Elsevier

Tectonophysics

Volume 386, Issues 3–4, 16 August 2004, Pages 157-175
Tectonophysics

Crustal structure in the Changbaishan volcanic area, China, determined by modeling receiver functions

https://doi.org/10.1016/j.tecto.2004.06.001Get rights and content

Abstract

During 1998–1999, we installed a temporary broadband seismic network in the Changbaishan volcanic region, NE China. We estimated crustal structure using teleseismic seismograms collected at the network. We detected a near surface region of strong anisotropy directly under the main volcanic edifice of the volcanic area. We modeled 109 receiver functions from 19 broadband stations using three techniques. First we used a “slant-stacking” method to model the principal crustal P reverberation phases to estimate crustal thickness and the average crustal P to S speed ratio (vp/vs), assuming an average P-wave velocity in the crust. We then estimated crustal S-wave velocity (vs) and vp/vs profiles by modeling stacked receiver functions using a direct search. Finally, we inverted several receiver functions recorded at stations closest to the main volcanic edifice using least squares to estimate vs velocity profiles, assuming a vp/vs value. The results from the three estimation techniques were consistent, and generally we found that the receiver functions constrained estimates of changes in wave speeds better than absolute values. We resolved that the crust is 30–39 km thick under the volcanic region and 28–32 km thick away from the volcanic region, with a midcrust velocity transition at about 10–15 km depth. We estimated that the average crust P-wave velocity is about 6.0–6.2 km/s surrounding the main volcanic region, while it is slightly lower in the vicinity of the main volcanic edifice. The estimates of vp/vs were more ambiguous, but we inferred that the bulk crustal Poisson's ratio (which is related to vp/vs) ranges between 0.20 and 0.30, with a suggestion that the Poisson's ratio is lower under the central volcanic region compared to the surrounding areas. We resolved low S-wave velocities (down to about 3 km/s) in the middle crust in the region of the main volcanic edifice. The low velocity anomaly extends from about 5–10 to 15–25 km below the surface, probably indicating a region of elevated temperatures. We were unable to determine if partial melt is present with the data we considered in this paper.

Introduction

The Changbaishan volcanic region is located in Jilin Province in northeastern China, along the border between China and North Korea (Fig. 1). There are two main volcanic edifices in the Changbaishan region, the Wangtian and the Tianchi, which are of basaltic and andesitic to rhyolitic composition underlain by a large Cenozoic basaltic shield (Liu, 1983, Liu et al., 1998). The Wangtian was last active 2.12 Ma and has approximately 400 km2 of exposed volcanic rock associated with it (Fan et al., 1999). The Tianchi volcano became active about 1.66–1.48 Ma years ago (Jin and Wei, 1994) and is still active, with the most recent activity in 1903 (Cui et al., 1995). The Tianchi eruption of 1000AD was one of the largest recorded in China in the past 2000 years (Liu et al., 1997).

Until recently, little geophysical research has been conducted in the Changbaishan region. Regional S-wave tomography by Guo et al. (1996) indicated a low velocity anomaly at 25–75 km depth below the Tianchi volcano (also see Liu et al., 1998). Additionally, a magnetotelluric study done by Tang (1998) revealed a region of low resistivity below 10–15 km depth extending from the Tianchi volcanic crater to about 30 km to the north. The region of low resistivity might indicate a region of partial melt or a region of high fluid content, but was interpreted to be most probably a magma chamber by Tang (1998). The Moho depth was determined to be about 30–35 km by modeling surface and Moho reflected phases from a regional earthquake (Shin and Baag, 2000). Based on geochemical analysis, Shangguan and Sun (1997) have determined that the gases released from local hot springs are from the top of a residual, mantle-derived magma chamber in the crust, located possibly as shallow as 5 km below the Tianchi crater. The water from hot springs in the Changbaishan region are about 80.75 °C and have been determined to be meteoric water circulating through regions of hot rock (Shangguan et al., 1997).

In the summer of 1998, the State University of New York at Binghamton, in collaboration with the Research Center of Geophysical Exploration (RCGE) of the Chinese Seismological Bureau, installed 19 portable, broadband seismic stations in NE China, in the region of the Changbaishan volcanic area (Table 1 and Fig. 1). The stations were borrowed from IRIS PASSCAL and each station comprised a Guralp 3T sensor and a six-channel Ref Tek recorder. We established stations at 18 sites in late June 1998. Eight sites were occupied until late September 1998, while 10 sites were occupied until March 1999. The one remaining station was installed in September 1998 and removed in March 1999. In August 1998, the RCGE conducted an active source, wide-angle reflection/refraction experiment, and analysis of profiles near the Tianchi volcano indicated a low P-wave velocity anomaly under the Tianchi volcano at about 10–20 km depth, and resolved fairly uniform Moho depths of about 30–35 km away from the volcanic center, thickening to about 40 km under the Tianchi edifice (Zhang et al., 2002, Song et al., 2003).

In this study, we present a determination of crustal structure using teleseismic receiver functions throughout the Changbaishan region. The receiver function method has been applied to many tectonic problems (e.g., Owens and Zandt, 1997, Julià et al., 1998, van der Hilst et al., 1998, Chevrot and van der Hilst, 2000, Juliá et al., 2000, Zhu and Kanamori, 2000) and volcanic regions (e.g., Langston, 1979, Darbyshire et al., 2000, Du and Foulger, 2001).

Section snippets

Source equalization

The first 20 or so seconds following direct P in a teleseismic seismogram principally contains rupture effects, source-side structure, and receiver-side structure. The source-side signal results from near source reflected P waves and S to P conversions. Receiver-side signal consists of P to S conversions and P and S reflections near the receiver, principally in the crust (Fig. 2). Direct P is incident on the Moho nearly vertically for a teleseismic distance source. Therefore, the amplitude of S

Crustal thickness and bulk velocity structure

Using the slant-stacking algorithm, we found the Moho to be at about 28–32 km depth away from the volcanic region, and 30–39 km in the volcanic region, with the thickest crust closest to the Tianchi crater (Fig. 7, Fig. 13). All of the events recorded at CANY station are from the south to southeast, so the region of Moho sampled is to the south and southeast. Thus, receiver functions recorded at CANY station constrain crustal thickness south of the Tianchi crater, under the Wangtian volcanic

Conclusions

In this paper, we presented the crustal structure in the Changbaishan region based on modeling teleseismic receiver functions. The Moho depths we obtained in this study are consistent with those obtained by other researchers (Shin and Baag, 2000, Zhang et al., 2002, Song et al., 2003). Researchers modeling wide-angle reflection/refraction data also detected a relatively thick crust (38–40 km) and a low velocity layer in the midcrust under the center of the Changbaishan volcanic region (Zhang et

Acknowledgments

This was a cooperative project between the Research Center of Geophysical Exploration, Chinese Seismological Bureau, and the State University of New York at Binghamton, covered under the Sino-US Seismological Research Protocol signed between USGS/NSF and CSB in 1980. We would like to thank IRIS/PASSCAL for providing the seismometers, as well as software, technical assistance and field support. Installation of the broadband network would not have been possible without the field assistance of

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    Now at US Geological Survey, 384 Woods Hole Road, Woods Hole, MA 02543 USA.

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