Coseismic Ground Displacement Research Articles

Structural Controls on Fault Slip Models of the 6 February 2023 Kahramanmaraş, Türkiye Earthquake Doublet With Finite Element Analyses

AbstractTwo major earthquakes of Mw7.8 and Mw7.5 ruptured the Southern East Anatolian Fault (SEAF) and the Savrun‐Çardak‐Sürgü fault (SCSF), devastating southeast Türkiye and northwest Syria on 6 February 2023. We adopt innovative nonlinear and linear approaches to analyze the coseismic ground displacements and estimate the complex slip geometry. Unlike conventional analytical solutions that simplify crust heterogeneity, finite‐element fault models invert the displacement data and simulate the dual‐fault geometry with non‐uniformly distributed shallow crustal materials. Our results suggest the west‐dipping SEAF and north‐dipping SCSF accommodate earthquake slips of >10 m. Their respective slip distributions and proximal aftershocks correlate spatially with local seismic velocity anomalies (i.e., ΔVp and ΔVs), which implies differences in structural control along these two faults and provides insights into assessing the seismic hazard of mixed incipient‐mature fault systems.

TraceNet: An Effective Deep-Learning-Based Method for Baseline Correction of Near-Field Acceleration Records

AbstractIn strong ground-motion observations, accelerograms are an important material in both seismic research and earthquake engineering. However, the ubiquitous baseline drift in near-field acceleration records has a large impact on the integrated velocity and double-integrated displacement with linear and parabolic drift, respectively. Conventionally, high-pass filtering and two-stage baseline fitting methods are commonly applied in baseline corrections to obtain reliable strong-motion records. However, these filtering methods exclude low-frequency components from acceleration records and cause unexpected waveform loss. The baseline fitting method, which is based on the experiential selection of intersection moments, is easily affected by external factors and requires a large amount of time for operations. Currently, as the number of accelerometers grows, conventional methods are insufficient in both efficiency and precision to process vast acceleration records. Here, we propose TraceNet, a deep-learning-based method, to correct baseline drifts in velocity records integrated from accelerograms. The training data set is developed with the fusion of artificial baselines and nondrift velocities from corrected accelerations and displacements from events. TraceNet extracts the baseline from the input velocity trace. After TraceNet prediction, the drift can be corrected by subtracting the extracted baseline. In addition, the potential coseismic ground displacement can be recovered from the integration in the corrected velocity. In this study, we used acceleration records and continuous Global Positioning System observations from the 2008 Wenchuan earthquake to demonstrate the ground offset recovery. As a deep learning application, TraceNet can extract and correct the baseline drifts automatically without subjective factors. The coseismic displacements estimated from accelerograms can provide additional insight into the ground deformation.

A new approach for 2-D and 3-D precise measurements of ground deformation from optimized registration and correlation of optical images and ICA-based filtering of image geometry artifacts

High resolution satellite images with improved spatial and temporal resolution provide unprecedented opportunities to monitor Earth Surface changes in 2D and 3D due, for example, to earthquakes, sand dune migration, ice flow, or landslides. The volume of imagery available for such measurements is rapidly growing but the exploitation of these data is challenging due to the various sources of geometric distortions of the satellite imagery. Here we propose a new approach to extract high-quality surface displacement in 3D based on the correlation of multi-date and multi-platform high resolution optical imagery. We additionally show that when a large enough volume of data is available, it is possible to separate the deformation signal from the artifacts due to the satellite jitter and misalignment of the CCDs, which, together with topographic artifacts, are the main source of noise in the measurements. Our method makes use of a reference DEM, but the outcome is independent of the characteristics of the chosen DEM. We use the case-example of the ground deformation caused by the Ridgecrest earthquake sequence to assess the performance of our proposed approach. We show that it outperforms the more standard approach which combines 2-D correlation and DEM differencing. With our technique, we were able to generate high quality measurements of coseismic ground displacement with GSD of 2.4 m, and uncertainties at the 90% confidence level on the NS, EW and vertical displacement measurements of 0.6 m, 0.7 m, and 0.6 m respectively.

