Global Centroid Moment Tensor Research Articles

Erratum toRevised Tectonic Forecast of Global Shallow Seismicity Based on Version 2.1 of the Global Strain Rate Map: Table 1

In our previous article (Bird and Kreemer, 2015), we presented a set of six long‐term stationary forecasts of global shallow seismicity, identified as the model series SHIFT‐GSRM2a–f. As preliminary validation, we also presented retrospective tests against global shallow seismicity in the years 2005–2012, as recorded in the Global Centroid Moment Tensor catalog. A smoothed‐seismicity forecast computed by Yan Kagan and David Jackson (Kagan and Jackson, 1994, 2010a,b, 2011, 2014) was …

Precise Relative Earthquake Magnitudes from Cross Correlation

Abstract We present a method to estimate precise relative magnitudes using cross correlation of seismic waveforms. Our method incorporates the intercorrelation of all events in a group of earthquakes, as opposed to individual event pairings relative to a reference event. This method works well when a reliable reference event does not exist. We illustrate the method using vertical strike‐slip earthquakes located in the northeast Pacific and Panama fracture zone regions. Our results are generally consistent with the Global Centroid Moment Tensor catalog, which we use to establish a baseline for the relative event sizes. Online Material: Supplementary description and formulation of uncertainty, and tables of relative magnitudes.

Geodetic slip solutions for the Mw = 7.4 Champerico (Guatemala) earthquake of 2012 November 7 and its postseismic deformation

As the first large subduction thrust earthquake off the coast of western Guatemala in the past several decades, the 2012 November 7 Mw = 7.4 earthquake offers the first opportunity to study coseismic and postseismic behaviour along a segment of the Middle America trench where frictional coupling makes a transition from weak coupling off the coast of El Salvador to strong coupling in southern Mexico. We use measurements at 19 continuous GPS sites in Guatemala, El Salvador and Mexico to estimate the coseismic slip and postseismic deformation of the November 2012 Champerico (Guatemala) earthquake. An inversion of the coseismic offsets, which range up to ∼47 mm at the surface near the epicentre, indicates that up to ∼2 m of coseismic slip occurred on a ∼30 × 30 km rupture area between ∼10 and 30 km depth, which is near the global CMT centroid. The geodetic moment of 13 × 1019 N m and corresponding magnitude of 7.4 both agree well with independent seismological estimates. Transient postseismic deformation that was recorded at 11 GPS sites is attributable to a combination of fault afterslip and viscoelastic flow in the lower crust and/or mantle. Modelling of the viscoelastic deformation suggests that it constituted no more than ∼30 per cent of the short-term postseismic deformation. GPS observations that extend six months after the earthquake are well fit by a model in which most afterslip occurred at the same depth or directly downdip from the rupture zone and released energy equivalent to no more than ∼20 per cent of the coseismic moment. An independent seismological slip solution that features more highly concentrated coseismic slip than our own fits the GPS offsets well if its slip centroid is translated ∼50 km to the west to a position close to our slip centroid. The geodetic and seismologic slip solutions thus suggest bounds of 2–7 m for the peak slip along a region of the interface no larger than 30 × 30 km.

Regional Moment Tensor Analysis in the Philippines: CMT Solutions in 2012–2013

Recently, the Philippine Institute of Volcanology and Seismology (PHIVOLCS) has upgraded its seismic network, equipping it with accelerometers and broadband seismometers for intensity and focal mechanism determinations. As part of this upgrade, PHIVOLCS adapted the use of a source analysis system called SWIFT to determine the centroid moment tensor (SWIFT CMT). SWIFT CMT solutions were estimated for medium to large size earthquakes (4.1 ≤Mw≤ 7.6) in the Philippines for the period of January 2012 to November 2013 and were statistically evaluated with respect to the CMT solutions of the Global Centroid Moment Tensor (GCMT) Project. The seismic moments, moment magnitudes, centroid locations, depths and focal mechanisms of most of the SWIFT CMT solutions are found to be consistent to those of the GCMT solutions for earthquakes withMw≥ 4.6. The SWIFT system with the new broadband seismographic network provides more CMT solutions for moderate size earthquakes (Mw≥ 4.1) than GCMT. SWIFT proves to be useful in the development of the Philippines CMT catalogue that will lead to a better understanding of seismotectonics in the Philippines.

