Harvard CMT Catalogue Research Articles

Strain rate and stress fields in the West and South Lut block, Iran: Insights from the inversion of focal mechanism and geodetic data

The active tectonic deformation and hazardous earthquakes in the south and west of the Lut block have been investigated for a long time. In this study, we compute the geodetic and seismic strain rates using focal mechanism data from the Harvard CMT catalogue and various other sources including the published GPS velocities. Moreover, we also perform Focal Mechanism Stress Inversion (FMSI) to deduce a stress model for the region. Our study shows an expected correlation between the stress orientations, seismic and geodetic strain rates. Our results show that the south and west of the Lut block is generally exposed as a compressional strike-slip tectonic regime. The tectonic convergence in this area is taken up not only by motions along and across the faults but also by the rotation of those blocks which bounded by these faults. The maximum amount of rotation rate is observed where there are the main right lateral strike slip fault systems such as Sabzevaran, Gowk, Nayband, Bam, Kuhbanan, and Kahurak. The orientation of the mean stress direction, obtained from the FMSI results in the west and south of the Lut block, is approximated ~N19 E. In this area, faults are almost oblique relative to the tectonic motion direction. Moreover, there are right-lateral and left-lateral shears, in addition to the dip movements in different parts of the south and west of the Lut block. Our analyses show three main categories of the stress regimes including strike-slip faulting (43.2%), thrust faulting (38.6%), and unknown or oblique faulting (18.2%).We also calculated seismic and geodetic moment rates for this area, which indicate the seismic moment rate is relatively high between Bam and Shahdad where there are some segments of the Gowk fault.

Seismic status in Bangladesh

Seismic status in Bangladesh

Contemporary tectonic stress field in China

The contemporary tectonic stress field in China is obtained on the basis of Chinese stress field database and Harvard CMT catalogue. Result of the inverted tectonic stresses shows that the maximum principal stress axis strikes nearly north-south direction in the west part of Tibet plateau, ENE direction in North China. In Central China, its strikes show a radiated pattern, i.e., NNE in north part and NNW in south part. The detailed stress field parameters of nearly whole China are given and can be used in geodynamic stress field simulation and earthquake prediction.

Scaled Energies of ML > 5.1 After shocks of the 1999 Chi-Chi, Taiwan, Earthquake Measured from Local Seismograms

In this study, we measure the seismic radiation energy, E(subscript s), and seismic moment, M(subscript o), of twenty-two larger-sized after shocks with 5.1≤M(subscript L)≤6.5 of the 1999 M(subscript s) 7.6 Chi-Chi, Tai wan, earth quake from high-quality digital strong-motion data recorded at stations with epicentral distances of less than 50 km through a method pro posed by Andrews (1986). We also eliminate the effects on the measures of E(subscript s) and M(subscript o) due to site amplification and finite frequency band width limitation. Comparison of the values of M(subscript o) obtained in this study and those listed in the Harvard CMT catalogue shows that Andrews' method to mea sure M(subscript o) from lo cal seismograms is acceptable. The measured values are E(subscript s)=2.0×10^18-8.9×10^21 g cm^2 sec^(-2) and M(subscript o)=1.3×10^23-1.4×10^26 g cm sec^(-2) cm^(-1), which give the scaled energy to be E(subscript s)/M(subscript o)=7.4×10^(-6)-2.6×10^(-4). The scaled energies of the 22 events are de pendent upon earth quake magnitude, M(subscript s), when both E(subscript s) and M(subscript o) are evaluated from lo cal seismograms; yet, in de pendent of M(subscript s) when M(subscript o) is estimated from teleseismic data. Scaled energy slightly depends on the depth, h (in km), through the following form: E(subscript s)/M(subscript o)=1.92×10^(-5)e(superscript 0.09h). In addition, the corner frequency, f(subscript c), is also measured. Its value ranges from 0.15 to 1.34. The scaling law between M(subscript o) and f(subscript c) is: M(subscript o) ~ f(subscript c)^(-3.65).