Coseismic Ground Displacement after the Mw6.2 Earthquake in NW Croatia Determined from Sentinel-1 and GNSS CORS Data

At the very end of the year 2020, on 29 December, a hazardous earthquake of Mw = 6.2 hit the area of Petrinja and its surroundings, in the NW of Croatia. The earthquake was felt across the area of 400 km, leaving an inconceivable damage in the vicinity of the epicenter, devastated towns and ruined lives. In order to map the spreading of earthquake waves and to determine the coseismic ground displacement after the mainshock, we have analyzed open satellite radar images of Sentinel-1 and the GNSS data from the nearest CORS station related to the epicenter, along with the seismic faults. In this paper, we addressed and mapped the displacement linear surface ruptures detected by the SAR interferometry. The results show the vertical ground displacement to the extent of −12 cm in the southern area and up to 22 cm in the north-western part of a wide area struck by the earthquake impact, related to the epicenter. Subsidence and uplift in a range of ±5 cm over a wider affected area indicate a spatial extent and hazardous impact made by the earthquake. The ground displacement of 30 cm to the West and 40 cm to the East has been identified considering the intersection of Pokupsko and Petrinja strike-slip fault system in the seismic zone of Pannonian basin. Accordingly, we obtained matching results of 5 cm south-easting shift and −3 cm subsidence on Sisak GNSS CROPOS station, addressing the tectonic blocks movement along the activated complex fault system. The results compared with the geology data confirm the existence of two main faults; the Pokupsko and the Petrinja strike-slip faults and interpret the occurrence of secondary post-seismic events over the observed area.

Association of Ionospheric Signatures to Various Tectonic Parameters During Moderate to Large Magnitude Earthquakes: Case Study

AbstractThe sudden ground movement associated with Mw > 6.5 earthquakes is considered a potential source of ionospheric electron density perturbations over the fault region. Coseismic ground displacement is a function of various seismic source parameters such as moment magnitude, focal depth, and focal mechanism etc. We study here the distinct effects of vertical ground displacement, moment magnitude and focal depth on coseismic ionospheric perturbation (CIP) amplitudes during moderate‐to‐large earthquakes. We analyze GPS‐total electron content variations during 59 dip‐slip earthquakes that occurred in the last 20 years. Our study reveals that though CIP amplitudes are primarily controlled by moment magnitude, they are also sensitive to the earthquake focal depth. To understand the influence of focal depth on the displacement field and therefore on CIP amplitudes, we present a simple synthetic test, for a depth range of 0–200 km, highlighting that the maximum vertical ground displacement decreases logarithmically with increasing focal depth while the volume (i.e., integrated vertical ground displacement) of uplifted/subsided material varies very marginally. We conclude that CIP is sensitive to the wavelength of co‐seismic vertical displacement field and that seismic energy propagation to the overlying atmosphere during deep earthquakes is not adequate to generate detectable CIP.

Sentinel optical and SAR data highlights multi-segment faulting during the 2018 Palu-Sulawesi earthquake (Mw 7.5)

The main active tectonic structure in the western part of Central Sulawesi (Indonesia) is the left-lateral Palu-Koro strike-slip fault. Its offshore section was thought not to have broken during the Mw 7.5 Palu Earthquake on 28 September 2018, challenging the established knowledge of the tectonic setting at this location. Here, we use Sentinel-1 SAR interferometry to produce a map of the ground velocities in the area of the Mw 7.5 earthquake for the seven months following the 2018 earthquake. We show evidence of surface deformation along the western coast of the Palu bay, indicating that the Palu Koro offshore fault section might have contribute to or been affected by the earthquake. As the possibility of multi-segment ruptures is a high concern in the area because of the high seismic and tsunami hazard, we present here, a fault model that includes the offshore section of the Palu-Koro fault. Thanks to four independents space-based geodetics measurements of the co-seismic displacement (Sentinel-1 and Sentinel-2 correlograms) we constrain the 3D co-seismic ground displacements. The modeling of these displacements allows us to estimate the co-seismic fault slip amplitude and geometry at depth. At the end, we consider the multi-segment faulting scenario, including the offshore section of the Palu-Koro fault, as a plausible model to explain the submarine landslides and the tsunamis. This study also gives the opportunity to observe a super-shear earthquake in the context of a complex fault network and implies an increase in the probability of submarine landslides within the bay in the forthcoming years.

