Sismology

Cooperation between France and Nepal in sismologie.

The National Seismological Network of Nepal D.M.G., Nepal – D.A.S.E., France 1979-2005 – 25 years of cooperation

As a continuation of geological research in the Himalayas, the geodynamic study of Nepal was started in October 1978 with the installation of the first telemetric station at Pulchoki in the south of Katmandu, under a collaborative project in seismology between the department of Mines and geology, Kathmandu and Laboratoire de Geophysique.

The first telemetric network was installed in 1980 and by 1985 a fifth station was operated in Central Nepal. 1998, the Udayapur earthquake (M=6.5) triggered the national network program with the creation, in 1994, of the National Seismological Network under French cooperation.

The Network

JPEG The network consists of 17 short period seismic stations in Lesser and Sub-Himalaya and it has a capability to monitor events with a magnitude as low as 2 occurring in Nepal. The addition of four more stations in the Karnali basin in 1998 is the latest landmark. By the end of 1998 the network has recorded 65,000 earthquakes out of which 26,000 are local and regional earthquakes (Fig.1).

This network provides an exceptional view of the microseimic activity over one third of the Himalayan arc, including the only segment, between longitudes 78 ° E and 85 ° E, that has not produced any M>8 earthquake over the last century, and thus stands as a potential location for the next large Himalayan earthquakes (Fig.2).

Seismicity

JPEG A major feature of Nepal seismicity is the intense microseismicity and frequent medium-size occurrence earthquakes (Ml<4) which tend to cluster beneath the topographic front of the Higher Himalaya. This 10-30 km deep seismicity (Fig.3) also correlates with a zone of localised uplift that has been revealed from geodetic data. Both these evidences indicate strain accumulation on the mid-crustal ramp that has been previously inferred from geological and geophysical evidence. This ramp connects a flat decollement under the Lesser and Sub-Himalaya with a deeper decollement under the Higher Himalaya, and act as a geometric asperity were strain and stress build up during interseismic period. The large Himalayan earthquakes could nucleate there and activate the whole flat-and ramp system up to the blind thrusts of the Sub-Himalayan.

What is an earthquake?

JPEG Earthquakes are the most evident demonstration of active deformation associated with the drifting of continental plates. This deformation occurs mainly along major fractures of the earth crust called faults (fig.4).

JPEG Mechanically, an earthquake is the expression of a sudden displacement occurring along a fault (fig.5).

The consequent rupture is accompanied by a relaxation of the stress accumulated during decades to centuries by elastic deformation of the rocks. The center location of rupture of rupture is called hypocenter (or focus); its projection on the earth surface is epicenter. A fraction of thereleased energy is propagated as elastic waves, which can travel considerable distances (up to tens of thousands of kilometers), and be recorded by a seismic station.

Magnitude, Seismic Moment and Seismic Gap

The magnitude scale, introduces in 1935 by C.F.Richter, quantifies the size of an earthquake. It’s calculated from the amplitude of the ground displacement, measured at a certained distance from the epicenter. The scale of magnitude is a logarithm one. Forexample, a magnitude 8 earthquake is one thousand time stronger in wave amplitude than an earthquake of magnitude 5. However, the magnitude is not well adapted to the measurement of big earthquake, because of the saturation of instrumental scale. The concept of seismic moment was defined in 1967 K.Aki, allowing measurement of energy released during the earthquake.The calculation of the moment is done knowing its source parameter: rupture area, displacement, geometry of the fault etc. The biggest earthquake recorded instrumentally was the Chile event (May 1960),where the fault had more than 10 meters displacement, with a rupture length of about 1200 km. The shock was felt for almost 400 sec ... The study of the distribution in time and space of seismic gaps, permit to identify area where a higher probability of next big event could be expected. In the context of Himalaya, a seismic gap is conspicuous in western Nepal (fig.2).

How is an earthquake recorded?

The seismograph is able to detect seismic waves coming from a distant earthquake. A set of seismographs, located at different place, allows to localize the earthquake, providing latitude, longitude, depth, arrival time and magnitude information. A typical station of the National Seismological Network of Nepal (fig.6) consist of :

JPEG A short period vertical component seismograph, recording signals in the 0.2 to 17 Hz band. Low level of local noise, cultural and natural, allows again of 500 000 to 1 000000 in graphical recording. In another words, a ground vibration of 40 angstrom (1m = 10E-10 Å) amplitude is represented as 2 millimeters in graphical record, for 1 Hz frequency and 500 000 gain. The seismograph is contained within a metallic vault, which is well cemented with the ground (fig.7). a transmitting antenna (supplemented by a receiving one if it is a relay station), from wherethe seismic signals are transmitted to be received at the processing center.

A 12V battery power supply charged by solar panels and lighting protection device. All the transmitter, receiver and electronic equipment are kept in a metallic shelter.

Seismograms and seismic waves

JPEG An earthquake generates an elastic waves which will propagate from the source to the seismograph, traveling through the earth interior. Such elastic waves can be either of longitudinal (like sound wave) type, or transversal (like optical wave) type. The velocity of the wave depends upon the composition of the rocks and depth. It varies from 4 to 14 km/sec for P-type and 2 to 8 Km/sec for S-type. Some special types of surface wave can also be detected in seismogram, known as Love and Raleigh waves. They are dispersive waves and have velocity less than S-wave. Thus, in a seismogram of an earthquake, we see first the P-wave arriving, then the transversal S-wave and then the surface wave (fig.8).

Processing of the data

The signal received in the recording center (Fig.9) is digitized with 50 Hz frequency, to generate seismic waveform database. The recorded signals are analyzed, event-by-event, picking up the readable P and S arrival phases in each station.
As an output, the location of the seismic events, the phase arrival time and the amplitude of waves are reported in weekly bulletin. The bulletins are exchanged with NEIC, Colorado, USA and ISC, UK, though which all the international seismological community benefits.

Seismic Hazard

The quicker the epicenter and magnitude of a local big earthquake are computed and communicated to concerned authorities, the sooner the rescue operations can be lauched effectively. It is one of the main task of the National Seismic center of Nepal. The intensity (Modified Mercalli: MM scale, graduated between I and XII), reflects the level of the seismic effects to the natural environment and human settlement. Intensity depends upon magnitude, distance to the epicenter, the hypocenter depth and the local geological factors. An earthquake is characterized by a unique magnitude, but it can give rise to a range of intensity. The highest intensity is generally observed in the epicenter area. It decrease rapidly beyond the rupture area as we move away from the epicenter. For example, the 1934 earthquake (estimated magnitude 8.3) resulted to X MM intensity in Bhojpur. The rupture area was characterized by an intensity of VIII MM. However, because of the prevailing geological conditions, intensity was as high as X MM in katmandu valley (fig.10a, 10b).

JPEG JPEG

Earthquake can also cause liquefaction of loose sandy sediments saturated with water, and massive landslides. Knowledge of the distribution of the earthquake sources and the potential magnitude is necessary to assess seismic hazard of many places. This can be supplemented with the past history of seismic events to project probable time of occurence for earthquake of different magnitudes. The overall scientific knowledge is directed to help, plan, design and construct earthquake safe premises and projects for the community.

Dernière modification : 19/01/2012

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