Checkerboard Resolution Test Research Articles

4D time lapse tomography for monitoring cave propagation and stress distribution in Deep Mill Level Zone (DMLZ) PT Freeport Indonesia

Caving activity results in an increased induced seismicity which should be monitored to avoid massive and uncontrolled rock damage. This research was conducted at the Deep Mill Level Zone (DMLZ) underground mine, the deepest underground mine in Indonesia operated by PT Freeport Indonesia. This research aims to monitor cave propagation by using 4D tomography with a catalogue of microseismic for 57 days with a total of 14,821 events recorded by 84 stations consisting of 176,265 P phases and 133,472 S phases. The data is divided into four subsets to see the velocity evolution related to cave progress. Checkerboard Resolution Test (CRT) and Derivative Weight Sum (DWS) are used to assess the resolution of the inversion. 3D initial velocity model is constructed based on geological information and coring data. We have succeeded in identifying the interpreted cave propagation of a 60 m extension to the NW at around 100 m above undercut level based on 4D changes in velocity tomogram validate by Time Domain Reflectometry data. The decrease of Vp and Vs in subset 3 is interpreted due to the fracturing processes as the cave progresses. Furthermore, we observe a stress redistribution along with the progress of the cave, which is characterized by high velocities (Vp and Vs) due to compensation for changes in low velocity values ​​in the area in front of the cave, which is starting to collapse. We suggest that a considerable change in the velocity tomogram as an indicator of impending caving.

An updated 3D crustal velocity model in Cyprus by non-linear traveltime tomography

We have used >6000 regional earthquakes in the broader Cyprus island area, with 100,000+ P and S-phases recorded by 26 local seismological stations to obtain a 3D velocity model for the Cyprus area using a non-linear seismic tomography approach. Due to the limited aperture of the recording network, the tomographic model mainly recovers the dominant geophysical features of the upper crust of the Cyprus island. The obtained results reveal new information on the 3D structure of the Troodos ophiolithic complex, identifying a high velocity anomaly that coincides with the NW-SE trending ophiolitic units of the Troodos mountain. Furthermore, they clearly show that the anomaly associated with the ophiolite zone of Troodos is well preserved up to a depth of at least 10 Km, dipping towards the Northeast. The Mesaoria basin in central Cyprus is recognized as a large low-velocity feature, with the results suggesting a significant thickness of the basin, exceeding 5 km. A similar low-velocity pattern can be observed for the Circum Troodos formations (Pakhna and Lefkara) to the southwest of the Troodos mountain, while the Kyrenia range is not associated with a crustal velocity anomaly. To assess the reliability of the results we have conducted checkerboard resolution tests, which suggest that the uppermost crustal features are well reconstructed, while for larger depths (>10 km) we cannot recover mid-crustal anomalies due to the insufficient traveltime data coverage and quality (e.g., traveltime picking errors).

Early Modeling of Seismic Velocity Structure P, S, and Vp/Vs Ratio in Bengkulu Region and its Surroundings

Bengkulu is located in the southern region of Sumatra, where the Indo-Australian and Eurasian plates are subducted. As a consequence of the subduction, seismicity in the area increases, as well as volcanic activity and the appearance of structures such as the Sumatran and the Mentawai faults. Seismic velocity structures were reconstructed using earthquake tomography in Bengkulu and the surrounding areas using data from the International Seismological Center (ISC) and the Meteorology, Climatology, and Geophysics Agency (BMKG). The earthquake data collection period was from January 1, 2010 to December 31, 2018. 19 stations recorded 1721 earthquakes. LOTOS-12 is used to do a tomography inversion, starting with a 1D velocity model and wadati’s Vp/Vs ratio and iteratively inverting for Vp, Vs, and Vp/Vs ratios. The bending algorithm is used to calculate the arrival time, and the LSQR method is used to invert it. The Checkerboard Resolution Test (CRT) yielded Tomography results. The results revealed the existence of zones with low-velocity anomalies and low-values of Vp/Vs ratios, indicating the presence of weak zones caused by Sumatra faults, Mentawai faults, and other local faults. Low-velocity anomalies with high-values of Vp/Vs ratios are related to volcanic activity, hereas high-velocity anomalies are associated with the existence of subduction plates and compact rocks. The low-velocity anomalies in the fore-arc at 30-50 km depths coincide with fluid circulation caused by slab dehydration.

