MANILA - Zambales’ topography spared the province from being devastated by a 6.1-magnitude tremor that flattened some structures of neighboring Pampanga province, state seismologists said Tuesday.

The April 22 quake that shook parts of Luzon around 5 p.m., centered in Castillejos, Zambales, but local authorities have yet to receive reports of fatalities or major damage there.

• No fatality, big damage reported so far in quake’s Zambales epicenter, mayor says

In contrast, authorities in Pampanga are racing to save survivors from a supermarket flattened by the quake that also cracked several roads and the belfry of a 17th century church.

• Quake-hit Pampanga sifts through rubble, readies state of calamity

The ground is hard in mountainous Castillejos, while soil in Pampanga towns of Porac, Floridablanca, Lubao and Angeles are soft due to lahar deposits, Phivolcs chief Renato Solidum told DZMM.

“Iyung mga lugar na malalambot, na-amplify o napalakas [ang lindol] kaya nagkaroon ng mga damage," he said.

(Areas where the ground is soft amplify the quake, resulting to damage in infrastructure.)

Infrastructure development was also a factor why Pampanga bore the brunt of the temblor, Phivolcs research specialist Henry Peñarubia said in a separate interview.

“Kung hindi developed yung area, walang mga structures, walang mga buildings, wala siyang risk o mababa ang risk,” he said.

(If the area is not developed, if there are no structures, no buildings, there is no risk or at least the risk is lower.)

At least 11 people were killed by Monday's quake, which also shuttered a secondary airport in Pampanga, grounded train services in the capital, and prompted the suspension of classes in some areas.

At least 437 aftershocks have been recorded, mostly around Zambales, but only 8 ground movements were felt on the surface, state seismologists said.