high-resolution crustal deformation observation …
TRANSCRIPT
HIGH-RESOLUTION CRUSTAL DEFORMATION
OBSERVATION USING BOREHOLE STRAINMETERS :
AN OVERVIEW IN TAIWAN
Change in rock length
~ ΔL/L
Change in rock volume
~ ΔV/V
(contraction if compressivestress applied, expansionif opposite)
Rock distortion (changein angle) : no area orvolume change
How to record strain ?
Sacks-Evertson borehole strainmeter(rock volume change)
Laser strainmeter : interferometry(linear strain at 90° : areal strain)
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Sacks-Evertson borehole strainmeter
InstallationDilatometer (SES-1)
3-component (SES-3)
SES-3Expansive groutε
V = ε
E + ε
N +
ε
Z
γ1 = ε
E – ε
N
γ2 = 2.ε
EN
(differential extension)
(engineering shear)
(dilatation)
Installation in a borehole
Advantages : isolation, noise reduction, ...
x3
Disadvantages : borehole relaxation, pore pressure, ...
[Johnston & Linde, 2002]
[Roeloffs, 2005]
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Borehole strainmeter : the most sensitive sensor in geodesy !
● Sensitivity of ~10-10 to 10-5 (i.e., 0.1 to > 10,000 nanostrain (nε)) from seconds to year
● About 100 to 1000 times more sensitive than GNSS at periods from hours to weeks
● Strainmeters have allowed to detect and model processes previously unrecognizedin Taiwan
What does record a strainmeter ?
normal modes
(local, regional, typhoons, ...)
Groundwater level variations(pore pressure, reservoirs, extraction, ...)
Earth's free oscillations
Network in the Longitudinal Valley
TAROKO
CHIMEI-RUEISUEI
CHIHSHANG-CHENGKUNG
ZANB
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Solid-Earth and ocean tides : reference for calibration
A
B
C
SES-3
Ev
ν1
ν2
Sensor orientation + calibration
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tides
seiches (free oscillations)
Large oceanic tides(example in Greece)
Strain response to groundwater level changes
Annual variations due to hydrological cycles in Taiwan
Crustal response due to hydrological cycles is still poorly understood (pore pressurediffusion, elastic response, dual processes ? ...)
Clear modulation of the strain signal by hydrological forcing at diverses periods (year,months, …) → Modeling of hydrology induced-strain should provide useful constraintson hydrological cycles in Taiwan.
SJNB station (Taroko)
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Modeling ground deformation induced
by tropical typhoons
➢ Air pressure variations is one of the largestsource of deformation recorded by strainmeters
➢ Tropical typhoons strongly impact Taiwan andlarge amount of rainfall (> 1m within 24 hrs) andlarge depression (> 100 hPa)
➢ Deformation induced by typhoons are difficultto detect with GNSS/InSAR but are well recorded by strainmeters
2009-2019
October 2008
Dila
tati
on
(n
ε)Typical strain response to typhoons
June 2008
10 days
KALMAEGI ~970 hPa FUNG-WONG
~950 hPa
50 nε
expansion
SINLAKU~925 hPa
JANGMI~ 925 hPa
Atmospheric reference (stable conditions) ~ 1005 hPa
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Typhoon's strain signature
Expansion
Compression
AP (hPa)
Hourly rain (mm)
FANAPI (19/09/2010) FBRB
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How typhoon deforms the ground ?
« Funnel effect »
A : direct water loading effect (mostly in the region directly above the sensor)
B : delayed loading effect (10-20 hours) : water runoff from hillslopes and concentrates above the sensor
Trade-off between 2 loading effects :
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Detection and modeling of aseismic
sources of deformation
➢ Aseismic sources of deformation play an important role in the earthquake cycle : howmuch they contribute to seismic budget ? Howthey interact with large earthquakes ? Do they occurred spontaneously ? Are they triggered(static, dynamic) ?
➢ Slow slip events (SSE) have been discoveredabout 20 years ago in Cascadia and they arenow observed in many subduction regions worldwide (mostly using GNSS) (M> 6-7).
