Masatoshi Miyazawa

Deep low-frequency tremor that correlates with passing surface waves

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B01307, doi:10.1029/2006JB004890, 2008 Deep low-frequency tremor that correlates with passing surface waves Masatoshi Miyazawa 1 and Emily E. Brodsky 2 Received 5 December 2006; revised 23 July 2007; accepted 28 September 2007; published 24 January 2008. [ 1 ] The large surface waves from the 2004 Sumatra-Andaman earthquake dynamically perturbed the upper mantle structure in Japan and triggered periodic deep low-frequency seismic tremor in eastern and western Shikoku, western and central Tokai, and the Kii peninsula. We use the relationship between the amplitude of the triggered tremor and the stresses of the seismic waves to investigate the mechanism of deep low-frequency seismic tremor. Volumetric strain changes from the 15–30 s Rayleigh waves play an important role in the strong triggering, likely via Coulomb failure stress changes. Building on previous results that the tremor signals become increasingly strong with increasing dilatation, we observe a clear increase in the triggered tremor with an increase in the dilatation due to the Rayleigh waves at the 30 km depth source regions. We also observe a correlation with the Coulomb failure stress change resolved on an appropriate plane. There is an exponential relationship between the signal amplitude from triggered tremor and both the dilatation and the Coulomb shear stress at the source region. This combined with the shape of the tremor packets implies that the tremor amplitude is predictable based on the amplitude of the incoming waves. The amplitude variations can be explained by a distribution of sources in the tremor source region. Citation: Miyazawa, M., and E. E. Brodsky (2008), Deep low-frequency tremor that correlates with passing surface waves, J. Geophys. Res., 113, B01307, doi:10.1029/2006JB004890. 1. Introduction [ 2 ] Deep low-frequency tremor on subducting slabs is a recently discovered and novel seismic signal [Obara, 2002]. The waves are unusually long-period for their amplitude and the signal is often continuous with little punctuation over times ranging from tens of minutes to a few days. These seismic features are often seen in volcanic regions and usually interpreted as signs of fluid-filled resonant conduits in that setting. Therefore early work on deep, slab tremor suggested a fluid source by analogy. Geochemical and petrological constraints supported the inference by suggesting that fluids are released from the subducting slab in a series of dehydration reactions at the depth of the tremor [e.g., Toriumi and Inui, 2001; Omori et al., 2004]. The coincidence of the tremor with slow slip also suggests that there is some physical connection with the large-scale slab processes at the depth of 30 –40 km. [ 3 ] More recent work has suggested that low-frequency earthquakes and tremor can be generated by simple shear failure in both the subduction and the volcanic settings. A low rupture velocity or low-stress drop can result in the low- frequency waves and overlapping earthquakes can generate the apparently continuous signal [Harrington and Brodsky, Disaster Prevention Research Institute, Kyoto University, Kyoto, Japan. Department of Earth Sciences, University of California, Santa Cruz, California, USA. Copyright 2008 by the American Geophysical Union. 0148-0227/08/2006JB004890

Application of seismic interferometry to extract P- and S-wave propagation and observation of shear-wave splitting from noise data at Cold Lake, Alberta, Canada

09.00 2007; Shelly et al., 2006]. Still, the occurrence of the tremor and slow slip at the depth of dehydration suggests that fluids may be an important component. [ 4 ] Here we study the origin of slab tremor by using a constraint posed by special cases of slab tremor that are triggered by the seismic waves from distant earthquakes. The 2004 Sumatra-Andaman earthquake (M w 9.2) was enor- mous and the surface wave amplitudes measured in Japan were comparable to or a few times as large as those from the Denali earthquake (M w 7.9) in 2002 that contributed to the well-studied earthquake triggering around North America [e.g., Prejean et al., 2004]. Miyazawa and Mori [2006] showed that periodic triggering of deep low-frequency events in western Japan was due to the Rayleigh waves from the Sumatra earthquake, and suggested that the triggering is well correlated with the large tensile dilatation at the source regions (Figures 1 and 2). Similar triggered tremor occurred after small local earthquakes and large teleseisms [Obara, 2003], and during the surface waves of the 2003 Tokachi-oki earthquake (M w 8.1) [Miyazawa and Mori, 2005]. These initial observations were thought to distinguish the tremor from the ordinary earthquakes triggered in Alaska that were promoted by shear failure [West et al., 2005]. Recently, Rubinstein et al. [2007] found that the bursts of similar nonvolcanic tremor in Cascadia subduction zone were trig- gered by Love waves from the 2002 Denali earthquake. [ 5 ] More careful analysis of the relationship between the input strains of the seismic waves and the resultant tremor may help us untangle the mechanisms of deep-slab tremor in western Japan. B01307 1 of 17

