Checkerboard Resolution Test Research Articles

Wave-equation-based travel-time seismic tomography – Part 2: Application to the 1992 Landers earthquake (<i>M</i><sub>w</sub> 7.3) area

Abstract. High-resolution 3-D P and S wave crustal velocity and Poisson's ratio models of the 1992 Landers earthquake (Mw 7.3) area are determined iteratively by a wave-equation-based travel-time seismic tomography (WETST) technique. The details of data selection, synthetic arrival-time determination, and trade-off analysis of damping and smoothing parameters are presented to show the performance of this new tomographic inversion method. A total of 78 523 P wave and 46 999 S wave high-quality arrival-time data from 2041 local earthquakes recorded by 275 stations during the period of 1992–2013 are used to obtain the final tomographic models, which cost around 10 000 CPU hours. Checkerboard resolution tests are conducted to verify the reliability of inversion results for the chosen seismic data and the wave-equation-based travel-time seismic tomography method. Significant structural heterogeneities are revealed in the crust of the 1992 Landers earthquake area which may be closely related to the local seismic activities. Strong variations of velocity and Poisson's ratio exist in the source regions of the Landers and three other nearby strong earthquakes. Most seismicity occurs in areas with high-velocity and low Poisson's ratio, which may be associated with the seismogenic layer. Pronounced low-velocity anomalies revealed in the lower crust along the Elsinore, the San Jacinto, and the San Andreas faults may reflect the existence of fluids in the lower crust. The recovery of these strong heterogeneous structures is facilitated by the use of full wave equation solvers and WETST and verifies their ability in generating high-resolution tomographic models.

Localized fault slip to the trench in the 2010 Maule, Chile Mw = 8.8 earthquake from joint inversion of high‐rate GPS, teleseismic body waves, InSAR, campaign GPS, and tsunami observations

AbstractThe 27 February 2010, Mw 8.8 Maule earthquake ruptured ~500 km along the plate boundary offshore central Chile between 34°S and 38.5°S. Establishing whether coseismic fault offset extended to the trench is important for interpreting both shallow frictional behavior and potential for tsunami earthquakes in the region. Joint inversion of high‐rate GPS, teleseismic body waves, interferometric synthetic aperture radar (InSAR), campaign GPS, and tsunami observations yields a kinematic rupture model with improved resolution of slip near the trench. Bilateral rupture expansion is resolved in our model with relatively uniform slip of 5–10 m downdip beneath the coast and two near‐trench high‐slip patches with >12 m displacements. The peak slip is ~17 m at a depth of ~15 km on the central megathrust, located ~200 km north from the hypocenter and overlapping the rupture zone of the 1928 M ~8 event. The updip slip is ~16 m near the trench. Another shallow near‐trench patch is located ~150 km southwest of the hypocenter, with a peak slip of 12 m. Checkerboard resolution tests demonstrate that correctly modeled tsunami data are critical to resolution of slip near the trench, with other data sets allowing, but not requiring slip far offshore. Large interplate aftershocks have a complementary distribution to the coseismic slip pattern, filling in gaps or outlining edges of large‐slip zones. Two clusters of normal faulting events locate seaward along the plate motion direction from the localized regions of large near‐trench slip, suggesting that proximity of slip to the trench enhanced extensional faulting in the underthrusting plate.

Three-Dimensional Compressional Attenuation Model (QP) for the Salton Trough, Southern California

Abstract I present a frequency‐independent 3D seismic attenuation model (indicated by ) for the crust of the Salton trough and the adjacent regions in southeastern southern California. The simul2000 tomographic algorithm was used to invert the frequency‐independent attenuation operator t * values measured from amplitude spectra of 23,378 P ‐wave arrivals of 1203 events through a recently developed 3D velocity model. The Q P model has a uniform horizontal grid spacing of 5 km, and the vertical‐node intervals range between 2 and 5 km down to 27 km depth. In general, the Q P values increase with depth and agree with the surface geology in the shallow depth layers. Low Q P values are observed in the Imperial Valley, California, which are consistent with the sedimentary deposits and may also reflect the presence of pore fluid in the active fault zones, whereas greatly elevated Q P values are shown in the surrounding crystalline ranges. The new Q P model provides an important complement to the existing velocity models for interpreting structural heterogeneity and fluid saturation of rocks in the study area. Online Material: Figures of checkerboard resolution tests, Q P versus V P , and Q P versus heat flow.

