skip to main content


Title: Precision Seismic Monitoring and Analysis at Axial Seamount Using a Real‐Time Double‐Difference System
Abstract

Seven three‐component ocean bottom seismometers (OBS) of the Ocean Observatories Initiative (OOI) Cabled Array on top of Axial Seamount are continuously streaming data in real time to the Incorporated Research Institutions for Seismology (IRIS). The OBS array records earthquakes from the submarine volcano which last erupted on 24 April 2015, about 4 months after the array came online. The OBS data have proven crucial in providing insight into the volcano structure and dynamics (Wilcock et al., 2016,https://doi.org/10.1126/science.aah5563). We implemented a real‐time double‐difference (RT‐DD) monitoring system that automatically computes high‐precision (tens of meters) locations of new earthquakes. The system's underlying double‐difference base catalog includes nearly 100,000 earthquakes and was computed using kurtosis phase onset picks, cross‐correlation phase delay times, and 3‐DPandSvelocity models to predict the data. The relocations reveal the fine‐scale structures of long‐lived, narrow (<200 m wide), outward dipping, convex faults on the east and west walls of the caldera that appear to form a figure 8‐shaped ring fault system. These faults accommodate stresses caused by the inflation of magma prior to and deflation during eruptions. The east fault is segmented and pulled apart in east‐west direction due to its interaction with the Juan de Fuca Ridge, which at this location forms an overlapping spreading center. The RT‐DD system enables the monitoring and rapid analysis of variations in fine‐scale seismic and fault properties and has the potential to improve prediction of timing and location of the next Axial eruption expected to occur in the 2022–2023 time frame.

 
more » « less
NSF-PAR ID:
10452703
Author(s) / Creator(s):
 ;  ;  ;  ;  ;  
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Journal of Geophysical Research: Solid Earth
Volume:
125
Issue:
5
ISSN:
2169-9313
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Abstract

    Axial Seamount is an active submarine volcano located at the intersection of the Cobb hot spot and the Juan de Fuca Ridge (45°57′N, 130°01′W). Bottom pressure recorders captured co‐eruption subsidence of 2.4–3.2 m in 1998, 2011, and 2015, and campaign‐style pressure surveys every 1–2 years have provided a long‐term time series of inter‐eruption re‐inflation. The 2015 eruption occurred shortly after the Ocean Observatories Initiative (OOI) Cabled Array came online providing real‐time seismic and deformation observations for the first time. Nooner and Chadwick (2016,https://doi.org/10.1126/science.aah4666) used the available vertical deformation data to model the 2015 eruption deformation source as a steeply dipping prolate‐spheroid, approximating a high‐melt zone or conduit beneath the eastern caldera wall. More recently, Levy et al. (2018,https://doi.org/10.1130/G39978.1) used OOI seismic data to estimate dip‐slip motion along a pair of outward‐dipping caldera ring faults. This fault motion complicates the deformation field by contributing up to several centimeters of vertical seafloor motion. In this study, fault‐induced surface deformation was calculated from the slip estimates of Levy et al. (2018,https://doi.org/10.1130/G39978.1) then removed from vertical deformation data prior to model inversions. Removing fault motion resulted in an improved model fit with a new best‐fitting deformation source located 2.11 km S64°W of the source of Nooner and Chadwick (2016,https://doi.org/10.1126/science.aah4666) with similar geometry. This result shows that ring fault motion can have a significant impact on surface deformation, and future modeling efforts need to consider the contribution of fault motion when estimating the location and geometry of subsurface magma movement at Axial Seamount.

     
    more » « less
  2. Abstract

    Hundreds of earthquakes were recorded during a nine‐month ocean bottom seismometer deployment surrounding Lō'ihi submarine volcano, Hawai'i. The 12‐station ocean bottom seismometer network widened the aperture of earthquake detection around the Big Island, allowing better constraints on the location of seismicity offshore Hawai'i. Although this deployment occurred during a time of volcanic quiescence for Lō'ihi, it establishes an important basis for background seismicity of the volcano. Offshore seismicity during this study was dominated by events located in the mantle fault zone at depths of 25–40 km. These events reflect rupture on preexisting faults in the lower lithosphere caused by stresses induced by volcano loading and flexure of the Pacific Plate (Pritchard et al., 2007,https://doi.org/10.1111/j.1365‐246X.2006.03169.x; Wolfe et al., 2004,https://doi.org/10.1029/2003GC000618). Tomography was performed using double‐difference seismic tomography and showed shallow velocities to be slower than the regional velocity model (HG50; Klein, 1981,https://pubs.geoscienceworld.org/ssa/bssa/article/71/5/1503/118231/A‐linear‐gradient‐crustal‐model‐for‐south‐Hawaii). A broad, low‐velocity anomaly was observed from 20–40‐km depth, and is suggestive of the central plume conduit that supplies magma to Lō'ihi and the active volcanoes of the Big Island. A localized high‐velocity body is observed 4–6‐km depth beneath Lō'ihi's summit, extending 10 km to the north and south. Following Lō'ihi's active rift zones and crossing the summit, this high‐velocity body is characteristic of intrusive material. Two low‐velocity anomalies are observed below the oceanic crust, interpreted as melt accumulation beneath Lō'ihi and magmatic underplating beneath Hawai'i Island.

