Susan Y. Schwartz

Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones

Deep tremor under Shikoku, Japan, consists primarily, and perhaps entirely, of swarms of low-frequency earthquakes (LFEs) that occur as shear slip on the plate interface. Although tremor is observed at other plate boundaries, the lack of cataloged low-frequency earthquakes has precluded a similar conclusion about tremor in those locales. We use a network autocorrelation approach to detect and locate LFEs within tremor recorded at three subduction zones characterized by different thermal structures and levels of interplate seismicity: southwest Japan, northern Cascadia, and Costa Rica. In each case we find that LFEs are the primary constituent of tremor and that they locate on the deep continuation of the plate boundary. This suggests that tremor in these regions shares a common mechanism and that temperature is not the primary control on such activity. Citation: Brown, J.R., G. C. Beroza, S. Ide, K. Ohta, D. R. Shelly, S. Y. Schwartz, W. Rabbel, M. Thorwart, and H. Kao (2009), Deep low-frequency earthquakes in tremor localize to the plate interface in multiple subduction zones, Geophys. Res. Lett., 36, L19306, doi:10.1029/2009GL040027.

Control of seafloor roughness on earthquake rupture behavior

Earthquake rupture complexity is described for three recent large underthrusting earthquakes along the Costa Rican subduction zone, the 1983 Osa, 1990 Nicoya Gulf, and 1999 Quepos events. These earthquakes occurred in regions characterized by distinctly different morphologic features on the subducting plate. The 1990 and 1999 events occurred along linear projections of subducting seamount chains and had fairly simple earthquake rupture histories. Both events are interpreted as failure of the basal contact of closely spaced isolated seamounts acting as asperities. In contrast, the 1983 event occurred along the subducting Cocos Ridge and had a complex rupture history. Comparison of rupture characteristics of these large underthrusting earthquakes with size and location of subducting features provides evidence that seamounts can be subducted to seismogenic depths and that variations in seafloor bathymetry of the subducting plate strongly influence the earthquake rupture process.

Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica

New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 ± 2 and 39 ± 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (∼1 year) observation period. We also estimate northwest motion of a coastal “sliver block” at 8 ± 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with ∼8 cm/yr on the Cocos-Panama block boundary (including a component of permanent shortening across the Fila Costena fold and thrust belt) and ∼1 cm/yr on the Panama block–Caribbean boundary. The GPS data suggest that the Cocos plate–Panama block boundary is completely locked from ∼10–50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.

Analysis of seismic and acoustic observations at Arenal Volcano, Costa Rica, 1995–1997

In November 1995, we installed five, three-component broadband seismometers and electronic tiltmeters around the circumference of Arenal Volcano, a young stratovolcano in Costa Rica that exhibits strombolian activity. With the addition of two continuous-recording GPS receivers deployed in May 1995, these instruments provide continuous monitoring of seismicity and ground deformation at an active volcano over a very wide bandwidth. In addition, during April‐May 1997, we deployed a small, linear array of co-located three-component seismometers and broadband microphones. This paper presents a comprehensive analysis of all the seismic and acoustic data collected thus far. Seismic signals are primarily of two types: (1) longperiod (1‐3 Hz) transients associated with summit explosions; and (2) harmonic tremor that contains regularly spaced spectral peaks (0.9, 1.8, 2.7, 3.6, 4.5, 5.4, 6.3, and 7.1 Hz) and lasts up to several hours. The explosion signals appear to originate in a small volume that is located at shallow depth beneath the vent and does not migrate with time. No unambiguous long-period seismic signalsOT . 5 sUassociated with volcanic processes at Arenal have been observed during the three-year deployment period. The spectra of summit explosions show distinct signatures at each site, suggesting significant path and/or site modification of the waveforms. In contrast, the harmonic tremor signals show no variation in the frequency content at the five sites, nor on the three seismic components at each site (Hagerty et al., 1997). This, and the fact that harmonic tremor is recorded in the acoustic channels as well, demonstrates that harmonic tremor is not a seismic propagation effect and that pressure disturbances propagate within the magma‐gas mixture inside of volcanic conduits. These pressure waves are sensitive to the flow velocity and to small changes in the gas content of the magma‐gas mixture. Observations and synthetic tests are presented that challenge the notion that harmonic tremor is a superposition of repeated gas explosions at shallow depth. We propose that equilibrium degassing of the melt creates a stable, stratified magma column where the void fraction increases with decreasing depth and that disruption of this equilibrium stratification is responsible for observed variations in the seismic efficiency of explosions.q 2000 Elsevier Science B.V. All rights reserved.

