Centroid Moment Tensor Catalog Research Articles

Comparative analysis of the characteristics of global seismic wave propagation excited by the Mw9.0 Tohoku earthquake using numerical simulation

Giant earthquakes generate rich signals that can be used to explore the characteristics of the hierarchical structure of the Earth’s interior associated with the eigenfrequencies of the Earth. We employ the spectral element method, incorporated with large-scale parallel computing technology, to investigate the characteristics of global seismic wave propagation excited by the 2011 Mw9.0 Tohoku earthquake. The transversely isotropic PREM model is employed as a prototype of our numerical global Earth model. Topographic data and the effect of the oceans are taken into consideration. Wave propagation processes are simulated by solving three-dimensional elastic wave governing equations with the seismic moment tensor obtained from the Global Centroid Moment Tensor Catalog. Three-dimensional visualization of our computing results displays the nature of the global seismic wave propagation. Comparative analysis of our calculations with observations obtained from the Incorporated Research Institutions for Seismology demonstrates the reliability and feasibility of our numerical results. We compare synthetic seismograms with incorporated and unincorporated ocean models. First results show that the oceans have obvious effects on the characteristics of seismic wave propagation. The peak displacement and peak velocity of P waves become relatively small under the effect of the ocean. However, the effect of the ocean on S-waves is complex. The displacement and velocity of S waves decrease rapidly over time using an unincorporated ocean model. Therefore, the effects of the ocean should be incorporated when undertaking quantitative earthquake hazard assessments on coastal areas. In addition, we undertake comparative analysis on the characteristics of the Earth’s oscillation excited by the 2004 Sumatra-Andaman, 2008 Wenchuan, and 2011 Tohoku earthquakes that incorporate the effect of the Earth’s gravitational potential. A comparison of the amplitude spectra of the numerical records indicates that energy released by the three big earthquakes is different. Our comparative analysis realizes that the computing results can accurately reproduce some eigenfrequencies of the Earth, such as toroidal modes 0T2 to 0T13 and spheroidal modes 0S7 to 0S31. These results demonstrate that numerical simulations can be successfully used to investigate the Earth’s oscillations. We propose that numerical simulations can be used as one of the major tools to further reveal how the Earth’s lateral heterogeneities affect the Earth’s oscillations.

Estimated likelihood of observing a large earthquake on a continental low-angle normal fault and implications for low-angle normal fault activity

The lack of observed continental earthquakes that clearly occurred on low-angle normal faults (LANFs) may indicate that these structures are not seismically active or that these earthquakes are simply rare events. To address this, we compile all potentially active continental LANFs (24 in total) and calculate the likelihood of observing a significant earthquake on them over periods of 1-100 years. This probability depends on several factors including the frequency-magnitude distribution. For either a characteristic or Gutenberg-Richter distribution, we calculate a probability of about 0.5 that an earthquake greater than M6.5 (large enough to avoid ambiguity in dip angle) will be observed on any LANF in a period of 35 years, which is the current length of the global centroid moment tensor catalog. We then use Bayes' Theorem to illustrate how the absence of observed significant LANF seismicity over the catalog period moderately decreases the likelihood that the structures generate large earthquakes.

Crustal earthquake triggering by modern great earthquakes on subduction zone thrusts

Among the many questions raised by the recent abundance of great (M > 8.0) subduction thrust earthquakes is their potential to trigger damaging earthquakes on crustal faults within the overriding plate and beneath many of the world's densely populated urban centers. We take advantage of the coincident abundance of great earthquakes globally and instrumental observations since 1960 to assess this triggering potential by analyzing centroids and focal mechanisms from the centroid moment tensor catalog for events starting in 1976 and published reports about the M9.5 1960 Chile and M9.2 1964 Alaska earthquake sequences. We find clear increases in the rates of crustal earthquakes in the overriding plate within days following all subduction thrust earthquakes of M > 8.6, within about ±10° of the triggering event centroid latitude and longitude. This result is consistent with dynamic triggering of more distant increases of shallow seismicity rates at distances beyond ±10°, suggesting that dynamic triggering may be important within the near field too. Crustal earthquake rate increases may also follow smaller M > 7.5 subduction thrust events, but because activity typically occurs offshore in the immediately vicinity of the triggering rupture plane, it cannot be unambiguously attributed to sources within the overriding plate. These observations are easily explained in the context of existing earthquake scaling laws.

