April-May 2015 (Mw 7.8, 7.4) Nepal earthquakes: A perspective

Kusala Rrajendran CEaS

Studies for the past 15 years (major collaborator, C.P, Rajendran). Other

collaborators: Jaishri Sanwal, B. Kotlia, Mike Sandiford and Kristin Morel, MU,

Australia)

Students participating in the research work and field work to Nepal

• Ms. Revathy Parameswaran (Ph. D student, works on source mechanisms of plate

boundary earthquakes) : Field surveys, source models

• Mr. Thusasiraman (Ph.D student, works on site response of earthquakes): Field

surveys, Site response studies, tectonic Geomprphology.

• Mr. Rishav Mallick (M.Tech student, works on source mechanisms): Field surveys

Others who participated in the field work

• C.P. Rajendran, Faculty, JNCASR

• Mathew Wood (Ph.D student with Mike Sandiford; works on landslides,

professional photographer)

What has led to our understanding (whatever little) has come out of collaborative effort

We acknowledge the financial support from MOES and IISc

Three great earthquakes (1905, 1934, 1950) have occurred along the Himalaya in the 1900s. The segment between the ruptures of 1905 and 1934 is considered as a gap, due for a great earthquake. (Rajendran et al 2015)

Estimated rupture area of major earthquakes along the Himalayas. The 1100 and

1413 AD events were both documented from paleoseismic studies. Bilham and

some coworkers believe that the 1505 historical earthquake must have had a

magnitude close to Mw 9 and with a renewal time of ~ 500 years (based on slip

estimates) they believe that a great earthquake is imminent.

Avouac, 2015

Results from other studies

Our studies have used various proxies to infer the chronology of past earthquakes

1. Heritage structures (damage reports and observations on temples)

2. Paleoliquefaction features

3. Slip on faults observed in trench sections

4. Cave features (tilt on stalagmite deposits)

Rajendran et al., 2015 concluded that “the frontal thrust in central Himalaya may

have remained seismically inactive during the last ~700 years. Considering this long

elapsed time, a great earthquake may be due in the region”.

Related publications: • Rajendran and Rajendran (2005, 2011); • Rajendran et al., (2013, SRL; 2015, JGR)

Major points of dissention with Bilham’s group

• On the estimated size and location of the 1505 earthquake, which finds no mention in historic records

• Believe that every large earthquake may not originate on the detachment

• Missing slip may be overestimated as partitioning on the splay faults are ignored in the geodetic slip models.

With these basic notions, let us approach the recent Nepal earthquakes.

Organization of this talk

• A brief overview of the tectonics of the Himalaya

• Introduction to some terms that we need to understand the source properties of earthquakes

• The Nepal earthquake: field observations, damage,

deformation features

• Source models and relation to seismogenic structures

• Remarks on why the threat of an impending earthquake continue to remain

Himalaya is the result of collision of India-Asia plates. Prior to collision, the Tethys

Sea, separated the two. The southern margin of Asia was an active margin with a

subduction zone (like Sumatra subduction zone). Age of the onset of collision is

estimated to be between 65 and 45 Ma. Paleolatitudes (variety of data including

paleimagnetic data) from sites just north and south of the suture zone suggest that

the collision occurred 44 and 55Ma.

Sketch from Avouac, 2015

Convergence rates over the last 80 Ma;

kinematics obtained from the synthesis

of the magnetic anomalies of the

Indian Ocean (Royer and Patriat, 2002)

At present, northern India is moving ~35 mm

year-1 along a N15° E azimuth (31 mm year -1

along a N10° E azimuth at the western

Himalayan syntaxis and 38 mm year-1 along a

N20°E azimuth at the eastern Himalayan

syntaxis (Bettinelli et al., 2006). Crustal

shortening across the Himalayas, estimated

at 19 ± 2.5 mm year-1 absorbs nearly half of

the current convergence rate between India

and Eurasia.

India-Eurasia collision zone showing location of major topographic features. The red arrow represents convergence between Bangalore (IISC) in southern India with stable Eurasia. This convergence is characterized by shortening across the Himalaya, Tibetan Plateau, and Tien Shan [Zhang et al., 2004].

And the resulting features: ~ 2500 km long plate boundary, the uplifted mountain ranges an the Tibetan Plateau…..