Origin and significance of deep earthquake clusters surrounding a pronounced seismic gap in northeast China

From analysis of earthquakes in the Global centroid moment tensor (GCMT) catalog, we identify an intermediate depth (∼250–350km) seismic gap having dimensions of ∼300km×500km within the subducting northwestern Pacific plate (NPP), and surrounded by several groups of isolated earthquakes near northeast China. Normally, groups of unusual deep earthquakes may represent a detached slab segment, as in Tonga–Vanuatu; and a deep seismic gap maybe either be the aseismic continuation of a subducting slab or a possible gap in the subducting plate. We evaluate the characteristics of deep earthquakes in the NPP and focus on three isolated clusters, particularly one directly beneath the seismic gap and northeast China. This sub-gap cluster has a peak in the number of quakes near depth ∼580km, unlike seismicity in the NPP as a whole. Its b-value (∼0.60) is significantly smaller than b-values in two other nearby clusters (∼0.81 and ∼0.86) in the NPP. Focal mechanisms in this sub-gap cluster also show a variation with depth, and projections of their T, P, and B axes differ from what is observed for the other isolated clusters. We speculate why these sub-gap earthquakes are isolated and suggest two possible models explaining the presence of the gap and the characteristics of the underlying cluster beneath northeast China. In the first model, the seismic gap is attributable to aseismic deformation caused by abnormally high temperatures in the subducting lithosphere. And, the characteristics of the sub-gap clusters and its related slab structures are attributed to slab buckling caused by resistance to subduction. In the second model, the seismic gap represents absent subducting lithosphere and the missing material has migrated to the bottom of subducting slab. Although we favor the second model, both models are consistent with our knowledge of deep earthquakes and the results of recent studies near northeast China.

Seismotectonics of Mindoro, Philippines

Mindoro Island is located where the Palawan Continental Block indented into the Philippine Mobile Belt during the Early Miocene and where the Manila Trench terminates, having ceased convergence due to collision. The seismotectonics of Mindoro are crucial to understanding the processes of arc-continent collision and slab dynamic responses of convergence cessation. This study was conducted using data from both the EHB and Global Centroid Moment Tensor catalogues. It is shown that following the southeasterly reduction of convergence rates from the Manila Trench offshore NW Mindoro to onshore SW Mindoro, the slab dipping angles steepen, were initiated at depth (~200km) and propagate upwards. The horizontal distances of the trench and slab, as measured from the Wadati–Benioff zone at 200km depth, also reduce in a southeasterly direction. Observations of intermediate-depth earthquakes that exhibit predominantly down-dip extensional stress patterns attest that the steepening of slab dipping angles is due to the negative buoyancy of the slab. In contrast, a broad region covered by central and south Mindoro, the Romblon group, and NW Panay, is characterized by a sporadic distribution of shallow earthquakes of mostly strike-slip type. Among these, the significantly sized events (Mw>5.5) tend to distribute around the borders suggesting that the region roughly acts as a unit block, moving or rotating coherently. Events in the vicinity of the Wawa–Mamburao valley exhibiting normal faulting of NW–SE horizontal extension suggest rifting of the Macolod Corridor extending southwestward to NW Mindoro.

Application of P-wave Mwp magnitude to earthquakes for tsunami early warning in and around South-Western Turkey

Tsunami warning centres inform the population who are located near and around coastal areas by determining the size and location of an earthquake. The Pacific Tsunami Early Warning System has used P-wave moment magnitude (Mwp) for the warning system for many years. A moment magnitude determination from the P-wave arrivals has the advantage of estimating the size of an earthquake within a couple of minutes, compared to other estimation procedures. The Eastern Mediterranean region is tectonically active area and has experienced many tsunami earthquakes in history. In this paper, the moment magnitude determination technique was used to calculate the moment magnitudes of events that occurred in the Eastern Mediterranean and near the coasts of Turkey with magnitudes between 5.5 and 6.8 and epicentral distance between about 100 and 1700 km. Mwp was calculated using seismic stations that were installed in the area and the number of stations used changed event by event. Their numbers were in the range of 11 to 71. Calculated Mwp values showed good compatibility with Global Centroid Moment Tensor Mw. The results of this study are encouraging for the early warning system that will be set up in the near future in the region.

