Geological Survey National Earthquake Research Articles

North Korean nuclear test: Seismic discrimination low yield

North Korea carried out a widely reported nuclear explosion on 9 October 2006 at 0135 UTC at a location about 40 kilometers northwest of the city of Kilju (Figure 1). The location of the test determined from seismic signals recorded at 31 stations around the world was given as 41.294°N, 129.094°E in Quick Determination of Epicenters (QED) by the U.S. Geological Survey's National Earthquake Information Center (NEIC), very close to Mantap‐san (Mount Mantap, 2205 meters, see Figure lc). Further, since the late 1990s, surveillance satellites have detected tunneling activity in this area suspected to be indicative of North Korea preparing to conduct nuclear tests at this site [Broad et al, 2005].

Chinese National Network Magnitudes, Their Relation to NEIC Magnitudes, and Recommendations for New IASPEI Magnitude Standards

We investigate the linear regression relationships between common seismic magnitudes determined by the Chinese Earthquake Network Center (CENC) and compare them with related magnitude determinations for the same events at the U.S. Geological Survey's National Earthquake Information Center (NEIC). Despite their generally good agreement some systematic differences are revealed. These dif- ferences are due to differences in seismograph response (shape and bandwidth), the time window for measurement of maximum P-wave amplitudes, the period and dis- tance ranges used, and, in part, also the different calibration functions applied. Chi- nese broadband body-wave magnitude mB, compared with the NEIC short-period P- wave magnitude mb, is much less prone to magnitude saturation. Thus it is more suitable to assess the size of large earthquakes from P waves. Also, following Inter- national Association of Seismology and Physics of the Earth's Interior (IASPEI) recommendations of 1967, Chinese surface-wave magnitude MS is determined in a wider distance (1 D 180) and period range (3 sec T 30 sec) than MS(20) at NEIC (20 D 160 and 18 sec T 22 sec, respectively). Chinese MS for small and medium earthquakes at regional distances between 2 D 10 scales well with local magnitude ML. In contrast, NEIC MS(20) tends to underestimate the magnitude of regional events when the IASPEI-recommended MS calibration function by Vane yk et al. (1962) is used. These findings support some of the new standards for magnitude measurements from digital data adopted at the IASPEI meeting in 2005. They include, complementary to band-limited m b and M S (20), the determination of m B and M S (BB) measured on unfiltered broadband records.

Taiwan shaken by an uncommon earthquake

Exactly 2 years to the date after the great Sumatra earthquake, the island of Taiwan was shaken by an uncommon magnitude 7.1 earthquake and a series of aftershocks centered just off her southwestern coast on the continental shelf. The event is the largest in a century in that particular area, technically controlled by the subduction of the Eurasia plate under the Philippine plate, a rare and peculiar case of a lighter continental plate subducting under a heavier oceanic plate.The main shock, at 1226 UT, 26 December 2006, had a moment magnitude of 7.1 as determined by the U.S. Geological Survey's National Earthquake Information Center (NEIC/USGS), followed by a 7.0 aftershock 8 minutes later, and three more subsequent aftershocks above magnitude 5 within a day. Relatively minor damage was reported on land, while underwater communication cables were severed at as many as seven locations at different times. A nearby nuclear power plant experienced a g‐force as large as 0.16g. A tsunami warning was issued in Vietnam and the Philippines but was later called off. The Broadband Array in Taiwan for Seismology (BATS) put the main shock depth at somewhat over 20 kilometers and deeper for the large aftershocks. The water depth at the epicenter region is no more than 600 meters.

The Mw7 Machaze, Mozambique, Earthquake of 23 February 2006

A major ( Mw 7.0) earthquake occurred on 22 February 2006 at 22:19 UTC (shortly after midnight the following day in local time) in the western province of Manica in Mozambique, Southern Africa. The epicenter was located in Machaze, the southernmost district of Manica province, about 45 km due south of the district capital of Chitobe (figure 1). The earthquake was felt throughout eastern Southern Africa but caused surprisingly little damage and a very small number of casualties. Figure 1. Location map of the epicentral region, showing settlements and observed surface rupture features, the main shock locations, and the aftershocks for a period of one month, as determined by the U.S. Geological Survey's National Earthquake Information Center (NEIC). Mozambique is not generally considered a country with significant seismic hazard, but its population is affected by many other serious threats, both natural and anthropogenic: life expectancy is close to 40 years. Despite improved economic growth following liberalization in 1994, Mozambique remains a poor country with a GDP per capita of just US $1,300. The country has appreciable mineral resources but the principal economic activity is agriculture, with seafood (shrimp and lobster) being the major export. The only major industrial activity is a large aluminum smelting plant, with other activities related mainly to food processing. As much as 80% of the population lives on subsistence farming or fishing. Mozambique is still suffering from the effects of a brutal civil war that began two years after the country achieved independence from Portugal in 1975 and continued until 1992, leaving at least 100,000 dead and one-third of the population displaced or forced into exile. In common with so many African countries, Mozambique is heavily affected by HIV/AIDS, with at least 12% of the population infected; we were informed that in Machaze district the infection rate …

