Evidence for Anthropogenic Surface Loading as Trigger Mechanism of the 2008 Wenchuan Earthquake

Two and a half years prior to China's M7.9 Wenchuan earthquake of May 2008, at least 300 million metric tons of water accumulated with additional seasonal water level changes in the Minjiang River Valley at the eastern margin of the Longmen Shan. This article shows that static surface loading in the Zipingpu water reservoir induced Coulomb failure stresses on the nearby Beichuan thrust fault system at <17km depth. Triggering stresses exceeded levels of daily lunar and solar tides and perturbed a fault area measuring 416+/-96km^2. These stress perturbations, in turn, likely advanced the clock of the mainshock and directed the initial rupture propagation upward towards the reservoir on the "Coulomb-like" Beichuan fault with rate-and-state dependent frictional behavior. Static triggering perturbations produced up to 60 years (0.6%) of equivalent tectonic loading, and show strong correlations to the coseismic slip. Moreover, correlations between clock advancement and coseismic slip, observed during the mainshock beneath the reservoir, are strongest for a longer seismic cycle (10kyr) of M>7 earthquakes. Finally, the daily event rate of the micro-seismicity (M>0.5) correlates well with the static stress perturbations, indicating destabilization.Comment: 22 pages, 4 figures, 3 table

Applying the Coulomb Failure Function with an optimally oriented plane to the 2008 Mw 7.9 Wenchuan earthquake triggering

The Coulomb failure function (CFF) quantitatively describes static stress changes in secondary faults near the source fault of an earthquake. CFF can be employed to monitor how static stress transfers and then shed some light on the probability of successive events occurring around a source fault. In this paper we focus on the CFF and particularly on optimally oriented planes. We present a unified model to determine an optimally oriented plane and its corresponding Coulomb stress, then apply the model to the 2003 Mw 6.6 Bam (Iran) earthquake and the 2008 Mw 7.9 Wenchuan (China) earthquake, thereby checking its effectiveness. Our results show that spatial correlation between positive Coulomb stress changes and aftershocks are, for the 2003 Bam earthquake, 47.06% when elastic Coulomb stress changes are resolved on uniform planes and 87.53% when these are resolved on optimally oriented planes at depth; for the 2008 Wenchuan earthquake the correlations are 45.68% and 58.20%, respectively. It is recommended that account be taken of optimally oriented planes when drawing a Coulomb stress map for analyzing earthquake triggering effects

Magnitude estimation for early warning applications using the initial part of P waves: A case study on the 2008 Wenchuan sequence

A period parameter τ_c and an amplitude parameter Pd determined from the very beginning of P wave are important for earthquake early warning (EEW), yet their dependence on source mechanism, focal depth and epicentral distance has not been fully studied. After the devastating Mw7.9 Wenchuan earthquake, hundreds of M4-6 earthquakes occurred with diverse focal mechanisms and depth range of 2–20 km. We calculate τ_c and Pd of these aftershocks and examine their dependence on magnitude, τ_c, distance, and depth. We find that τ_c correlates well with magnitude, but joint regression including distance and depth does not significantly improve the correlation. The effect of focal mechanism on the τ_c-magnitude correlation is not obvious. When P wave is nodal, τ_c measurement becomes inaccurate. Also, τ_c is systematically greater for slow earthquakes, leading to a possible false alarm. Thus, more studies are required to discriminate slow earthquakes for robust early warning

Longer aftershocks duration in extensional tectonic settings

Aftershocks number decay through time, depending on several parameters peculiar to each seismogenic regions, including mainshock magnitude, crustal rheology, and stress changes along the fault. However, the exact role of these parameters in controlling the duration of the aftershock sequence is still unknown. Here, using two methodologies, we show that the tectonic setting primarily controls the duration of aftershocks. On average and for a given mainshock magnitude (1) aftershock sequences are longer and (2) the number of earthquakes is greater in extensional tectonic settings than in contractional ones. We interpret this difference as related to the different type of energy dissipated during earthquakes. In detail, (1) a joint effect of gravitational forces and pure elastic stress release governs extensional earthquakes, whereas (2) pure elastic stress release controls contractional earthquakes. Accordingly, normal faults operate in favour of gravity, preserving inertia for a longer period and seismicity lasts until gravitational equilibrium is reached. Vice versa, thrusts act against gravity, exhaust their inertia faster and the elastic energy dissipation is buffered by the gravitational force. Hence, for seismic sequences of comparable magnitude and rheological parameters, aftershocks last longer in extensional settings because gravity favours the collapse of the hangingwall volumes

Earthquake Risk and Earthquake Catastrophe Insurance for the People's Republic of China