Coseismic Ground Deformation Reproduced through Numerical Modeling: A Parameter Sensitivity Analysis

Coseismic ground displacements detected through remote sensing surveys are often used to invert the coseismic slip distribution on geologically reliable fault planes. We analyze a well-known case study (2009 L’Aquila earthquake) to investigate how three-dimensional (3D) slip configuration affects coseismic ground surface deformation. Different coseismic slip surface configurations reconstructed using aftershocks distribution and coseismic cracks, were tested using 3D boundary element method numerical models. The models include two with slip patches that reach the surface and three models of blind normal-slip surfaces with different configurations of slip along shallowly-dipping secondary faults. We test the sensitivity of surface deformation to variations in stress drop and rock stiffness. We compare numerical models’ results with line of sight (LOS) surface deformation detected from differential SAR (Synthetic Aperture Radar) interferometry (DInSAR). The variations in fault configuration, rock stiffness and stress drop associated with the earthquake considerably impact the pattern of surface subsidence. In particular, the models with a coseismic slip patch that does not reach the surface have a better match to the line of sight coseismic surface deformation, as well as better match to the aftershock pattern, than models with rupture that reaches the surface. The coseismic slip along shallowly dipping secondary faults seems to provide a minor contribution toward surface deformation.

The Relationship between InSAR Coseismic Deformation and Earthquake-Induced Landslides Associated with the 2017 Mw 3.9 Ischia (Italy) Earthquake

We investigated the contribution of earthquake-induced surface movements to the ground displacements detected through Interferometric Synthetic Aperture Radar (InSAR) data, after the Mw 3.9 Ischia earthquake on 21 August 2017. A permanent displacement approach, based on the limit equilibrium method, allowed estimation of the spatial extent of the earthquake-induced landslides and the associated probability of failure. The proposed procedure identified critical areas potentially affected by slope movements partially overlapping the coseismic ground displacement retrieved by InSAR data. Therefore, the observed ground displacement field is the combination of both fault slip and surficial sliding caused by the seismic shaking. These findings highlight the need to perform preliminary calculations to account for the non-tectonic contributions to ground displacements before any estimation of the earthquake source geometry and kinematics. Such information is fundamental to avoid both the incorrect definition of the source geometry and the possible overestimation of the coseismic slip over the causative fault. Moreover, knowledge of the areas potentially affected by slope movements could contribute to better management of a seismic emergency, especially in areas exposed to high seismic and hydrogeological risks.

Estimating coseismic ground displacement during the 2015 Gorkha, Nepal, earthquake from accelerometric data at KATNP station

This paper presents an investigation into the residual ground displacement during the 2015 Gorkha, Nepal earthquake. The available accelerometric data recorded from a near field recording station (KATNP) are processed using two different techniques. The techniques include a method based on pulse identification after low pass filtering and modifications to an empirical regression method to remove some of the subjectivity. The results are compared to both high-rate (5Hz) global positing system (GPS) coseismic displacement data and interferometric synthetic aperture radar (InSAR) satellite imaging. Since the GPS sensors are not collocated with the acceleration sensors, the InSAR data is used to determine displacement trends to facilitate comparison. It is shown that both methods investigated in this paper provide similar coseismic displacement waveforms matching those of the high-rate GPS. In addition, the vertical and east displacements derived from accelerometric data match well to the quasi up-down and quasi east-west displacement trends from InSAR.

Fractal dimension and b value of the aftershock sequence of the 2008 M S 8.0 Wenchuan earthquake

This study analyzed the seismicity of aftershock events recorded around the earthquake fault area following the 2008 Ms 8.0 Wenchuan earthquake (China). We calculated the fractal dimension (D) and b value of the Gutenberg–Richter law for the aftershocks that occurred within one year of the original earthquake. The results showed that the D values of the earthquake fault ruptures reflected the complexities of the fault structures, and that higher D values were obtained for areas with larger fault slip and larger coseismic ground displacement. The b values for the Wenchuan aftershock sequence were relatively small, which indicated that the larger stress changes occurred in the area of the earthquake faults. Generally, b values are associated with the rupture mechanism of earthquake faults. Here, lower b values were correlated with those regions where thrust ruptures occurred and higher b values were found for those areas with strike-slip ruptures. More importantly, medium- and large-sized aftershocks appeared to occur primarily within areas with small D and b values. A positive correlation between b and D was established from the aftershock sequence, which is described by the relation D = 2b.