Did a submarine landslide contribute to the 2011 Tohoku tsunami?

Many studies have modeled the Tohoku tsunami of March 11, 2011 as being due entirely to slip on an earthquake fault, but the following discrepancies suggest that further research is warranted. (1) Published models of tsunami propagation and coastal impact underpredict the observed runup heights of up to 40 m measured along the coast of the Sanriku district in the northeast part of Honshu Island. (2) Published models cannot reproduce the timing and high-frequency content of tsunami waves recorded at three nearshore buoys off Sanriku, nor the timing and dispersion properties of the waveforms at offshore DART buoy #21418. (3) The rupture centroids obtained by tsunami inversions are biased about 60 km NNE of that obtained by the Global CMT Project. Based on an analysis of seismic and geodetic data, together with recorded tsunami waveforms, we propose that, while the primary source of the tsunami was the vertical displacement of the seafloor due to the earthquake, an additional tsunami source is also required. We infer the location of the proposed additional source based on an analysis of the travel times of higher-frequency tsunami waves observed at nearshore buoys. We further propose that the most likely additional tsunami source was a submarine mass failure (SMF—i.e., a submarine landslide). A comparison of pre- and post-tsunami bathymetric surveys reveals tens of meters of vertical seafloor movement at the proposed SMF location, and a slope stability analysis confirms that the horizontal acceleration from the earthquake was sufficient to trigger an SMF. Forward modeling of the tsunami generated by a combination of the earthquake and the SMF reproduces the recorded on-, near- and offshore tsunami observations well, particularly the high-frequency component of the tsunami waves off Sanriku, which were not well simulated by previous models. The conclusion that a significant part of the 2011 Tohoku tsunami was generated by an SMF source has important implications for estimates of tsunami hazard in the Tohoku region as well as in other tectonically similar regions.

The relationship between seismicity and fault structure on the Discovery transform fault, East Pacific Rise

There is a global seismic moment deficit on mid-ocean ridge transform faults, and the largest earthquakes on these faults do not rupture the full fault area. We explore the influence of physical fault structure, including step-overs in the fault trace, on the seismic behavior of the Discovery transform fault, 4S on the East Pacific Rise. One year of microseismicity recorded during a 2008 ocean bottom seismograph deployment (24,377 0 ≤ ML ≤ 4.6 earthquakes) and 24 years of Mw ≥ 5.4 earthquakes obtained from the Global Centroid Moment Tensor catalog, are correlated with surface fault structure delineated from high-resolution multibeam bathymetry. Each of the 15 5.4 ≤ Mw ≤ 6.0 earthquakes that occurred on Discovery between 1 January 1990 and 1 April 2014 was relocated into one of five distinct rupture patches using a teleseismic surface wave cross-correlation technique. Microseismicity was relocated using the HypoDD relocation algorithm. The western fault segment of Discovery (DW) is composed of three zones of varying structure and seismic behavior: a zone with no large events and abundant microseismicity, a fully coupled zone with large earthquakes, and a complex zone with multiple fault strands and abundant seismicity. In general, microseismicity is reduced within the patches defined by the large, repeating earthquakes. While the extent of the large rupture patches on DW correlates with physical features in the bathymetry, step-overs in the primary fault trace are not observed at patch boundaries, suggesting along-strike heterogeneity in fault zone properties controls the size and location of the large events.

Seismic Intensities, PGA, and PGV for the 20 April 2013, Mw 6.6 Lushan, China, Earthquake, and a Comparison with North America