The highest mountain range + the largest high-altitude plateau on earth; seismically most active continental collision boundary

Tapponier et al.,2001

The youngest fault system that accommodates the slip

And how are these thrust systems evolving?

Simplified geologic map of Himalayan arc (Avouac, 2015). Section A A’ (next slide) shows structures in section

Himalaya is seen as an analogue to the

wedge of sand that forms in front of a

moving bulldozer (Davis et al., 1983).

This wedge is critical, meaning that any

point within the wedge is at the verge

of failure.

Schematic geologic section across the Himalayas of central Nepal reflecting early

interpretation of the major thrust faults as crustal-scale parallel faults. Analogy with

the geometry of a sand wedge at front of a bulldozer

Critical Taper

Applied to the Himalaya

In geodynamics the

concept is used to explain

tectonic observations in

accretionary wedges.

Landscape carved by such propagating wedges

ITSZ – Indus-Tsangpo suture zone; STD – South Tibetan detachment; MCT – Main Central thrust; MBT – Main Boundary thrust; MFT – Main Frontal thrust

MCT: 24–21 Ma; MFT: < 2 Ma

Aseismic slip

Locked, breaks in great eqs

section

EQ

• The faults get progressively younger towards south, but there are out-of-

sequence thrusts (change in order of age) that are mapped.

• For example, a physiographic break south of MCT has been identified as an out of

sequence thrust (younger than MBT).

• Seismogenic potential of such thrusts have not been understood hitherto, as

there was no known association with large/great earthquakes.

• The 2015 sequence may provide the first example.

View of a thrust sheet in the Nepal sub-Himalaya

The South Tibetan detachment (north of

Mount Everest). Tethyan Sedimentary

Series in the hanging wall with marble,

calc-silicates, and more in the footwall

(Jessup et al., 2006).

Section of detachment

Geologic cross-section of

central Nepal Himalaya,

showing location of

tectonic units and major

thrust faults. Figure from

Pandey et al. [1995].

On which structure did the earthquake occur?

The earthquake was shallow (<15 km); with a thrust faulting a 10 degree dipping plane.

The rupture (of the first eq) propagated from west to east.

So, what is the causative structure? Is there a surface rupture?

Did it propagate all the way to the Indogangetic plains as was expected?

Second earthquake occurred where the first rupture terminated.

It had a short rupture duration and aftershocks clustered in a small area.

Source of 2015 eq?

Motion of India relative to

Eurasia by two different

models Bettinelli (2006).

Frontal fault defined by thrust

earthquakes on shallow

dipping planes (10-15°);

shallow, within 30 km.

Summary of plate motions and background seismicity

Great earthquakes originate on the detachment and propagate to the surface (eg.

1903, 1934). Surface ruptures have not been observed as the fault-propagation

folds absorb them before reaching the surface. Lack of surface ruptures is also

linked to the difficult field conditions and rapid weathering of outcrops. A limitation

for paleoseismologic studies in the Himalaya

A brief on focal mechanism

P-axis (~ Shmax)

T-axis (~ Shmin)

Beach Ball solution: A way of representing the style of faulting. Point source assumption, match on the P-waves. Does not work well with large ruptures. Hence, moment tensor inversions and teleseismic waveform modeling.

P- P axis (direction of compression) T- T axis (direction of tension axis)

Normal fault: σ1 vertical, e.g

gravitational load.

Thrust fault: σ1 horizontal; σ3

vertical. (e.g. Himalaya).

Strike slip fault: σ1 and σ3 are also horizontal. (e.g., San Andreas Fault USA)

• Basic platform: Seismic Analysis Code (SAC)

• Technique: construction of synthetic wave forms giving best match

with the observed seismogram

• Stations are selected from II (IRIS/IDA), IU (IRIS/USGS) and GT (Global

Telemetric seismograph network) or Global Seismic Network (GSN)

• Selection criteria: azimuth, epicentral distance (35°-100°) and signal

quality

• Teleseismic Body wave inversion program by Prof. Kikuchi and Prof.

Kanamori, is used to calculate moment rate, maximum dislocation,

focal mechanism, rupture extent and moment magnitude.

