 and 5.1 occurred at the Obsidian Buttes, near the southern San Andreas Fault. This swarm provides the best opportunity to date to assess the mechanisms driving seismic swarms along transform plate boundaries. The recorded seismicity can only explain 20% of the geodetically observed deformation, implying that shallow, aseismic fault slip was the primary process driving the Obsidian Buttes swarm. Models of earthquake triggering by aseismic creep can explain both the time history of seismic activity associated with the 2005 swarm and the ∼1 km/h migration velocity exhibited by this and several other Salton Trough earthquake swarms. A combination of earthquake triggering models and denser geodetic data should enable significant improvements in time-dependent forecasts of seismic hazard in the key days to hours before significant earthquakes in the Salton Trough.
  • Article
    Earthquake swarms on transform faults
    (John Wiley & Sons, 2009-06-04) Roland, Emily C. ; McGuire, Jeffrey J.
    Swarm-like earthquake sequences are commonly observed in a diverse range of geological settings including volcanic and geothermal regions as well as along transform plate boundaries. They typically lack a clear mainshock, cover an unusually large spatial area relative to their total seismic moment release, and fail to decay in time according to standard aftershock scaling laws. Swarms often result from a clear driving phenomenon, such as a magma intrusion, but most lack the necessary geophysical data to constrain their driving process. To identify the mechanisms that cause swarms on strike-slip faults, we use relative earthquake locations to quantify the spatial and temporal characteristics of swarms along Southern California and East Pacific Rise transform faults. Swarms in these regions exhibit distinctive characteristics, including a relatively narrow range of hypocentral migration velocities, on the order of a kilometre per hour. This rate corresponds to the rupture propagation velocity of shallow creep transients that are sometimes observed geodetically in conjunction with swarms, and is significantly faster than the earthquake migration rates typically associated with fluid diffusion. The uniformity of migration rates and low effective stress drops observed here suggest that shallow aseismic creep transients are the primary process driving swarms on strike-slip faults. Moreover, the migration rates are consistent with laboratory values of the rate-state friction parameter b (0.01) as long as the Salton Trough faults fail under hydrostatic conditions.
  • Article
    The Cascadia Initiative : a sea change In seismological studies of subduction zones
    (The Oceanography Society, 2014-06) Toomey, Douglas R. ; Allen, Richard M. ; Barclay, Andrew H. ; Bell, Samuel W. ; Bromirski, Peter D. ; Carlson, Richard L. ; Chen, Xiaowei ; Collins, John A. ; Dziak, Robert P. ; Evers, Brent ; Forsyth, Donald W. ; Gerstoft, Peter ; Hooft, Emilie E. E. ; Livelybrooks, Dean ; Lodewyk, Jessica A. ; Luther, Douglas S. ; McGuire, Jeffrey J. ; Schwartz, Susan Y. ; Tolstoy, Maya ; Trehu, Anne M. ; Weirathmueller, Michelle ; Wilcock, William S. D.
    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.
  • Article
    (American Geophysical Union, 2019-10-14) Fan, Wenyuan ; McGuire, Jeffrey J. ; de Groot‐Hedlin, Catherine D. ; Hedlin, Michael A. H. ; Coats, Sloan ; Fiedler, Julia W.
    Seismic signals from ocean‐solid Earth interactions are ubiquitously recorded on our planet. However, these wavefields are typically incoherent in the time domain limiting their utilization for understanding ocean dynamics or solid Earth properties. In contrast, we find that during large storms such as hurricanes and Nor'easters the interaction of long‐period ocean waves with shallow seafloor features located near the edge of continental shelves, known as ocean banks, excites coherent transcontinental Rayleigh wave packets in the 20‐ to 50‐s period band. These “stormquakes” migrate coincident with the storms but are effectively spatiotemporally focused seismic point sources with equivalent earthquake magnitudes that can be greater than 3.5. Stormquakes thus provide new coherent sources to investigate Earth structure in locations that typically lack both seismic instrumentation and earthquakes. Moreover, they provide a new geophysical observable with high spatial and temporal resolution with which to investigate ocean wave dynamics during large storms.