Online Material: Seismic intensity map with observation points; table of geographical coordinates, MMI values. The 20 April 2013 M w 6.6 Lushan earthquake occurred at the geologic boundary between the western Sichuan basin and the eastern flank of the Tibetan plateau. This earthquake occurred 100 km south‐southwest of the epicenter of the 2008 M w 7.9 Wenchuan earthquake (Fig. 1). In response to the deadly Wenchuan earthquake, there has been a significant increase in the density of seismographic, strong‐motion, and Global Positioning System (GPS) stations throughout western Sichuan Province, including the vicinity of the Lushan earthquake. This increased level of earthquake monitoring has provided a dense array of coseismic recordings of the Lushan earthquake. These data have been used for numerous studies of the source properties, aftershock distribution, and the crustal strain field associated with this earthquake (Fang et al. , 2013; Liu et al. , 2013; Shan et al. , 2013; Xie et al. , 2013, 2014; Xu et al. , 2013; Han et al. , 2014; Zhang et al. , 2014). Figure 1. (a) Topographic map of Longmen Shan, western Sichuan Province, with the epicenters of the 2008 Wenchuan and 2013 Lushan earthquakes obtained from China Earthquake Network Center and the focal mechanisms (Global Centroid Moment Tensor project). The black lines show the extent of the rupture planes of the Wenchuan earthquakes (Zhang et al. , 2010; Hartzell et al. , 2013). (b) Finite‐fault slip model for the Lushan earthquake (Liu et al. , 2013); maximum slip is 150 cm and the rupture dimension for slip >25 cm is 20 km by 20 km. We report a field investigation of seismic intensities in the immediate vicinity of the epicenter of the Lushan earthquake. We compare these results with the recorded seismic ground accelerations and ground …

New Tsunami Forecast Tools for the French Polynesia Tsunami Warning System

This paper presents the tsunami warning tools, which are used for the estimation of the seismic source parameters. These tools are grouped under a method called Preliminary Determination of Focal Mechanism_2 (PDFM2), that has been developed at the French Polynesia Warning Center, in the framework of the \(SEISCOMP3\) system, as a plug-in concept. The first tool determines the seismic moment and the focal geometry (strike, dip, and slip), and the second tool identifies the ”tsunami earthquakes” (earthquakes that cause much bigger tsunamis than their magnitude would imply). In a tsunami warning operation, initial assessment of the tsunami potential is based on location and magnitude. The usual quick magnitude methods which use \(P\) waves, work fine for smaller earthquakes. For major earthquakes these methods drastically underestimate the magnitude and its tsunami potential because the radiated energy shifts to the longer period waves. Since French Polynesia is located far away from the subduction zones of the Pacific rim, the tsunami threat is not imminent, and this luxury of time allows to use the long period surface wave data to determine the true size of a major earthquake. The source inversion method presented in this paper uses a combination of surface waves amplitude spectra and P wave first motions. The advantage of using long period surface data is that there is a much more accurate determination of earthquake size, and the advantage of using P wave first motion is to have a better constrain of the focal geometry than using the surface waves alone. The \(PDFM2\) method routinely gives stable results at \(T_0 + 45\) minutes, with \(T_0\) being the origin time of an earthquake. Our results are then compared to the Global Centroid Moment Tensor catalog for validating both the seismic moment and the source geometry. The second tool discussed in this paper is the slowness parameter \(\theta,\) and is the energy-to-moment ratio. It has been used to identify tsunami earthquakes, which are characterized by having unusual slow rupture velocity and release seismic energy that has been shifted to longer periods and, therefore, have low \(\theta\) values. The slow rupture velocity would indicate weaker material and bigger uplift and, thus, bigger tsunami potential. The use of the slowness parameter \(\theta\) is an efficient tool for monitoring the near real-time identification of tsunami earthquakes.

Rapid Estimates of the Source Time Function and Mw using Empirical Green's Function Deconvolution

The U.S. Geological Survey National Earthquake Information Center (NEIC) uses a variety of classical network‐averaged magnitudes (e.g., m b and M s) and waveform modeling procedures to determine the moment magnitude ( M w) of an earthquake from teleseismic observations. Initial magnitude estimates are often inaccurate because of poor azimuthal control (sampling of the focal sphere) and/or intrinsic limitation of each method to a specific range of event size. To provide faster and more accurate estimates of the moment magnitude, source duration, and source complexity, NEIC is exploring the use of a variation of the empirical Green’s function (EGF) deconvolution procedure. This approach uses a predicted focal mechanism derived from the Global Centroid Moment Tensor Catalog to compute teleseismic P ‐wave synthetic seismograms, which are then deconvolved from observed P and SH waveforms to determine station‐specific M w, source time function, and a network‐averaged M w. Our EGF approach is validated using broadband waveforms from 246 earthquakes in the magnitude range M w 6.0–9.1. Within approximately 13 min of earthquake origin time, our procedure using teleseismic P waves only computes an M w that lies within ±0.25 of the final W ‐phase M w in the magnitude range 6–8. Using later arriving teleseismic SH phases results in an M w that lies within ±0.12 of the W ‐phase M w. For magnitude 8 or larger earthquakes, we underestimated the moment magnitude by up to 0.8 magnitude units, primarily due to the initial P phase not containing the total seismic moment release. Long‐period phases such as the W ‐phase and surface waves that better characterize total moment release can also be incorporated in the processing.