Source model of the 2015 Mw 6.4 Pishan earthquake constrained by interferometric synthetic aperture radar and GPS: Insight into blind rupture in the western Kunlun Shan

AbstractThe Pishan, Xinjiang, earthquake on 3 July 2015 is the one of largest events (Mw 6–7) that has occurred along the western Kunlun Shan, northwestern edge of the Tibetan Plateau in recent time. It involved blind thrusting at a shallow depth beneath the range front, providing a rare chance to gain insights into the interaction between the Tarim Basin and the Tibetan Plateau. Here we present coseismic ground displacements acquired by high‐resolution ALOS‐2 SAR imagery and derived from GPS resurveys on several near‐field geodetic markers after the event. We observed a maximum displacement exceeding 10 cm in the epicentral region. Analysis of the data based on a finite fault model indicates that coseismic slip occurred on a subsurface plane of 22 km × 8 km in size with a dip of about 27° to the north and a strike of 114°, representing partial break of one ramp fault buried in Paleozoic strata at 8–16 km depths beneath the foothill of the western Kunlun Shan. This blind rupture is characterized largely by a compact thrusting patch with a peak slip of 0.63 m, resulting in a stress drop of 2.3 MPa. The source model yields a geodetic moment of 5.05 × 1018 N · m, corresponding to Mw 6.4. The Pishan earthquake suggests a northward migration of deformation front of the Tibetan Plateau onto the Tarim Basin. Our finding highlights slip along ramp‐décollement faults to build up the western Kunlun Shan as the Tarim slab is subducting beneath western Tibet.

Precise Positioning of BDS, BDS/GPS: Implications for Tsunami Early Warning in South China Sea

Global Positioning System (GPS) has been proved to be a powerful tool for measuring co-seismic ground displacements with an application to seismic source inversion. Whereas most of the tsunamis are triggered by large earthquakes, GPS can contribute to the tsunami early warning system (TEWS) by helping to obtain tsunami source parameters in near real-time. Toward the end of 2012, the second phase of the BeiDou Navigation Satellite System (BDS) constellation was accomplished, and BDS has been providing regional positioning service since then. Numerical results indicate that precision of BDS nowadays is equivalent to that of the GPS. Compared with a single Global Satellite Navigation System (GNSS), combined BDS/GPS real-time processing can improve accuracy and especially reliability of retrieved co-seismic displacements. In the present study, we investigate the potential of BDS to serve for the early warning system of tsunamis in the South China Sea region. To facilitate early warnings of tsunamis and forecasting capabilities in this region, we propose to distribute an array of BDS-stations along the Luzon Island (Philippines). By simulating an earthquake with Mw = 8 at the Manila trench as an example, we demonstrate that such an array will be able to detect earthquake parameters in real time with a high degree of accuracy and, hence, contribute to the fast and reliable tsunami early warning system in this region.

Synthetic Hybrid Broadband Seismograms Based on InSAR Coseismic Displacements

Abstract Conventional acceleration records do not properly account for the observed coseismic ground displacements, thus leading to an inaccurate definition of the seismic demand needed for the design of flexible (long period) structures. Large coseismic displacements observed during the 27 February 2010 Maule earthquake suggest that this effect should be included in the design of flexible structures by modifying the design ground motions and spectra considered. Consequently, Green’s functions are used herein to compute synthetic low‐frequency seismograms that are consistent with the coseismic displacement field obtained from interferometry using synthetic aperture radar (SAR) images. In this case, the coseismic displacement field was determined by interfering twenty SAR images of the Advanced Land Observation Satellite (ALOS)/PALSAR satellite taken between 12 October 2007 and 28 May 2010. These images cover the region affected by the 2010 M w 8.8 Maule earthquake. Synthetic broadband seismograms are built by superimposing the low‐pass filtered synthetic low‐frequency seismograms with high‐frequency strong‐motion data. The broadband seismograms generated are then consistent with the coseismic displacement field and the high‐frequency content of the earthquake. A sensitivity analysis is performed using three different fault and slip parameters, the rupture velocity, the corner frequency, and the slip rise time. Results show that the optimal corner frequency of the low‐pass filter f c =1/ T c , leads to a trade‐off between acceleration and displacement accuracy. Furthermore, spectral response for long periods, say T ≥8 s, is relatively insensitive to the value of T c , whereas shorter periods are strongly dependent on both the slip rise time and T c . In general, larger displacements consistent with coseismic data are obtained using this technique instead of digitally processing the acceleration ground‐motion records.

Analysis of large, seismically induced, gravitational deformations imaged by high-resolution COSMO-SkyMed synthetic aperture radar

Using differential synthetic aperture radar interferometry (DInSAR) on COSMO-SkyMed (Constellation of Small Satellites for the Mediterranean Basin Observation) data, we obtain an accurate and spatially continuous measure of the coseismic ground displacement due to the Mw = 6.3 L9Aquila (central Italy) earthquake. We identify two local deformation patterns associated with long-term gravitational mass movements, and interpret the deformation mechanisms by integrating geological analysis and simple analytical modeling. These subtle deformations, the role of which in landscape evolution and earthquake-induced hazard needs to be fully assessed, may only be identified using high-resolution DInSAR.

