Sea Of Okhotsk Earthquake Research Articles

Subslab ultra low velocity anomaly uncovered by and facilitating the largest deep earthquake

It is enigmatic that M8+ earthquakes can take place at depth greater than 600 km inside the slab, where the P-T conditions generally do not favor seismic slip rate (~m/s) on faults. Here we provide fresh insights to the initial rupture and mechanism of the Mw 8.3 Sea of Okhotsk earthquake by analyzing high-frequency (up to 0.8 Hz) teleseismic array data. We determine the relative location and timing of two early subevents, and the geometry and velocity perturbation of a nearby structure anomaly. We found a small-scale (~30 × 60 × 60 km) ultralow (−18 ± 2%) P-wave velocity anomaly located beneath the Pacific slab around the 660 km discontinuity. The volatile-bearing highly melted nature of the anomaly provides significant buoyancy, stressing the slab dramatically closer to the critical condition for thermal runaway weakening that allows the rupture to propagate beyond the metastable olivine wedge, forming M8+ events. Enormous velocity reduction urges for further mineral physics and geodynamic investigations.

Deep Dehydration as a Plausible Mechanism of the 2013 Mw 8.3 Sea of Okhotsk Deep-Focus Earthquake

The rupture mechanisms of deep-focus (>300 km) earthquakes in subducting slabs of oceanic lithosphere are not well understood and different from brittle failure associated with shallow (<70 km) earthquakes. Here, we argue that dehydration embrittlement, often invoked as a mechanism for intermediate-depth earthquakes, is a plausible alternative model for this deep earthquake. Our argument is based upon the orientation and size of the plane that ruptured during the deep, 2013 Mw 8.3 Sea of Okhotsk earthquake, its rupture velocity and radiation efficiency, as well as diverse evidence of water subducting as deep as the transition zone and below. The rupture process of this earthquake has been inferred from back-projecting dual-band seismograms recorded at hundreds of seismic stations in North America and Europe, as well as by fittingP-wave trains recorded at dozens of globally distributed stations. If our inferences are correct, the entirety of the subducting Pacific lithosphere cannot be completely dry at deep, transition-zone depths, and other deep-focus earthquakes may also be associated with deep dehydration reactions.

Peculiarities of the Earth’s Free Oscillations after the Sea of Okhotsk Deep-Focus Earthquake of May 24, 2013

A description of the initial excitation of spheroidal mode 0S2 initiated by the 2013 Sea of Okhotsk earthquake (Mw = 8.3) is presented. It is based on the newly revealed fact of the origination of a standing P-wave in the epicenter of the Sea of Okhotsk earthquake and excitation of the traveling surface P-waves on the Earth’s surface by this wave. A new interpretation of the nature of the Earth’s free spheroidal oscillations in the case of a strong earthquake (M ≥ 7.5) is proposed. The spheroidal mode 0S2 is interpreted as a combination of standing and traveling P-waves, transferring the energy of Earth’s deformations.

The Nature of Coseismic Ground Vibrations in the Epicentral Zone of the Sea of Okhotsk Earthquake, May 24, 2013, and Estimation of P-Wave Amplitudes at Teleseismic Distances

The GPS-based coseismic observations in the epicentral zone of the earthquake on the Sea of Okhotsk of May 24, 2013, are analyzed. The peculiarities of the changes in amplitudes of seismic waves with distance are revealed on the basis of records from Russian and worldwide seismic stations. The global propagation of P-waves is investigated for the Sea of Okhotsk and Bolivian (June 9, 1994) earthquakes, and the respective amplitudes are estimated at teleseismic distances.