Spatial variation in seismotectonics and stress conditions across the Kurile and Japan trenches inferred from the analysis of focal mechanism data in Hokkaido, northern Japan

Abstract We analyzed seismotectonic characteristics of focal mechanism data across the Kurile and Japan trenches in the Hokkaido region, north Japan, to check for spatial variation in seismotectonics. For this purpose we selected highly constrained focal mechanism data from the Harvard CMT catalog and Japanese National Research Institute for Earth Science and Disaster Prevention (NIED) for the period 1976 to July 2007. Further, we selected data with high double couple components within a depth range of ±7.5 km from the inferred boundary of the subducting Pacific slab. We found a systematic variation in seismotectonic affinity of the data exhibiting eight distinct zones in the region. Among them, the outer rise region exhibits a normal fault regime with biaxial extension along and across the trenches. In the regions hosting the Pacific slab at shallow depths, most of the focal mechanism data lie in the thrust regime and the odd-event regime. This is due to a complicated stress condition with a major component of seismic deformation being accommodated by extensional and strike-slip movements in a thrust regime. Dominantly uniaxial compression acting downdip of the subducted Pacific slab is found in the regions hosting the rupture zones of large subduction earthquakes such that during an interseismic period tectonic stress is loaded by subduction. In the proximity of the Hidaka collision zone the stress conditions suggest that the impact of collision on ongoing seismotectonics is inevitable. In such regions we found thrust regimes with biaxial compression along the collision and subduction directions that favor bulging and/or contortion of the subducted slab.

Low detection capability of global earthquakes after the occurrence of large earthquakes: investigation of the Harvard CMT catalogue

SUMMARY We investigated the detection capability of global earthquakes immediately after the occurrence of a large earthquake. We stacked global earthquake sequences after occurrences of large earthquakes obtained from the Harvard centroid-moment tensor catalogue, and applied a statistical model that represents an observed magnitude–frequency distribution of earthquakes to the stacked sequence. The temporal variation in model parameters, which corresponds to the detection capability of earthquakes, was estimated using a Bayesian approach. We found that the detection capability of global earthquakes is lower than normal for several hours after the occurrence of large earthquakes; for instance, the duration of lowered detection capability of global earthquakes after the occurrence of an earthquake with a magnitude of seven or larger is estimated to be approximately 12 hr.

Measurement of the duration of high-frequency energy radiation and its application to determination of the magnitudes of large shallow earthquakes

Abstract We have developed an algorithm to measure the duration of high-frequency energy radiation by processing the first arriving P-waves and then applied it to analyses of 69 large shallow earthquakes that occurred between 1995 and 2006. We compared our estimates to time differences between centroid and origin times in the Harvard CMT catalog and found that there was a good correlation between them, thereby suggesting that the measured durations are good estimates of source durations. We show that it is possible to determine magnitudes consistent with moment magnitudes in the Harvard CMT catalog using the durations of high-frequency energy radiation and maximum displacement amplitudes of the first arriving P-waves.

Analysis of Shallow and Deep Earthquake Doublets in the Fiji-Tonga-Kermadec Region

For the Fiji-Tonga-Kermadec area and for the period from January 1977 to July 2003, the Harvard CMT catalogue lists 1022 shallow, 410 intermediate and 633 deep earthquakes of moment magnitude from 4.9 to 8.0. The magnitude threshold, above which the catalogue is complete, is 5.3–5.4, and the number of earthquakes of magnitude above this value is 691 for shallow, 329 for intermediate and 476 for deep events, respectively. The proportion of earthquakes associated with doublets and multiplets against the total number of earthquakes is approximately the same in both data sets and therefore all earthquake pairs were considered regardless of their magnitude. We investigated all the pairs of earthquakes that occurred at a centroid distance of less than 40, 60 or 90 km from each other and within a time interval of 200, 300 or 450 days, depending on their magnitude. We found 208 pairs of shallow, 31 of intermediate and 92 of deep events. To ascertain whether these earthquakes in pairs are not connected by chance, the possibility of their occurrence in an uncorrelated Poissonian catalogue was considered. It was assumed that in such a catalogue the inter-event time is exponentially distributed, the earthquake magnitude follows the Gutenberg-Richter relation, and the distribution of centroid distances between the events in pairs is controlled by its non-parametric kernel estimate. The probability of the appearance of the observed proportion of doublets of shallow earthquakes in the Poissonian catalogue was found to be very low. The low probability of occurrence in a semi-random catalogue, created by randomising centroid locations in the actual data set, also indicates major importance of the distance criterion used for a doublet specification. In general, shallow earthquakes tend to form pairs at shorter distances and within shorter time intervals than deep earthquakes. Both the distance and the time intervals do not depend on the magnitude of involved events. The largest number of pairs of deep earthquakes is observed at a depth of about 600 km, and the proportion of deep events associated with doublets against the number of all events increases with depth. From comparison of the focal mechanism of earthquakes in pairs, measured by the 3-D rotation angle, it follows that deep earthquakes forming pairs have a more diverse focal mechanism than shallow events; the rotation angle for three quarters of shallow pairs and only for about one third of deep pairs is reasonably small. The azimuth between two events forming a doublet is in about 60–65% of cases close to the strike of one of nodal planes of the first or the second event.