Guest editorial: L’Aquila earthquake: seismic sequence of 6th April 2009, Abruzzo, Italy

On April 6th 2009, a Mw 6.3 earthquake struck the town of L’Aquila and surroundings in Central Italy, causing 308 deaths. The mainshock was preceded by a swarm which started at the end of 2008; the largest earthquake of this swarm had magnitude 4.1 and occurred on March 30th 2009. The mainshock was followed by thousands of aftershocks, some of which with magnitudes greater than Mw 5.0 (Chiarabba et al. 2009). Since the first hours after the mainshock, the national and international scientific community approached the L’Aquila earthquake from various points of view and using different methodologies, in order to achieve a complete understanding of the phenomenon as soon as possible. After more than one year from the earthquake, it is possible to outline some first research milestones reached through the work carried out by the scientific community since that 6th April and some future possible advances, resulting from more in depth studies exploiting data collected after the event. From a seismotectonic point of view, in addition to the classical geological, geomorphological and paleoseismological investigations (e.g., EMERGEO Working Group 2009), which have reached a very high level of accuracy, the L’Aquila earthquake has been especially characterized by a massive and mature use of technology. Fully developed seismic and strong motion national networks have provided huge, highquality datasets available in quasi-real time, allowing for a rapid response of the seismologists’ community, also suitable for operational and decisional purposes. Moreover, a great contribution has come from the satellite technology. Apart from the GPS-based estimates of coseismic ground displacement, which are well-established (Anzidei et al. 2009), in this case also interferometric investigations have reached a very high level of accuracy for the Italian

Two-dimensional Co-Seismic Surface Displacements Field of the Chi-Chi Earthquake Inferred from SAR Image Matching.

The Mw=7.6 Chi-Chi earthquake in Taiwan occurred in 1999 over the Chelungpu fault and caused a great surface rupture and severe damage. Differential Synthetic Aperture Radar Interferometry (DInSAR) has been applied previously to study the co-seismic ground displacements. There have however been significant limitations in the studies. First, only one-dimensional displacements along the Line-of-Sight (LOS) direction have been measured. The large horizontal displacements along the Chelungpu fault are largely missing from the measurements as the fault is nearly perpendicular to the LOS direction. Second, due to severe signal decorrelation on the hangling wall of the fault, the displacements in that area are un-measurable by differential InSAR method. We estimate the co-seismic displacements in both the azimuth and range directions with the method of SAR amplitude image matching. GPS observations at the 10 GPS stations are used to correct for the orbital ramp in the amplitude matching and to create the two-dimensional (2D) co-seismic surface displacements field using the descending ERS-2 SAR image pair. The results show that the co-seismic displacements range from about -2.0 m to 0.7 m in the azimuth direction (with the positive direction pointing to the flight direction), with the footwall side of the fault moving mainly southwards and the hanging wall side northwards. The displacements in the LOS direction range from about -0.5 m to 1.0 m, with the largest displacement occuring in the northeastern part of the hanging wall (the positive direction points to the satellite from ground). Comparing the results from amplitude matching with those from DInSAR, we can see that while only a very small fraction of the LOS displacement has been recovered by the DInSAR mehtod, the azimuth displacements cannot be well detected with the DInSAR measurements as they are almost perpendicular to the LOS. Therefore, the amplitude matching method is obviously more advantageous than the DInSAR in studying the Chi-Chi earthquake. Another advantage of the method is that the displacement in the hanging wall of the fault that is un-measurable with DInSAR due to severe signal decorrelation can almost completely retrieved in this research. This makes the whole co-seismic displacements field clearly visible and the location of the rupture identifiable. Using displacements measured at 15 independent GPS stations for validation, we found that the RMS values of the differences between the two types of results were 6.9 cm and 5.7 cm respectively in the azimuth and the range directions.

