By Tabrez Ali | February 18, 2021

On February 13 at 11:07 p.m. local time, an M7.1 earthquake struck the seismically active region of northeastern Japan where the Pacific Plate is subducting beneath the Okhotsk Plate at a velocity of ~80 mm/year. The earthquake caused widespread shaking throughout northern Honshu, triggering landslides and damaging buildings and critical infrastructure, which disrupted electrical and transportation networks in Fukushima and Miyagi prefectures.

According to the United States Geological Survey (USGS), the hypocenter was located at a depth of ~50 km, about 65 km from the coast of Fukushima. The earthquake occurred close to the downdip edge of the subducting interface, which ruptured during the M9.0 Tohoku earthquake almost exactly 10 years ago on March 11, 2011. The Japan Meteorological Agency (JMA) views the recent quake as an aftershock of the Tohoku quake.

Preliminary results indicate reverse faulting on a 60 km x 40 km westward dipping fault plane. As fault plane orientations inferred from focal mechanisms are ambiguous, the possibility of an eastward dipping plane within the subducting Pacific slab cannot be ruled out.

With most small-to-moderate size earthquakes, aftershock activity decays relatively quickly over a few days or weeks following the mainshock, but for great M9.0 class earthquakes it can take many years. Following the 2011 M9.0 Tohoku earthquake, 21 earthquakes with magnitudes exceeding M6.5 have been recorded in the region—seven of them, including this most recent one on February 13, have had magnitudes exceeding M7.0.

These numbers are consistent with those estimated from ETAS (Epidemic Type Aftershock Sequence) type models [Ogata, 1988] in which the probability of occurrence of an earthquake at any time and location depends on events preceding it. While the timing of such relatively long-delayed aftershocks remains elusive, the mechanism has been attributed to long-lasting transient processes (around the rupture zone) that follow the mainshock.

Ground motion intensities from this recent earthquake were recorded by the Japanese strong-motion seismograph network, which consists of more than 1,000 stations. According to the observations, peak ground acceleration (PGA) values exceeded 0.5g (~500 gals) at several locations in Fukushima and Miyagi and exceeded 1g (~1000 gals) in at least one location.

Figure 1
Figure 1: Observed vs modeled ground motion intensity (PGA) estimated from GMPE. (Source: AIR)

These observed values are systematically higher than those expected from ground motion prediction equations (GMPEs) developed for Japan. For example, Figure 1 shows simulated PGA values calculated using three different GMPEs using event source parameters along with recorded observations. All three GMPEs underestimate ground motion at distances up to ~500 km from the fault. This discrepancy is likely due to unmodeled source and/or propagation effects. It is therefore extremely important to account for uncertainty while modeling ground motion for the purpose of seismic hazard analysis.

  Read “After Tohoku, AIR’s View of Hazard from Megathrust Earthquakes and Insights into Vulnerability to Tsunami

Categories: Earthquake

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