Automatic Detection of Slow Slip Events Using the PICCA: Application to Chilean GNSS Data

The detection of transient events related to slow earthquakes in GNSS positional time series is key to understanding seismogenic processes in subduction zones. Here, we present a novel Principal and Independent Components Correlation Analysis (PICCA) method that allows for the temporal and spatial detection of transient signals. The PICCA is based on an optimal combination of the principal (PCA) and independent component analysis (ICA) of positional time series of a GNSS network. We assume that the transient signal is mostly contained in one of the principal or independent components. To detect the transient, we applied a method where correlations between sliding windows of each PCA/ICA component and each time series are calculated, obtaining the stations affected by the slow slip event and the onset time from the resulting correlation peaks. We first tested and calibrated the method using synthetic signals from slow earthquakes of different magnitudes and durations and modelled their effect in the network of GNSS stations in Chile. Then, we analyzed three transient events related to slow earthquakes recorded in Chile, in the areas of Iquique, Copiapó, and Valparaíso. For synthetic data, a 150 days event was detected using the PCA-based method, while a 3 days event was detected using the ICA-based method. For the real data, a long-term transient was detected by PCA, while a 16 days transient was detected by ICA. It is concluded that simultaneous use of both signal separation methods (PICCA) is more effective when searching for transient events. The PCA method is more useful for long-term events, while the ICA method is better suited to recognize events of short duration. PICCA is a promising tool to detect transients of different characteristics in GNSS time series, which will be used in a next stage to generate a catalog of SSEs in Chile.

Slow periodic oscillation without radiation damping: New evolution laws for rate and state friction

Summary The dynamics of sliding friction is mainly governed by the frictional force. Previous studies have shown that the laboratory-scale friction is well described by an empirical law stated in terms of the slip velocity and the state variable. The state variable represents the detailed physicochemical state of the sliding interface. Despite some theoretical attempts to derive this friction law, there has been no unique equation for time evolution of the state variable. Major equations known to date have their own merits and drawbacks. To shed light on this problem from a new aspect, here we investigate the feasibility of periodic motion without the help of radiation damping. Assuming a patch on which the slip velocity is perturbed from the rest of the sliding interface, we prove analytically that three major evolution laws fail to reproduce stable periodic motion without radiation damping. Furthermore, we propose two new evolution equations that can produce stable periodic motion without radiation damping. These two equations are scrutinized from the viewpoint of experimental validity and the relevance to slow earthquakes.

Development of a Compact Broadband Ocean-Bottom Seismometer

Studies of very-low-frequency earthquakes and low-frequency tremors (slow earthquakes) in the shallow region of plate boundaries need seafloor broadband seismic observations. Because it is expected that seafloor spatially high-density monitoring requires numerous broadband sensors for slow earthquakes near trenches, we have developed a long-term compact broadband ocean-bottom seismometer (CBBOBS) by upgrading the long-term short-period ocean-bottom seismometer that has seismic sensors with a natural frequency of 1 Hz and is being mainly used for observation of microearthquakes. Because many long-term ocean-bottom seismometers with short-period sensors are available, we can increase the number of broadband seafloor sensors at a low cost. A short-period seismometer is exchanged for a compact broadband seismometer with a period of 20 or 120 s. Because the ocean-bottom seismometers are installed by free fall, we have no attitude control during an installation. Therefore, we have developed a new leveling system for compact broadband seismic sensors. This new leveling system keeps the same dimensions as the conventional leveling system for 1 Hz seismometers so that the broadband seismic sensor can be installed conveniently. Tolerance for leveling is less than 1°. A tilt of up to 20° is allowed for the leveling operation. A microprocessor controls the leveling procedure. Some of the newly developed ocean-bottom seismometers were deployed in the western Nankai trough, where slow earthquakes frequently occur. The data from the ocean-bottom seismometers on the seafloor were evaluated, and we confirmed that the long-term CBBOBS is suitable for observation of slow earthquakes. The developed ocean-bottom seismometer is also available for submarine volcanic observation and broadband seafloor observation to estimate deep seismic structures.

Shallow slow earthquakes to decipher future catastrophic earthquakes in the Guerrero seismic gap

The Guerrero seismic gap is presumed to be a major source of seismic and tsunami hazard along the Mexican subduction zone. Until recently, there were limited observations at the shallow portion of the plate interface offshore Guerrero, so we deployed instruments there to better characterize the extent of the seismogenic zone. Here we report the discovery of episodic shallow tremors and potential slow slip events in Guerrero offshore. Their distribution, together with that of repeating earthquakes, seismicity, residual gravity and bathymetry, suggest that a portion of the shallow plate interface in the gap undergoes stable slip. This mechanical condition may not only explain the long return period of large earthquakes inside the gap, but also reveals why the rupture from past M < 8 earthquakes on adjacent megathrust segments did not propagate into the gap to result in much larger events. However, dynamic rupture effects could drive one of these nearby earthquakes to break through the entire Guerrero seismic gap.

