Over 12 years of continuous monitoring of Changbaishan volcano in the border region of China and North Korea by means of volcanic seismicity, ground deformation, and volcanic gas geochemistry yields new evidence for magmatic unrest of the volcano between 2002 and 2006. In this so‐called “active period,” the frequency of volcanic earthquakes increased by about 2 orders of magnitude compared to that of the background “inactive periods.” The active period was also accompanied by ground inflation, high values of CO2, He, H2, and high ratios of N2/O2 and 3He/4He in volcanic gases released from three hot springs near the caldera rim. The monitoring evidence implies pressurization of the magma chamber, possibly caused by incremental magma recharge. The ground deformation data from both GPS and precise leveling are modeled to suggest the corresponding deformation source is at 2–60 km depth beneath the volcano's summit, where earthquake swarms were detected in 2002 and 2003. Our findings suggest that the magma chamber beneath Changbaishan volcano has awakened and resumed activity after remaining dormant since AD 1903. There is an urgent need to keep close watch on this active and very hazardous volcano in northeast China.

3‐D P‐wave velocity structure in crust and uppermost mantle in and around Wenchuan earthquake source region was studied using long‐term accumulated seismic travel time data in Sichuan–Yunnan region and aftershock data of Wenchuan earthquake. The result shows that shallow P‐wave velocity structure has good correlation with surface geology. Longmenshan fault zone is imaged as high P‐wave velocity region in 0~20 km depth. Pengguan complex and Baoxing complex are imaged as two local high velocity anomaly bodies. The upper to middle crustal high velocity anomaly bodies in Longmenshan fault zone control the distribution of aftershocks. At the southern part of the aftershock zone, aftershocks occurred only in the northeast of the high velocity body related with Baoxing complex; In the middle part, the distribution of aftershocks seems to be controlled by the high velocity anomaly body corresponding to Pengguan complex to some extent; In the northeast part, the high velocity body around Ningqiang–Mianxian may prevent the further extension of aftershocks to the northeast. The existence of upper crustal high P‐wave velocity zone in Longmenshan fault zone implies that the upper crust has relatively high strength, which may play an important role in obstructing the extrusion of Tibet Plateau material to east, and is prone to accumulate energy in deep depth. Yangzi Block is characterized by obvious high velocity region below 30 km depth, and its front edge extends to Tibetan Plateau with depth increasing and reaches to the west side of Longmenshan fault zone in the lower crust and upper mantle.

The S wave velocity structure within the depth of 0-100 km beneath digital seismic stations of Yunnan Province was obtained from teleseismic receiver function modeling. The results show that the crustal thickness changes greatly in Yunnan, reaching about 62 km in Zhongdian and Lijiang, and decreasing to 32-34 km in Jinghong, Simao and Changyuan in south. The thick crust extends from northwest to southeast, decreases in thickness and range, and is about 42 km thick around Tonghai. Its shape and range consist with Chuandian diamond block bounded by Xiaojiang and Yuanjiang faults. In the eastern and southern region, the crust is relatively thin, and the velocity contrast across Moho is obvious. In the areas with great variation of crustal thickness or thick crust area, the Moho is characterized by transition zone with high velocity gradient. In Yunnan region, S wave velocity structures show strong horizontal heterogeneity. Above the depth of 10 km, the S wave velocity in north area is obviously lower than that in south area, while within the depth of 10-20 km, the S wave velocity in north is higher than that in south. The velocity interface within crust is discontinuous, and the depth and range of lower velocity zones change with different seismic stations. There is no obvious crustal low velocity zone beneath nearly half of the stations. Influenced by the upper mantle in south area, within the depth of 40-50 km, S wave velocity in south area is higher than that in north, high velocity area extends to north, and the shape of low velocity zone tends to consist with Chuandian diamond block. The upper mantle velocity distribution within the depth of 70-80 km seems to be correlated to the distribution of strong earthquakes.

The level of seismicity in Chanbaishan Tianchi Volcano has increased obviously since June of 2002. In this paper, the seismic activity of the Changbaishan Tianchi volcano is studied by using the data recorded at 15 broad‐band portable seismometers operating in the volcanic region in the summer of 2002. The result shows that the average rate of seismicity is more than 30 events per day in this area during summer of 2002. Most earthquakes occurred in two areas, southwest and northeast of the Tianchi caldera. The focal depths are shallow, and usually less than 5km. The b values of events occurred in southwest and northeast areas are quite different from each other. Seismic spectral and time spectral analyses show that the events occurred in the summer of 2002 are all volcano‐tectonic earthquakes. The obvious 2Hz low frequency component in seismic records at stations HSZ and DZD is mainly caused by local variations of media and low velocity fault zones near the two stations. We infer that the large number of earthquakes and earthquake swarms occurred in the summer of 2002 might be caused by local faulting induced by deep volcanic activities.