Investigation of the earthquake sequence off Miyagi prefecture with historical seismograms

We extract downward-propagating P- and S-waves from industrial noise generated by human and/or machine activity at the surface propagating down a borehole at Cold Lake, Alberta, Canada, and measure shear-wave splitting from these data. The continuous seismic data are recorded at eight sensors along a downhole well during steam injection into a 420–470-m-deep oil reservoir. We crosscorrelate the waveforms observed at the top sensor and other sensors to extract estimates of the direct P- and S-wave components of the Green’s function that account for wave propagation between sensors. Fast high-frequency and slow low-frequency signals propagating vertically from the surface to the bottom are found for the vertical and horizontal components of the wave motion, which are identified with P- and S-waves, respectively. The fastest S-wave polarized in the east-northeast–west-southwest direction is about 1.9% faster than the slowest S-wave polarized in the northwest-southeast direction. The direction of polarization...

Omori-Utsu Law c-Values Associated with Recent Moderate Earthquakes in Japan

We investigated the old seismograms of several earthquakes which occurred offshore of Miyagi prefecture in 1933 (June 18, 21:37 UT), 1936 (Nov. 2, 20:45 UT), 1937 (July 26, 19:56 UT), 1978 (June 12, 08:14 UT), and 2005 (Aug. 16, 02:46 UT). A characteristic earthquake model for the sequence of the 1936, 1978, and several other earlier events is the basis of the recent forecast by the Earthquake Research Committee (2003) that “The probability of the occurrence of another similar earthquake in the next 30 years is 99%”. To assess the validity of the characteristic earthquake model, we compared the waveforms, size and other characteristics of these earthquakes recorded at Pasadena, DeBilt, Abuyama, Aso, Weston, Strasbourg, and Christchurch. We conclude that (1) The 1978 event is 3 to 4.5 times larger (in seismic moment) than the 2005 event; (2) The 1936 and the 2005 events are about the same size and are fairly close in location; (3) The 1937 event is smaller than the 1936 event, and is significantly deeper, possibly as deep as 90 km. In contrast, the 1933 event is significantly shallower than the 1936 event. The differences between these events are too large to justify the use of a simple characteristic earthquake model for the probabilistic forecast. The seismic slip rate in this area and along the adjacent subduction zone to the south is about 1/4 of the plate convergence rate, which has an important implication for the long-term seismic hazard in this area.The 2005 West Off Fukuoka Prefecture earthquake caused serious damage to and on Genkai Island as well as to downtown Fukuoka City. There were no strong motion instruments on the island, therefore no one knows how the strong ground motion occurred during the mainshock. The ground motion simulation on Genkai Island is very important to our understanding of earthquake damage at the near-source region. We have conducted an aftershock observation on the island in order to verify site amplification due to steep topography and to record aftershocks for reproducing ground motion during the mainshock by the empirical Green’s function method. The observed records of aftershocks show small variations in the input motions in the island, indicating that the amplification due to the topography seems to be small below 2 Hz. We first estimated the strong motion generation area for the mainshock using the observation records at stations surrounding the source region. We then carried out broadband ground motion simulation on Genkai Island by using the aftershock records as empirical Green’s functions. The simulated ground velocities exceed 1 m/s with a dominant period of 1–2 s due to the forward rupture directivity, and the instrumental seismic intensity reaches 6.6.