The year 2008 witnessed the renewed interests in earthquake risk management and insurance in the People's Republic of China (PRC), after the Wenchuan earthquake hit the country in May. Located along the southeastern edge of the Euro-Asian Plate, the PRC has a relatively high seismicity, which is manifested by the frequent occurrence of large and disastrous earthquakes. Buildings and infrastructure in the earthquake-prone regions of the PRC have relatively low earthquake resistance levels. Hence, disastrous earthquakes result not only in large numbers of injuries and fatalities but also in huge economic losses from property damages. While the PRC began testing earthquake insurance programs in the late 1980s, the overall penetration rate remains very low. The low penetration rate not only creates disruptions for the government after a major earthquake but also, in some cases, delays the reconstruction efforts. Moreover, as a result of the low penetration of earthquake insurance in the PRC, the government serves as the predominant bearer of financial risk from earthquake catastrophes. This paper discusses historical earthquakes and earthquake risk in the PRC and the recent developments of PRC's earthquake risk reduction efforts. The general principles of earthquake programs are explained and the critical issues of formulating earthquake programs in the PRC are discussed, including lessons from earthquake insurance in other countries and other catastrophe insurance in the PRC, data issues, loss risk modeling issues, financial risk modeling issues, legislative issues, and public awareness issues. The paper concludes with several policy directions that the Asian Development Bank can take to help the PRC in its design and implementation of earthquake insurance.earthquake insurance; China; earthquake risk; Wenchuan earthquake; catastrophe insurance

Comparative analysis of the seismic hazard of Central China

Seismic hazard assessment is globally recognised as a tool in identifying levels of earthquake ground shaking within an area. However, methodologies for seismic hazard calculation are wide ranging and produce variations in results and maps. As a case study seismic hazard and results from Gumbel’s method of extremes are determined for the area of greatest intraplate seismicity in China covering the provinces of Gansu, Sichuan and Yunnan. This area is termed the North-South Seismic Zone. Devastating earthquakes in this zone include the 8.4 MS 1920 Haiyuan earthquake causing over 220,000 deaths and the 1996 Lijiang earthquake. Most recently the 2008 Wenchuan earthquake caused over 69,000 deaths with more than 18,000 people missing. These results and seismic hazard maps are compared with the publicly available maps of GSHAP and the national seismic hazard map of China at the level of 10% probability of exceedance in a 50 year period. The distributions of high and low hazard areas are similar and adjacent to the major thrust and strike-slip faults dominating in this area. However, results from the Gumbel method of extremes suggest that the hazard levels within certain areas are slightly different compared to the other two models. This is primarily because the Gumbel methodology is based on determining hazard from earthquakes that have already taken place whereas the other two models determine maximum hazard levels in areas which may exhibit no previous strong hazard. Additionally the Chinese national hazard map does not indicate levels of ground shaking intensity greater than IX in detail, whereas such zones are identified using the extreme value method. This work should be used to strengthen the seismic hazard analysis of this area of China

Kajian Bencana Tanah Longsor dan Mitigasinya di Desa Bojongkoneng, Kecamatan Kandangserang, Kabupaten Pekalongan

Bencana tanah longsor terjadi pada hari Minggu, 2 Februari 2014 di Desa Bojongkoneng, Kecamatan Kandangserang, Kabupaten Pekalongan. Akibat dari gerakan tanah tersebut, sebanyak 28 rumah mengalami rusak berat. Oleh sebab itu, diperlukan metode identifikasi jenis gerakan tanah sebagai antisipasi kejadian gerakan tanah berikutnya. Metode yang digunakan dalam penelitian ini berupa analisis data secara kualitatif yaitu penilaian faktor penyebab gerakan tanah, kemudian analisis kuantitatif berupa pembobotan dan observasi lapangan untuk validasi data. Berdasarkan pengamatan lapangan di dapatkan data bahwa lokasi penelitian memiliki morfologi agak curam. Litologi penyusun dari lokasi penelitian berupa litologi Batulempung sisipan batupasir. Tipe gerakan tanah pada lokasi penelitian merupakan tipe aliran tanah. Penyebab terjadinya gerakan tanah akibat dari relief yang terjal, litologi batulempung yang menjadikanya bidang gelincir, aliran hidrologi yang tidak baik dan curah hujan yang sangat tinggi. Selain itu, adanya pembebanan di atas lereng dimana tata guna lahannya berupa jalan desa yang dilalui kendaraan setiap saat. Metode teknis penanggulangan longsor yang ada di lokasi penelitian dapat berupa Soil Cut Slopes, Retaining Walls dan Pengendalian Saluran Drainase. Kata Kunci: Mitigasi, Gerakan Tanah, Aliran Tanah, Dinding Penaha

Human-triggered earthquakes and their impacts on human security

A comprehensive understanding of earthquake risks in urbanized regions requires an accurate assessment of both urban vulnerabilities and earthquake hazards. Socioeconomic risks associated with human-triggered earthquakes are often misconstrued and receive little scientific, legal, and public attention. However, more than 200 damaging earthquakes, associated with industrialization and urbanization, were documented since the 20th century. This type of geohazard has impacts on human security on a regional and national level. For example, the 1989 Newcastle earthquake caused 13 deaths and US$3.5 billion damage (in 1989). The monetary loss was equivalent to 3.4 percent of Australia&#x2019;s national income (GDI) or 80 percent of Australia&#x2019;s GDI per capita growth of the same year. This article provides an overview of global statistics of human-triggered earthquakes. It describes how geomechanical pollution due to large-scale geoengineering activities can advance the clock of earthquakes or trigger new seismic events. Lastly, defense-oriented strategies and tactics are described, including risk mitigation measures such as urban planning adaptations and seismic hazard mapping