Seismicity before and after the two great Sumatra earthquakes of 2004 and 2005

The Harvard CMT catalogue contains 481 shallow earthquakes that occurred between 1 January 1977 and 30 November 2005 within a broad region defined by the geographical latitude from 3°S to 14°N and by the longitude from 91°E to 102°E. There are 230 events that occurred before the great earthquake of 26 December 2004. Their surface distribution is not uniform and the source area of the 2004 great event appears as an area of seismic quiescence with a radius of about 100 km. There are 186 events that occurred between the two great earthquakes of 26 December 2004 and 28 March 2005. Practically all of them are located to the northwest from the great earthquake of 2005, that in turn was followed by 63 events, mostly located to the southeast.

Test of the Preshock Accelerating Moment Release (AMR) in the Case of the 26 December 2004 Mw 9.0 Indonesia Earthquake

A case study of the 26 December 2004 M w 9.0 earthquake off the west coast of northern Sumatra, Indonesia, was conducted to explore whether there was a preshock accelerating moment release (amr) process for the intermediate timescale. The Harvard cmt catalog was used to calculate the cumulative moment tensor directly, with clearer physical significance with regard to the deformation prior to the great earthquake. We observed that average moment tensors at different times over the last decade before the great earthquake are consistent, and are similar to the focal mechanism of the great earthquake. However, the widely used cumulative scalar seismic moment and cumulative Benioff strain are only an approximation of the preshock deformation. To test the robustness of the accelerating property with respect to the selection of spatiotemporal parameters, we calculated the scaling coefficient m for different spatiotemporal ranges. The curvature parameter q was used to quantify the difference between the power-law fit and the linear fit to ensure the statistical significance of the power-law-like accelerating behavior. Grid searching over the ( t f , m ) space was conducted to explore the global stability of the solution. The result showed that there existed a reliable preshock amr process before this great earthquake, with duration of a quarter of a century and a spatial range from 800 to 1500 km, providing seemingly positive evidence for the amr model. However, the failure time t f was not well constrained by the amr analysis, and the amr model may be problematic for a longer timescale.

The December 26, 2004, off the west coast of northern Sumatra, Indonesia, M W=9.0, earthquake and the critical-point-like model of earthquake preparation

Long-term seismic activity prior to the December 26, 2004, off the west coast of northern Sumatra, Indonesia, M sub-W=9.0 earthquake was investigated using the Harvard CMT catalogue. It is observed that before this great earthquake, there exists an accelerating moment release (AMR) process with the temporal scale of a quarter century and the spatial scale of 1500 km. Within this spatial range, the M sub-W=9.0 event falls into the piece-wise power-law-like frequency-magnitude distribution. Therefore, in the perspective of the critical-point-like model of earthquake preparation, the failure to forecast/predict the approaching and/or the size of this earthquake is not due to the physically intrinsic unpredictability of earthquakes. Language: en

Focal mechanism dependence of static stress triggering of earthquakes

We perform a global statistical investigation into the problem of Coulomb stress triggering of earthquakes by using the Harvard CMT catalogue of shallow earthquakes from 1976 to 2001. We consider ‘earthquake pairs’, i.e., pairs of successive earthquakes occurring near each other with similar focal mechanisms, and address the problem of whether or not the change of the Coulomb failure stress (CFS) produced by the first earthquake of the pair ‘encourages’ the occurrence of the second one. An interesting feature is that such a Coulomb stress-triggering phenomenon has an apparent focal mechanism dependence: for thrust earthquakes, a more evident triggering effect can be observed.