Application of Cross-hole Seismic Tomography to Inferred Cavities Condition.

The construction of the Rajamandala hydroelectric power plant civil works has experienced critical problems that arise in the implementation process, with the occurrence of cracks cavities conditions in the headrace tunnel locations. We applied the cross-hole seismic tomography to detect important heterogeneities and mechanical proprieties of the formations between two boreholes in this case inferred cracks cavities condition. The seismic source is inside the borehole “a sparker” and the receivers “hydrophones” are in an adjacent one, the sparker is lowered into the borehole one step at the time. Seismic waves, propagating in the tunnel wall rock, spread widely, and are reflected and refracted when encountering interfaces between rocks with different acoustic impedances. Reflected waves returning to the receivers are recorded. After the processing of the first arrivals of the P waves we obtain a cross section of the seismic velocities in between the two wellbores. With the cross-hole seismic tomography, the basis could be provided to the tunnel construction and parameter adjustments to guarantee the construction safety. Our result shown that there is a weak zone above the tunnel with low Vp zone (~1.2 km/s) may be related to weak zone (may be fracture, unconsolidated rock, and fluid-filled rock or landslide caving). Checker-board resolution test is also conducted to determine the tomogram areas that are reliable to be interpreted. Our challenges are we have poor resolution due to not enough raypath. The Cross-hole seismic tomography can effectively and safely guide the excavation of the tunnel section working surface in combination with reconstructed images and excavation technology.

Directivity of Coseismic Ionospheric Disturbances Propagation Following the 2016 West Sumatra Earthquake Using Three-Dimensional Tomography GNSS-TEC

Ionospheric disturbances caused by the 2016 West Sumatra earthquake have been studied using total electron content (TEC) measurements by Global Navigation Satellite System (GNSS) observation stations evenly distributed in Sumatra and Java, Indonesia. Previous observation focused on the coseismic ionospheric disturbances (CID) detected 11–16 min after the earthquake. The maximum TEC amplitude measured was 2.9 TECU (TEC Unit) with speed between 1 and 1.72 km/s. A comprehensive analysis needs to be done to see how the growth and direction of the movement of the CID due to the earthquake is using the 3D tomography method. The dimensions of 3D tomographic model are setup to 1° × 1.2° × 75 km. The continuity constraints were used to stabilize the solution, and multiple resolution tests with synthetic data were conducted to evaluate the precision of the results. This research focuses on the anomalous movement of the ionosphere observed in three dimensions. From the model, the positive anomaly initially appeared 11 min after the earthquake at the altitude of 300 km, which is the highest ionization layer and correspond to the electron density profile using IRI model. The anomalous movement appeared 12 min after the mainshock and moved 1° toward the geomagnetic field every minute. The density anomaly of the ionosphere began to weaken 8 min after the appearance of CID. To check the accuracy of the 3D tomography model, we carried out two types of tests, namely checkerboard resolution test and the second resolution test.

Seismogenesis of the 2021 Mw 7.1 earthquake sequence near the northeastern Japan revealed by double-difference seismic tomography

Recently, a Mw 7.1 earthquake struck the Pacific coast of northeastern Japan on 2021/02/13. To investigate the cause of this earthquake and its aftershocks, we applied a Vp/Vs model-consistency constrained double-difference (DD) seismic tomography method to the earthquake arrival time data from the dense seismic networks in Japan to image the velocity structures at the hypocenter areas. Compared to the conventional DD tomography, the new method can determine the Vp/Vs model with high resolution and precision. The reliability of the seismic tomography results is testified by checkerboard resolution test. The velocity profiles show that the oceanic crust and uppermost part of the oceanic mantle are characterized by low Vs and high Vp/Vs due to hydration reaction, while the lower oceanic mantle is associated with high Vs and low Vp/Vs anomalies that are likely caused by serpentine dehydration. The mainshock of the Mw 7.1 earthquake sequence is located at the boundary between the low velocity layer and underlying high velocity zone. The aftershocks are mainly distributed within the low velocity layer. The rupture plane inferred from the aftershock distribution and mainshock focal mechanism solution has penetrated ∼10 km into the slab and extended ∼50 km along a direction nearly parallel to the trench. These observations have suggested that the Mw 7.1 earthquake sequence occurs as a result of reactivation of a pre-existing fault likely created at the outer-rise before subduction. The association of the mainshock with the velocity anomaly edge suggests that this earthquake sequence is probably caused by dehydration of the mantle serpentines, which increases the pore pressure on the fault. Besides, the variation of rock mechanics at the hypocenter area of the mainshock also facilitates the nucleation of the whole earthquake sequence.