The S wave velocity structure in Changbaishan volcanic region was obtained from teleseismic receiver function modeling. The results show that there exist distinct low velocity layers in crust in volcano area. Beneath WQD station near to the Tianchi caldera the low velocity layer at 8 km depth is 20 km thick with the lowest S-wave velocity about 2.2 km/s. At EDO station located 50 km north of Tianchi caldera, no obvious crustal low velocity layer is detected. In the volcanic region, the thickness of crustal low velocity layer is greater and the lowest velocity is more obvious with the distance shorter to the caldera. It indicates the existence of the high temperature material or magma reservoir in crust near the Tianchi caldera. The receiver functions and inversion result from different back azimuths at CBS permanent seismic station show that the thickness of near surface low velocity layer and Moho depth change with directions. The near surface low velocity layer is obviously thicker in south direction. The Moho depth shows slight uplifting in the direction of the caldera located. We consider that the special near surface velocity structure is the main cause of relatively lower prominent frequency of volcanic earthquake waveforms recorded by CBS station. The slight uplifting of Moho beneath Tianchi caldera indicates there is a material exchanging channel between upper mantle and magma reservoir in crust.

In the paper, source mechanisms of 33 small-moderate earthquakes occurred in Yunnan are determined by modeling of regional waveforms from Yunnan digital seismic network. The result shows that most earthquakes occurred within or near the Chuandian rhombic block have strike-slip mechanism. The orientations of maximum compressive stresses obtained from source mechanism are changed from NNW-SSN to NS in the areas from north to south of the block, and tensile stresses are mainly in ENE-WSW or NE-SE. In the eastern Tibetan Plateau, the orientations of maximum compressive stress radiate toward outside from the plateau, and the tensile stress orientations mostly parallel to arc structures. Near 28°N the orientations of both maximum compressive stress and tensile stress changed greatly, and the boundary seems to correspond to the southwestern extended line of Longmenshan fault. Outside of the Chuandian rhombic block, the orientations of P and T axes are some different from those within the block. The comparison shows that the source mechanism of small-moderate events presented in the paper is consistence with that of moderate-strong earthquakes determined by Harvard University, which means the source mechanism of small-moderate events can be used to study the tectonic stress field in this region.

In summers of 2002 and 2003, temporary seismic stations had recorded a large number of small earthquakes and a series of swarms near Changbaishan Tianchi volcano. Earthquake location result shows that these earthquakes mainly occurred near the Tianchi caldera. Almost all swarms were concentrated at southwest of the caldera, and no swarms occurred in northeast area where earthquakes densely concentrated. In other seasons, the permanent station CBS is the only seismic station to monitor volcanic earthquakes in Tianchi volcano. Waveform correlation analysis of seismic data recorded at CBS in different time shows that earthquake swarms occurred in other seasons also concentrated in the southwest Tianchi Lake. High‐precision relative location shows that hypocenters of earthquake swarm distributed along northwest with a dip angle of 80° to southwest. During the July 13, 2003 earthquake swarm the hypocenters migrated from deep to shallow and deeper events had upward first motion directions almost at all stations. That means they had obvious expansion component in source mechanism. Taking into account of significant surface deformation in Changbaishan Tianchi volcano since 2002, geochemical anomalies and harmonic‐spectral earthquakes, we believe that the earthquake swarm activity was caused by magmatic and hydrothermal activity and pressure increase of magma near the depth of 5km.

21 earthquakes recorded by a temporary seismic network in the Changbaishan Tianchi volcanic area in Northeast China operated during the summer of 2002 and 2003 were analyzed to estimate the S coda attenuation. The attenuation quality factor Q c was estimated using the single scattering attenuation model of Sato (1977) in the frequency band from 4 to 24 Hz. All the events studied in this paper occurred at depths from 2 to 6 km with ML of 1.4-2.8. The epicentral distances are less than 25 km. For all events which occurred near the Tianchi Lake (caldera), the Q c patterns obtained at the stations near the lake are similar, and the Q c values are relatively small. At the stations located about 15 km east of the Tianchi Lake, however, the average Q c is significantly higher. For an event which occurred 25km from the lake to the west, Q c patterns derived at the stations near the lake are quite similar to the above mentioned Q c for stations located in the east. Further study shows that Q c value in the north and central areas of the volcano is relatively lower than that in the surrounding area. Compared to other volcanic areas in the world, the average Q c of the Changbaishan Tianchi volcanic area is obviously lower. The deep seismic sounding and teleseismic receiver function studies indicated more than one lower velocity layer in the crust. The MT studies suggested the presence of high conductive bodies beneath the area. We interpret the strong attenuation of coda waves near the Changbaishan Tianchi volcano as being possibly related to high temperature medium caused by shallow magma chambers.