Assessment of SCARDEC source parameters of global large (Mw ≥ 7.5) subduction earthquakes

Rapid and accurate source characterizations of large, shallow subduction earthquakes are key for improved tsunami warning efforts. We assess the quality of source parameters of large magnitude (Mw ≥ 7.5) shallow subduction earthquakes of the past 20 yr determined using SCARDEC, a recent fully automated broad-band body-wave source inversion technique for the fast estimation of the moment magnitude, depth, focal mechanism and source time functions of global events. We find that SCARDEC source parameters agree well with those reported in the global centroid moment tensor (GCMT) catalogue, with only the fault dip angle showing a tendency for steeper SCARDEC dip values than GCMT. We investigate this discrepancy through independent validation tests of the source models by: (i) testing how well they explain data not used in their construction, notably low-frequency normal mode data; and, (ii) assessing the data fit using 3-D forward modelling tools more sophisticated than those used to build the source models; specifically, we use a spectral element method for a 3-D earth model. We find that SCARDEC source parameters explain normal mode data reasonably well compared to GCMT solutions. In addition, for the 3-D earth model used in our experiments, SCARDEC dip angles explain body-wave data similarly or slightly better than GCMT. Moreover, SCARDEC dip angles agree well with results from individual earthquake studies in the literature and with geophysical constraints for different subduction zones. Our results show that SCARDEC is a reliable technique for rapid determinations of source parameters of large (Mw ≥ 7.5) subduction earthquakes. Since the SCARDEC method provides realistic source time functions allowing the fast identification of long-duration tsunami earthquakes, it is complementary to existing methods routinely used for earthquake monitoring and suitable for ocean-wide tsunami warning purposes

Earthquake Focal Mechanisms in Yunnan and their Inference on the Regional Stress Field

Using the broadband records from a regional permanent network, we determined the focal mechanisms of 174 small and moderate earthquakes (3.4≤ M w≤6.0) from January 2008 to March 2011 in Yunnan, China, by fitting the three‐component waveforms of P and S waves. Results show that the earthquakes predominantly have strike‐slip mechanisms, with occasional normal and very few thrust‐faulting events. Most earthquakes occur in the upper crust above 16 km depth. We then derive the regional distribution of the stress field through a damped linear inversion using the focal mechanisms obtained in this study and augmented by the solutions in the Global Centroid Moment Tensor (GCMT) catalog and in Xu et al. (2010). Inversion results suggest that the entire region is predominantly under a strike‐slip faulting regime. Orientations of both maximum and minimum principle axes exhibit a clockwise rotational pattern. The axes of maximum compression are oriented roughly north–south, with a gradual change across the region from trending northwest–southeast in the South China block to trending northeast–southwest in the Indo‐China block. This pattern is consistent with the surface horizontal velocity derived from geodetic observations. Localized normal faulting stress regions are observed in the Tengchong volcanic zone and in the Qiao‐Jia segment of the Xianshuihe–Xiaojiang fault zone. Online Material: Tables of earthquake catalogs and figures describing stress inversion tests.

Convergence of the frequency‐size distribution of global earthquakes

The Gutenberg‐Richter (GR) frequency‐magnitude relation is a fundamental empirical law of seismology, but its form remains uncertain for rare extreme events. Here, we show that the temporal evolution of model likelihoods and parameters for the frequency‐magnitude distribution of the global Harvard Centroid Moment Tensor catalog is inconsistent with an unbounded GR relation, despite if being the preferred model at the current time. During the recent spate of 12 great earthquakes in the last 8 years, record‐breaking events result in profound steps in favor of the unbounded GR relation. However, between such events the preferred model gradually converges to the tapered GR relation, and the form of the convergence cannot be explained by random sampling of an unbounded GR distribution. The convergence properties are consistent with a global catalog composed of superposed randomly‐sampled regional catalogs, each with different upper bounds, many of which have not yet sampled their largest event.