The relationship between seismic deformation and deep-seated gravitational movements during the 1997 Umbria–Marche (Central Italy) earthquakes

This paper re-evaluates the origin of some peculiar patterns of ground deformation in the Central Apennines, observed by space geodetic techniques during the two earthquakes of the Colfiorito seismic sequence on September 26th, 1997. The surface displacement field due to the fault dislocation, as modelled with the classic Okada elastic formulations, shows some areas with high residuals which cannot be attributed to non-simulated model complexities. The residuals were investigated using geomorphological analysis, recognising the geologic evidence of deep-seated gravitational slope deformations (DSGSD) of the block-slide type. The shape and direction of the co-seismic ground displacement observed in these areas are correlated with the expected pattern of movement produced by the reactivation of the identified DSGSD. At least a few centimetres of negative “Line of Sight” ground displacement was determined for the Costa Picchio, Mt. Pennino, and Mt. Prefoglio areas. A considerable horizontal component of movement in the Costa Picchio DSGSD is evident from a qualitative analysis of ascending and descending interferograms. The timing of the geodetic data indicates that the ground movement occurred during the seismic shaking, and that it did not progress appreciably during the following months. This work has verified the seismic triggering of DSGSD previously hypothesized by many researchers. A further implication is that in the assessment of DSGSD hazard seismic input needs to be considered as an important cause of accelerated deformation.

Seismogenic structure in a tectonic suture zone: With new constraints from 2006 Mw6.1 Taitung earthquake

A Mw 6.1 earthquake occurred on April 1st, 2006 near Taitung, eastern Taiwan. It produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month. This event provides an opportunity to diagnose the seismogenic structure in the southern Longitudinal Valley (LV) of eastern Taiwan, long viewed as one of the collision sutures between the Philippine Sea and the Eurasian plates. With precisely relocated main‐ and aftershock hypocenters, focal mechanisms for M ≥ 3.8 events, and coseismic ground displacements from strong motion records, we determine a main shock dislocation model. Our results indicate that the main shock occurred on a high angle fault (azimuth 198°, dip 77°). The model comprises a fault with two segments; the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left‐lateral strike‐slip motion, in agreement with P‐wave first motions and waveform (USGS) solutions. The southern segment exhibits a slightly larger thrust component, in agreement with CMT solutions. Tectonically, this event highlights a NNE‐trending fault on the west side of the LV, which is predominantly strike‐slip. The aftershocks clustered to the east of the main shock, which exhibit mainly thrust mechanisms, indicate that shortening is still acting on the sedimentary materials deposited between the Coastal and Central ranges prior to collision. As a result, the southern LV is undergoing slip partitioning along different faults, which has never been specified before.

Coseismic versus interseismic ground deformations, fault rupture inversion and segmentation revealed by 2003 Mw 6.8 Chengkung earthquake in eastern Taiwan

The 2003 Chengkung earthquake (Mw 6.8) provided diagnostic evidence for a source model showing the deformation process of the seismogenic Chihshang fault in eastern Taiwan. The aftershocks show a fault‐bend at a depth of 18 km. Coseismic ground displacements recorded by strong‐motion records allow us to deduce instant rupturing of this event. Our resulting model shows a fault length of ∼33 km and dip‐slip dominant rupture on fault‐plane deeper than 18 km. Estimated coseismic displacements constrain two fault planes: one at 5–18 km depth dipping 60°E and 18–36 km depth dipping 45°E. The uppermost fault‐plane of the Chihshang Fault (0–5 km) did not break immediately after the main shock; however, it may have a major role in after‐slip and even interseismic ground deformation. The Taiyuan basin developed in the hanging wall is a geomorphic feature consistent with and adequately explained by coseismic ground displacements.

Measuring ground deformations with 1-Hz GPS data: the 2003 Tokachi-oki earthquake (preliminary report)

We analyzed 1-Hz GPS data observed at 14 stations of the GPS Earth Observation Network (GEONET) of the Geographical Survey Institute, Japan, associated with the 2003 M JMA 8.1 Tokachi-oki earthquake, which occurred at the Kurile Trench. The GPS stations are located 70–240 km away from the epicenter. GPS data clearly captured rapid co-seismic ground displacements. At a GPS station 70 km away from the epicenter, coseismic displacements started 15 seconds after the origin time, and after 40 seconds at the stations 240 km away. Observed displacement amplitude exceeded 20 cm at GPS sites 240 km away from the epicenter. Displacement amplitudes attenuate with time and distance from the epicenter, oscillating with periods of 40–60 seconds. We compared the 1-Hz GPS data and displacement seismogram integrated from strong ground motion data, which showed fairly good agreements. In spite of careful screening of 1-Hz GPS data during 30 minutes preceding the main shock, no significant preseismic deformation over 1 cm in the horizontal components was recorded. 30 second sampling GPS data at 14 sites during 20 hours preceding the main shock did not show any significant pre-seismic deformation, either. These results indicate that pre-seismic strain change, if any, was smaller than 0.5–1.0×10−7 before the 2003 Tokachi-oki earthquake.