Qi-Fu Chen

Predicting the 1975 Haicheng Earthquake

The publicized four-stage (long-term, middle-term, short-term, and imminent) prediction of the M 7.3 1975 Haicheng, China, earthquake once generated worldwide fascination. Yet the prediction process has remained mysterious because of lack of reports on real-time documentation and details of how warnings were issued. In the present work, study of declassified Chinese documents and interviews of key witnesses have allowed us to reconstruct this important history. Our findings indicate that there were two official middle-term predictions but no official short- term prediction. On the day of the earthquake, a county government issued a specific evacuation order, and actual actions taken by provincial scientists and government officials also effectively constituted an imminent prediction. These efforts saved thousands of lives, but the local construction style and time of the earthquake also contributed to minimizing fatalities. Evacuation was extremely uneven across the disaster region, and critical decisions were often made at very local levels. The most important precursor was a foreshock sequence, but other anomalies such as geodetic deformation, changes in groundwater level, color, and chemistry, and peculiar animal behavior also played a role.

Quick and Approximate Estimation of Earthquake Loss Based on Macroscopic Index of Exposure and Population Distribution

In the traditional method of earthquake loss estimation, all exposed facilities are classified according to their structural type and/or occupancy. Inventory data is collected and the total loss is estimated as the aggregate of all facility losses from each facility class separately. For many regions of the world, however, the vast amount of data required for this method is difficult or impossible to obtain. The traditional method is also unable to estimate quickly the loss from an unexpected catastrophic earthquake. It is difficult to give the necessary risk information to help the government with rescue and relief after the earthquake disaster.In this paper, we propose a quick and approximate estimation method of earthquake loss based on a macroscopic index of exposure and population distribution from GIS. This method was applied to analyze several earthquake scenarios with World Bank and CIESIN data. The preliminary analysis and comparison results show that our method is effective and reasonable for quick assessment of earthquake risk.

The 2008 Wenchuan Earthquake and Earthquake Prediction in China

Abstract For historical reasons, earthquake prediction in China is government sanctioned and law regulated. During 1966–1976, coinciding with the political and social turmoil of the Cultural Revolution, there was an explosion of earthquake prediction. Enthusiasm climaxed with the prediction of the 1975 Haicheng earthquake, a prediction that was due mainly to the occurrence of an unusually pronounced foreshock sequence and the extraordinary readiness of some local officials of the time to issue imminent warning. Since the catastrophic Tangshan earthquake in 1976, there has been diminishing confidence in prediction and increasing emphasis on seismic risk mitigation. The M w xa07.9 Wenchuan earthquake of 12 May 2008 once again demonstrated the gloomy reality of earthquake prediction. In spite of the installation of massive precursor monitoring networks and an elaborate schedule of prediction conferences, no anomalous pattern was identified before this earthquake that would have enabled its prediction. Instead, the Wenchuan earthquake overwhelmingly demonstrated the vital importance of seismic risk mitigation. Different from Tangshan 1976, where seismic design was not required, such design was required for the Wenchuan area in 2008. Buildings that met the design standard suffered much less damage than those that did not. Notwithstanding, progress in practice lagged far behind the promulgation of regulations; stricter enforcement of seismic design provisions and wiser selection of construction sites would have prevented many deaths and greatly reduced the destruction of property. The prediction program in China has provided useful experience and taught many lessons. The most important lesson is that, regardless of its future potential, it is presently impractical to rely on prediction to prevent earthquake disasters. The practical approach is to strengthen the resilience of our built environment based on an assessment of seismic hazard.