  • Article
    A slow slip event in the south central Alaska Subduction Zone and related seismicity anomaly
    (American Geophysical Union, 2012-08-11) Wei, Meng ; McGuire, Jeffrey J. ; Richardson, Eliza
    We detected a slow slip event in the south central Alaska Subduction Zone by analyzing continuous GPS data from the Plate Boundary Observatory (PBO) network. The slow slip event started in early 2010 at a depth of 35 km beneath the Cook Inlet, near the down-dip end of the locked zone, and is ongoing as of November 2011 with an accumulated magnitude of Mw 6.9. Analysis of the earthquake catalog in the same area using the stochastic Epidemic Type Aftershock Sequence model (ETAS) shows a small but detectable seismicity increase during the slow slip event. We also find a change in seismicity rate around 1998, that may suggest an earlier slow slip event in the same region. Slow slip events in Alaska appear more widespread than previously thought but have remained undetected due to their long durations, the time intervals between them, and the limited time records from the continuous GPS.
  • Article
    Frictional behavior of oceanic transform faults and its influence on earthquake characteristics
    (American Geophysical Union, 2012-04-26) Liu, Yajing ; McGuire, Jeffrey J. ; Behn, Mark D.
    We use a three-dimensional strike-slip fault model in the framework of rate and state-dependent friction to investigate earthquake behavior and scaling relations on oceanic transform faults (OTFs). Gabbro friction data under hydrothermal conditions are mapped onto OTFs using temperatures from (1) a half-space cooling model, and (2) a thermal model that incorporates a visco-plastic rheology, non-Newtonian viscous flow and the effects of shear heating and hydrothermal circulation. Without introducing small-scale frictional heterogeneities on the fault, our model predicts that an OTF segment can transition between seismic and aseismic slip over many earthquake cycles, consistent with the multimode hypothesis for OTF ruptures. The average seismic coupling coefficient χ is strongly dependent on the ratio of seismogenic zone width W to earthquake nucleation size h*; χ increases by four orders of magnitude as W/h* increases from ∼1 to 2. Specifically, the average χ = 0.15 ± 0.05 derived from global OTF earthquake catalogs can be reached at W/h* ≈ 1.2–1.7. Further, in all simulations the area of the largest earthquake rupture is less than the total seismogenic area and we predict a deficiency of large earthquakes on long transforms, which is also consistent with observations. To match these observations over this narrow range of W/h* requires an increase in the characteristic slip distance dc as the seismogenic zone becomes wider and normal stress is higher on long transforms. Earthquake magnitude and distribution on the Gofar and Romanche transforms are better predicted by simulations using the visco-plastic model than the half-space cooling model.
  • Article
    Complex and diverse rupture processes of the 2018 Mw 8.2 and Mw 7.9 Tonga-Fiji deep earthquakes
    (American Geophysical Union, 2019-02-20) Fan, Wenyuan ; Wei, S. Shawn ; Tian, Dongdong ; McGuire, Jeffrey J. ; Wiens, Douglas A.
    Deep earthquakes exhibit strong variabilities in their rupture and aftershock characteristics, yet their physical failure mechanisms remain elusive. The 2018 Mw 8.2 and Mw 7.9 Tonga‐Fiji deep earthquakes, the two largest ever recorded in this subduction zone, occurred within days of each other. We investigate these events by performing waveform analysis, teleseismic P wave backprojection, and aftershock relocation. Our results show that the Mw 8.2 earthquake ruptured fast (4.1 km/s) and excited frequency‐dependent seismic radiation, whereas the Mw 7.9 earthquake ruptured slowly (2.5 km/s). Both events lasted ∼35 s. The Mw 8.2 earthquake initiated in the highly seismogenic, cold core of the slab and likely ruptured into the surrounding warmer materials, whereas the Mw 7.9 earthquake likely ruptured through a dissipative process in a previously aseismic region. The contrasts in earthquake kinematics and aftershock productivity argue for a combination of at least two primary mechanisms enabling rupture in the region.
  • Article
    Aseismic transient slip on the Gofar transform fault, East Pacific Rise
    (National Academy of Sciences, 2020-04-28) Liu, Yajing ; McGuire, Jeffrey J. ; Behn, Mark D.