Testing the Utilization of a Seismic Network Outside the Main Mining Facility Area for Expanding the Microseismic Monitoring Coverage in a Deep Block Caving

In the case of mining in an inclined intrusion using the block caving method, the highest stress is usually concentrated in the seismogenic and abutment zones, especially in the front of the sloping area. In an inclined intrusion of more than 40°, the seismometer network is usually distributed in the facility area where the footwall area is also located. This causes a limitation in microseismic monitoring due to ray coverage. In this study, we conduct a seismometer deployment outside a mining facilities area with borehole seismometers. The study aims to maximize the resolution and minimize the monitoring uncertainty of underground mines. We created two scenarios of seismometer deployment: (i) seismometers are deployed following the intrusion mining level in the mining facility area; and (ii) additional seismometers are deployed in off-facilities areas. Both areas were tested for their raypath responses and sensitivity using the Checkerboard Resolution Test (CRT). The monitoring resolution influenced by the additional borehole seismometers in the off-facilities area can be quantified. The results suggest that the additional seismometers in the off-facilities areas can increase resolution by 30% in the seismogenic and abutment zones.

Travel Time Tomography to Delineate 3-D Regional Seismic Velocity Structure in the Banyumas Basin, Central Java, Indonesia, Using Dense Borehole Seismographic Stations

The Banyumas Basin is a tertiary sedimentary basin located in southern Central Java, Indonesia. Due to the presence of volcanic deposits, 2-D seismic reflection methods cannot provide a good estimation of the sediment thickness and the subsurface geology structure in this area. In this study, the passive seismic tomography (PST) method was applied to image the 3-D subsurface Vp, Vs, and Vp/Vs ratio. We used 70 seismograph borehole stations with a recording duration of 177 days. A total of 354 events with 9, 370 P and 9, 368 S phases were used as input for tomographic inversion. The checkshot data of a 4, 400-meter deep exploration well (Jati-1) located within the seismic network were used to constrain the shallow crustal layer of the initial 1-D velocity model. The model resolution of the tomographic inversions was assessed using the checkerboard resolution test (CRT), the diagonal resolution element (DRE), and the derivative weight sum (DWS). Using the obtained Vp, Vs, and Vp/Vs ratio, we were able to sharpen details of the geological structures within the basin from previous geological studies, and a fault could be well-imaged at a depth of 4 km. We interpreted this as the main dextral strike-slip fault that controls the pull apart process of the Banyumas Basin. The thickness of the sediment layers, as well as its layering, were also could be well determined. We found prominent features of the velocity contrast that aligned very well with the boundary between the Halang and Rambatan formations as observed in the Jati-1 well data. Furthermore, an anticline structure, which is a potential structural trap for the petroleum system in the Banyumas Basin, was also well imaged. This was made possible due to the dense borehole seismographic stations which were deployed in the study area.