Remotely Triggered Seismicity in Continental China following the 2008 Mw 7.9 Wenchuan Earthquake

We perform a systematic search of remotely triggered seismicity in con- tinental China following the 2008 Mw 7.9 Wenchuan earthquake. We visually identify earthquakes as impulsive seismic energy with clear P and S arrivals on 5 Hz high-pass filtered three-component velocity seismograms 1 hr before and after the Wenchuan earthquake. Out of the 271 stations in the updated Chinese digital seismic network (CSN), 17 stations show statistically significant seismicity increase with β-statistic values larger than 2, following the Wenchuan earthquake. These include 11 stations in the north China block, which is seismically active and is in the rupture propagation direction of the Wenchuan earthquake, three stations along the coastal lines in the relatively stable south China block, one station near the Haiyuan fault zone in north- west China, and two stations near the Tanlu fault zone and the Longgang volcano in northeast China. These observations suggest that dynamic triggering in intraplate regions tends to occur near active faults that have ruptured in historic times and in the rupture propagation directions of the mainshock. However, it is worth noting that many sites that satisfy the criteria are not triggered, suggesting that these conditions would help but are not sufficient enough to guarantee remote triggering in intraplate regions. The tectonic environments near the sites with clear triggered activity range from transpressional to tranextentional, and most regions are not associated with active geothermal or volcanic activity, indicating that dynamic triggering could occur in a wide range of tectonic environments. Online Material: Tables of station information and measured parameters and two supplementary figures showing locally triggered earthquakes.

Spatial clustering and repeating of seismic events observed along the 1976 Tangshan fault, north China

[1]xa0Spatial and temporal features of the seismicity occurring along the Tangshan fault in 2001–2006 were investigated with data recorded by the Beijing metropolitan digital Seismic Network. The relocated seismicity with the double difference method clearly exhibits a dextral bend in the middle of the fault. More than 85% of the earthquakes were found in the two clusters forming the northern segment where relatively small coseismic slips were observed during the 1976 M7.8 earthquake. The b values calculated from the seismicity occurring in the northern and southern segment are 1.03 ± 0.02 and 0.85 ± 0.03, respectively. The distinct seismicity and b values are probably the collective effect of the fault geometry and the regional stress field that has an ENE-WSW oriented compression. Using cross-correlation and fine relocation analyses, we also identified a total of 21 doublets and 25 multiplets that make up >50% of the total seismicity. Most of the sequences are aperiodic with recurrence intervals varying from a few minutes to hundreds of days. Based on a quasi-periodic sequence, we obtained a fault slip rate of ≤2.6 mm/yr at ∼15 km, which is consistent with surface GPS measurements.

Anisotropic Rayleigh wave phase velocity maps of eastern China

We explore the variations of Rayleigh wave phase velocity beneath eastern China in a broad period range (20–200u2009s). Rayleigh wave dispersion curves are measured by the two-station technique for a total of 734 interstation paths using vertical component broadband waveforms at 39 seismic stations in eastern China from 466 global earthquakes. In addition, 599 waveform inversion interstation measurements were added to this data set. The interstation dispersion curves are then inverted for high-resolution isotropic and azimuthally anisotropic phase velocity maps at periods between 20 and 200u2009s. At shorter periods sampling the crustal depth range, phase velocities are higher in the southeastern part of the region, reflecting the thinner crust there. The Jiangnan Belt separates Cathaysia from the Yangtze Craton, the latter with thicker crust and a deep, high-velocity cratonic root. The eastern part of Yangtze Craton, however, east of 115–116°E, does not display a deep root and has a thin lithosphere. Azimuthal anisotropy at long periods (>120u2009s) shows fast propagation directions broadly similar to that of the absolute plate motion. Beneath Cathaysia and eastern Yangtze Craton, anisotropy in the asthenosphere is strong and suggests coast-perpendicular flow. Asthenospheric flow from beneath Chinas thick continental lithosphere toward the thinner lithosphere of the margin and the resulting decompression melting may be the fundamental causes of the intraplate basaltic volcanism along the eastern coast of China.

Topography of the 660‐km discontinuity beneath northeast China: Implications for a retrograde motion of the subducting Pacific slab

[1]xa0Clear S to P converted waves at the 660-km discontinuity (S660P) are observed at stacked seismograms of deep earthquakes occurring in northeast China and Japan Sea recorded by broadband seismic arrays in North America and Europe. Differential travel times between the S660P and P waves are used to constrain depth variations of the 660 beneath northeast China. A rapid change in the depth of the 660 is observed within a narrow longitudinal range of 130.8°E–131.4°E, where the lower boundary of the subducting slab encounters the 660. Towards the west, the 660 deepens steadily as it approaches the coldest core of the slab. The maximum depression occurs at the west end of the studied region with an amplitude of ∼20 km. To the east, the 660 appears to be flat, showing no obvious effect of the subducting slab. These observations are consistent with a scenario for a retrograde moving Pacific slab lying over the upper and lower mantle boundary progressively from west to east.