An Assessment of Seismic Body-wave Magnitudes Published by the Prototype International Data Centre

A significant initiative in the reporting of global seismicity began in January 1995 with the publication of daily “Reviewed Event Bulletins” (REB's), a few days in arrears. This work was and still is done in the context of international efforts aimed at verification of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). When it was finalized in 1996 the CTBT specified that the seismic monitoring network of the International Monitoring System (IMS) would consist of 50 primary stations contributing data continuously to an International Data Centre (IDC) in Vienna, plus 120 auxiliary stations that record continuously but contribute data to IDC only on request (Richards, 2002). Until IDC began daily operations on 21 February 2000, the daily REB was published by the Prototype International Data Centre (PIDC) and a predecessor organization. We shall refer to all the REB's from 1 January 1995 to 20 February 2000 as having been published by PIDC. This paper focuses on the PIDC teleseismic body-wave magnitudes ( mb ). They are important as a modern effort to measure short-period magnitudes based on what is expected to become a fixed network of global stations, all operating in a standardized way. Although seismic magnitudes are intended to be a characteristic of the source, it is well known that in practice they are influenced by numerous additional factors such as the particular set of stations reporting individual magnitudes, the instrument responses, and the details of how each station magnitude is assigned. Global bulletins of seismicity, including mb values, have long been reported, months in arrears, by the Preliminary Determination of Epicenters (PDE) and Earthquake Data Report (EDR) of the U.S. Geological Survey's National Earthquake Information Center (USGS NEIC). They have also been reported, about two years in arrears, by the International Seismological Centre (ISC), based in England. To obtain its …

Global teleseismic earthquake relocation with improved travel times and procedures for depth determination

AbstractWe relocate nearly 100,000 events that occurred during the period 1964 to 1995 and are well-constrained teleseismically by arrival-time data reported to the International Seismological Centre (ISC) and to the U.S. Geological Survey's National Earthquake Information Center (NEIC). Hypocenter determination is significantly improved by using, in addition to regional and teleseismic P and S phases, the arrival times of PKiKP, PKPdf, and the teleseismic depth phases pP, pwP, and sP in the relocation procedure. A global probability model developed for later-arriving phases is used to independently identify the depth phases. The relocations are compared to hypocenters reported in the ISC and NEIC catalogs and by other sources. Differences in our epicenters with respect to ISC and NEIC estimates are generally small and regionally systematic due to the combined effects of the observing station network and plate geometry regionally, differences in upper mantle travel times between the reference earth models used, and the use of later-arriving phases. Focal depths are improved substantially over most other independent estimates, demonstrating (for example) how regional structures such as downgoing slabs can severely bias depth estimation when only regional and teleseismic P arrivals are used to determine the hypocenter. The new data base, which is complete to about Mw 5.2 and includes all events for which moment-tensor solutions are available, has immediate application to high-resolution definition of Wadati-Benioff Zones (WBZs) worldwide, regional and global tomographic imaging, and other studies of earth structure.

Nepal quake doesn't fit

An earthquake in the foothills of the Himalaya Mountains in Nepal August 21 killed more than 700 people and caused landslides and damage to buildings over a wide area. Preliminary information on the location of the magnitude (Ms)‐6.6 quake may also send a tremor or two through generally accepted theories about the geometry and mechanics of India's subduction beneath Asia.The earthquake occurred near the geologic boundary between the Indo‐Gangetic basin and the Himalayas, 200 km southeast of Katmandu. The epicenter of the quake, as determined by the U.S. Geological Survey's National Earthquake Information Service in Denver, Colo., was at 26.6°N, 86.6°E, nearly coincident with that of a magnitude‐8.3 earthquake that rocked the region in 1934. A series of great earthquakes [M⩾8] has taken place during the past 2 centuries along the length of the range, near the topographic break that marks the southern limit of Himalayan deformation. Most earthquakes in the region occur under the high Himalaya, 100–200 km farther north, and are typically of small to moderate size.