Pacific Plate

The interface between these two plates is complicated and changes along the length of the country.

But off the east coast of the North Island, the Pacific Plate dives under the Australian one in what's called the Hikarangi subduction zone.

This continues off the coast at the top of the South Island.

Along the length of the plate boundary, on either side of it, are crustal faults, basically stress fractures in Earth's crust, the topmost

Each big earthquake changes the stresses in the crust around and can have influence on the occurrence of the next earthquakes.

So a question they're working on is whether deeper faults play a role.

Remember the Hekarangi subduction zone that I mentioned, where the Pacific plate goes under the Australian plate?

That's actually New Zealand's biggest fault, but it's one that you can't see at the surface, but we know is there from our geophysical studies.

To understand how an earthquake can happen where no earthquake should be possible, we must turn time back to the early years of the 20th century, when a German meteorologist proposed an idea that the scientific world ridiculed.

alfred wegener studying a map of the world noticed that the two shores of the atlantic fit together like the torn pieces of a picture he proposed that the continents move the science of his day waved the idea away because no one could imagine a force strong enough to shove an entire continent and wegener died on the greenland ice in nineteen thirty without seeing himself proved right but then the seafloor was surveyed undersea mountain ranges were discovered the ancient magnetic fields recorded in seafloor rock were mapped and by the middle of the twentieth century the truth flipped the continents do indeed move though what moves is not the continents alone but the great rigid plates of the crust and the utmost mantle drifting on the softer rock below like sheets of ice on a river the earth's surface is divided into about a dozen major plates and the largest of them wider than the pacific itself carries a name that will become the protagonist of to-day's story the pacific plate the pacific plate does not sit still it is born at the mid-ocean ridges where hot rock wells up cools freezes into new crust and pushes the plate upward on both sides at a rate of a few centimeters per year at precisely the speed at which human finger-nails grow a few centimeters a year sounds small but compounded over tens of millions of years, that pace is enough to carry an entire sheet of ocean crust on a grand voyage, borne on one side of an ocean, sliding all the way across the Pacific, and arriving at the far side, where it meets other plates, there is nowhere left to go, at that point only one ending is possible, the dense cold oceanic plate, heavier than the rock beneath it, sinks, the phenomenon is called subduction, and the line where it dives is called a trench the deepest gashes on the planet beneath the sea off ogosawara the izuogosawara trench is exactly such a line where the pacific plate bows down beneath the philippine sea plate today's noontime tremor occurred within the reach of this colossal tectonic machine but at a paradoxical position not where the two plates grind against each other but deep inside the body of the plate that has already sunk at the boundary between two plates earthquakes are easy to understand rock on either side accumulates stress for hundreds of years then breaks past its threshold and snaps into a slip the energy radiating outward as seismic waves nine out of ten earthquakes in the world are of this kind but from the early twentieth century onward ever more sensitive seismographs began recording an entirely different kind ruptures with focal depths of three hundred four hundred five hundred even seven hundred kilometers too deep to lie within the earth's crust at all located squarely in the mantle they came to be called deep focus earthquakes For decades they were a question without an answer.

The problem was a stubborn physical contradiction.

Brittle fracture, the kind of breaking that produces earthquakes, only occurs when rock is deformed quickly at low temperature and pressure, allowing stress to exceed its strength and crack it open.

The deeper you go, the hotter the rock, the more ductile it becomes, deforming smoothly like wax instead of cracking.

calculations showed that below a few dozen kilometers in ordinary mantle rock brittle failure and therefore earthquakes should be impossible yet deep earthquakes kept happening recorded beyond all doubt like a taunt left lying in the data In 1994, a magnitude 8.2 earthquake beneath Bolivia, its focus 637 kilometers down, was felt as far away as Toronto, in Canada, thousands of kilometers from the epicenter, closing every remaining doubt about whether the phenomenon was real, and opening the hunt for its explanation.

The explanations proposed today revolve around a subtle idea, the rock making up a sunken plate is not the same as the mantle around it as the oceanic plate descends it carries a dominant mineral called olivine a structure stable at surface conditions but one that transforms under sufficient compression its atoms rearranging into a denser structure called spinu the mantle transition zone bounded at depths of four hundred ten and six hundred sixty kilometers is where these phase changes take place but the core of a sinking plate is colder than its surroundings and that cold slows the transformation down inside the plates core there remain pockets of olivine in a metastable state existing past their deadline in a pressure regime where they should already have transformed within that metastable wedge a tiny crack can trigger a chain reaction olivine converts to spinel with a sudden collapse in volume but crack races along the phase boundary stress is released in the form of fracture and if a broad expanse of plate joins in the result is an earthquake this hypothesis called transformational foving may not be the final word scientists still debate still measure still run laboratory experiments at pressures a hundred thousand times that of the atmosphere but whatever the precise mechanism one thing is clear deep earthquakes are the testimony of sunken plates and each time the earth lets out a tremor from this depth it is a message from the body of a plate drifting down into the mantle transmitted back to the surface like a trace of a journey that began on the ocean floor hundreds of millions of years ago today's magnitude seven point zero earthquake its focus four hundred seventy kilometers down sits squarely inside this transition zone close to the four hundred ten kilometer boundary where the minerals in the body of the pacific plate are in the middle of their atomic rearrangement Put figuratively, this midday humanity heard the cracking of an aged plate, born on the ocean floor hundreds of millions of years ago, now sinking into the Earth's interior on its way back to eternity.

That crack, on the scale of geologic time, is but an instant, yet the instant has one property that seismologists treasure.