Global Seismic Hazard Assessment Based on Area Source Model and Seismicity Data

A global seismic hazard assessment was conducted using the probabilistic approach in conjunction with a modified means of evaluating the seismicity parameters. The earthquake occurrence rate function was formulated for area source cells from recent instrumental earthquake catalogs. For the statistical application of the G–R relation of each source cell, the upper- and lower-bound magnitudes were determined from, respectively, historical earthquake data using a Kernel smoothing operator and detection thresholds of recent catalogs. The seismic hazard at a particular site was obtained by integrating the hazard contribution from influencing cells, and the results were combined with the Poisson distribution to obtain the seismic hazard in terms of the intensity at 10% probability of exceedance for the next 50 years. The seismic hazard maps for three countries, constructed using the same method, agree well with the existing maps obtained by different methods. The method is applicable to both oceanic and continental regions, and for any specific duration of time. It can be used for those regions without detailed geological information or where the relation between existing faults and earthquake occurrence is not clear.

Rayleigh-wave dispersion reveals crust-mantle decoupling beneath eastern Tibet

The Tibetan Plateau results from the collision of the Indian and Eurasian Plates during the Cenozoic, which produced at least 2,000 km of convergence. Its tectonics is dominated by an eastward extrusion of crustal material that has been explained by models implying either a mechanical decoupling between the crust and the lithosphere, or lithospheric deformation. Discriminating between these end-member models requires constraints on crustal and lithospheric mantle deformations. Distribution of seismic anisotropy may be inferred from the mapping of azimuthal anisotropy of surface waves. Here, we use data from the CNSN to map Rayleigh-wave azimuthal anisotropy in the crust and lithospheric mantle beneath eastern Tibet. Beneath Tibet, the anisotropic patterns at periods sampling the crust support an eastward flow up to 100°E in longitude, and a southward bend between 100°E and 104°E. At longer periods, sampling the lithospheric mantle, the anisotropic structures are consistent with the absolute plate motion. By contrast, in the Sino-Korean and Yangtze cratons, the direction of fast propagation remains unchanged throughout the period range sampling the crust and lithospheric mantle. These observations suggest that the crust and lithospheric mantle are mechanically decoupled beneath eastern Tibet, and coupled beneath the Sino-Korean and Yangtze cratons.

Increasing background seismicity and dynamic triggering behaviors with nearby mining activities around Fangshan Pluton in Beijing, China

Dynamic triggering in western Fangshan Pluton, Beijing, China, has been repeatedly identified, but previous studies are limited by sparse seismic station coverage. Here we systematically analyze continuous waveforms recorded by both permanent stations and a temporary seismic network 40u2009days before and after the 11 March 2011 Mwu20099.1 Tohoku-Oki and the 14 April 2012 Mwu20098.6 Indian Ocean earthquakes. We first build a template database using a short-term average to long-term average method. Next, we apply the matched filter technique that cross correlates the template waveforms with continuous data to detect additional seismic events. Overall, we detect 1956 and 950 seismic events around the Tohoku-Oki and Indian Ocean main shocks, respectively. Most detected events are shallow (<5u2009km) and clustered at Beiling Syncline in western Fangshan Pluton, which is adjacent to a running coalmine. Seven and 10 events are detected during the large-amplitude surface waves of the two main shocks, respectively, but no similar burst is detected following their major foreshock and aftershocks. Multiple statistical tests indicate that the short-term bursts after the two main shocks are dynamically triggered. We suggest that mining-related activities may perturb the subsurface stress conditions and hence make the region more susceptible for dynamic triggering than other places.