Crustal velocity and Vp/Vs structures beneath central Anatolia from local seismic tomography

We applied a seismic tomography technique to arrival time data generated by local crustal earthquakes in central Anatolia in order to study the three-dimensional velocity and Vp/Vs structures and their relation with the complex tectonic processes and seismic activity occurring in the study region. The relatively equal and large number of both P- and S-wave arrival times comprising a total of 51,650 arrivals and the relatively uniform distribution of the recording stations imply that the obtained velocity anomalies are reliable features down to a depth of 40 km. This is also evident from the results of the checkerboard resolution test, hit count, and the ray-path coverage. The inversion results indicate the existence of strong lateral heterogeneities in the crust and uppermost mantle beneath central Anatolia. Prominent low-velocity anomalies are clearly imaged at all layers especially beneath existing volcanoes and the active fault segments. Higher-than-average Vp/Vs ratios are widely distributed, indicating the possible existence of over-pressurized fluids that may be responsible for the triggering of the large crustal earthquakes along the north and east Anatolian fault zones. We noticed that the seismic activity occurs mainly at the low-velocity areas and to a lesser extent in some high-velocity zones, perhaps because of the complex tectonics and geological structures. These observations imply that all the zones with velocity anomalies—either low or high—are potential sites for strain energy accumulation and subsequent release. The obtained velocity and Vp/Vs models are consistent with previous geophysical measurements conducted beneath central Anatolia and give much deeper understanding of the current seismotectonic processes occurring in the region.

Determining Velocity and Q-factor Structure using Crosshole Tomography

In this study, we have conducted a crosshole tomography survey to obtain seismic data from two boreholes on the ITB campus. The first borehole was 39 meters deep while the second was 19 meters deep. The aim of the study was to determine the subsurface velocity and Q-factor for the region between the two boreholes for geotechnical purposes. Sources of seismic waves were produced by an impulse generator and sparker and were recorded by 12 channels of borehole hydrophones. In the tomography inversion, the pseudo-bending ray tracing method was employed to calculate travel times. The initial velocity model was a 1-D model with 1x1 m 2 block dimensions. The non-linear inversion problem was solved by delay-time tomography with the LSQR method. Also, a checkerboard resolution test (CRT) was conducted to evaluate the resolution of the tomography inversion. Using the velocity structure results, a LSQR Q-tomography inversion was carried out using spectral curve fitting to obtain the attenuation structure ( t * values). The resulting tomogram shows that there are 3 layers, with an unconsolidated layer (down to 8 meters), a consolidated layer (from 8 meters deep to 20 meters), and bedrock (more than 20 meters). From the results, the ground water level is estimated at a depth of 14 meters.

Tomographic Imaging of the Seismic Structure Beneath the East Anatolian Plateau, Eastern Turkey

The high level of seismic activity in eastern Turkey is thought to be mainly associated with the continuing collision of the Arabian and Eurasian tectonic plates. The determination of a detailed three-dimensional (3D) structure is crucial for a better understanding of this on-going collision or subduction process; therefore, a body wave tomographic inversion technique was performed on the region. The tomographic inversion used high quality arrival times from earthquakes occurring in the region from 1999 to 2001 recorded by a temporary 29 station broadband IRIS-PASSCAL array operated by research groups from the Universities of Bogazici (Turkey) and Cornell (USA). The data was inverted and consisted of 3,114 P- and 2,298 S-wave arrival times from 252 local events with magnitudes (MD) ranging from 2.5 to 4.8. The stability and resolution of the results were qualitatively assessed by two synthetic tests: a spike test and checkerboard resolution test and it was found that the models were well resolved for most parts of the imaged domain. The tomographic inversion results reveal significant lateral heterogeneities in the study area to a depth of ~20 km. The P- and S-wave velocity models are consistent with each other and provide evidence for marked heterogeneities in the upper crustal structure beneath eastern Turkey. One of the most important features in the acquired tomographic images is the high velocity anomalies, which are generally parallel to the main tectonic units in the region, existing at shallow depths. This may relate to the existence of ophiolitic units at shallow depths. The other feature is that low velocities are widely dispersed through the 3D structure beneath the region at deeper crustal depths. This feature can be an indicator of the mantle upwelling or support the hypothesis that the Anatolian Plateau is underlain by a partially molten uppermost mantle.