     
    more » « less
  3. Abstract

    The West Pacific Warm Pool (WPWP)'s response to increasedpCO2during the Pliocene is a key model validation target. Different temperature proxies show different trends: The foraminiferal Mg/Ca sea surface temperature (SST) record shows Pliocene WPWP temperatures ~1.2°C cooler than today (Wara et al., 2005,https://doi.org/10.1126/science.1112596), whereas a TEX86study finds a cooling trend and claims the Pliocene WPWP was warmer than today (Zhang et al., 2014,https://doi.org/10.1126/science.1246172). We focus on understanding biases in Mg/Ca data as the best way to constrain the temperature of the Pliocene WPWP. The strongest nonthermal controls on foraminiferal Mg/Ca are Mg/Ca of seawater and dissolution. Dissolution, which imparts a cool bias to Mg/Ca temperatures, depends on Δ[CO32−], the difference from the carbonate ion concentration needed for calcite saturation. Thus, Pliocene proxy discrepancies might stem from varying Δ[CO32−] over time. To constrain the effect of changing dissolution on the Mg/Ca data, we collected benthic foraminiferal B/Ca data (a proxy for Δ[CO32−]) from the WPWP spanning 0–5.5 Ma. We find no long‐term trend in Δ[CO32−], but variations above and below the threshold of foraminiferal dissolution yield an ~0.4°C cold bias when averaged over the middle to early Pliocene. Changes in seawater Mg/Ca create an ~0.6°C cold bias in the Pliocene Mg/Ca data. After accounting for these biases, we find that the Pliocene WPWP was ~0.1°C cooler than the late Holocene, ranging from −0.5°C to +0.5°C including all uncertainties. Our reconstruction shows a much lower east‐west temperature gradient in the Pliocene tropical Pacific than today, supporting a permanent El Niño‐like “El Padre” state.

     
    more » « less
  4. Abstract

    Synchronization behavior of large earthquakes (rupture of nearby faults close in time for many cycles) has been reported in many fault systems. The general idea is that the faults in the system have similar repeating intervals and are positively coupled through stress interaction. However, many details of such synchronization remain unknown. Here, we built a numerical model in the framework of rate‐and‐state friction to simulate earthquake cycles on the west Gofar fault, East Pacific Rise. Our model consists of two seismic patches separated by a barrier patch, which are constrained by seismic observations. We varied the parameters in the barrier to understand its role on earthquake synchronization. First, we found that when the barrier is relatively weak, synchronization can be achieved by afterslip or post‐seismic creep in the barrier patch. Second, static stress transfer can lead to synchronization, opposite to the suggestion by Scholz (2010,https://doi.org/10.1785/0120090309), which was based on results from a spring‐slider model using rate‐and‐state friction. Third, the width of the barrier is more important than its strength. When the barrier is narrow enough (no more than half the width of the seismic patch in our model), the system can achieve synchronization even with a very strong barrier. Fourth, for certain simulations, the interaction between the two seismic patches promotes partial rupture in the seismic patches and leads to complex behavior: the system switches from synchronized to unsynchronized over 10–20 cycles.

     
    more » « less
  5. Abstract

    Earthquake focal mechanisms, determined with P‐wave polarities and S/P amplitude ratios, are primary data for analyzing fault zone geometry, sense of slip, and the crustal stress field. Solving for the focal mechanisms of small earthquakes is often challenging because phase arrivals and first‐motion polarities are hard to be separated from noise. To overcome this challenge, we implement convolutional‐neural‐network algorithms (Ross, Meier, & Hauksson, 2018, Ross, Meier, Hauksson, & Heaton, 2018,https://doi.org/10.1029/2017jb015251,https://doi.org/10.1785/0120180080) to detect additional phases and polarities. Using both existing and these new data, we build a high‐quality focal mechanism catalog of 297,478 events that occurred from 1981 to 2021 in southern California with the HASH method of Hardebeck and Shearer (2002),https://doi.org/10.1785/0120010200, Hardebeck and Shearer (2003),https://doi.org/10.1785/0120020236. The new focal mechanism catalog is overall consistent with the standard catalog (Yang et al., 2012,https://doi.org/10.1785/0120110311) but includes 40% more focal mechanisms, and is more consistent with moment tensor solutions derived using waveform‐fitting methods. We apply the new catalog to identify changes in focal mechanism properties caused by the occurrences of large mainshocks such as the 2010Mw7.2 El Mayor‐Cucapah and 2019Mw7.1 Ridgecrest earthquakes. Such changes may be associated with co‐seismic stress drops, post‐seismic deformation processes, and static stress changes on a regional scale. The new high‐resolution catalog will contribute to improved understanding of the crustal stress field, earthquake triggering mechanisms, fault zone geometry, and sense of slip on the faults in southern California.

     
    more » « less