3D coseismic deformations and source parameters of the 2010 Yushu earthquake (China) inferred from DInSAR and multiple-aperture InSAR measurements

The coseismic deformation field related to the 2010 Mw6.9 Yushu earthquake (Qinghai, China) has been investigated by previous studies using the satellite differential interferometric synthetic aperture radar (DInSAR) method. However, one of the limiting factors in the conventional DInSAR is that it can measure only one-dimensional (1D) ground motion along the radar line of sight (LOS) direction, thus resulting in anamorphic coseismic displacements and biased source parameters. In this paper, the three-dimensional (3D) deformation maps for this earthquake were reconstructed over an area of about 100×70km2 for the first time by a combined use of the DInSAR method for detecting LOS motion and the multiple-aperture interferometry (MAI) method for detecting along-track (AT) motion. The fault slip map for this earthquake was generated using the 3D deformation maps. Based on the fault traces on the fault slip map, we constructed a fault model with five segments and preformed a two-step inversion using the 3D deformation data as the constraint. Our results show that the maximum coseismic displacements in up–down, north–south and east–west directions are −40.4, 113.8, and −65.7cm, respectively, and the peak-to-peak slip offset between the southern and northern fault wall is 1.82m. Our model reveals a shallow dislocation on a NW–SE trending fault extending about 74km, along which the four fault segments are nearly vertical (84°–88°) and the one fault segment dips 66.2° to the southwest. The estimated seismic moment is 2.43×1019Nm (Mw6.92), which is close to the solution provided by the Global Centroid Moment Tensor (GCMT) Project. The slip distribution derived for each fault patch shows that the slip occurrence extends down to 12km beneath the ground surface, and a peak slip of 2.23m appears at the depth of about 5km right near the epicenter. Comparison with the results derived from the seismic waves, field investigation, GPS and conventional DInSAR observations indicates that the source parameters of the Yushu earthquake can be improved due to the combined use of DInSAR and MAI measurements.

Constraining the focal mechanism of the Lushan earthquake with observations of the Earth’s free oscillations

The amplitudes of the Earth’s free oscillations have a close relationship to earthquake focal mechanisms. Focal mechanisms of large earthquakes can be well analyzed and constrained with observations of long period free oscillations. Although the 2013 Lushan earthquake was only moderately sized, observable spherical normal modes were excited and clearly observed by a superconductive gravimeter and a broadband seismometer. We compare observed free oscillations with synthetic normal modes corresponding to four different focal mechanisms for the Lushan earthquake. The results show that source parameters can be analyzed and constrained by spherical normal modes in a 2.3–5 mHz frequency band. The scalar seismic moment M0 has a major influence on the amplitudes of free oscillations; additionally, the strike, dip, rake and depth of the hypocenter have minor influences. We found that the synthetic modes corresponding to the focal mechanism determined by the Global Centroid Moment Tensor show agreement to the observed modes, suggesting that earthquake magnitudes predicted in this way can readily reflect the total energy released by the earthquake. The scalar seismic moment obtained by far-field body wave inversion is significantly underestimated. Focal mechanism solutions can be improved by joint inversion of far- and near-field data.

Explanation of Temporal Clustering of Tsunami Sources Using the Epidemic-Type Aftershock Sequence Model

Temporal clustering of tsunami sources is examined in terms of a branching process model. It previously was observed that there are more short interevent times between consecutive tsunami sources than expected from a stationary Poisson process. The epidemic‐type aftershock sequence (ETAS) branching process model is fitted to tsunami catalog events, using the earthquake magnitude of the causative event from the Centennial and Global Centroid Moment Tensor (CMT) catalogs and tsunami sizes above a completeness level as a mark to indicate that a tsunami was generated. The ETAS parameters are estimated using the maximum‐likelihood method. The interevent distribution associated with the ETAS model provides a better fit to the data than the Poisson model or other temporal clustering models. When tsunamigenic conditions (magnitude threshold, submarine location, dip‐slip mechanism) are applied to the Global CMT catalog, ETAS parameters are obtained that are consistent with those estimated from the tsunami catalog. In particular, the dip‐slip condition appears to result in a near zero magnitude effect for triggered tsunami sources. The overall consistency between results from the tsunami catalog and that from the earthquake catalog under tsunamigenic conditions indicates that ETAS models based on seismicity can provide the structure for understanding patterns of tsunami source occurrence. The fractional rate of triggered tsunami sources on a global basis is approximately 14%. Online Material: Figures of residual analysis for tsunami and earthquake catalogs.