Spatio-temporal distribution of seismic moment release near the source area of the 2011 Tohoku-Oki earthquake

To understand the generation mechanism of the 2011 Tohoku-Oki earthquake, we analyzed the spatial and temporal distribution of the cumulative seismic moment (ΣM0) for all earthquakes along the Japan Trench listed in the global centroid moment tensor catalog between January 1976 and November 2011. Three areas with distinct characteristics of ΣM0 are identified: (1) in the 2011 Tohoku-Oki source area, the ΣM0 released in the down-dip portion (≥30 km) was greater than that in the up-dip portion (<30 km) before the mainshock; (2) the Σ M0 of the up-dip portion in the region where slow slip activity prevails is greater than that of the down-dip portion throughout the study period; (3) in the surroundings of the source area, where interplate coupling is relatively low and the largest foreshock occurred, values of ΣM0 for the down-dip and up-dip portions are both intermediate. Our results show that a megathrust rupture could be generated by large accumulations of strain energy at the subduction interface, resulting from the differential strain energy released in the down-dip and up-dip portions during the interseismic interval. We propose that the variation pattern of ΣM0 may offer spatial constraints for seismic hazard assessment in the future.

A Quantitative Comparison between 1D and 3D Source Inversion Methodologies: Application to the Middle East

We present a quantitative comparison between seismic moment tensor (MT) inversion solutions using 1D and 3D synthetic seismograms at two frequency bands for events in the Middle East, to assess the effects of 3D models on source studies. Complex geology associated with the active continental convergent margin leads to a scarcity of reliable, available data, necessitating a thorough examination of solution stability and robustness to assure an accurate description of sources with well‐characterized source parameters. Solutions were calculated for 195 events ( M w>5.5) using a full‐waveform MT inversion matching both phase and amplitude. Seismic data processed at two frequency bands compares short‐ and long‐period performance for 1D and 3D synthetic seismograms. An improvement in fit between data and synthetics is seen using 3D over 1D synthetic seismograms, especially for complex body‐wave propagation and surface‐wave dispersion. At short periods, 3D synthetics provide a more robust solution compared with 1D, showing a reduction in error of the source mechanism. The percentage of double‐couple components increases with the addition of 3D structure and suggests the percentage of non‐double‐couple components is a result of poorly constrained Earth structure. Event solutions contained in the catalog (![Graphic][1] Table S1 in the electronic supplement to this paper) have an average cross‐correlation value of 0.87, with good amplitude ratios, and are improved (i.e., increased variance reduction) yet consistent with longer period solutions from the Global Centroid Moment Tensor (CMT) catalog. Online Material: Table of 184 event solutions from 3D moment tensor inversion. [1]: /embed/inline-graphic-1.gif

GPS constraints on current tectonics of Albania

Current tectonics of Albania is documented by neotectonics evidences and by a large number of medium size earthquakes. Focal mechanisms suggest a current shortening across the external Albanides whereas internal Albanides are affected by extension with direction varying from E–W to N–S extension. To investigate the kinematics of Albanides, we integrate continuous and episodic GPS velocity measurements with focal mechanisms of the regional centroid moment tensor catalog. This study has allowed distinguishing the followings: 1) western Albania affected by southwestward motions relative to Apulia microplate, the foreland of Albanides, illustrating the ongoing collision of external Albanides; 2) the inner Albanides affected by a southward motion with a southward increase, relative to both Apulia and stable Eurasia. Active thrusts and backthrusts of external Albanides are segmented by strike-slip faults; the fault between the external Albanides and the Ionian zone appears to be particularly active. The fault zone forming the boundary between Dinarides and Albanides, is identified as the northern limit of an area including Albania and western Greece, affected by a clockwise rotation relative to Apulia and also the northern limit of the area (inner Albanides, Macedonia, Bulgaria) affected by southward motion relative to Apulia and stable Eurasia. The ENE–WSW transverse fault zone in Central Albania and western Macedonia (Diber-Elbasani fault) appears to be mainly affected by a moderate extension. CGPS data confirm that the external-inner Albanides transition is the western border of the domain (inner Albania, northern Greece, Macedonia, Bulgaria) affected by southward displacements relative to stable Eurasia and Apulia, whereas the fault zone forming the boundary between Dinarides and Albanides constitutes probably its northern limit.