Seismic anisotropy in the shallow crust of the Loma Prieta segment of the San Andreas Fault System

We study seismic anisotropy beneath the Santa Cruz Mountains using digital recordings of S waves from 60 aftershocks of the 1989 Loma Prieta earthquake (Mω = 7.0) recorded at 18 local stations. Three-component particle motion covariance matrix decomposition combined with a cross-correlation method is used to estimate the polarization direction of the fast shear wave arrival and the delay time between the split shear waves. Results from this procedure are compared with those obtained through visual inspection of horizontal particle motions. Inclusion of the vertical component in the particle motion analysis allows the recognition and elimination of S to P converted phases from our study. Uncertainties in the polarization of the fast shear wave and in the delay time between arrival of the fast and slow waves are estimated to be about 20° and 10 ms, respectively. About half of the S wave records examined show clear evidence of shear wave splitting. Two persistent polarization directions of fast shear wave arrivals are observed regardless of the focal mechanism and hypocenter location of the individual events; 52% of the polarizations are parallel to the strike of the San Andreas fault (NW-SE), and 24% are parallel to the probable direction of local maximum horizontal compressive stress (NE-SW). The fault-parallel fast polarization direction dominates at 10 stations and may result from mineral or fracture alignment caused by shearing along the plate boundary. The fast polarization direction perpendicular to the local axes of minimum horizontal compressive stress dominates at one station farthest from the San Andreas fault and is consistent with the hypothesis of extensive dilatancy anisotropy (EDA), where parallel alignment of fluid-filled fractures produces the anisotropy. The predominance of fault-parallel fast shear wave polarizations indicates that the use of shear wave splitting results to deduce the orientation of local axes of principal stress may not always yield accurate results. The splitting time between fast and slow shear waves varies from 10 to 125 ms and shows no systematic relationship with either hypocentral distance or focal depth. This suggests that the seismic anisotropy is no deeper than 2 km, the depth of the shallowest earthquakes analyzed. For an anisotropic zone 2 km thick, the average velocity anisotropy is about 5% in our study region.

Lithospheric structure of the Qiangtang Terrane, northern Tibetan Plateau, from complete regional waveform modeling: Evidence for partial melt

We report models of P and S wave velocity and attenuation for the the crust and uppermost mantle of the Qiangtang Terrane, northern Tibetan Plateau, inferred by fitting reflectivity synthetic seismograms to observed complete regional waveforms. The data are three-component broadband seismograms recorded by the 1991–1992 IRIS-PASSCAL (Incorporated Research Institutions for Seismology-Program for Array Seismic Studies of the Continental Lithosphere) Tibetan Plateau Experiment and Global Seismic Network stations in the region. The Qiangtang Terrane has thick crust (65±5 km) with P and S wave velocities of 6.1–6.3 and 3.34–3.43 km/s, respectively, yielding an anomalously high crustal Poissons ratio of 0.29±0.02. Seismic velocities of the upper mantle of the Qiangtang Terrane are normal for P waves and slow for S waves (8.10 and 4.35–4.41 km/s, respectively) with a high mantle Poissons ratio of 0.29±0.01. Attenuation in the crust and upper mantle is high (QP = 100–200 and QS = 44–89). Modeling of the broadband P waveforms suggests that a decrease in mantle velocity occurs at about 160 km depth in the mantle; however, this is not unambiguously supported by the data and modeling. The crust and uppermost mantle of the Qiangtang Terrane probably contains partial melt based on the high Poissons ratio, low shear wave velocities, and low Q. The absence of high-frequency Sn and the presence of volcanism of mantle lithospheric origin support the presence of partial melt. Crustal and uppermost mantle structure in the Qiangtang Terrane is different from that for the Lhasa Terrane (immediately to the south) based on results of our previous studies. The average crustal P and S wave velocities are 4% faster and 2% slower, respectively, in the Qiangtang Terrane relative to the Lhasa Terrane. This yields a significant difference in the crustal Poissons ratio with values of 0.29 for the Qiangtang Terrane and 0.25 for the Lhasa Terrane. Differences in the uppermost mantle P and S wave velocities and Poissons ratios of these two adjacent terranes cannot be explained by temperature differences alone. Using the mantle temperature estimates of McNamara et al.. [1997] we suggest that partial melt of an ultramafic composition beneath the Qiangtang Terrane fits the velocity and Poissons ratio estimates.

