Qua uantification n of of Top opography Ef Effect on on Se - PowerPoint PPT Presentation

Qua uantification n of of Top opography Ef Effect on on Se Seismi mic Grou ound nd M Mot otion on: A Case se Study i y in Norther ern T Taiwan Yi-Ching Lo 1 , Li Zhao 2 , and Shu-Huei Hung 1 1. Department of Geosciences, National Taiwan University, Taiwan 2. Institute of Earth Sciences, Academia Sinica, Taiwan August 15, 2016

Motiva vations s for S Studying T Topography E y Effect ct  The Earth is not flat. Some regions have drastic variation in surface relief, e.g. ± 4 km in Taiwan region over ~100-km distance.  Topography affects wave propagation and therefore arrival times and amplitudes of seismic waves  Neglecting topography: • Topography-induced travel time anomalies introduce biases in seismic tomography models • Topography-induced amplitude anomalies lead to unrealistic ground motion predictions

Stu Studying T g Topo pogr graph aphy Effec ects ts o on Ground M d Moti tion by Nu Numerical S l Sim imula latio ions Southern California Northern Taiwan SAF • Surface topography SGM influences the intensity of ground motion by focusing, defocusing and scattering of SAF SGM seismic waves. • Topography has been ignored in most ground SAF SGM motion studies, leading to biases in PGV and PGA predictions. Lee et al. (2009) Ma et al. (2007) Numerical simulations show that the effect of topography on PGV predictions can be up to ± 50% for ground motion of ~0.5 Hz (Ma et al., 2007; Lee et al., 2009).

Fin init ite-difference M ce Method ( (FDM) • Newton’s second law: ρ ∂ v = ∇⋅ + σ f • FDM code by Zhang et al. (2012) , ∂ t • Grid spacing: 300 m horizontally and • Hooke’s Law: variable vertically (171.4 m near surface and increasing to 782.6 m at ~ 60-km) . ∂ σ 1 ′ = ∇ + ∇ − T C : [ v ( v ) ] m , • Accurate waveforms up to 0.8 Hz ∂ t 2 • FD simulations on the IES HPC cluster • Surface topography : curvilinear grids transformed to Cartesian grids Zhang et al. (2012) ∂ ∂ ∂ ∂ U U U U    = + + + A B C F , ∂ ∂ ξ ∂ η ∂ ζ t

Taiwan: 3 Ta 3D Mode del a and To d Topo pography (km/s) V P 8 3 ETOPO1 2.5 7 2 0 6 Depth (km) 10 1.5 20 5 30 40 1 4 0.5 3 0 2 -0.5 (km/s) V S -1 5 -1.5 4.5 4 -2 0 3.5 Depth (km) 10 • Regional model from travel time tomography 20 3 30 (Kuo-Chen et al., 2012). 40 2.5 • ETOPO1 topography: 1 arc-minute (~1.85 km). 2 Up to ~6 km topography contrast over 100-km 1.5 distance in northern Taiwan.

Sources a Sour and nd Statio tions 38 Stations (BATS and TAIGER) • Use 54 x 3 sources (at depths 7 , 15 and 23 km) to examine P and S waves coming from a variety of directions. • Compute waveforms from the 162 sources to all 38 stations with and without topography and measure the differences in their P- and S- wave travel times and amplitudes.

Compar ariso son o of W Wave veforms ( s (Z componen ent) T G N 0 1 1 2 8 / 8 S B C B 1 2 6 / 7 1 T G N 0 2 1 2 1 / 4 7 T G N 0 3 1 1 2 / 2 4 8 1 1 1 / 8 4 2 T G N 1 5 T G N 1 6 1 0 7 / 7 9 9 T G N 0 4 1 0 4 / 2 1 8 9 8 / 7 8 1 A N P B 9 6 / 9 2 1 T G N 0 5 9 6 / 1 6 9 8 9 6 / 1 6 9 8 N S N 1 5 T G N 1 9 T G N 2 2 9 2 / 1 0 4 6 N S N 1 4 9 0 / 2 0 3 4 8 5 / 1 2 3 T A T O Y H N B 8 5 / 9 3 5 T G N 1 8 8 4 / 1 3 2 7 T G N 0 6 8 0 / 1 7 7 9 N S N 1 3 T G N 1 7 7 9 / 9 2 7 7 9 / 9 2 7 N S N 1 2 7 6 / 8 1 7 N S N 1 1 7 4 / 6 7 9 W F S B 7 2 / 4 3 5 N S N 1 0 6 9 / 4 0 6 T G N 1 1 6 6 / 7 1 6 T G N 1 2 6 4 / 2 9 4 N S N 0 9 6 1 / 2 7 5 6 0 / 1 8 1 9 T G N 0 8 5 6 / 1 2 7 5 5 / 1 1 1 N S N 0 8 N S N 0 7 5 5 / 1 3 4 N S N 0 6 5 4 / 2 1 6 N S N 0 1 N S N 0 2 N S N 0 5 T G N 1 3 5 3 / 2 3 6 5 3 / 7 1 5 3 / 7 3 4 N S N 0 3 5 1 / 8 9 5 1 / 3 7 N S N 0 4 4 8 / 3 8 3 T G N 0 9 T G N 1 4 4 0 / 2 2 3 0 10 20 30 40 50 60 70 80 9 0 distance (km)/station elevation (m) T i m e ( s e c ) Black: “ record ” (with topography) Red: “ Synthetics ” (flat surface)

