Microtremor recordings in Northern Mississippi: Evaluating site effect and correlating with wave climate

Site effects is considered as one of the main reasons that cause enormous damages in earthquakes and it is mainly controlled by the local geological condition. The main research area, Northern Mississippi, is located in Mississippi Embayment (ME) where is mainly composed of unconsolidated sediments which can be as thick as 1400m along the Mississippi River floodplain, and is favorable to have significant site effects. Meanwhile Northern Mississippi is also located in the moderate to heavy potential damage area of the New Madrid Seismic Zone, which is in the northern section of ME. Therefore, it is very necessary to evaluate the site effect in Northern Mississippi area in order to prepare for the earthquake. In this research, microtremor recordings are used to evaluate the site effects. A total of 14 continuous long-term microtremor recordings (LTRs), each lasting at least 6 hours, at 8 selected locations in Mississippi (MS), Louisiana (LA) and Alabama (AL) states, and a large number (305) of systematic single-point short-term recordings (STRs), each lasting 15-30 minutes, in Northern Mississippi area are collected using a LE-3D/20s seismometer with Eigen-frequency of 0.05Hz and RefTek 130-01/3 data logger with a sampling rate of 100Hz. With these recordings, the horizontal to vertical spectral ratio (HVSR) method is applied to find the predominant frequency (f0) and roughly estimate amplification factor as HVSR value at f0 (HVSR@f0). Within Northern Mississippi area, the f0 is tightly correlated with unconsolidated sediments thickness (UST) and average shear wave velocity (Vs) are also estimated. Within the low frequency range (<0.2Hz), high HVSRs are observed in most LTRs and STRs, which is possibly caused by the wind either directly blow on the seismometer or on the buildings around the recording location, or human’s activities. The spectra of these recordings all show high power spectral density (PSD) energy level at frequency of around 0.2Hz, which is known as double-frequency peak based on observations on ocean bottom. By correlating the PSD level at DF peaks (PSD@fp) of LTRs with the simultaneous ocean data (significant ocean wave frequency, significant ocean wave height, wind speed, and atmosphere pressure) of Atlantic Ocean and Gulf of Mexico as well as the local wind speed and atmosphere pressure, it is concluded that the DF peaks observed in Northern Mississippi are combined impact of wave climate in both Atlantic Ocean and Gulf of Mexico. The particle motion analysis and calculation of vibration angle strengthen this conclusion. The plots of DFs and PSD@fp of STRs vs. UST in horizontal plane are significantly different from the plots in vertical direction, which indicated that the shear wave resonance in thick sediments modifies the DF microseism more obviously in horizontal direction than in vertical direction. From this research in Northern Mississippi, it can be observed that the predominant frequency of the area where UST > 200m are within DF range, and it can be concluded that DF microseisms are strongly influenced by ocean activities. Therefore, the possible influence from ocean activities on the estimation of f0 and amplification factor using HVSR method is examined by correlating the HVSR@f0 to the ocean wave climate (significant wave height, wind speed and atmosphere pressure) and projecting the microtremor spatial spectral vectors on stereographic net. These analysis show that the estimation of f0 value is not related to either the vibration direction or the energy level of the ocean wave, but the HVSR@f0 values are significantly affected by the energy level of ocean wave climate. To improve the estimation of amplification factor, a modified HVSR method is proposed, with which, a more reliable amplification factor is obtained by calculating the average spatial spectral vector based on the stereographic projection method. Three main strong conclusions can be drawn from this research: 1) the HVSR method is a fast and reliable method to estimate the predominant frequency; 2) the double-frequency microseism in northern Mississippi is a combined impact of both Atlantic Ocean and Gulf of Mexico; and 3) the estimated amplification factor by HVSR method should be modified according to the microtremor recording condition especially the noise level at around predominant frequency.

Name thesis:degree_name

Ph.D. in Engineering Science

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Geology and Geological Engineering

Year dc:date.available

2015

Author dc:creator

• Guo, Zhen

Contributors dc:contributor

• Adnan Aydin
• Roger Waxler
• Terry Panhorst

• ambient noise
• double frequency
• microtremor
• site effect
• wave climate
• Geology

Repository record dc:identifier

https://egrove.olemiss.edu/etd/1401

OAI identifier oai:identifier

oai:egrove.olemiss.edu:etd-2400