Surface wave tomography for southeastern Asia using IRIS-FARM and JISNET data

Previous seismic studies for southeastern Asia around Indonesia have used global seismic data and thus included few regional broadband seismic stations. To better understand seismic structure of this area, we temporarily deployed Japan–Indonesia Seismic NETwork (JISNET) which was composed of 23 broadband seismic stations in central to western Indonesia since 1998 under the Super Plume Project of Japan. In this study we analyse regional JISNET data and global Incorporated Research Institute of Seismology (IRIS)-Fast Archive Recovery Method (FARM) data, in order to obtain detailed images of 3D S-wave velocity (Vs) of this area. We use vertical component LP seismograms of FARM data recorded from 1989 to 1993 (13471 seismograms) and vertical component of broadband JISNET data recorded from 1998 to 2000 (1782 seismograms). We select seismic events with body wave magnitudes greater than 5.5, and take hypocenters and centroid moment tensor solutions from the Harvard CMT catalogue. Phase velocity anomalies of fundamental mode Rayleigh waves (period from 34 to 197 s) relative to Preliminary Reference Earth Model (PREM) are determined by measuring phase differences between the synthetic waveforms and the observations. Global distribution of Rayleigh wave phase velocities are determined by block inversions using damped least squares with a finer block size of 5°×5° for the Indonesia region than the surrounding area. Firstly, we compare the results obtained from the inversion using the FARM data alone (called “FARM inversion”) and those from the inversion using both the FARM and the JISNET data (called “JISNET + FARM inversion”). For the fundamental Rayleigh waves of short periods, phase velocities obtained from the JISNET + FARM inversion show greater degrees of negative anomaly throughout Indonesia than those from the FARM inversion. The clearest difference is seen for the region around Kalimantan and Java: slower phase velocities of less than 1% relative to PREM at periods of about 70 s are obtained by the FARM inversion, whereas including JISNET data requires lower phase velocity anomalies of 2.5%. Secondly, we compare the resolution kernels of both data set. Around Java, Sumatra, Kalimantan, and Sulawesi the resolution of the JISNET + FARM inversion is substantially better than that of the FARM data alone, with the peaks of resolution kernels up to five times as high as those for the FARM inversion. The best resolution improvement is obtained for west Kalimantan to Java Sea. Thirdly, we invert the phase velocity perturbations for the modes to obtain the upper mantle 3D S-wave velocity (Vs) structure of the study area. The JISNET + FARM inversion tends to give larger (up to four times) negative Vs anomalies when compared with the FARM inversion. The Vs heterogeneities concentrate in the uppermost mantle shallower than 200 km throughout Indonesia. Beneath Sumatra, Java, and Kalimantan, the uppermost 100 km of the mantle is slower than PREM by about 2–3%. A prominent high Vs anomaly (2–4%) around the depths from 100 to 200 km extends from northern Australia, through the Banda Sea, to east Sulawesi and Halmahera. The subducted Indian plate is visible as a nearly 1% high Vs anomaly.