    Oceanic transform faults display a unique combination of seismic and aseismic slip behavior, including a large globally averaged seismic deficit, and the local occurrence of repeating magnitude (M) ∼6 earthquakes with abundant foreshocks and seismic swarms, as on the Gofar transform of the East Pacific Rise and the Blanco Ridge in the northeast Pacific Ocean. However, the underlying mechanisms that govern the partitioning between seismic and aseismic slip and their interaction remain unclear. Here we present a numerical modeling study of earthquake sequences and aseismic transient slip on oceanic transform faults. In the model, strong dilatancy strengthening, supported by seismic imaging that indicates enhanced fluid-filled porosity and possible hydrothermal circulation down to the brittle–ductile transition, effectively stabilizes along-strike seismic rupture propagation and results in rupture barriers where aseismic transients arise episodically. The modeled slow slip migrates along the barrier zones at speeds ∼10 to 600 m/h, spatiotemporally correlated with the observed migration of seismic swarms on the Gofar transform. Our model thus suggests the possible prevalence of episodic aseismic transients in M ∼6 rupture barrier zones that host active swarms on oceanic transform faults and provides candidates for future seafloor geodesy experiments to verify the relation between aseismic fault slip, earthquake swarms, and fault zone hydromechanical properties.
  • Article
    High-resolution imaging of the Bear Valley section of the San Andreas fault at seismogenic depths with fault-zone head waves and relocated seismicity
    (Blackwell Publishing, 2005-09-02) McGuire, Jeffrey J. ; Ben-Zion, Yehuda
    Detailed imaging of fault-zone (FZ) material properties at seismogenic depths is a difficult seismological problem owing to the short length scales of the structural features. Seismic energy trapped within a low-velocity damage zone has been utilized to image the fault core at shallow depths, but these phases appear to lack sensitivity to structure in the depth range where earthquakes nucleate. Major faults that juxtapose rocks of significantly different elastic properties generate a related phase termed a fault-zone head wave (FZHW) that spends the majority of its path refracting along the fault. We utilize data from a dense temporary array of seismometers in the Bear Valley region of the San Andreas Fault to demonstrate that head waves have sensitivity to FZ structure throughout the seismogenic zone. Measured differential arrival times between the head waves and direct P arrivals and waveform modelling of these phases provide high-resolution information on the velocity contrast across the fault. The obtained values document along-strike, fault-normal, and downdip variations in the strength of the velocity contrast, ranging from 20 to 50 per cent depending on the regions being averaged by the ray paths. The complexity of the FZ waveforms increases dramatically in a region of the fault that has two active strands producing two separate bands of seismicity. Synthetic waveform calculations indicate that geological observations of the thickness and rock-type of the layer between the two strands are valid also for the subsurface structure of the fault. The results show that joint analysis of FZHWs and direct P arrivals can resolve important small-scale elements of the FZ structure at seismogenic depths. Detailed characterization of material contrasts across faults and their relation to earthquake ruptures is necessary for evaluating theoretical predictions of the effects that these structures have on rupture propagation.
  • Article
    Seismicity of the Atlantis Massif detachment fault, 30°N at the Mid-Atlantic Ridge
    (American Geophysical Union, 2012-10-09) Collins, John A. ; Smith, Deborah K. ; McGuire, Jeffrey J.
    At the oceanic core complex that forms the Atlantis Massif at 30°N on the Mid-Atlantic Ridge, slip along the detachment fault for the last 1.5–2 Ma has brought lower crust and mantle rocks to the seafloor. Hydroacoustic data collected between 1999 and 2003 suggest that seismicity occurred near the top of the Massif, mostly on the southeastern section, while detected seismicity along the adjacent ridge axis was sparse. In 2005, five short-period ocean bottom seismographs (OBS) were deployed on and around the Massif as a pilot experiment to help constrain the distribution of seismicity in this region. Analysis of six months of OBS data indicates that, in contrast to the results of the earlier hydroacoustic study, the vast majority of the seismicity is located within the axial valley. During the OBS deployment, and within the array, seismicity was primarily composed of a relatively constant background rate and two large aftershock sequences that included 5 teleseismic events with magnitudes between 4.0 and 4.5. The aftershock sequences were located on the western side of the axial valley adjacent to the Atlantis Massif and close to the ridge-transform intersection. They follow Omori's law, and constitute more than half of the detected earthquakes. The OBS data also indicate a low but persistent level of seismicity associated with active faulting within the Atlantis Massif in the same region as the hydroacoustically detected seismicity. Within the Massif, the data indicate a north-south striking normal fault, and a left-lateral, strike-slip fault near a prominent, transform-parallel, north-facing scarp. Both features could be explained by changes in the stress field at the inside corner associated with weak coupling on the Atlantis transform. Alternatively, the normal faulting within the Massif might indicate deformation of the detachment surface as it rolls over to near horizontal from an initial dip of about 60° beneath the axis, and the strike-slip events may indicate transform-parallel movement on adjacent detachment surfaces.