ISC-GEM: Global Instrumental Earthquake Catalogue (1900–2009), III. Re-computed MS and mb, proxy MW, final magnitude composition and completeness assessment

This paper outlines the re-computation and compilation of the magnitudes now contained in the final ISC-GEM Reference Global Instrumental Earthquake Catalogue (1900–2009). The catalogue is available via the ISC website (www.isc.ac.uk/iscgem/) and lists in a comma separated format the location and magnitude parameters (with corresponding uncertainties) of global large earthquakes. In this work we report on the procedures adopted to obtain the final magnitude composition of the nearly 20,000 earthquakes processed. We have made every effort to use uniform procedures of magnitude determination throughout the entire period of the catalogue. The re-computation of the surface wave MS and short-period body-wave mb values benefitted from new hypocentres (Bondár et al., 2015), previously unavailable amplitude-period data digitized during this project (Di Giacomo et al., 2015), and a more reliable algorithm for magnitude estimation based on a 20% alpha-trimmed median magnitude (Bondár and Storchak, 2011). In particular, for MS until the end of 1970 we have processed an unprecedented amount of data and obtained several thousands of station magnitudes not available before.The available re-computed MS and mb provided an ideal basis for deriving new conversion relationships to moment magnitude MW. Therefore, rather than using previously published regression models, we derived new empirical relationships using both generalized orthogonal linear and exponential non-linear models to obtain MW proxies from MS and mb. The new models were tested against true values of MW, and the newly derived exponential models were then preferred to the linear ones in computing MW proxies.For the final magnitude composition of the ISC-GEM catalogue, we preferred directly measured MW values as published by the Global CMT project for the period 1976–2009 (plus intermediate-depth earthquakes between 1962 and 1975). In addition, over 1000 publications have been examined to obtain direct seismic moment M0 and, therefore, also MW estimates for 967 large earthquakes during 1900–1978 (Lee and Engdahl, 2015) by various alternative methods to the current GCMT procedure. In all other instances we computed MW proxy values by converting our re-computed MS and mb values into MW, using the newly derived non-linear regression models. The final magnitude composition is an improvement in terms of magnitude homogeneity compared to previous catalogues.The magnitude completeness is not homogeneous over the 110years covered by the ISC-GEM catalogue. Therefore, seismicity rate estimates may be strongly affected without a careful time window selection. In particular, the ISC-GEM catalogue appears to be complete down to MW 5.6 starting from 1964, whereas for the early instrumental period the completeness varies from ∼7.5 to 6.2. Further time and resources would be necessary to homogenize the magnitude of completeness over the entire catalogue length.

First geodetic observations of a deep earthquake: The 2013 Sea of Okhotsk Mw 8.3, 611 km-deep, event

We analyze the first ever GPS observations of static surface deformation from a deep earthquake: the 24 May 2013 Mw 8.3 Sea of Okhotsk, 611 km-deep, event. Previous studies of deep earthquake sources relied on seismology and might have missed evidence for slow slip in the rupture. We observed coseismic static offsets on a GPS network of 20 stations over the Sea of Okhotsk region. The offsets were inverted for the best fitting double-couple source model assuming a layered spherical Earth. The seismic moment calculated from static offsets is only 7% larger than the seismological estimate from Global Centroid Moment Tensor (GCMT). Thus, GPS observations confirm shear faulting as the source model, with no significant slow-slip component. The relative locations of the U.S. Geological Survey hypocenter, GCMT centroid, and the fault from GPS indicate slip extending for tens of kilometers across most of the slab thickness.

The 1 April 2014 Iquique, Chile,Mw8.1 earthquake rupture sequence

The 1 April 2014 Iquique, Chile,<i>M_w</i>8.1 earthquake rupture sequence

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.