Magma acoustics and time-varying melt properties at Arenal Volcano, Costa Rica

The similarity of acoustic and seismic spectra recorded during Strombolian activity of Arenal Volcano provides conclusive evidence that pressure waves are generated and propagated within the magma-gas mixture inside volcanic conduits. These pressure waves are sensitive to the flow velocity and to small changes in the gas content of the magma-gas mixture, and thus can provide useful indicators of the time-varying properties of the unsteady flow regime and the chemical composition of the melt. The dominant features of the observed explosion and tremor signals are attributed to the source excitation functions and the acoustic resonance of a magma-gas mixture inside the volcanic conduit. We postulate that explosions are triggered in the shallow parts of the magma conduit, where a drastic pressure drop with depth creates a region where violent degassing can occur. Tremor may be sustained by unsteady flow fluctuations at depth. Equilibrium degassing of the melt creates a stable, stratified magma column where the void fraction increases with decreasing depth. Disruption of this equilibrium stratification is thought to be responsible for observed variations in the seismic efficiency of explosions and enhanced acoustic transmission from the interior of the conduit to the atmosphere.

Subduction‐induced strain in the upper mantle east of the Mendocino triple junction, California

We observe splitting of teleseismic shear waves at five stations of the Berkeley Digital Seismic Network located east of the Mendocino triple junction in northeastern California that is dependent on the arrival direction of the seismic phases. The observed variations with back azimuth cannot be explained with laterally varying anisotropy with a horizontal symmetry axis and are attributed to the presence of fabric with an inclined symmetry axis. We assume that the anisotropy is caused by the preferred alignment of olivine crystals. A grid search over possible orientations of the olivine a axes reveals that south of the Mendocino triple junction they dip to the east, whereas north of the triple junction they dip to the northeast. On the basis of a comparison of the ray paths of our data to the spatial distribution of fast and slow P wave velocity anomalies in the upper mantle, we conclude that the anisotropy is located within seismically slow regions and that the directions are controlled by the geometry of a steeply dipping fast P wave velocity anomaly. Assuming that the fast P velocity anomaly represents subducted slab material, we conclude that the fabric beneath the stations north of the triple junction is most likely caused by the differential motion between this rigid, strong down going plate and the surrounding mantle. South of the triple junction the fabric may have developed while subduction of the Farallon plate was still ongoing in this region (prior to 6 Ma). However, we prefer to attribute the observations to more recent asthenospheric flow associated with the opening of a slabless window beneath the North American lithosphere. The flow is modulated by the presence of rigid lithosphere to the north and east.

Slow slip near the trench at the Hikurangi subduction zone, New Zealand

Applying pressure to plate tectonics The full range of deformation behavior of subduction zone faults that are responsible for great earthquakes and tsunamis is now clearer. Wallace et al. observed the heave of the ocean floor near the Hikurangi trench, off the east coast of New Zealand, with a network of absolute pressure gauges (see the Perspective by Tréhu). The gauges sit on the ocean floor and detect changes in pressure generated from slow-slip deformation events. Detailed geodetic observation of deformation events will finally clarify the role that such aseismic events play at major plate boundaries. Science, this issue p. 701; see also p. 654 Absolute pressure gauges detect a slow-slip event near the Hikurangi trench. The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world’s largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

A tremor and slip event on the Cocos-Caribbean subduction zone as measured by a global positioning system (GPS) and seismic network on the Nicoya Peninsula, Costa Rica

In May 2007 a network of global positioning systems (GPS) and seismic stations on the Nicoya Peninsula, of northern Costa Rica, recorded a slow-slip event accompanied by seismic tremor. The close proximity of the Nicoya Peninsula to the seismogenic part of the Cocos-Caribbean subduction plate boundary makes it a good location to study such events. Several centimeters of southwest motion were recorded by the GPS stations over a period of several days to several weeks, and the seismic stations recorded three distinct episodes of tremor during the same time span. Inversion of the surface displacement data for the depth and pattern of slip on the plate interface shows peak slip at a depth of 25–30 km, downdip of the main seismogenic zone. Estimated temperatures here are ∼250°–300°C, lower than in other subduction zones where events of this nature have been previously identified. There may also be a shallower patch of slip at ∼6 km depth. These results are significant in that they are the first to suggest that slow slip can occur at the updip transition from stick slip to stable sliding, and that a critical temperature threshold is not required for slow slip. Tremor and low-frequency earthquake locations are more difficult to determine. Our results suggest they occur on or near the plate interface at the same depth range as the deep slow slip, but not spatially colocated.