SmartSolo Instruments: HVSR Method for Urban Fault Detection

Introduction to Urban Active Fault Detection

Urban active fault detection is crucial for understanding the spatial distribution of faults, assessing seismic risks, and planning effective disaster prevention measures. These efforts aim to reduce potential losses from earthquakes by accurately mapping fault zones and analyzing seismic backgrounds.

Study Area: Mingguang City and the Tanlu Fracture Zone

Mingguang City, Anhui Province, is located at the junction of the middle and southern segments of the Tanlu Fracture Zone, the largest seismically active zone in eastern China. This region features:

• High elevations in the north, transitioning to plains, hills, and low mountains.
• Complex lithology with significant Quaternary deposits in the basin areas.

Studying this area’s fault structures is scientifically and practically significant for disaster prevention.

Applying the HVSR Method

Advantages of HVSR for Seismic Studies

Traditional fault detection methods are costly, shallow in exploration depth, and limited in application. The Horizontal-to-Vertical Spectral Ratio (HVSR) method, which uses background noise surface waves, has emerged as a cost-effective and reliable technique for shallow surface structure exploration.

Equipment and Data Collection

Researchers deployed 133 SmartSolo IGU-16HR 3C three-component node seismometers to collect background noise data in the Mingguang section of the Tanlu Fault Zone. The data were used to:

• Calculate HVSR curves and peak frequencies.
• Invert shallow S-wave velocity structures using particle swarm algorithms.
• Verify results against boreholes, seismic reflection profiles, and geological data.

Key Findings and Conclusions

HVSR Profiles and Amplification Effects

1. Site Response Characteristics:
   • HVSR profiles showed complex patterns, including double-peak and multi-peak features.
   • Sites were classified as Class II medium-hard soils, with notable variations in peak frequencies.
   • Western depression areas exhibit significant amplification effects on ground shaking.
2. S-Wave Velocity Structure:
   • The soil-rock interface revealed by HVSR inversion matched borehole results and seismic reflection profiles.
   • The weathered bedrock-unweathered bedrock interface, undetected by active sources, was clearly delineated.
3. Fault Activity:
   • Two ruptures at the eastern boundary of the Tanlu Fracture Zone were identified.
   • The Chihuhe-Taihu Lake rupture was found to be more active than the Jashan-Lujiang rupture, suggesting it is a hidden active fault.

Broader Implications

The study confirms that the HVSR method is effective for detecting urban active faults and analyzing site response characteristics. Its ability to reveal hidden structures and seismic zones offers valuable insights for urban seismic zoning and disaster mitigation.

Conclusion

The deployment of SmartSolo IGU-16HR 3C seismometers and HVSR techniques has proven effective for urban fault detection. This approach offers a reliable, cost-efficient alternative for analyzing seismic risk and mitigating hazards in tectonically active regions.

Original source: Ni Hongyu, Zhang Ruohan, Li Junlun, Huang Xianliang, Zheng Haigang, Hong Dequan, Miao Peng, Peng Liuya, Bao Ziwen.2023.Application of seismic background noise HVSR method in response characterisation of urban sites and detection of active faults in Mingguang, Anhui Province. Geophysical Journal,66(11):4552-4571,doi:10.6038/cjg2023R0118