Development of a Trans‐Dimensional Fault Slip Inversion for Geodetic Data

AbstractGeodetic fault slip inversions have generally been performed by employing a least squares method with a spatially uniform smoothing constraint. However, this conventional method has various problems: difficulty in strictly estimating non‐negative solutions, assumption that unknowns follow the Gaussian distributions, unsuitability for expressing spatially non‐uniform slip distributions, and high calculation cost for optimizing many hyper‐parameters. Here, we have developed a trans‐dimensional geodetic slip inversion method using the reversible‐jump Markov chain Monte Carlo (rj‐MCMC) technique to overcome these problems. Because sub‐fault locations were parameterized by the Voronoi partition and were optimized in our approach, we can estimate a slip distribution without the need for spatially uniform smoothing constraints. Moreover, we introduced scaling factors for observational errors. We applied the method to the synthetic data and the actual geodetic observational data associated with the 2011 Tohoku‐oki earthquake and found that the method successfully reproduced the target slip distributions including a spatially non‐uniform slip distribution. The method provided posterior probability distributions with the unknowns, which can express a non‐Gaussian distribution such as large slip with low probability. The estimated scaling factors properly adjusted the initial observational errors and provided a reasonable slip distribution. Additionally, we found that checkerboard resolution tests were useful to consider sensitivity of the observational data for performing the rj‐MCMC method. It is concluded that the developed method is a powerful technique to solve the problems of the conventional inversion method and to flexibly express fault‐slip distributions considering the complicated uncertainties.

Lindu Software: A Free Seismological Data Processing Software For Traveltime Tomography Using Python Framework

Earthquake data can be used to infer some physical properties for representing the subsurface condition. The 3-Dimensional (3D) seismic velocity structure as a kind of these important properties contains the information of variation in lithology change and fluid saturation. The most common method for inverting from the travel time of seismic event into 3D seismic velocity structure is travel time tomography which is based on the relation between velocity and travel time of P- and S-wave. Based on this concept, we develop a module of Lindu software to infer this seismic velocity structure from travel time data. This module is a part of seismological data processing sequences that have been integrated into Lindu software. The Lindu software uses Python framework, a kind of high-level programming languages. The pseudo-bending raytracing method is employed to calculate the travel time between the event sources and stations and also to build the kernel matrix. The resolution test that relates density of rays and resulted tomogram uses the synthetic Checkerboard Resolution Test (CRT) by using Damped-Least Squares (DLS) method for the inversion. For validating this module, it has been tested by using both synthetic and real data.

Seismological Evidence for Lithospheric Low-Velocity Anomalies beneath the Eastern Mediterranean: Impact of Tectonics

Being a part of the eastern Mediterranean, northern Lebanon and northwestern Syria is a seismotectonically active region due to the juxtaposition of several plate boundaries, the existence of many active faults, and the intense seismicity generated along rifting, transform, and convergent plate boundaries. We applied a tomographic inversion on seismic body wave arrivals recorded at 12 seismic stations distributed in the northern part of Lebanon and northwestern Syria to study the P- and S-wave velocity (Vp, and Vs) structures and the seismogenic behavior of the upper portion of the lithosphere. The seismic data comprise 4855 and 2950 P- and S-wave arrivals, respectively, generated by 605 local crustal earthquakes which occurred mainly along the Yammouneh Fault and splays in the Eastern Mediterranean. The crustal structure is highly heterogeneous down to the upper mantle depths. Low-velocity (low-V) anomalies are widely distributed especially along the active faults delimiting the boundary between the Arabian and African plates and areas of thick sedimentary deposits and Cenozoic volcanics. Results of the checkerboard resolution test and hit count coverage indicate that the mapped velocity anomalies are reliable features. Furthermore, they are generally consistent with many geological and geophysical investigations which have been detected beneath the study area by other researchers including the emplacement of ophiolite rocks, Cenozoic volcanics, inefficient Sn propagation, low Lg Q values, and low seismic wave velocities.