Stress field along the Ryukyu Arc and the Okinawa Trough inferred from moment tensors of shallow earthquakes

We investigated the stress field along the Ryukyu Arc and the Okinawa Trough using regional (NIED MT) and global (Harvard CMT) moment tensor catalogs. Using the combined dataset of 4 years of data from the NIED MT catalog and 24 years of data from the Harvard CMT catalog, we found an arc-parallel extensional stress province along the forearc of the Ryukyu Arc and observed a detailed extensional stress field in the Okinawa Trough. This arc-parallel extension is observed in the entire region of the Ryukyu Arc except at its northeastern end. This stress field is clearly separated from the backarc extensional stress field in the Okinawa Trough by the volcanic chain. Since it exists in a normal subduction region as well as in a region of weakly coupled subduction, the effects of oblique subduction might not be enough to explain the formation of the arc-parallel extension. Thus, we consider that the backarc opening process should play an important role in this arc-parallel extension. Along the Okinawa Trough, an arc-perpendicular extensional stress field is observed in the southeastern and central Ryukyu Arc, while in the northeastern Okinawa Trough, the direction of the extensional axis is oblique to both the arc normal direction and the direction of plate motion. Two-dimensional mechanical interaction models which consider the interactions between the subducting plate, forearc and backarc are inadequate because an arc-parallel extensional stress field is observed in the entire Ryukyu Arc, and the extension axis in the northeastern Okinawa Trough is oblique to the arc trend. The oblique backarc stress may originate beneath the backarc or on the continental side of the backarc. One of the simplest ways to explain this observation might be active rifting in the northeastern Okinawa Trough.

Surface Rupture of the Kunlunshan Earthquake (Ms 8.1), Northern Tibetan Plateau, China

On 14 November 2001, an extraordinarily large earthquake occurred on the northern Tibetan Plateau. The event, known as the Kunlunshan earthquake, was the largest to strike the mainland of China over the past 50 years. The moment magnitude of the earthquake was 7.7 to 7.9 (USGS National Earthquake Information Center, 2001; Harvard CMT Catalog, 2001), and its surface wave magnitude was 8.1 (Chinese Digital Seismograph Network, 2001). Its epicenter was instrumentally located as close to Buka Daban Feng, a snowcapped summit with an altitude of 6,800 m (USGS National Earthquake Information Center, 2001; Chinese Digital Seismograph Network, 2001), or south of Hoh Sai Hu Lake (Harvard CMT Catalog, 2001) (Figure 1). Field investigation indicates that the earthquake surface rupture occurred on the two western segments of the Kunlun Fault with a total length of 350 to 426 km, but mainly on the Hoh Sai Hu segment, with a length of 350 km and a maximum left-lateral displacement of 7.6 m. The variation in length is due to uncertainty about whether breaks of the western section southwest of Buka Daban Feng are tectonic or not. The surface rupture includes shear fractures, transtensional fractures, tension gashes, and mole tracks. Together with horizontal slicksides on the steep shear planes, this surface pattern reveals a purely strike-slip rupture characterized by several meters of left-lateral offset. Reverse components are also noted, exhibiting northern side-up motion, but they are much smaller than the horizontal slip component at the same localities. The macroscopic epicenter of the Kunlunshan earthquake is inferred to be in the southern piedmont area to the northeast of Hoh Sai Hu Lake, about 80 to 90 km west of Kunlunshan Pass. The earthquake surface rupture zone is the most impressive example of recent faulting on the northern Tibetan Plateau. We speculate that a large …

Distribution of apparent stress in western China

Distribution of apparent stress in Chinese mainland and its surrounding regions was presented using the NEIC broadband radiated energy catalogue and the Harvard CMT catalogue from January 1987 to December 1998. Due to the limitation on the number of samples, reliable results are only for the western China. It is observed that the average apparent stress in Chinese mainland is 0.8 MPa; The maximum apparent stress to the east of the Tibetan plateau is 2.6 MPa; And the average apparent stress in the north-south seismic belt is more than one time higher than its adjacent regions. Distribution pattern of apparent stress seems to have a large-scale correlation with the cumulative energy release level in the 20th century.

Entropy, energy, and proximity to criticality in global earthquake populations

We examine the question of proximity of the global earthquake population to the critical point characterised by the energy E and entropy S based on annual frequency data from the Harvard CMT catalogue. The results are compared with a theoretical model corresponding to a Boltzmann probability density distribution of the form p(E) ∝ E−B−1e−E/θ. The data are consistent with the model predictions for fluctuations in the characteristic energy θ at constant B value, of the form S∼B〈lnE〉. This approximation is valid for large θ, relative to the maximum possible event size, confirming that the Earth is perpetually in a near‐critical state, reminiscent of self‐organized criticality. However, the results also show fluctuations of ±10% in entropy that may be more consistent with the notion of intermittent criticality. A more precise definition of the two paradigms, and a similar analysis of numerical models, are both needed to distinguish between these competing models.