  • Article
    The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise
    (John Wiley & Sons, 2014-09-29) Wolfson-Schwehr, Monica ; Boettcher, Margaret S. ; McGuire, Jeffrey J. ; Collins, John A.
    There is a global seismic moment deficit on mid-ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step-overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 inline image ML inline image 4.6 earthquakes) and 24 years of Mw inline image 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high-resolution multibeam bathymetry. Each of the 15 5.4 inline image Mw inline image 6.0 earthquakes that occurred on Discovery between 1 January 1990 and 1 April 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross-correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step-overs in the primary fault trace are not observed at patch boundaries, suggesting along-strike heterogeneity in fault zone properties controls the size and location of the large events.
  • Article
    Spatial and temporal variations in earthquake stress drop on Gofar Transform Fault, East Pacific Rise : implications for fault strength
    (John Wiley & Sons, 2018-09-07) Moyer, Pamela A. ; Boettcher, Margaret S. ; McGuire, Jeffrey J. ; Collins, John A.
    On Gofar Transform Fault on the East Pacific Rise, the largest earthquakes (6.0 ≤ MW ≤ 6.2) have repeatedly ruptured the same portion of the fault, while intervening fault segments host swarms of microearthquakes. These long‐term patterns in earthquake occurrence suggest that heterogeneous fault zone properties control earthquake behavior. Using waveforms from ocean bottom seismometers that recorded seismicity before and after an anticipated 2008 MW 6.0 mainshock, we investigate the role that differences in material properties have on earthquake rupture at Gofar. We determine stress drop for 138 earthquakes (2.3 ≤ MW ≤ 4.0) that occurred within and between the rupture areas of large earthquakes. Stress drops are calculated from corner frequencies derived using an empirical Green's function spectral ratio method, and seismic moments are obtained by fitting the omega‐square source model to the low frequency amplitude of the displacement spectrum. Our analysis yields stress drops from 0.04 to 3.2 MPa with statistically significant spatial variation, including ~2 times higher average stress drop in fault segments where large earthquakes also occur compared to fault segments that host earthquake swarms. We find an inverse correlation between stress drop and P wave velocity reduction, which we interpret as the effect of fault zone damage on the ability of the fault to store strain energy that leads to our spatial variations in stress drop. Additionally, we observe lower stress drops following the MW 6.0 mainshock, consistent with increased damage and decreased fault strength after a large earthquake.
  • Article
    Spatial and temporal evolution of stress and slip rate during the 2000 Tokai slow earthquake
    (American Geophysical Union, 2006-03-23) Miyazaki, Shin'ichi ; Segall, Paul ; McGuire, Jeffrey J. ; Kato, Teruyuki ; Hatanaka, Yuki
    We investigate an ongoing silent thrust event in the Tokai seismic gap along the Suruga-Nankai Trough, central Japan. Prior to the event, continuous GPS data from April 1996 to the end of 1999 show that this region displaced ∼2 cm/yr to the northwest relative to the landward plate. The GPS time series show an abrupt change in rate in mid-June 2000 that continues as of mid-2005. We model this transient deformation, which we refer to as the Tokai slow thrust slip event, as caused by slip on the interface between the Philippine Sea and Amurian plates. The spatial and temporal distribution of slip rate is estimated with Kalman filter based inversion methods. Our inversions reveal two slow subevents. The first initiated in late June 2000 slightly before the Miyake-jima eruption. The locus of slip then propagated southeast in the second half of 2000, with maximum slip rates of about 15 cm/yr through 2001. A second locus of slip initiated to the northeast in early 2001. The depth of the slip zone is about 25 km, which may correspond to the transition zone from a seismogenic to a freely sliding zone. The cumulative moment magnitude of the slow slip event up to November 2002 is M w ∼ 6.8. We calculate shear stress changes on the plate interface from the slip histories. Stress change as a function of slip rate shows trajectories similar to that inferred for high-speed ruptures; however, the maximum velocity is 8 orders of magnitude less than in normal earthquakes.
  • Article
    Observations of seismicity and ground motion in the Northeast U.S. Atlantic Margin from ocean‐bottom seismometer data
    (Seismological Society of America, 2016-11-02) Flores, Claudia H. ; ten Brink, Uri S. ; McGuire, Jeffrey J. ; Collins, John A.