Analysis of intermediate period correlations of coda from deep earthquakes

We aim at assessing quantitatively the nature of the signals that appear in coda wave correlations at periods >20 s. These signals contain transient constituents with arrival times corresponding to deep seismic phases. These (body-wave) constituents can be used for imaging. To evaluate this approach, we calculate the autocorrelations of the vertical component seismograms for the Mw 8.4 sea of Okhotsk earthquake at 400 stations in the Eastern US, using data from 1 h before to 50 h after the earthquake. By using array analysis and modes identification, we discover the dominant role played by high quality factor normal modes in the emergence of strong coherent phases as ScS-like, and P'P'df-like. We then make use of geometrical quantization to derive the constituent rays associated with particular modes, and gain insights about the ballistic reverberation of the rays that contributes to the emergence of body waves. Our study indicates that the signals measured in the spatially averaged autocorrelations have a physical significance, but a direct interpretation of ScS-like and P'P'df-like is not trivial. Indeed, even a single simple measurement of long period late coda in a limited period band could provide valuable information on the deep structure by using the temporal information of its autocorrelation, a procedure that could be also useful for planetary exploration.

Seismic anisotropy of the mantle beneath Eastern Asia based on ScS and S waves from deep-focus earthquakes

The seismic anisotropy of the mantle is studied based on the data of S and ScS waves from earthquakes occurred in the mantle transition zone over the period of 2007–2013 and recorded by seismic stations in the continental margin of Asia, on Sakhalin Island, and in the southern part of the Kamchatka Peninsula. The measurements of the azimuths of polarization of the fast S and ScS waves in the continental margin of Asia show that they are predominantly oriented in the E–SE directions. Based on the distribution of the shear wave splitting parameters, the symmetry of the medium can be described in terms of a transversely isotropic model with a horizontal symmetry axis and may correspond to horizontal flow in the upper mantle beneath the Amur Plate. The fast azimuths of polarization of ScS wave, which were determined to be of N–NE directions in the northern area of Sakhalin Island and in the continental part of Asia, may correspond to an inclined flow under the conditions of oblique subduction and complex geometry of the downgoing Pacific Plate. In the south of the Kamchatka Peninsula, the S- and ScS-wave azimuths of polarization from the M 8.4 Sea of Okhotsk earthquake are determined to be oriented along the direction of the Pacific Plate motion. The fast-S-wave azimuths of polarization from the aftershocks of the Sea of Okhotsk earthquake and from other large events of 2008–2009 are determined to be nearly parallel to the motion trend of the Pacific Plate, but orthogonal to it for the events of 2008–2009. On the basis of the distribution of azimuths of polarization of the fast S waves, the symmetry of the medium can be described in terms of a transversely isotropic model with the symmetry axis inclined orthogonally to the plane of downgoing plate and oriented westward orthogonally to the trench strike.

The impacts of the M W 8.3 Sea of Okhotsk earthquake of May 24, 2013 in Kamchatka and worldwide

A deep-focus magnitude M W 8.3 earthquake occurred off the Kamchatka Peninsula on May 24, 2013. This was named the “Sea-of-Okhotsk” event. The earthquake has attracted special attention because, among other issues, it was felt uncommonly far from the epicenter, to distances of nearly 10000 km. The present paper reports its felt effects for the area around the epicenter and worldwide. We quote peak acceleration values at many stations in Kamchatka and worldwide as derived from instrumental recordings at digital seismograph stations in the Russian Far East and from those due to the Global Seismographic Network (GSN). We discuss features that are present in the instrumental records of the Sea-of-Okhotsk earthquake.

Teleseismic Peak Ground Accelerations from the 24 May 2013 Sea of Okhotsk Deep Earthquake

Abstract The 2013 Sea of Okhotsk deep earthquake ( M w 8.3), which was recorded by many global seismic stations with adequate coverage of azimuth and distance, provided an opportunity to understand the global characteristics of ground shaking. Peak ground accelerations (PGAs) from the Sea of Okhotsk earthquake decreased with distance up to 120° and peaked at a distance of 140°–150°. The variation as a function of distance is similar to that of the 1994 Bolivia earthquake. PGA values at distances between 40° and 85° are associated with vertical components of direct P waves, and the values are in a range from 10 −4 to 10 −2 m/s. The average decay with distance agrees with that of the P ‐wave amplitude predicted by the ray theory using lower‐mantle attenuation in a range of previously obtained models. Frequencies characterizing the PGA decay are between 0.8 and 1.8 Hz. In agreement with observations from other deep earthquakes, the P ‐wave radiation pattern can affect the decay curves of PGA with distance by changing the amplitude of P waves in the frequency range. Spatial variations in PGA can depend on the tectonic setting; large values of PGA are observed in stable continents and old seas, whereas small values are observed in tectonically active regions. Considering observations from the Sea of Okhotsk deep earthquakes, it is likely that PGA values have a positive correlation with shear‐velocity variations at depths of around 100–150 km below the stations.