The Local Earthquake Tomography of Erzurum (Turkey) Geothermal Area

Erzurum and its surroundings are one of the seismically active and hydrothermal areas in the Eastern part of Turkey. This study is the first approach to characterize the crust by seismic features by using the local earthquake tomography method. The earthquake source location and the three dimensional seismic velocity structures are solved simultaneously by an iterative tomographic algorithm, LOTOS-12. Data from a combined permanent network comprising comprises of 59 seismometers which was installed by Ataturk University-Earthquake Research Center and Earthquake Department of the Disaster and Emergency Management Authority to monitor the seismic activity in the Eastern Anatolia, In this paper, three-dimensional Vp and Vp/Vs characteristics of Erzurum geothermal area were investigated down to 30 km by using 1685 well-located earthquakes with 29.894 arrival times, consisting of 17.298 P- wave and 12.596 S- wave arrivals. We develop new high-resolution depth-cross sections through Erzurum and its surroundings to provide the subsurface geological structure of seismogenic layers and geothermal areas. We applied various size horizontal and vertical checkerboard resolution tests to determine the quality of our inversion process. The basin models are traceable down to 3 km depth, in terms of P-wave velocity models. The higher P-wave velocity areas in surface layers are related to the metamorphic and magmatic compact materials. We report that the low Vp and high Vp/Vs values are observed in Yedisu, Kaynarpinar, Askale, Cimenozu, Kaplica, Ovacik, Yigitler, E part of Icmeler, Koprukoy, Uzunahmet, Budakli, Soylemez, Koprukoy, Gunduzu, Karayazi, Icmesu, E part of Horasan and Kaynak regions indicated geothermal reservoir.

3D crustal velocity and VP/VS structures beneath Southeast Anatolia and their geodynamic implications

We applied a seismic tomography method to arrival time data generated by local crustal earthquakes in Southeast Anatolia to study the shallow, three-dimensional, velocity and V P /V S structures beneath the area. Many of the previous seismological studies of the region are of a regional, or even global, scale. A total of 2150 carefully-selected events generating 13690 and 12560 P - and S - wave arrival times are finally used in the tomographic inversion. Results of the checkerboard resolution test imply that the obtained velocity and V P /V S anomalies are reliable features. In addition, hit count maps indicate that all parts are hit by an adequate number of rays to retrieve the crustal velocity structure. Strong lateral crustal heterogeneities are revealed beneath southeast Anatolia with many lower-than-average velocity anomalies. The low velocity anomalies are imaged especially near the active fault segments. In addition, high V P /V S ratios are mapped at most crustal layers especially at depths of 10 and 22 km which are consistent with the distribution of ophiolite belts. The high V P /V S zones are induced by the possible existence of over-pressurized fluids in the crust and perhaps the uppermost mantle. The existence of these fluids along with the intense tectonic activity could trigger large crustal earthquakes along the western segment of the East Anatolian fault zone. Although may occur in high velocity zones, the majority of the large crustal earthquakes are distributed near zones of average velocity/high V P /V S anomalies. Such mapped velocity and V P /V S zones are in agreement with many previous geophysical investigations beneath southeast Anatolia such as low P n and S n velocities, high S n attenuation, high heat flow, and low L g Q 0 values. Results beneath this region of the Anatolian Plateau are also similar to those observed beneath other continental plateaus such as the Tibet and point to a hot, partially-molten upper mantle.

Afterslip evolution on the crustal ramp of the Main Himalayan Thrust fault following the 2015 Mw 7.8 Gorkha (Nepal) earthquake

Current afterslip models of the Mw 7.8 Gorkha earthquake contradict with the depth ranges of relocated aftershock distribution and inter-seismic locked-creeping transition zone. We use both GPS and interferometric synthetic aperture radar (InSAR) data to image the afterslip evolution in the first 189 post-seismic days, i.e., from 25 April to 1 November 2015. In the study region, 51 GPS stations are divided into three categories based on their observation time: (1) 25 stations with both pre- and post-seismic observations, (2) 16 stations with only post-seismic observations, and (3) 10 stations with only pre-seismic observations. The secular tectonic motion and seasonal variations are corrected carefully and directly using the fitted functions from the pre-seismic position time series. During the inversions, we adopt the widely accepted flat–ramp–flat geometrical model of the Main Frontal Thrust fault (MFT)–Main Himalayan Thrust fault (MHT). The preferred weight ratios between GPS and InSAR observations are elaborately determined through comprehensive analysis of model resolution and fitting residuals. In contrast to previous afterslip models, our preferred afterslip distributes on the crustal ramp of the MHT from depths of 11 to 29 km, which is verified by further inversions using only GPS data and checkerboard resolution tests. The seismic moment released by the afterslip in the first six months is 3.42 × 1019 N m, only approximately 4% of the mainshock moment. The rupture gap between the mainshock and the 12 May aftershock is filled up with post-seismic aseismic slip.