Co‐seismic deformation of deep slabs based on summed CMT data

We assess the co‐seismic deformation inferred from earthquake moment tensor solutions for subducting slabs at depths greater than 50 km globally. We rotate each moment tensor into a local slab reference frame, then sum tensors within 50 km depth bins to approximate long term deformation characteristics. This builds upon previous analyses by using the up‐to‐date global Centroid Moment Tensor catalog, incorporating a more complete slab geometry, and focusing on the 3‐D aspects of slab deformation. Results show a general consistency with Isacks and Molnar (1969), who found that most slabs can be divided into intermediate‐extensional, intermediate‐extensional‐deep‐compressional, and intermediate to deep‐compressional categories. Exceptions to these three categories can be related to slab bending in the top 100 km, plate convergence that is oblique to the trench normal direction, and regions of higher focal mechanism heterogeneity. The regions of higher focal mechanism heterogeneity appear where there are along‐strike changes in slab geometry and/or evidence of double‐seismic zones. We find that the sense of deformation in the intermediate strain axis direction is opposite to that of the down‐dip direction, in agreement with Kuge and Kawakatsu (1993). By quantitative comparison to numerical models of global mantle flow, we show that these observations are consistent with deformation of viscous slabs responding to their own negative buoyancy and an upper to lower mantle viscosity increase.

Revisiting the global detection capability of earthquakes during the period immediately after a large earthquake: considering the influence of intermediate-depth and deep earthquakes

This study examines the global earthquake detection capability of the Global Centroid Moment Tensor (GCMT) catalogue during the periods immediately following large earthquakes, including intermediate-depth (70 ≤ depth &amp;lt; 300 km) and deep (300 km ≤ depth) events. We have already shown that the detection capability beyond an aftershock zone degrades remarkably and that this condition persists for several hours after the occurrence of large shallow (depth &amp;lt; 70 km) earthquakes. Because an intermediate-depth or deep earthquake occasionally generates seismic waves with significant amplitudes, it is necessary to investigate the change in the detection capability caused by such events. To this end, from the GCMT catalogue, we constructed the time sequences of the earthquakes that occurred immediately after the large earthquakes, and stacked these time sequences. To these stacked sequences, we then applied a statistical model representing the magnitude-frequency distribution of all observed earthquakes. This model has a parameter that characterizes the detection capability, and the temporal variation of the parameter is estimated by means of a Bayesian approach with a piecewise linear function. Consequently, we find that the global detection capability is lower after the occurrence of shallow earthquakes with magnitudes ≥ 5.45, intermediate-depth earthquakes with magnitudes ≥ 5.95, and deep earthquakes with magnitudes ≥ 6.95.