Me Measuri ring T Tim ime and A Amplit itude A Anomali lies P wave S wave • “ Record ” and “ synthetics ” with and without topography Broadband CC Broadband CC 0.1 Hz CC 0.1 Hz CC S P 0.2 Hz CC 0.2 Hz CC • Compute synthetic autocorrelation C A and record-synthetic crosscorrelation C C • Bandpass filter the correlations around 3 frequencies: 0.1Hz, 0.2Hz and 0.5Hz 0.5 Hz CC 0.5 Hz CC • Time anomaly: lag time of Amplitude anomaly:

Freq equency-dependence ce o of Topography E y Effec ect Z-comp. P (broadband) Z-comp. P (0.5Hz/2sec) Z-comp. P (0.1Hz/10sec) Z-comp. P (0.2Hz/5sec) δ t P-wave arrival time is more delayed at higher elevation and longer period, up to 0.4 sec. N-comp. S (broadband) N-comp. S (0.1Hz/10sec) N-comp. S (0.2Hz/5sec) N-comp. S (0.5Hz/2sec) δ t S-wave arrival time is more delayed at higher elevation and longer period, up to 0.7 s. N-comp. S (broadband) N-comp. S (0.2Hz/5sec) N-comp. S (0.5Hz/2sec) N-comp. S (0.1Hz/10sec) δ lnA Topography effect on S-wave amplitude has a complex pattern.

P-wav wave T Trav avel Time A e Anomalies d due to to T Topo pogr graph phy δ t vs. epicentral distance broadband 0.1 Hz 0.2 Hz 0.5 Hz δ t (sec) Epicentral distance (km) δ t vs. station elevation • P-wave delay times due to topography can be up to ~0.4 s. • Delay times from topography have no clear δ t (sec) relationship with epicentral distance; increase with elevation. broadband 0.1 Hz • Delay times have larger variation at longer 0.2 Hz 0.5 Hz periods (red and green symbols) due to finite- frequency effect. Station elevation (m)

S-wav wave T Trav avel Time A e Anomalies d due to to T Topo pogr graph phy δ t vs. epicentral distance broadband 0.1 Hz 0.2 Hz 0.5 Hz δ t (sec) Epicentral distance (km) δ t vs. station elevation • S-wave delay times due to topography can be up to ~0.7 s. • Delay times from topography have no clear relationship with epicentral distance; increase δ t (sec) with elevation. broadband • 0.1 Hz Delay times have larger variation at longer 0.2 Hz periods (red and green symbols) due to finite- 0.5 Hz frequency effect. Station elevation (m)

S-wav wave A Amplitude de Anomalies d due to to T Topo pogr graph phy δ lnA vs. epicentral distance broadband 0.1 Hz 0.2 Hz 0.5 Hz δ lnA Epicentral distance (km) δ lnA vs. station elevation broadband 0.1 Hz • S-wave amplitude anomalies due to 0.2 Hz topography have slightly negative mean, 0.5 Hz i.e. amplitude reduction. δ lnA • Topography effect on amplitudes has no clear relation with either distance or station elevation. Station elevation (m)

Topography E y Effects v s vs. Distan ance ce P wave travel time anomaly S wave travel time anomaly S wave amplitude anomaly • In northern Taiwan, topography induces up to 0.4 s and 0.7 s in P- and S-wave delay times, respectively, and up to 80% in S-wave amplitude anomaly. • P- and S-wave delay times have positive means and no dependence on distance • Amplitude anomalies for S wave have slightly negative mean and no dependence on distance. Source Depths: 7 km 15 km 23 km

Topography E Effects ts v vs. . Statio tion E Ele levatio tion P wave travel time anomaly S wave travel time anomaly S wave amplitude anomaly • Delay times for P and S waves increase with station elevation. • Increase of S-wave delay times with station elevation is nearly twice as fast as P wave delay times. • Amplitude anomalies for S wave have no dependence on station elevation. Source Depths: 7 km 15 km 23 km

Varia riatio ion wi with th B Bac ack A Azimuth: P-wav wave Trav avel T Times Source Depths 7 km 15 km 23 km SBCB : Western Foothills with a low elevation of ~70.6 m TATO : Taipei Basin with an elevation of ~123.0 m YHNB : Central Range with a relatively high elevation of 934.9 m. Topography-induced delay times δ t : • No clear variation in the topography-induced P-wave delay times with back azimuth. • Clearly positive with larger values at high-elevation YHNB and smaller values at low- elevation SBCB.

Varia riatio ion wi with th B Bac ack A Azimuth: S-wav wave A Amplitude des Source Depths 7 km 15 km 23 km Topography-induced amplitude anomalies δlnA : • Amplitude anomalies show complex variation with azimuth. • A majority of amplitude anomalies are negative (reduction).

Summa mmary  In northern Taiwan, topography induces up to 0.4 s and 0.7 s in P- and S-wave delay times, respectively, and up to 80% in S-wave amplitude anomaly.  Topography effect leads to positive anomalies in P- and S-wave travel times (delays), and slightly negative anomalies in S-wave amplitudes (reduction).  Delay times increase with station elevation but have no dependence on epicentral distance. Amplitude anomalies do not depend on either station elevation or epicentral distance.  Topography-induced travel time delays do not vary with azimuth; whereas topography effect on amplitude varies with azimuth in a complex fashion.