The 14 November 2001, Mw = 7.8 Kokoxili Earthquake in Northern Tibet (Qinghai Province, China)

On 14 November 2001, at 5:26 pm local time (9:26:10.3 GMT), a great earthquake ( Ms = 8.1) struck the Kokoxili region of the northwest part of Qinghai Province, China (Figure 1). Although this earthquake was felt in Sichuan Province, more than 1,000 km away from the epicentral area, it did not attract much media attention because it shook a thinly populated area on the northern margin of the Tibetan Plateau. The preliminary epicenter (U.S. Geological Survey-National Earthquake Information Center [NEIC]) is located near 90.5°E, about 300 km west of the Golmud-Lhasa road (Figures 2 and 3), across which surface rupture has been reported near Kunlun Pass. As mentioned in several Chinese newspapers, the famous milestone statue at Kunlun Pass broke and fell because of the earthquake, and the construction of the Golmud-Lhasa railroad has been put on hold since the occurrence of this unexpected event. The earthquake appears to have ruptured the left-lateral Kunlun Fault (Tapponnier and Molnar, 1977) or Kusai-Dongxi-Maqu Fault (Cui and Jiang, 1979; Jia et al. , 1988), which extends roughly east-west for about 1,600 km in north Tibet. This event follows, nearly four years later, the 8 November 1997, Mw = 7.6 Manyi earthquake, which broke another segment of the same active fault system farther west (Figure 2). The centroid (Harvard CMT Catalog), at 35.5°N 92.7°E, lies about 200 km east of the USGS (NEIC) and China Seismological Bureau (CSB) preliminary epicentral locations and about 35 km south of the fault, consistent with a south-dipping fault plane (Figure 3). Eighteen of the twenty-two aftershocks with magnitude Mb ≥ 3.8 that occurred during the first week are located another 100 km farther east, between 93°E and 94.7°E. The easternmost ones are located east of Kunlun Pass. Two focal mechanisms, one from the …

Scaling of apparent stress from broadband radiated energy catalogue and seismic moment catalogue and its focal mechanism dependence

Comparison between the NEIC broadband radiated energy catalogue and the Harvard CMT catalogue provides information about the properties of apparent stress. Based on the numerical studies of Shaw (1998) we examined the ratio of broadband radiated energy and scalar seismic moment of shallow earthquakes worldwide from 1987 to 1998. It is observed that for different focal mechanisms, scaling of apparent stress or ‘scaled energy’ with seismic moment is different. For thrust and normal events, apparent stress decreases with seismic moment; while for strike-slip events, apparent stress increases with seismic moment. On average, for thrust events, the apparent stress of aftershocks is higher than that of main shocks; for strike-slip events, the apparent stress of aftershocks is lower than that of main shocks; while the situation of normal events is in between.

Regional GPS data confirm high strain accumulation prior to the 2000 June 4Mw= 7.8 earthquake at southeast Sumatra

Summary Site velocities derived from repeated measurements in a regional GPS network in Southeast Asia help to constrain the motion of tectonic blocks as well as slip rates along major faults in the area. Using 3-D forward dislocation modelling, the influence of seismic elastic loading and unloading on the measured site motions are approximated. Results suggest that the northwestern Sunda arc is fully coupled seismogenically, whereas its eastern part along Java shows localized deformation. Higher horizontal velocity gradients than expected from the modelling of a fully coupled plate interface west of Manila in the Philippines suggest that deformation may be localized there. Assuming that geodetically derived convergence represents long-term rates, accumulated geodetic moments are compared to those derived using seismic data from 1977 to 2000 (Harvard CMT catalogue). If areas displaying localized deformation are dominated by creep processes, the largest difference between accumulated and seismically released deformation is located where the 2000 June 4 Mw = 7.8 Sumatra earthquake occurred.