Rupture complexity of the Mw 8.3 sea of okhotsk earthquake: Rapid triggering of complementary earthquakes?

AbstractWe derive a finite slip model for the 2013 Mw 8.3 Sea of Okhotsk Earthquake (Z = 610 km) by inverting calibrated teleseismic P waveforms. The inversion shows that the earthquake ruptured on a 10° dipping rectangular fault zone (140 km × 50 km) and evolved into a sequence of four large sub‐events (E1–E4) with an average rupture speed of 4.0 km/s. The rupture process can be divided into two main stages. The first propagated south, rupturing sub‐events E1, E2, and E4. The second stage (E3) originated near E2 with a delay of 12 s and ruptured northward, filling the slip gap between E1 and E2. This kinematic process produces an overall slip pattern similar to that observed in shallow swarms, except it occurs over a compressed time span of about 30 s and without many aftershocks, suggesting that sub‐event triggering for deep events is significantly more efficient than for shallow events.

Deep-Focus Earthquake Analogs Recorded at High Pressure and Temperature in the Laboratory

Phase transformations of metastable olivine might trigger deep-focus earthquakes (400 to 700 kilometers) in cold subducting lithosphere. To explore the feasibility of this mechanism, we performed laboratory deformation experiments on germanium olivine (Mg2GeO4) under differential stress at high pressure (P = 2 to 5 gigapascals) and within a narrow temperature range (T = 1000 to 1250 kelvin). We found that fractures nucleate at the onset of the olivine-to-spinel transition. These fractures propagate dynamically (at a nonnegligible fraction of the shear wave velocity) so that intense acoustic emissions are generated. Similar to deep-focus earthquakes, these acoustic emissions arise from pure shear sources and obey the Gutenberg-Richter law without following Omori's law. Microstructural observations prove that dynamic weakening likely involves superplasticity of the nanocrystalline spinel reaction product at seismic strain rates.

Energy Release of the 2013 M w 8.3 Sea of Okhotsk Earthquake and Deep Slab Stress Heterogeneity

Earth's deepest earthquakes occur in subducting oceanic lithosphere, where temperatures are lower than in ambient mantle. On 24 May 2013, a magnitude 8.3 earthquake ruptured a 180-kilometer-long fault within the subducting Pacific plate about 609 kilometers below the Sea of Okhotsk. Global seismic P wave recordings indicate a radiated seismic energy of ~1.5 × 10(17) joules. A rupture velocity of ~4.0 to 4.5 kilometers/second is determined by back-projection of short-period P waves, and the fault width is constrained to give static stress drop estimates (~12 to 15 megapascals) compatible with theoretical radiation efficiency for crack models. A nearby aftershock had a stress drop one to two orders of magnitude higher, indicating large stress heterogeneity in the deep slab, and plausibly within the rupture process of the great event.

The September 5, 1970 Sea of Okhotsk Earthquake: A multiple event with evidence of triggering

The September 5, 1970 Sea of Okhotsk earthquake consisted of two possibly causally related but dissimilar events: a small (Mag 4.5) event at the hypocenter given by the International Seismologic Centre (52.28 N, 151.49 E, 560 km) followed 5.16±.06 seconds later by a larger (Mag 5.7) event almost at the same epicenter but 23.0±1 km deeper. The fault planes and the principal stress axes of the two events are significantly different; furthermore, the second event does not lie on either of the nodal planes of the first event. We interpret this as a possible triggering of the second event by the smaller first event and not as a change in direction of rupture propagation during a single continuous faulting episode.