Seismicity around Lhasa Tsoja Wangmo 1) , Norsang Gelsor 1) and Jens Havskov 2)

1. Seismicity around Lhasa Tsoja Wangmo1), Norsang Gelsor1) and Jens Havskov 2) 1) Jiangsu Road No 36 Lhasa, Tibet, PRC2) University of Bergen, Department of Earth Science, Allégaten 41, N-5007 Bergen, Abstract The Tibetan Plateau is the highest plateau in the world. It is situated at an average altitude of approximately 5000 m above the sea level. Due to the collision of Indian and Eurasian plates, the elevation of Tibet has been slowly increasing with time, which cause series of tectonic activity within the Tibetan plateau. Tibet is very active seismic regions due to collision of India and Eurasian plates. The distribution of seismicity in Tibet is diffuse with shallow earthquakes occurring all over the high plateau and deep earthquakes intensively at the southern boundary of the Plateau, where the subduction zone is located. In the Lhasa area, the seismic network in Tibet do not show much seismicity since that network has to cover all of Tibet. But the Lhasa region has historically had significant earthquake activity and it is also one of the densest populated in Tibet, so it is important to have regular observations of seismicity around this area. The Science Faculty of Tibet University has operated small seismic network around Lhasa since 2004. The seismic network was funded by the Norwegian government through the University of Bergen. It includes three short period stations as well as data center at Tibet University. This work will use all data we have recorded at the three stations from June, 2004 to June, 2005 to present local seismicity in the Lhasa region.During a yare period, 600 earthquakes have been detected in the area around Lhasa. The largest event had magnitude 5.2 and the detection threshold is 1.5 as compared to 4.5 for the ISC catalog. The existing stations in Tibet locate very little seismicity in the Lhasa region. Thus the new small network has given significantly new seismicity information for the area and it is expected that it can give new information about the local tectonics. Stations and Equipment The sensors are three component 4.5 Hz geophones and the digitizers are 16 or 24 bit units from SARA. Timing is done by a GPS integrated in the digitizer. The recorders are standard PC’s using the SEISLOG windows recording system. Triggered data are collected at regular intervals (2 weeks to a month) using a memory stick and all data is processed with SEISAN. Some PC units have experienced problems with altitude and had to be replaced. There is also problems with frequent power failures. Some stations have difficult access and others have had to be moved due to difficulty in getting permission for operation. So for the first year, not all stations have been operating at all times, however operation is more stable now. Stations near Lhasa Stations operated by Tibet University. LHAS, JAMG and NHIM operated from June 2004. PALB operated from August 2004 to April 2005. CHUD and CHUS operated from August 2005. The current stations are CHUD, CHUS, NHIM and JAMG. See figure to the right for stations near Lhasa. Outside of station CHUS. The sensor is in an unused room on the rock at the back Outside of station JAMG Station CHUD. The big picture shows the recording system PC. Bottom left shows the GPS antanna for the seismometer and bottom right picture shows the sensor. Operation time for three stations for the first year of operation. X and Y axies refer to operation time and station, respectively . Examples of Recording The M 9.3 large Indonesia event of December 26, 2004 was also recorded on the network. It was large enough to produce clear 20 s surface waves recorded by the 4.5 Hz geophones. The Ms and mb calculated with the network for this event 3000 km away were 8.3 and 7.3, respectively. This is comparable to the PDE values of 8.8 and 7.0 respectively.  The original raw traces. The numbers above the traces to the right is maximum amplitude in counts. The displacement traces in the frequency band 0.05 to 0.1 Hz. The numbers above the traces to the right is maximum displacement in nm Example of a local event detected by three stations The Tibet earthquake of August 24, 2004 occurred 330 km N-NE of Lhasa. It was well recorded on the 2 stations in operation at the time, see figure to the left. Locating with 2 stations and azimuth gave a location about 100 km West of the PDE location. The magnitudes calculated by Tibet University (TBU) were moment magnitude Mw =5.4 (calculated from S-wave spectrum) and local magnitude Ml = 5.2. The PDE Mw magnitude was given to 5.5. Thus acceptable results have been obtained with this inexpensive network and of particular importance is that the magnitude can be correctly estimated. Seismograms from stations JAMG and LHAS for August 24 event . The time scale is in seconds. Signals have been filtered between 0.1 and 1 Hz. Comparison of PDE and Tibet University (TBU) epicenters for the August 24 event. LSA is Lhasa. Noise spectrum at best station CHUS Seismicity The Tibet University network (TBU) has recorded about 600 events the first year. In comparison, ISC reports about 70 events per year in the same area. The Gutenberg-Richter relation for the local events. The b-value is 0.52 and the detection threshold is about 2.0. The Gutenberg-Richter relation for the ISC events from 1998 to 2003. The b-value is 1.23 and the detection threshold is about 4.5 Magnitude scales used The magnitude scales used are: Mc = 2.6log(coda) + 0.001dist –3.0 Ml = log (amp) – 1.11 log (dist) – 0.00189 dist – 2.09 where Mc is coda magnitude, coda is coda length in seconds, dist is hypocentral distance in km, Ml is local magntude and amp is maximum S-amplitude in nm. ISC epicenters for the period 1998 to 2003. There are a total of events 415. Average is 70 events per year Seismicity around Lhasa located by TBU, using one or more stations, for the period June 2004 to June 2005. Blue triangles refer to stations. Red dote refer to events. Size of the dots refer to different magnitudes. Seismicity around Lhasa located by more than two stations of TBU for the period of 06.2004 to 06.2005. Blue triangles refer stations around Lhasa. Red dots refer to events. Size to the dots refer to different magnitudes. Number of events per month for June 2004 to June 2005. X and Y axes refer to time and number of events, respectively.