➢ What about inland Taiwan ? No sign of SSEs on GPSto date (detected offshore Taiwan)
interseismic
SS
E
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(a)
(b)
SSE's strain signatureSSE produce exponential-like strain signture and remain undetected by GNSS stations
M~ 4.5 (2-4 km)
M~ 5.5 (8-12 km)
~ 3.5 days
2 weeks
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Seismic-aseismic interplay : case of M5.5 SSE
Coseismic slip
Coulomb stress changes
Postseismic slip(afterslip)
~ 25 %Barrier ?
2003-2010
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Dynamic field (bandpassed 3-7 s)
(Canitano et al., 2017a)
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Detection of postseismic relaxation from small events (ML<6)Postseismic slip represents a significant fraction of the total slip budget of an earthquakesequence. If large afterslip are easily detectable by GNSS/InSAR, smaller deformation remains difficult to detect and estimate.
Aftershocks likelycontrolled by afterslip
1 month
M5 (6 km)
M5.7 (26 km) M5.9 (17 km)
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Fault zone frictional parameters
● Afterslip results from rate-strengthening frictional sliding on the fault plane
Rate-dependent friction laws :
2 days
1 month
(Perfettini & Avouac, 2004)
Strain (afterslip)
Seismicity rate R(t)
tr = Aσn/τ : relaxation time
d = exp(∆σ/Aσn) : velocity jump
tr = 35 daysd = 10³ε0 = 2x10³ nε/yr Aσn = 3x10 ² MPa, ⁻ A = 3x10⁻⁴R0 ~ 50 events/yrτ = 0.3 MPa/yr
● Good agreement with estimates from 2003Chengkung earthquake with GPS signals (Hsu et al., 2009)
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Seismic source analysis: the October 2013
Mw 6.2 Ruisui earthquake
Since strainmeters record seismic waves (dynamic strain) and permanent static deformation,they can be used to infer seismic source location and mechanisms
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Coseismic static offsets
SSNB
CHMB
HGSB
ZANB
-910 nε
-12 nε-300 nε
-380 nε
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Source fault model inferred from coseismic static signals
Okada (1992)
● Grid search approach for 6 parameters, (strike, dip, rake) and fault plane location(30 km x 30 km fault plane, slip ~ 0.1 m)
● Strike = 217° ± 2°, Dip = 48° ± 3°, Rake = 49° ± 4°
● Parameters are in good agreement with seismology(strike = 209°, dip = 59°, rake = 50°)
● Depth of the plane well constrained (± 500 m) (upper limit ~ 4.3 km)
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Dynamic rupture modeling : static strainP S
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Dynamic field (bandpassed 3-7 s)
(Canitano et al., 2017a)
Obs.
Model
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Observation and modeling of seismically-
triggered infrasound signals
➢ Infrasound correspond to the subaudible spectrum of acoustic waves (< 20 Hz)
➢ Triggered by various processes : volcaniceruption, earthquakes, explosions, ...
➢ Infrasound generated by earthquakesare of 3 kinds :
- Epicentral infrasound : generatednear the source region by large shaking
- Remote infrasound : far from sourcedue to wave coupling with topography
- Near-receiver infrasound : waves detected by colocated sensors when passing near observation site (usuallyusing seismometers)
2008 M7.9 Sichuan
2011 M9.1 Tohoku
P Rayleigh
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Strain-infrasound coupling relation
Experimental coupling ratio for east Taiwan 3.7 (~ 80 cases)
Amplitude
Phase delay
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Preliminary modeling of near-receiver infrasound with strain12 s period 15 s period
12 s period 8 s period
15 s period
2-3 s period 1 s period
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Summary
● Despite the effort require for their installation and calibration, strainmeters arecan benefit research in geodesy and seismology
● In the Longitudinal Valley, sensors have allowed us to observe and model awide range of geophysical phenomena and to uncover their physical processes
● Such sensors remain largely unknown due to the paucity of network worldwideand the relatively high cost of the sensor and the hole drilling (10M NT for a site,~ 50 % for each entity)
● A large variety of other phenomena can also be analyzed (tsunamis, landslides,….) and strainmeters also show great potential for Early Warning Systems
