Extreme Slip in the Tohoku : What Happened and A Role for Geodesy in Assessing Hazard from Extreme Events

Greg Beroza Stanford Geophysics Department School of Earth Sciences

PNT Challenges and Opportunities Symposium

Tectonic Setting

Japan Trench ~83 mm/yr convergence Old, cold Pacific Plate

Most of plate boundary is underwater so it’s hard to tell what’s going on out there.

Figure of GPS velocity vectors from GSI website Historical

Large (up to M 8+)

Not giganc (M 9)

1896 event (pre-instrumental era in seismology) weakly felt but big

Image: Chuck Ammon; Rupture outlines: Hiroo Kanamori; Earthquake Locaons: USGS ERC Segmentaon Model

Off Miyagi, 6 earthquakes – 1793(M8.2), 1835(M7.3), 1861 (M7.4), 1897(M7.4), 1936(M7.4), and 1978(M7.4) – occurred in the past 200 years with an average interval of 37.1 years.

30-year probability of 99% (highest in Japan).

M7.2 earthquake in 2005. Since less than 7.5, the “expected” earthquake was not considered to have occurred.

Adapted from Preliminary report of the 2011 off the Pacific coast of Tohoku Earthquake Mar. 25, 2011 Yoshimitsu Okada (President, NIED) Geologic Hints of Larger Earthquakes

Last earthquake of this size may have been 869 AD Jogan earthquake. From tsunami

modeled as Mw 8.3 (Minoura et al., 2001), but could have been larger.

Unclear how extensive tsunami from 1611 Keicho earthquake was.

Also noted deposits from two earlier indicang, “…giganc tsunamis occurred three mes during the last 3000 years.”

They wrote, “More than 1100 years have passed since the Jogan tsunami and, given the recurrence interval, the possibility of a large tsunami striking the Sendai Plain is high.” March 11, 2011 Mw 9.0 Mainshock Real-Time Seismology

0 minutes Earthquake began at 11 Mar 2011 05:46:23 UTC. (ruptured for ~160 seconds) 3 minutes JMA M 6.8

5 minutes PTWC M 7.3

10-20 minutes PTWC, ATWC, & JMA M 7.9 tsunami warnings (tsunami didn’t reach shore for ~30 minutes).

20 minutes USGS M 9.0 – clearly an extraordinary event.

Now M 9.0, but esmates range from 8.8 to 9.1.

Global Wavefield

M 7.9 aershock

Everywhere on Earth moved by several cm. Dip Ide and Takeo (1997) 15 deg Ide (2002) 440 440 x 240 km fault plane discretized using km spline basis with 10-km interval in space and 5-s interval in time for 160 s Linear Inversion (NNLS) 240 km Space & Time Smoothing (Bayesian) ABIC Minimum Solution

Slip Evoluon in the Tohoku-Oki Earthquake

Ide et al. (2011) ANIMATION OF SLIP EVOLUTION

Slip-rate distribution

Time in seconds relative to the mainshock origin time is shown in the legend in the lower right.

Slip-rate (in meters/second) is shown with colors as the earthquake evolves.

Ide et al. (2011) FINAL SLIP

Moment-rate function indicates rapid growth about 50 s after initiation.

Moment rate is equivalent to one

Mw 8.0 earthquake every 2 seconds for an extended period.

Exceeds 30 m near the trench.

Slip extends along strike near the trench.

Ide et al. (2011) Other Slip Models

Broadband P P + R1 + cGPS Other Slip Models Inial Updated Other Slip Models 1 sps GPS Measurements

Courtesy of Ronni Grapenthin and Jeff Freymueller, University of Alaska Preliminary GPS me series provided by the ARIA team at JPL and Caltech. Original GEONET RINEX data provided by the Geospaal Informaon Authority (GSI) of Japan. Permanent Displacements from GPS (Geospaal Informaon Authority of Japan) Seafloor Geodesy

Sato et al. (2011) Seafloor Geodesy Confirms Extreme Slip

Several cm accuracy achievable. Error for these measurements is about 10-20 cm for MYGW, MYGI, FUKU and 50-60 cm at KAMN and KAMS. Sato et al. (2011) Free Surface

As rupture approaches surface on a reverse fault, dynamic normal stress behind tip drops, causing higher stress drop. The effect propagates back down the fault, extends duration, and increases slip (Shi et al, 1998; Oglesby et al., 1998).

ShearStress

Excess stress drop

Ma and Beroza (2008) Oglesby et al. (1998) Slip on Fault with Shallow (15°) Dip

Tamura and Ide Free Surface

Ide et al. (2011) Extreme Shallow Slip  Exceptional Tsunami

Exceponal tsunami excitaon because largest displacement occurs near the trench where:

(1) it translates most directly into seafloor deformaon

(2) the displaced water column is deepest. Japan Coast Guard Ship Sailing Over Tsunami Extreme Shallow Slip  Exceptional Tsunami

Exceponal tsunami excitaon because largest displacement occurs near the trench where:

(1) it translates most directly into seafloor deformaon

(2) the displaced water column is deepest. May also have happened in the 1896 Sanriku earthquake. Another Possible Example: 1907 Sumatra

Near-trench slip, exceponal tsunami Kanamori et al. (2010) Another Possible Example: 2010 Mentawai Earthquake

Lay et al. (2011) Accurate Hazard Characterization is Required to Mitigate Risk Effectively

Extreme Events: Size of the earthquake was unancipated. The last earthquake of this size there may have been the 869 AD Jogan earthquake. Record in Japan is among the longest anywhere, yet earthquakes over the last 1000 years have been considerably smaller. Other parts of the world where unexpectedly large earthquakes might be ancipated?

40,000 km of subducon zones worldwide. Most are thought not to be capable of great earthquakes due to “segmentaon.” Examples include: Java, Central America, Mexico (1787?), Tonga, southern Europe, Makran.

In most places reliable historical records only for last few hundred years.

Most plate boundaries, parcularly in subducon zones are largely underwater. A global seafloor geodesy effort could dramacally increase our understanding of hazard from great earthquakes.