Evolution of the Southern Segment of the Philippine Trench: Constraints From Seismic Tomography

AbstractA high‐resolution model of three‐dimensional P wave tomography of the crust and upper mantle beneath the southern part of the Philippine Trench is determined by inverting 22,960 arrival times of 2,789 local earthquakes and 4,660 traveltime residuals of 751 teleseismic events simultaneously. The tomographic method TOMOG3D is used to obtain the tomographic model that has a lateral resolution of 0.6° or better, as shown by extensive checkerboard resolution tests. A model‐recovery synthetic test is also performed, which indicates that main features of our tomographic images are quite robust. Our results show that the Philippine Sea slab has subducted down to depths of 450–600 km with an overturned dip angle along the southern segment of the Philippine Trench, indicating that the subduction of the Philippine Sea Plate along the southern segment of the Philippine Trench initiated at 20–25 Ma, contemporaneous with the Sangihe Trench. The subduction initiation of the Philippine Sea Plate at the central Philippine Trench resulted from the collision between the Philippines and the Palawan block at ~10 Ma. The subduction completion of the Molucca Sea slab propagated from the south to the north at ~5 Ma, which presumably resulted in the subduction reorganization of the Philippine Sea Plate along the southern segment of the Philippine Trench and subduction initiation of the Philippine Sea Plate along the northern segment of the Philippine Trench.

Shallow/upper crustal shear wave structure of the Tehran region (Central Alborz, Iran) from the inversion of Rayleigh wave dispersion measurements

Surface wave dispersion curves from microearthquakes are used to obtain group velocity dispersion maps. The calculation of the local dispersion curves for each grid point from these maps then produces the input data to retrieve the 3D shear wave velocity model of the Tehran region. The group velocity maps indicate that the tomographic results agree well with the three main tectonic features and the geological units in the study area. The tomographic maps generally possess high-velocity structures across most of the mountain belts (Central Alborz and east-southeast mountains), whereas the Tehran Basin correlates to a low-velocity structure. Increasing the period in the study area highlights four independent low-velocity zones that reflect faults and fault junction systems. The shear wave velocity profiles indicate that the depth to bedrock exhibits southward variation ranging from ~ 300 m to ~ 1500 m. We also focus our analysis on the existence of faults within the shear wave profiles and discuss the low shear wave velocity anomalies deeper than 2 km result from the main fault structures (e.g., North Tehran, North-South Rey and Parchin). Furthermore, we argue that the dip angle of the North Tehran fault varies along fault strike, whereas the North-South Rey fault possesses a constant dip angle. Moreover, initial model uncertainties and checkerboard resolution tests are used to identify reliable and robust anomaly features in the 3D shear wave velocity model and 2D tomographic maps, respectively. Microearthquake analysis provides an effective approach for studying the upper crustal structure heterogeneity, especially the fault structure, of the Tehran region.

Time-lapse seismic tomography of an underground mining zone

In this study, we determined P-wave tomographic images of the Yongshaba deposit, an underground mining zone in Guizhou Province, China, by inverting arrival-time data of micro-seismic events and blasts recorded by a passive seismic array consisting of 28 sensors during January to April 2014 using an event location technique and a travel-time tomography method that can handle complex seismic discontinuities. The damping parameter used in the damped least-squares method is determined with the help of trade-off curve. To reveal internal P-wave velocity changes of the study area under severe mining activities, a time-lapse data partition scheme is used. To assess the human influence on the underground structure, we introduce a parameter to measure the relative velocity changes in different periods. The resolution of the tomographic images and the robustness of the obtained features are examined by conducting a series of checkerboard resolution tests and a restoring resolution test. Our tomographic results obtained from the entire data set reveal a prominent low-velocity (low-V) zone and many obvious high-velocity (high-V) zones. These features match well with the geological setting and the excavation plan carried out in the mine, according to in-site surveys. The low-V zone may reflect empty volumes, stress releases and rock breaking and cracking caused by mining activities, whereas the high-V zones are probably the consequences of local stress concentrations caused by regional stress redistribution. The time-lapse tomographic images may reveal the process of stress concentrations caused by rock breaking due to the continuing excavations in the entire observation period, indicating that the mining activities influenced the rock property and caused complex changes of the underground structure.