Learning from dynamic triggering of low-frequency tremor in subduction zones

We investigate the early aftershock activity associated with four moder- ate earthquakes (Mw 6.6-6.7) that occurred recently in Japan. For each aftershock sequence, we examine continuous high-pass filtered seismograms recorded at seismic stations nearby the main fault to identify as many early events as possible. The mag- nitude of these events is calibrated using aftershocks that are listed in the earthquake catalog of Japan Meteorological Agency (JMA). The analysis of the aftershock decay rates reveals a power-law time dependence with a scaling exponent close to 1.0 that starts from about one minute from the mainshock. Our results demonstrate that the c-value of the Omori-Utsu law is very small, although a lower bound is not estab- lished due to completeness problems in the first minute after the mainshock and sta- tistical fluctuations.

Aftershock distribution of the 2004 Mid Niigata Prefecture Earthquake derived from a combined analysis of temporary online observations and permanent observations

Remote triggering of small low-frequency seismic tremor (non-volcanic tremor) near the seismic-aseismic transition zone of subduction zones, by surface waves from large distant earthquakes, has been reported in southwest Japan and the Cascadia region. Recent observed triggering in southwest Japan from three large earthquakes (2003 Tokachi-oki, 2007 Solomon, and 2008 Wenchuan) covering wide azimuthal information provides strong evidence for the influence of fluids in the source area. The Coulomb failure stress analyses suggest the effective friction coefficient is large for southwest Japan and small for the Cascadia region, which could be related to the amount of fluid in the source regions of the low-frequency tremor.

Analysis of microearthquake data at Cold Lake and its applications to reservoir monitoring

The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj ≥5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.

Detection of seismic events triggered by P-waves from the 2011 Tohoku-Oki earthquake

More than four months of continuously recorded micro-earthquake data acquired at Cold Lake, Canada, was analyzed using advanced algorithms for microearthquake location and subsurface tomography. Robust determination of the spatial, temporal, and magnitude distribution of seismicity is the first step toward understanding the relationship between the stress perturbations caused by the cyclic steam stimulation (CSS) process and seismicity. Acquisition geometry was constrained because the receivers were located in a single vertical borehole. Despite this constraint, we were successful in improving event locations by use of the double-difference method, which highlights several tight event clusters. The deep cluster at a depth of 400 m , just above the oil reservoir, shows very high seismicity during the CSS processes. A second cluster is observed at shallower depths in the successive steam cycle. This suggests that repeated steaming causes the deformation to spread to shallower depths. The number of events, h...

Seismic fatigue failure may have triggered the 2014 Mw7.9 Rat Islands earthquake

Large-amplitude surface waves from the 2011 Tohoku-Oki earthquake triggered many seismic events across Japan, while the smaller amplitude P-wave triggering remains unclear. A spectral method was used to detect seismic events triggered by the first arriving P-waves over Japan. This method uses a reference event to correct for source and propagation effects, so that the local response near the station can be examined in detail. P-wave triggering was found in the regions where triggered non-volcanic tremor (NVT) has been observed, and some seismic and volcanic regions. The triggering strain due to P-waves is of the order of 10−8 to 10−7, which is 1 to 2 orders of magnitude smaller than the triggering strain necessary for the surface wave triggering. In the regions of NVT, the triggered event was not identified with slow events, but with other seismic events such as tectonic earthquakes. The sequence of triggering in the regions started with P-wave arrivals. The subsequent surface waves contributed to triggering of NVT, possibly together with slow slip, which resulted in the large amplitude of the NVT.

Building Damage Estimates Using Slowness Change in Propagating Waves

Seismic waves propagating from large earthquakes cause global transient stress changes capable of triggering other earthquakes at great distances. The study of such remote and dynamic triggering phenomena provides a better understanding of the mechanisms that generate earthquakes. I introduce an integrated seismicity model to stochastically evaluate the time intervals of consecutive earthquakes at global scales, making it possible to detect a pair of earthquakes possibly related to each other. I show a Mw7.9 intermediate-depth earthquake that occurred in the Rat Islands in 2014 is inferred to have been associated with a sequence of distant large (Mw≥6.5) earthquakes originating from the Kermadec Islands. The passage of seismic surface waves from the Kermadec events that produced small stress changes varying within at most 10 Pa at the hypocenter, probably caused a reduction in the faults strength by cyclic fatigue and eventually triggered its failure during their passage.