Spatial changes in inclusion band spacing as an indicator of temporal changes in slow slip and tremor recurrence intervals

Slow slip and tremor (SST) downdip of the seismogenic zones may trigger megathrust earthquakes by frequently transferring stress to seismogenic zones. Geodetic observations have suggested that the recurrence intervals of slow slip decrease toward the next megathrust earthquake. However, temporal variations in the recurrence intervals of SST during megathrust earthquake cycles remain poorly understood because of the limited duration of geodetic and seismological monitoring of slow earthquakes. The quartz-filled, crack-seal shear veins in the subduction mélange deformed near the downdip limit of the seismogenic zone in warm-slab environments record cyclic changes in the inclusion band spacing in the range from 4 ± 1 to 65 ± 18 μm. The two-phase primary fluid inclusions in quartz between inclusion bands exhibit varying vapor/liquid ratios regardless of inclusion band spacing, suggesting a common occurrence of fast quartz sealing due to a rapid decrease in quartz solubility associated with a large fluid pressure reduction. A kinetic model of quartz precipitation, considering a large fluid pressure change and inclusion band spacing, indicates that the sealing time during a single crack-seal event cyclically decreased and increased in the range from 0.16 ± 0.04 to 2.7 ± 0.8 years, with one cycle lasting at least 27 ± 2 to 93 ± 5 years. The ranges of sealing time and duration of a cycle may be comparable to the recurrence intervals of SST and megathrust earthquakes, respectively. We suggest that the spatial change in inclusion band spacing is a potential geological indicator of temporal changes in SST recurrence intervals, particularly when large fluid pressure reduction occurs by brittle fracturing.

3-D thermal regime and dehydration processes around the regions of slow earthquakes along the Ryukyu Trench

Several interplate seismic events, such as short-term slow slip events (S-SSEs) and low-frequency earthquakes (LFEs), have been identified in the Ryukyu Trench, southwestern Japan. As one of the specific characteristics of this seismicity, the depths at which S-SSEs occur at the plate interface beneath Okinawa Island are approximately 5–10 km shallower than those beneath the Yaeyama Islands. To elucidate the cause of this difference in depth, we constructed a three-dimensional, Cartesian thermomechanical subduction model and applied the subduction history of the Philippine Sea (PHS) plate in the model region. As a result, the interplate temperatures at which S-SSEs take place were estimated to range from 350 to 450 °C beneath Okinawa Island and from 500 to 600 °C beneath the Yaeyama Islands. The former temperature range is consistent with previous thermal modelling studies for the occurrence of slow earthquakes, but the latter temperature range is by approximately 150 °C higher than the former. Therefore, explaining how the depth difference in S-SSEs could be caused from the aspect of only the thermal regime is difficult. Using phase diagrams for hydrous minerals in the oceanic crust and mantle wedge, we also estimated the water content distribution on and above the plate interface of the PHS plate. Near the S-SSE fault planes, almost the same amount of dehydration associated with phase transformations of hydrous minerals from blueschist to amphibolite and from amphibolite to amphibole eclogite within the oceanic crust were inferred along Okinawa Island and the Yaeyama Islands, respectively. On the other hand, the phase transformations within the mantle wedge were inferred only beneath the Yaeyama Islands, whereas no specific phase transformation was inferred beneath Okinawa Island around the S-SSE occurrence region. Therefore, we conclude that dehydrated fluid derived from the oceanic crust at the plate interface would play a key role in the occurrence of S-SSEs.

Effect of Ocean Fluid Changes on Pressure on the Seafloor: Ocean Assimilation Data Analysis on Warm-Core Rings off the Southeastern Coast of Hokkaido, Japan on an Interannual Timescale

The relationship between sea surface height (SSH) and seawater density anomalies, which affects the pressure on the seafloor (PSF) anomalies off the southeastern coast of Hokkaido, Japan, was analyzed using the eddy-resolving spatial resolution ocean assimilation data of the JCOPE2M for the period 2001–2018. On an interannual (i.e., year-to-year) timescale, positive SSH anomalies of nearly 0.1 m appeared off the southeastern coast of Hokkaido, Japan, in 2007, associated with a warm-core ring (WCR), while stronger SSH anomalies (∼0.2 m) related to a stronger WCR occurred in 2016. The results show that the effects of such positive SSH anomalies on the PSF are almost canceled out by the effects of negative seawater density anomalies from the seafloor to the sea surface (SEP; steric effect on PSF) due to oceanic baroclinic structures related to the WCRs, especially in offshore regions with bottom depths greater than 1000 m. This means that oceanic isostasy is well established in deep offshore regions, compared with shallow coastal regions. To further verify the strength of the oceanic isostasy, oceanic isostasy anomalies (OIAs), which represent the barotropic component of SSH anomalies, are introduced and analyzed in this study. OIAs are defined as the sum of the SSH anomalies and SEP anomalies. Our results indicate that the effect of oceanic fluid changes due to SSH and seawater density anomalies (i.e., OIAs) on PSF changes cannot be neglected on an interannual timescale, although the amplitudes of the OIAs are nearly 10% of those of the SSH anomalies in the offshore regions. Therefore, to better estimate the interannual-scale PSF anomalies due to crustal deformation related to slow earthquakes including afterslips, long-term slow slip events, or plate convergence, the OIAs should be removed from the PSF anomalies.