    Earthquake data from two short‐period ocean‐bottom seismometer (OBS) networks deployed for over a year on the continental slope off New York and southern New England were used to evaluate seismicity and ground motions along the continental margin. Our OBS networks located only one earthquake of Mc∼1.5 near the shelf edge during six months of recording, suggesting that seismic activity (MLg>3.0) of the margin as far as 150–200 km offshore is probably successfully monitored by land stations without the need for OBS deployments. The spectral acceleration from two local earthquakes recorded by the OBS was found to be generally similar to the acceleration from these earthquakes recorded at several seismic stations on land and to hybrid empirical acceleration relationships for eastern North America. Therefore, the seismic attenuation used for eastern North America can be extended in this region at least to the continental slope. However, additional offshore studies are needed to verify these preliminary conclusions.
  • Article
    Analysis of seafloor seismograms of the 2003 Tokachi-Oki earthquake sequence for earthquake early warning
    (American Geophysical Union, 2008-07-31) McGuire, Jeffrey J. ; Simons, Frederik J. ; Collins, John A.
    Earthquake Early Warning (EEW) algorithms estimate the magnitude of an underway rupture from the first few seconds of the P-wave to allow hazard assessment and mitigation before the S-wave arrival. Many large subduction-zone earthquakes initiate 50–150 km offshore, potentially allowing seafloor instruments sufficient time to identify large ruptures before the S-waves reach land. We tested an EEW algorithm using accelerograms recorded offshore Hokkaido in the region of the 2003 Mw 8.1 Tokachi-Oki earthquake and its aftershocks. A wavelet transform of the first ∼4 s of the P-wave concentrates information about earthquake magnitude from both waveform amplitude and frequency content. We find that wavelets with support of a few seconds provide discriminants for EEW that are both accurate enough to be useful and superior to peak acceleration or peak velocity. Additionally, we observe a scaling of wavelet coefficient magnitude above Mw 6.0 indicating that, at least for the mainshock (Mw 8.1) and largest aftershock (Mw 7.1), the final size of a rupture could have been estimated from the initial portion of the seismogram.
  • Article
    Millimeter-level precision in a seafloor geodesy experiment at the Discovery transform fault, East Pacific Rise
    (John Wiley & Sons, 2013-10-07) McGuire, Jeffrey J. ; Collins, John A.
    Direct-path acoustic ranging is a promising seafloor geodetic technique for continuous high-resolution monitoring of geodynamical process such as fault slip and magma intrusion. Here we report on a yearlong acoustic ranging experiment conducted across the discovery transform fault at ∼4°S on the East Pacific Rise. The ranging instruments utilized a novel acoustic signal designed to enhance precision. We find that, after correcting for variations in sound speed at the path end-points, the ranging measurements have a precision of ∼1 mm over baselines approaching 1 km in length. The primary difficulty in this particular experiment was with the physical stability of the benchmarks, which were deployed free fall from a ship. Despite the stability issues, it appears that the portion of the transform fault that the array covered was locked during the year of our survey. The primary obstacle to continuous, high sample rate, high-precision geodetic monitoring of oceanic ridges and transform faults is now limited to the construction of geodetic monuments that are well anchored into bedrock.
  • Article
    Influence of fore-arc structure on the extent of great subduction zone earthquakes
    (American Geophysical Union, 2007-09-06) Llenos, Andrea L. ; McGuire, Jeffrey J.
    Structural features associated with fore-arc basins appear to strongly influence the rupture processes of large subduction zone earthquakes. Recent studies demonstrated that a significant percentage of the global seismic moment release on subduction zone thrust faults is concentrated beneath the gravity lows resulting from fore-arc basins. To better determine the nature of this correlation and to examine its effect on rupture directivity and termination, we estimated the rupture areas of a set of Mw 7.5–8.7 earthquakes that occurred in circum-Pacific subduction zones. We compare synthetic and observed seismograms by measuring frequency-dependent amplitude and arrival time differences of the first orbit Rayleigh waves. At low frequencies, the amplitude anomalies primarily result from the spatial and temporal extent of the rupture. We then invert the amplitude and arrival time measurements to estimate the second moments of the slip distribution which describe the rupture length, width, duration, and propagation velocity of each earthquake. Comparing the rupture areas to the trench-parallel gravity anomaly (TPGA) above each rupture, we find that in 11 of the 15 events considered in this study the TPGA increases between the centroid and the limits of the rupture. Thus local increases in TPGA appear to be related to the physical conditions along the plate interface that favor rupture termination. Owing to the inherently long timescales required for fore-arc basin formation, the correlation between the TPGA field and rupture termination regions indicates that long-lived material heterogeneity rather than short timescale stress heterogeneities are responsible for arresting most great subduction zone ruptures.