Teleseismic body-wave inversion allows modeling a longer part of

the waveform and is more representative of the rupture

Organization of this talk

• A brief overview of the tectonics of the Himalaya

• Introduction to some terms that we need to understand the source properties of earthquakes

• The Nepal earthquake: field observations, damage,

deformation features

• Source models and relation to seismogenic structures

• Remarks on why the threat of an impending earthquake continue to remain

April 25th, 2015 Mw 7.8

May 12th, 2015 Mw 7.3

Source mechanisms for the 7.8 and 7.3 earthquakes

Pink stars: Post May 12; Blue stars: Pre May 12 Scaled to size 3.3 to 7.8

Route map followed by the team during the course of the field work.

Arniko Highway

Bhaktapur Lumbini

Gorkha

Kathmandu city

East ward directed rupture

Bhaktapur

Kathmandu city

Swayambhunath Temple, Kathmandu

Outskirts of Kathmandu city

Outskirts of Kathmandu city

Pashupathinath Temple (15th century)

caught the attention mostly because

the main shrine survived both the

1934 and 2015 events. The only

notable damage was in the main

entrance where the wooden pillar has

detached from the brick-frame.

Damage to heritage structures

Poonsva Mahadev (believed to be more than 200 years old) stands adjacent to Pashupathinath. Sustained severe damage in 1934.

Sikhara- style Temple on Darbar Square; the largest

Sikhara tower ever built in Nepal, using bricks. Damaged

in 1934 and dome was replaced later.

Bhaktapur

Note: The tower was not affected by the 1833 earthquake, north of Kathmandu

On April 25th, there were 67-70 tourists within the Dharara and many perished.

Words of the shop-owners who work ~10 m away: “ We were inside, at the farther

end of the shop, when the shaking began. We could not move. However we managed

to somehow cross the length of the shop (~2m) and head to the door. We saw the

tower sway twice. And then it collapsed completely in the next 3-5 seconds… It killed

almost everyone inside…”

What remains

Kathmandu Valley

Liquefaction

Sites studied in some detail

Subsidence

Slumping on the Araniko Highway

The central part of the road slumped by almost 90 cm. Ground cracks could be traced

these cracks all the way into the alleys Kinematic GPS surveys on both sides of the road

so as to create a Digital Elevation Model (DEM) of the surface. We learned that the

slumped part of the road used to be a lake-bed many years ago, which was filled up to

make the highway. Numbers are GPS location numbers for reference.

Liquefaction

Sites studied in some detail

Subsidence of Highway

Extensive damage

Extensive damage

Ambient noise measurements for site response studies made at 10 locations including the sites shown here

Liquefactions at Chagunarayan

Kathmandu valley (600 sqkm): filled by lacustrine deposits derived from the surrounding mountains.

Some far field effects

Everest shaken but no change in height

http://cires1.colorado.edu/~bilham/2015%20Nepal/Nepal_2015_earthquake.html

The 25 April rupture did not get

close to Mt. Everest but the

elastic strain from distant

rupture did. The figure shows

that it lies close to the zero line

of height change. If anything its

height sank 1 or 2 mm in the

earthquake.

~ 600 km away

24 hour positions using final orbits (Blue and using rapid orbits, Majenta). Source: Nevada Geodetic Lab.

North: ~ 2mm

East: ~ 3mm

Vertical: ~ 8 mm

Source parameters

(Strike, dip, rake)

(H= source depth)

(var.=residual between original and synthetic waveforms)

Maximum slip is east of the source location

Maximum slip : 4.5 m

(Strike, dip, rake)

(H= source depth)

(var.=residual between original and synthetic waveforms)

Maximum slip is at the source location

USGS finite fault model Maximum slip : 3.41m

A potential surface Rupture? And where to look for?

• Surface Ruptures of great Himalayan earthquakes have not been mapped so far.

Only suggestion for a surface rupture comes from the recent palseoseimological

studies (Sapkota et al., 2013, Nature Geoscience) that suggest that the 1934

earthquake must have indeed produced a surface rupture. It is believed to have

ruptured along the Main Himalayan Thrust.

• If the 2015 earthquake also ruptured in a similar way (as proposed by current

geodetic models), we should find evidence for rupture in the plains, but the initial

aerial surveys and satellite images could not suggest any.

