The Prediction of Earthquake Ground Motions by Regression Model

1. Introduction

Southeast Asia was shaped as a result of the interaction between the Indo-Australian, Eurasian, Philippine, and West Pacific tectonic plates. Thailand located inside the Eurasian plate, is delimited by the Andaman thrust within the west, the Sunda Arc within the south, and also the Philippine trench within the east [1]. As a result of recent awareness of earthquake hazards, the Thai Meteorological Department has established quite 20 seismometer stations Thailand. Figure 1 shows samples of ground motion records at a Chiang Mai station (in northern Thailand) from an earthquake event in the regions around Chiang Rai (M 5.9). The distance from the epicenter of the earthquake is about 128 km. One will observe a small peak ground acceleration (PGA) (≈2.29 × 10⁻³ g) in the vertical component, and a peak horizontal ground acceleration in the horizontal components (E-W component ≈ 1.63 × 10⁻³ g and N-S component ≈ 1.19 × 10⁻³ g). The researcher calculated average peak horizontal ground acceleration (PHA_avg) ≈ 1.04 × 10⁻³ g.

The attenuation relationship for PHA_avg for the Chiang Mai station in Thailand has been proposed in this investigation. The attenuation relationship for PHA_avg was determined based on the multiple linear regression (MLR) models by using 132 components of strong motion data from 66 earthquake events. By using the MLR models, the researcher constructed site-specific attenuation relation based on a previously proposed attenuation relation [2, 3]. The specific site in this investigation is a Chiang Mai seismic station (CMMT) station located on the rock site.

2. Characteristics of ground motion in Thailand

Because Thailand is not a country prone to earthquakes, most of the earthquakes felt in Thailand occur outside the country, in places such as Sumatra, Nicobar Island, the Andaman Sea, Myanmar, Laos, and in the west and south areas of China. However, inside the country, Thailand has active faults, which can cause small-to-moderate earthquakes in areas such as Chiang Mai.

Pairojn and Wasinrat [4] presented the observed earthquakes recorded by the Chiang Mai station in northern Thailand from 07/10/2006 to 11/11/2012, as shown in Figure 2.

3. Site classification

In this study, the researcher chose to investigate data from shear wave velocity (Vs) recorded at the Chiang Mai seismic station (CMMT) in northern Thailand, located on a rocky site (NEHRP site class). The CMMT station used Trillium 120 velocity sensors and TSA100S accelerometers, as seen in Figure 3.

4. Ground motion prediction

Recently, several ground-motion prediction equations (GMPEs) have been developed. Douglas [6] summarizes all empirical ground-motion prediction equations (GMPEs) used to estimate earthquake peak ground acceleration (PGA) and elastic response spectral ordinates published between 1964 and 2017. Most empirical ground motion attenuation relations are derived from numerical analyses.

For Thailand, Idriss’s model [2, 3] was selected because it has an appropriate attenuation relation. Idriss suggested that the attenuation relation for motions in western North America [2, 3]. The available data included 572 individual horizontal components in rock sites that were used to derive this attenuation model. This study focuses on M ≤ 6.0 and M > 6.0 using local M_L and surface wave magnitude M_S scales, respectively. The range of applicability is 1–100 km for distance and 4.6–7.4 for M [2, 3]. The peak ground acceleration (Y) at rock sites was derived as following equation:

where Y is in g (m/s²), a = 0.2, for M ≤ 6 𝛼₀ = −0.150, 𝛼₁ = 2.261, 𝛼₂ = −0.083, 𝛽₀ = 0, 𝛽₁ = 1.602, 𝛽₂ = −0.142 and 𝜎 = 1.39–0.14 M and for M > 6 𝛼₀ = −0.050, 𝛼₁ = 3.477, 𝛼₂ = −0.284, 𝛽₀ = 0, 𝛽₁ = 2.475, 𝛽₂ = −0.286 and for M < 7.25 𝜎 = 1.39–0.14 M, and for M ≥ 7.25 𝜎 = 0.38. (F = 0 for Strike slip, F = 0.5 for Oblique, F = 1 for Reverse).

This chapter uses the method of ground-motion prediction under regression analysis [5].

4.2. The multiple linear regression for predicting earthquake ground motion

Pairojn and Wasinrat [4] proposed the MLR model to predict average peak horizontal ground acceleration, PHA_avg (Ŷ), in Thailand by including magnitude, and distance as predictor variables. The model was conducted from Table 1. The MLR model can be generalized as follows:

lnŶ=−5.4239+1.7410M−2.3469lnRE8

where M represents the magnitude and R represents the distance. The natural log (ln) of Y and R are used to transform the linear relationship between Y and R, and lnY and M. The value of R2=0.8438indicated that the earthquake ground motion is predicted by magnitude and distance equal to 84.38%. The results showed that the MLR model is able to predict the average of PHA in Thailand to a greater degree than Idriss’s model (Figure 4).

No.	Date	Time UTC	Epicenter	Magnitude, M (Mw)	Distance, R (km)	PHAavg, Y (g)
1	5/5/2014	12:06:19	Chiang Rai	5.1	121	0.0002990
2	5/5/2014	12:20:57	Chiang Rai	5.2	140	0.0003755
3	5/5/2014	21:17:05	Chiang Rai	5.1	119	0.0005555
4	5/5/2014	23:04:55	Chiang Rai	5.2	121	0.0003380
5	6/5/2014	00:50:16	Chiang Rai	5.9	128	0.0014140
6	6/5/2014	0:58:19	Chiang Rai	5.6	116	0.0010405
7	12/5/2014	11:05:29	Chiang Rai	5.0	137	0.0002955

Table 1.

Seven new earthquake events.

5. The practicality of ground motion prediction

The MLR model was used on seven new earthquake events that occurred from 05/05/2014 to 05/12/2014. The data are shown in Table 1.

The predicted PHA values from Eq. (8) and Eq. (1) are presented in Table 2 (Figures 5, 6, 7).

The root mean square error (RMSE) is used for comparing the predicted PHA values from MLR model to the observed PHA values. The RMSE measures the error between a predicted value and an observed value, defined as:

where Ŷiis the predicted values from MLR model and Yiis the observed values from nobservations. The RMSE value is 0.00009.

6. Conclusions

The MLR model is presented for developing a previous attenuation relationship based on observations. The MLR model is suitable for measuring the attenuation relationship for Thailand because Thailand has few instances of motion data, with most peak ground acceleration being measured at less than 0.1 g. It is expected that this method can be applied to observation sites throughout the country. The MLR model has been used for probabilistic hazard analysis, risk analysis, building design analysis, and many other fields, that is, and construction of nuclear power plants, dams, bridges, and high-rise buildings.

Acknowledgments

The authors would like to acknowledge the Chandrakasem Rajabhat University and Thai Meteorological Department, which collectively supported this project.