Lithospheric structure of Southeast Anatolia from joint inversion of local and teleseismic data

A joint tomographic inversion of local and teleseismic arrival times recorded at 41 seismic stations in southeast Anatolia is conducted to study the 3-D lithospheric velocity structure and its relation to the prevailing tectonic processes. A total of 21300 arrivals from local and teleseismic events are used in the final inversion. The tomographic model reveals prominent lower crustal/uppermost mantle low-velocity anomalies. High-velocity zones are imaged in the western part of the study area. The background seismic activity occurs mainly at the low-velocity areas and to a lesser extent in some high-velocity zones. Large crustal earthquakes occur in average velocity zones, but not in high-velocity areas that can resist stress. Results of the checkerboard resolution test indicate the reliability of the obtained images; while the large hit counts at most depth slices denote reasonable ray-path coverage for most parts of the study area. The obtained velocity anomalies are generally consistent with many previous geophysical measurements and give much deeper understanding of the current seismotectonic processes occurring in the region.

Seismic Travel-time Tomography beneath Merapi Volcano and its Surroundings: A Preliminary Result from DOMERAPI Project

Many geophysical methods at various scales have been applied to understand the internal structure beneath Merapi volcano and its magmatic process. As part of the DOMERAPI project, a seismic experiment was conducted from October 2013 to mid April 2015 in order to determine the deep magma source beneath the volcano through seismic travel-time tomography. The earthquake events were identified and picked manually and carefully to determine the hypocenters. They were then relocated to get precise hypocenter locations before running the seismic tomographic imaging. The data from the BMKG network from the same period of time as mentioned above were also incorporated to minimize azimuthal gap, because the majority of events occurred outside the DOMERAPI network. The checkerboard resolution test result depicts that the area around the network can be well resolved. Compared to previous studies, our result shows a higher resolution at shallow depths, i.e., less than 35 km and a low velocity material imaged to ascend diagonally from the deeper area.

On the use of sensitivity tests in seismic tomography

Sensitivity analysis with synthetic models is widely used in seismic tomography as a means for assessing the spatial resolution of solutions produced by, in most cases, linear or iterative nonlinear inversion schemes. The most common type of synthetic reconstruction test is the so-called checkerboard resolution test in which the synthetic model comprises an alternating pattern of higher and lower wave speed (or some other seismic property such as attenuation) in 2-D or 3-D. Although originally introduced for application to large inverse problems for which formal resolution and covariance could not be computed, these tests have achieved popularity, even when resolution and covariance can be computed, by virtue of being simple to implement and providing rapid and intuitive insight into the reliability of the recovered model. However, checkerboard tests have a number of potential drawbacks, including (1) only providing indirect evidence of quantitative measures of reliability such as resolution and uncertainty, (2) giving a potentially misleading impression of the range of scale-lengths that can be resolved, and (3) not giving a true picture of the structural distortion or smearing that can be caused by the data coverage. The widespread use of synthetic reconstruction tests in seismic tomography is likely to continue for some time yet, so it is important to implement best practice where possible. The goal of this paper is to develop the underlying theory and carry out a series of numerical experiments in order to establish best practice and identify some common pitfalls. Based on our findings, we recommend (1) the use of a discrete spike test involving a sparse distribution of spikes, rather than the use of the conventional tightly spaced checkerboard; (2) using data coverage (e.g. ray-path geometry) inherited from the model constrained by the observations (i.e. the same forward operator or matrix), rather than the data coverage obtained by solving the forward problem through the synthetic model; (3) carrying out multiple tests using structures of different scale length; (4) taking special care with regard to what can be inferred when using synthetic structures that closely mimic what has been recovered in the observation-based model; (5) investigating the range of structural wavelengths that can be recovered using realistic levels of imposed data noise; and (6) where feasible, assessing the influence of model parametrization error, which arises from making a choice as to how structure is to be represented. © The Authors 2016. Published by Oxford University Press on behalf of The Royal Astronomical Society.