用近震波形分析 2013 年 6 月 2 日南投地震的震源過程

謝銘哲 1, 趙里 2, 馬國鳳 1

1國立中央大學地球物理研究所2 中央研究院地球科學研究所

Outline

• Real-time Focal Mechanism Determination– Generalized Cut and Paste method (gCAP) in 1D structure

• Revised Focal Mechanism in 3D Structure– Improve focal mechanism solution in 3D structure

• Real-time Finite-fault Determination– Determine average rupture property – Resolve fault-plane ambiguity

Point Source Focal Mechanism Determination in 1D Structure

CWB EarthquakeEarly WarningSystem (EEWS)

EEWS Report(Eq. long/lat)

Focal Mechanism DeterminationSystem Triggered

BATS WaveformRecords Cut

SAC Files PreparedRotation toR/T components

Waveform TracesSelected

Grid Search forBest Solution

Generate Output and Exit

Iteration

gCAP

gCAP: generalized Cut And Paste (Zhu & Ben-Zion, 2013)

1D Point Source Solution of 02 June 2013 Nantou Earthquake

http://tecdc.earth.sinica.edu.tw/FM

Real-time gCAP solution failed for this event due to the saturation of records at SSLB station (Δ≈10.3km).

Source grids to 60-km depth

(2.4km spacing)

2282 Receiver gridson the surface (4 km spacing)

3D Green’s Function Database

Model: Kuo-Chen et al. (2012) + ETOPO1Green’s functions calculated by finite-difference (Zhang and Chen, 2006).

h=5km

h=10km

h=15km

h=20km

YULB-HHE

Broadband Record and 3D Synthetics (≤ 0.8Hz)

Broadband 3D synthetics calculated using gCAP mechanism agree with records very well. Visual comparison suggests a source depth of 10-15 km?

sec

Upper-Band Record and 3D Synthetics (0.05Hz-0.3Hz)

h=5km

h=10km

h=15km

h=20km

YULB-HHE

Source depth 10-15 km?sec

Lower-Band Record and 3D Synthetics (0.02Hz-0.1Hz)

h=5km

h=10km

h=15km

h=20km

YULB-HHE

Source depth 5-10 km?sec

h=5km

h=10km

h=15km

h=20km

TWGB-HHZ

Broadband Record and 3D Synthetics (≤ 0.8Hz)

Ratio of surface and P wave amplitudes is also diagnostic of source depth (10 km?).

sec

Upper-Band Record and 3D Synthetics (0.05Hz-0.3Hz)

h=5km

h=10km

h=15km

h=20km

TWGB-HHZ

sec

Source depth 5-10 km?

Lower-Band Record and 3D Synthetics (0.02Hz-0.1Hz)

h=5km

h=10km

h=15km

h=20km

TWGB-HHZ

secSource depth 10-15 km?

• gCAP result as initial model

• Grid search parameters in 3D model– Depth: 10 – 30 km– Strike: 337° - 47°, dip: 4° - 44°, rake: 55° - 125° each at an interval of 5°– Grid search of 18,225 point sources

• Waveforms fitting are measured by L2-norm errors and an iterative scheme

• Moment magnitude (Mw) is solved by balancing amplitudes of available stations

• 6 stations (20 3-comp. P and surface wave traces)– 0.02Hz-0.1Hz for surface wave– 0.05Hz-0.3Hz for P-wave (first arrival)

Revising Focal Mechanism in 3D Structure

Depth 20k m, Mw 6.04Plane-A: S/D/R= 12°/24°/90°Plane-B: S/D/R=192°/66°/90°

gCAP

Teleseismic Regional

Mw 6.2Depth 25km

Mw 5.73Depth 23km

Mw 6.0Depth 16km

gCAP

Point Source in 3D

Depth 12.0 km, Mw 6.05Plane-A: S/D/R= 342°/29°/55°Plane-B: S/D/R=201°/67°/108°

Teleseismic Regional

Mw 6.2Depth 25km

Mw 5.73Depth 23km

Mw 6.0Depth 16km

Focal Mechanismin 3D Structure

Simplified Models for Finite-Source Rupture Scenario Search

Circular (6)(2 planes)

Possible Types

55% β75% β

95% β

(2 planes)

Unilateral (48)

3 possible rupture speeds for each direction

Bilateral (72)(2 planes)

Possible Rupture Speeds

9 possible rupture speeds for each direction

Fault plane dimension is estimated by scaling-law study (Yen and Ma, 2011)

• The best average finite-fault model shows a rupture on the east-dipping plane propagating unilaterally from NE to SW with an upward component at 75% of shear-wave speed.

STR/DIP/RAK342°/29°/55°

Average Rupture Model for 0602 Event

RMS errors of 126 finite-fault models

75% β

AU420SW NE

STR/DIP/RAK42°/22°/114°

Average Rupture Model for 0327 Event

95% β 75% β

AB323SW NE• The best average finite-fault model shows a rupture on the east-dipping plane propagating horizontally with 95% of shear-wave speed to SW and 75% of shear-wave speed to NE.

RMS errors of 126 finite-fault models

Comparisons with Aftershock Distribution- 0327 and 0602 events -

From CWB

Comparisons with Shake Intensity

CWB Intensity Observation

Point Source Synthetic PGV

Finite-Fault Synthetic PGV

Conclusion

• Overall comparisons of synthetics and records show that the 3D model of Kuo-Chen et al. (2012) predicts good P waveforms up to 3-sec period.

• Relative amplitudes between surface and P waves suggest that 0602 Nantou earthquake has a centroid depth of ~10 km.

• The 0602 Nantou earthquake is a unilateral rupture on the east-dipping fault plane propagating NE-to-SW with an upward component at 75% shear-wave speed. The 0327 and 0602 Nantou earthquakes have similar rupture scenarios.

Finite-Fault SimulationZ-component

- Finite-fault waveform simulation (2-sec Gaussian STF, ~ 0.8 Hz )- 3-D velocity model- ETOPO1 topography

2013.06.02

Thanks for your attention.