The 3 August 2009 Mw 6.9 Canal de Ballenas Region, Gulf of California, Earthquake and Its Aftershocks

On 3 August 2009 an earthquake of magnitude Mw 6.9 occurred near Canal de Ballenas, in the north-central region of the Gulf of California, Mexico. The focal mechanism of the main event, reported in the Global Centroid Moment Tensor (CMT) catalog, is right lateral strike-slip with a strike of 216° and a dip of 78°. The initial location reported by the National Seismological Service of Mexico (Servicio Sismologico Nacional (SSN)) and the Array Network Facility (ANF) sug- gested that the epicenter was on the North American plate near the Tiburon fault, which is considered inactive. This earthquake was preceded by a magnitude mb 5.5 event that occurred about 5 min before. In the next 40 min after the main event two aftershocks with magnitudes mb 4.9 and Mw 6.2 occurred, and on 5 August a third aftershock of Mw 5.7 was located in the Canal de Ballenas region. The events of August 2009 were recorded by the regional stations of the broadband network Red Sismologica de Banda Ancha (RESBAN) that Centro de Investigacion Cientifica y de Educacion Superior de Ensenada (CICESE) operates and by stations of the SSN also located in the region of the Gulf of California. We used body-wave arrivals to determine precise epicentral locations and to estimate the rupture area of this impor- tant sequence of earthquakes. The resulting hypocentral coordinates indicate that the main event of this sequence occurred along the Canal de Ballenas transform fault, with a rupture length of 50 km. Based on the aftershock distribution, we estimate that the main event had a rupture area of approximately 600 km 2 , an average slip of 1.3 m, and

Rapid full-wave centroid moment tensor (CMT) inversion in a three-dimensional earth structure model for earthquakes in Southern California

SUMMARY A central problem of seismology is the inversion of regional waveform data for models of earthquake sources. In regions such as Southern California, preliminary 3-D earth structure models are already available, and efficient numerical methods have been developed for 3-D anelastic wave-propagation simulations. We describe an automated procedure that utilizes these capabilities to derive centroid moment tensors (CMTs). The procedure relies on the use of receiver-side Green’s tensors (RGTs), which comprise the spatial-temporal displacements produced by the three orthogonal unit impulsive point forces acting at the receivers. We have constructed a RGT database for 219 broad-band stations in Southern California using a tomographically improved version of the 3-D SCEC Community Velocity Model Version 4.0 (CVM4) and a staggered-grid finite-difference code. Finite-difference synthetic seismograms for any earthquake in our modelling volume can be simply calculated by extracting a small, source-centred volume from the RGT database and applying the reciprocity principle. The partial derivatives needed for the CMT inversion can be generated in the same way. We have developed an automated algorithm that combines a grid-search for suitable focal mechanisms and hypocentre locations with a Gauss–Newton optimization that further refines the grid-search results. Using this algorithm, we have determined CMT solutions for 165 small to medium-sized earthquakes in Southern California. Preliminary comparison with the CMT solutions provided by the Southern California Seismic Network (SCSN) shows that our solutions generally provide better fit to the observed waveforms. When applied to a large number of earthquakes, our algorithm may provide a more robust CMT catalogue for earthquakes in Southern California.

European–Mediterranean Regional Centroid Moment Tensor catalog: Solutions for 2005–2008

Since 1997 we maintain and systematically update the European–Mediterranean Regional Centroid Moment Tensor (RCMT) catalog, which contains seismic-moment tensors for earthquakes with moderate magnitude (4.5 < M < 5.5) in the Mediterranean and European regions. We present 354 new solutions for the period 2005–2008. The Catalog now spans 12 years and contains more than 1000 definitive RCMT solutions. In addition to definitive solutions, we maintain and update a dataset of recent ‘quick’ solutions that are computed soon after an earthquake occurs using the more limited set of data available in quasi-real time. We investigate the reliability of the moment tensor results in the RCMT catalog. Comparison with Global CMT results, which is possible for the larger events ( M < 5.0) in the RCMT catalog, shows that for 75% of the events, the Kagan angle measure of the similarity of two moment tensors is smaller than 20°, reflecting good agreement of the results. We describe improvements and enhancements in the dissemination of the RCMT results through our web site.