• Our search for surface rupture was based on our understanding of the geometry of

faults and the INSAR images that marked out uplift and subsidence.

http://cires1.colorado.edu/~bilham/2015%20Nepal/Nepal_2015_earthquake.html

The region near the star (mainshock) slipped up to 4 m displacing the boundary between Nepal and Tibet decimeters southward. A striking feature of the earthquake is the apparent absence of slip on the main frontal thrust faults in the northern plains of Nepal.

Geometry of Fault

Uplift: 1 m

Subsidence : 0.5 m

Expected rupture along the uplifted area; we search along accessible sites.

Mapped surface rupture (~ 100 m)

Source: http://topex.ucsd.edu/nepal/

INSAR image of the source of the 2015 earthquake

This geodetic method uses two or more synthetic aperture images to generate maps of surface deformation.

Beaumont et al., 2001

Coupled thermal- mechanical models show that channel flow and ductile extrusion

may be dynamically linked through the effects of surface denudation focused at the

edge of a plateau that is underlain by low-viscosity material.

Uplift- followed by denudation- followed by extrusion of underlying, partially molten

material shapes the tall mountain ranges.

Uplift-Erosion-extrusion driven model for the tectonic evolution of Himalaya

Chanel Flow

(Hodges et al., 2004).

PT2 is defined as a sharp physiographic break characterized by expressed by a

variety of landform changes: (1)Narrow and steep-walled gorges in the north to

alluviated valleys in the south (2) abrupt decrease in hill-slope gradient (3)

abrupt transition from landslide covered hill-slopes to weathered hillslopes.

Wobus et al., (Nature, 2005) proposed this as an out-of sequence thrust ( Out of

sequence in age), but its seismogenic potential not known.

Earthquakes

PT2: Physiographic break controlled by extrusion

Chanel Flow

Slope map showing Physiographic transition, PT2, is prominent break in hillslope gradient between yellow arrows (Wobus et al., Geology 2003).

Mw 7.8

Mw 7.3

Mapped Rupture

N N

w

E

Two-dimensional north-south structure of Vp and Vp /Vs . White dots: microseismicity.

(base map from Monsalve et al., 2008)

Projected station locations

Vp/Vs image shows a region of relatively low ratio north of latitude 27.5 N at depths

from the surface to 40 km BSL. Note that both the earthquakes occurred on the

boundary of high-low velocity. The aftershocks of the second earthquake did not

propagate towards the low velocity region. Are we seeing evidence for channel flow?

PT2

MSL Himalayan crust above

Implications….

• Rupture propagated along the emerging out-of-sequence thrust. The first large

earthquake for which slip is modeled at source (for other earthquakes slip is

inferred)

• If indeed there are such out-of-sequence thrusts, it would imply that the ~ 21 mm

year-1 slip estimated to be absorbed at the MFT, is underestimated and the total

shortening rate across the range would subsequently exceed the geodetic rate.

• The lesson from the 2015 earthquake, to improve on the geodetic models.

Accomodate for potential previous 2015-type earthquakes.

(~8000 m) PT2?

Organization of this talk

• A brief overview of the tectonics of the Himalaya

• Introduction to some terms that we need to understand the source properties of earthquakes

• The Nepal earthquake: field observations, damage,

deformation features

• Source models and relation to seismogenic structures

• Remarks on why the threat of an impending earthquake continue to remain

• Pink zones : potential for great earthquakes.

• Indian Kashmir is sufficiently stressed to host one or more Mw8 or greater

earthquakes (Mmax=8.9).

• Similarly, Himalayan segments in Sikkim, Bhutan and Arunachal have not

sustained rupture for many hundreds of years.

http://cires1.colorado.edu/~bilham/2015%20Nepal/Nepal_2015_earthquake.html

What next? Projections by the Bilham’s group

And what we think

• The 1505 earthquake has not ruptured the central segment (Rajendran et al., 2015); last earthquake here was ~ 700 years B.P. Slip potential exists.

• Slip potential also for Kashmir Himalaya, Bhutan and Arunachal segments.

• Not all large earthquakes propagate to the plains, they can emerge through out-of sequence thrusts, as we believe it happened in 2015. Reassessment of missing slip?

• Many more known unknowns could surprise us in future?

Nature continues with its acts..

But life goes on…