HIGH-RESOLUTION CRUSTAL DEFORMATION OBSERVATION …

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

THANK YOU !

canitano@earth.sinica.edu.tw