Centroid moment tensor catalogue for Indonesia

We developed a centroid moment tensor (CMT) catalogue of earthquakes in and around Indonesia (InaCMT) using data from the nationwide broadband seismograph network in Indonesia. We obtained CMT solutions for about 500 earthquakes that occurred in and around Indonesia between 2006 and 2009. The moment magnitudes ranged from 4.5 to 8.3. We examined the accuracy of the CMT solutions of the InaCMT catalogue by comparing them with those obtained by the Global CMT (GCMT) Project. The seismic moments and focal mechanisms of these catalogues were highly consistent with each other, but we found systematic differences between the catalogues in the source centroid locations of earthquakes off Sumatra. The InaCMT source centroid locations were closer to the hypocenter locations in the Preliminary Determination of Earthquakes (PDE) of the U.S. Geological Survey compared to those of GCMT. The systematic deviations in the GCMT source centroid locations may mainly reflect insufficient azimuthal coverage of the stations used for the inversions as well as uncertainties in the Earth model. Using the InaCMT catalogue, we investigated seismic activity related to the off Bengkulu seismic sequence on 12 September 2007 ( M w = 8.3, 7.9, and 6.8), southwest of Sumatra, and the earthquakes northwest of the island of New Guinea on 3 January 2009 ( M w = 7.7 and 7.4). In the aftershock activity of the 2007 off Bengkulu seismic sequence, we found that shallow earthquakes were aligned along the eastern coast of Siberut Island, located between the Sunda trench and Sumatra. These earthquakes may have occurred along the Mentawai fault or another unknown fault. The focal mechanisms of the earthquakes were dominantly reverse slip, although the Mentawai fault has been considered to be a strike-slip fault. Shallow large earthquakes along this fault may cause damage above the source region and generate large tsunamis. We found that the 2009 earthquakes northwest of New Guinea occurred along the Manokwari trough and that the aftershocks were aligned along a plane dipping about 30° to the south, indicating that the Pacific and/or Philippine Sea plate subducts below the Australian plate at the Manokwari trough. The InaCMT catalogue enables us to perform detailed analyses of seismic activity in and around Indonesia, which contributes to better understanding of tectonic features and potential seismic and tsunami hazards in Indonesia as well as the coastal regions of the Pacific and Indian Oceans.

The Puzzle of the 1996 Bardarbunga, Iceland, Earthquake: No Volumetric Component in the Source Mechanism

A volcanic earthquake with M w 5.6 occurred beneath the Bardarbunga caldera in Iceland on 29 September 1996. This earthquake is one of a decade-long sequence of ![Graphic][1] events at Bardarbunga with non-double-couple mechanisms in the Global Centroid Moment Tensor catalog. Fortunately, it was recorded well by the regional-scale Iceland Hotspot Project seismic experiment. We investigated the event with a complete moment tensor inversion method using regional long-period seismic waveforms and a composite structural model. The moment tensor inversion using data from stations of the Iceland Hotspot Project yields a non-double-couple solution with a 67% vertically oriented compensated linear vector dipole component, a 32% double-couple component, and a statistically insignificant (2%) volumetric (isotropic) contraction. This indicates the absence of a net volumetric component, which is puzzling in the case of a large volcanic earthquake that apparently is not explained by shear slip on a planar fault. A possible volcanic mechanism that can produce an earthquake without a volumetric component involves two offset sources with similar but opposite volume changes. We show that although such a model cannot be ruled out, the circumstances under which it could happen are rare. [1]: /embed/inline-graphic-1.gif

GPS-derived motion of the Adriatic microplate from Istria Peninsula and Po Plain sites, and geodynamic implications

We studied the motion of the Adriatic microplate using Eurasian-referenced GPS-derived velocities from Istria Peninsula (Slovenia, Croatia) and Po Plain (Italy) sites and earthquake slip vectors around its edges from a Regional Centroid Moment Tensor catalogue. We explored kinematic parameters by inverting GPS velocities using a variety of site combinations and comparing results. Our best-fitting GPS Adria–Eurasia angular velocity vector (Euler pole) comes from 7 Istria Peninsula (Slovenia, Croatia) and 10 Po Plain (Italy) sites; it locates at 45.03°N, 6.52°E, with a 0.297 ± 0.116°/Myr counterclockwise rotation rate. This new GPS-derived pole locates and overlaps with our earthquake slip-vector-derived pole. An Adriatic microplate interpretation is at odds with Neogene geologic features that indicate recent convergence across the Apennines and Alps. The neotectonics–geology mismatch probably signals the recent birth of the Adria microplate upon termination of the Nubia–Eurasia Alpine collison and Adria slab break-off beneath the Apennines.

Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M w>∼3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude ( M L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M w with M L for New Zealand earthquakes. In addition to M w and M L, there are more than 330 teleseismic body-wave magnitude ( m b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M w. These are used to examine the relationship between M w, M L, and m b for New Zealand earthquakes. There is a clear distinction in the relationship of M L to M w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M L to be fairly consistent with M w, particularly for events with M w>∼4.5 and with ![Graphic][1] . Deep focus earthquakes have M L consistently larger than M w (more than a full magnitude unit for some events) with ![Graphic][2] . M w and m b are in fairly good agreement regardless of the depth, whereas m b estimates are consistently smaller than M L for deep events. This suggests that M L is overestimated for deep focus events and that the large M Ls are not the result of physical properties of the source. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif

Multifractal Omori law for earthquake triggering: new tests on the California, Japan and worldwide catalogues

SUMMARY The Multifractal Stress-Activated model is a statistical model of triggered seismicity based on mechanical and thermodynamic principles. It predicts that, above a triggering magnitude cut-off M 0, the exponent p of the Omori law for the time decay of the rate of aftershocks is a linear increasing function p(M) = a 0 M + b0 of the main shock magnitude M. We previously reported empirical support for this prediction, using the Southern California Earthquake Center (SCEC) catalogue. Here, we confirm this observation using an updated, longer version of the same catalogue, as well as new methods to estimate p. One of this methods is the newly defined Scaling Function Analysis (SFA), adapted from the wavelet transform. This method is able to measure a mathematical singularity (hence a p-value), erasing the possible regular part of a time-series. The SFA also proves particularly efficient to reveal the coexistence and superposition of several types of relaxation laws (typical Omori sequences and short-lived swarms sequences) which can be mixed within the same catalogue. Another new method consists in monitoring the largest aftershock magnitude observed in successive time intervals, and thus shortcuts the problem of missing events with small magnitudes in aftershock catalogues. The same methods are used on data from the worldwide Harvard Centroid Moment Tensor (CMT) catalogue and show results compatible with those of Southern California. For the Japan Meteorological Agency (JMA) catalogue, we still observe a linear dependence of p on M ,b ut with a smaller slope. The SFA shows however that results for this catalogue may be biased by numerous swarm sequences, despite our efforts to remove them before the analysis.

Subduction of the Pacific plate under Kamchatka: Seismic velocity of underthrust

The results of studying the process of underthrusting in the Pacific plate under Kamchatka are presented, and the phenomena associated with this process are described. The seismic component of the velocity of the plate underthrust is estimated on the basis of (1) data from the CMT (Centroid Moment Tensor) catalog and (2) the sequence of the strongest Kamchatka earthquakes. A flat site with a strike azimuth of 217° and a dip angle of 25° located at depths of 30–70 km is assumed to be the interface between the plunging plate and Kamchatka. From CMT focal mechanisms, the underthrust velocity is estimated at V = 0.7 cm/yr for the southern zone (located south of Cape Shipunski) and at V = 1.1 cm/yr for the central zone (from Cape Shipunski to Cape Kronotski). From pairs of the strongest earthquakes that have occurred successively since 1737, the underthrust velocity for the southern zone is estimated at V = 6.6–7.1 cm/yr (from two pairs) and for the central zone, at V = 6.6 cm/yr. The creep portion of the underthrust amounts to 5–15% of the total velocity (the velocity of motion of the Pacific plate is 8 cm/yr).