Joint inversion of strong motion, teleseismic, geodetic, and tsunami datasets for the rupture process of the 2011 Tohoku earthquake

[1] The 2011 Tohoku earthquake was observed by dense strong motion, teleseismic, geodetic, and tsunami networks. We first inverted each of the datasets obtained by the networks separately, for the rupture process of the earthquake. We then performed checkerboard resolution tests for assessing the resolving power of these datasets. In order to overcome the limited resolutions of the separate inversions and differences in their results, we performed a quadruple joint inversion of all these data to determine a source model most suitable for explaining all the datasets. In the obtained source model, the maximum coseismic slip was approximately 35 m, and the total seismic moment was calculated to be 4.2 × 1022 Nm, which yielded Mw = 9.0. The main rupture propagated not only in the strike direction but also in the dip direction and included both the deep area called the Miyagi-oki region and the compact shallow area near the Japan Trench.

Seismic velocity and Poisson’s ratio tomography of the crust beneath East Anatolia

Eastern Anatolia is a region in the early stages of continent–continent collision and so provides a unique opportunity to study the early development of continental plateau. Located within the Alpine–Himalayan fold-thrust fault belt, the Anatolian plateau is geologically very complex, with over half of the surface area covered with late Cenozoic volcanics of diverse composition. The plateau is also seismically active and is dissected by numerous seismogenic faults predominantly of strike-slip motion. In this study, we determine 3-D tomographic images of the crust beneath eastern Anatolia by inverting a large number of arrival time data of P- and S-waves. From the obtained P- and S-wave velocity models, we estimated the Poisson’s ratio structures for a more reliable interpretation of the obtained velocity anomalies. Our tomographic results are generally consistent with the major tectonic features of the region. High P- and S-wave velocity anomalies are recognized near the surface, while at deeper crustal layers, low seismic wave velocities are widely distributed. Poisson’s ratio exhibits significant structural heterogeneities compared to the imaged velocity structure. The seismic activity is intense along highly heterogeneous zones and is closely associated with pre-existing faults in the central and western parts of the study area. Results of the checkerboard resolution test indicate that the imaged anomalies are reliable features down to a depth of about 40 km. The low-velocity/high Poisson’s ratio zones in the middle to lower crust are consistent with many geophysical observations such as strong Sn attenuation, low Pn and Sn velocity, and the absence of mantle lid, implying the presence of partial melt in the uppermost mantle.

Three-dimensional seismic modelling of the crustal structure between East European Craton and the Carpathians in SE Poland based on CELEBRATION 2000 data

SUMMARY In this paper, we present results of 3-D first-arrival tomography applied to data recorded in SE Poland during the CELEBRATION 2000 seismic experiment. The target area covers ca. 500 × 500 km 2 and represents a complex geological setting from old Precambrian platform (East European Craton, EEC), through the crustal blocks (terranes) that form the TransEuropean Suture Zone (TESZ) to the young Alpine orogen—the Carpathians. Contrasting velocity distributions in various geological units makes the tomographic inversion challenging. For this reason, much attention was paid to the inversion methodology. We tested ‘multioffset’ and ‘multiscale’ approaches. By increasing the offset range in the ‘multioffset’ inversion we have independently constrained different depth ranges of our model. We have found that the ‘multiscale’ method, that is, the gradual stepping from bigger to smaller model cells produced the preferred solution. Resolution of the resultant crustal model was determined by performing checkerboard and restoration resolution tests. Lateral resolution of the order of 25 km is inferred down to 10 km depth and 50 km down to 25‐30 km depth. We have also applied a quasi-Monte Carlo analysis to determine the absolute errors of model parameters, which to our knowledge, is the first such attempt in case of a 3-D wide-angle data set. The mean uncertainties of our tomographic velocities are 0.1 km s −1 , but some anomalies are recovered with

Evolution of 3D velocity structure before Dayao M=6.2 and M=6.1 earthquakes in 2003

In this paper the crustal velocity structure are imaged at the annual scale for determining the evolution of 3D velocity structure before the Dayao earthquakes in 2003, and the relation between physical variation of medium and cause of earthquake occurrence is discussed. Checkerboard resolution tests show that the spatial resolution at 15 km depth is approximate 0.6 around Dayao. Error analyses show that the error (Approximate 0.02 km/s) at the focus area is far smaller than the magnitude (approximate 0.15 km/s) of velocity variation. Results of the studies show that a NNW-striking juncture zone of high-and low-velocity is formed before Dayao earthquake, whose strike is basically consistent with that of aftershock distribution of Dayao earthquakes and the focal mechanism, and the hypocenters lie on the high-velocity side of juncture zone. Furthermore, there is a low-velocity body under the hypocenters after Dayao earthquakes. The evolution of velocity structure provides restriction for discussing the circumstances of earthquake gestating and occurring.

High precision slip distribution of the 1944 Tonankai earthquake inferred from tsunami waveforms: Possible slip on a splay fault

Inversion of tsunami waveforms is a well-established technique for estimating the slip distributions of subduction zone earthquakes, with some of the most detailed results having been obtained for earthquakes in the Nankai Trough, SW Japan. The present study, although it uses a method and tsunami waveform data set almost identical to previous study, aims to improve on previous work by using a more precise specification of initial conditions for the calculation of tsunami Green's functions. Specifically, we incorporated four improvements in the present study: (1) we used a realistic plate model based only on seismic survey results, and assumed it to be the fault plane of the 1944 Tonankai earthquake; (2) the smallest subfaults consistent with the long wavelength approximation were used in the tsunami inversion analysis; (3) we included the effect of horizontal displacement of the ocean bottom on tsunami generation; and (4) we performed a checkerboard resolution test. As obtained in previous studies, a zone of high slip (> 2.0 m) was resolved off the Shima Peninsula. However, the more precise calculation of tsunami Green's functions has revealed additional detail that was not evident in previous studies, which we demonstrate is resolvable and correlates with the position of known faults in the accretionary prism. While there was little or no slip near the trench axis in the eastern part of the rupture zone, there was up to 1.5 m of slip resolved within 30 km of the trough axis in the western part, along the coast of the Kii Peninsula. This troughward slip zone coincides with the position of a large splay fault mapped in multichannel reflection surveys. Furthermore, it is also clear that the upper edge of the Enshu fault off Shima and Atsumi peninsulas is consistent with the up-dip limit of slip in the eastern part of our model. We tested the possibility that slip occurred on the former splay fault instead of on the plate interface during the 1944 Tonankai earthquake, and find that slip on this splay fault is also consistent with the data, although we cannot distinguish whether slip was dominant on the splay fault or on the plate interface. We further suggest that the position of the Enshu fault may be determined by the subduction of topographic highs, and that such faults may have an important influence on the up-dip rupture limit of the 1944 Tonankai and, potentially, other subduction zone earthquakes.

Offshore geodetic data conducive to the estimation of the afterslip distribution following the 2003 Tokachi-oki earthquake

A cable-based seafloor observatory located offshore of Hokkaido, Japan, provides the world's first direct observation of offshore post-seismic displacement. After removing thermal noise, this data is used to constrain the distribution of offshore afterslip from a great interplate earthquake (Mw 8.0) that occurred during 2003 along the southern Kuril Trench. A checkerboard resolution test showed a great contribution of the offshore data to improving the spatial resolution on the plate interface slip near the trench. The 1-yr afterslip is distributed in a U-shaped pattern encircling its co-seismic slip area. This result indicates that the stick-slip frictional properties of the plate interface are distributed in a patch-like pattern as opposed to a banding pattern as previously assumed in young subduction zones.

Crustal heterogeneity beneath southwest Japan estimated from direct and Moho-reflected waves

In this study, we have used later phases reflected at the Moho discontinuity in addition to direct arrival time data to study the 3-D crustal velocity structure beneath southwest (SW) Japan. We have collected more than 6000 direct and Moho-reflected phases and used them to obtain P- and S-wave velocity structures in the study area. Our results revealed significant low-velocity anomalies beneath the active and Quaternary volcanoes in SW Japan. Results obtained using each data set ( P, PmP, S, and SmS) are generally consistent with each other and all showed low-velocity anomalies beneath most of the active and Quaternary volcanoes, western Kii Peninsula and eastern Shikoku. Ray-path analysis showed that the data set we used has a good ray criss-crossing that is sufficient to accurately locate the velocity anomalies in the study area. In addition, results of the checkerboard resolution test indicate that these structures are generally well recovered and that adding later phases could significantly improve the resolution scale of the obtained tomographic images in comparison with those from first arrivals alone.

Three-dimensional crustal structure of Ascension Island from active source seismic tomography

We present a crustal velocity and Moho depth model of the Ascension volcanic edifice using one of the most densely sampled 3-D wide-angle data sets for volcanic islands currently available. We invert traveltimes of first and second arrivals by seeking a layer-interface minimum structure model, and then test the resulting velocity model by gravity modelling and a 3-D checkerboard resolution test. Within the shallow extrusive part of the crust, two main high-velocity regions coincide with the highest topography on land and the gravity maximum off the west coast of the island, respectively. These features are connected with a high-velocity intrusive core that is created either within or on the top of oceanic layer 3 and is interpreted as a possible relic magma chamber. The thickness of the surface low-velocity region is similar to that observed at Hawaii, Jasper seamount and Great Meteor seamount, suggesting a similar process of edifice construction. The mean density of the volcanic edifice is significantly less than that of normal oceanic crust and than load densities typically assumed in studies of flexure as a result of seamount loading. There is no simple flexural model that explains the shape of the Moho beneath the island, perhaps because of the long-lived volcanism and the proximity of the island to the Mid-Atlantic Ridge (MAR) and the Ascension fracture zone. There is no evidence for magmatic underplating beneath Ascension Island.

A low velocity zone beneath the Hida Mountains derived from dense array observation and tomographic method

Seismic waves suffer strong attenuation when propagating beneath the Hida Mountains in Central Honshu, Japan. In order to study this region in detail, we conducted three kinds of dense seismic array observations in and around the Hida Mountains in the summer of 1996. Picking P- and S-wave arrival time data from 54 events at 101 stations, 3175 P- and 2335 S-wave arrival time data were obtained for our tomographic study. Hypocenter locations and velocity structure were determined simultaneously. We assessed ray coverage and resolution of the velocity structure with checkerboard resolution tests. Ray paths for the model of the obtained velocity structure were examined in detail. There are two zones of low P-wave velocity, one at a depth of 4 km and the other at 15 km. The resolution is good at depths of 0–20 km for P-wave velocities and at depths of 0–15 km for S-wave velocities. A high VP/VS ratio (2.7) indicates that a partially melting rock exists beneath the Hida Mountains. The deep low velocity zone is located just above the upper/lower crustal boundary. These observations indicate that a magma reservoir exists in the upper crust beneath the Hida Mountains.

Three‐dimensional velocity structure and earthquake locations beneath the northern Tien Shan of Kyrgyzstan, central Asia

We used the arrival times of local earthquakes and quarry blasts recorded by the Kyrgyzstan Broadband Network (KNET) to obtain three‐dimensional (3‐D) P and S wave velocity models of the upper crust beneath an actively deforming mountain front and its associated foreland in the Kyrgyz Tien Shan. The continuous velocity models, described by cubic B spline interpolation of the squared slowness over a regular 3‐D grid, were computed by simultaneous inversion of hypocenter and medium parameters. Exact ray tracing was done in the smooth 3‐D medium by shooting rays from the sources to the stations by an analytical perturbation method based on the paraxial ray theory. The deduced large, sparse, linear system was solved using the damped, iterative, least squares algorithm LSQR. The stability and resolution of the result was qualitatively tested by two synthetic tests: the spike test and the checkerboard resolution test. We found that the models are well resolved up to a depth of ∼27 km for most parts of our image domain. The P and S wave velocity models are consistent with each other and provide evidence for marked heterogeneity in the upper crustal structure beneath the northern Tien Shan. At shallower depths (<7 km) the sediment‐filled foreland is imaged as a relatively lower velocity feature compared to the mountains, which are cored by crystalline basement rocks. In contrast, at midcrustal depths the mountains are underlain by relatively lower velocity materials compared to the foreland. A distinct contrast in velocity structure is also observed between the eastern and western parts of the Kyrgyz Range at midcrustal depths, with the presence of relatively higher velocities toward the east. The seismicity is concentrated near the traces of major active faults and extends deeper beneath the foreland compared to the mountains. The regional compression in the Tien Shan is accommodated along a series of high‐angle reverse faults distributed throughout the orogenic system that extend from the surface down to midcrustal depths. The range‐bounding fault zone can be identified by a sharp lateral gradient in seismic velocities with a pronounced southward dip combined with a zone of seismicity that also deepens to the south and reverse fault source mechanisms from moderate‐sized events. A pronounced low‐velocity zone (LVZ) is imaged in the P wave field, at midcrustal depths, beneath the western part of the Kyrgyz Range. This LVZ is presumably correctable with reported high‐conductivity zones in this region that have been proposed to mark active fault zones along which fluid migration occurs. The location of the LVZ, which is closely coincident with the depth of maximum earthquake generation, might imply that it is a crustal decoupling zone at the brittle‐ductile transition.