As the 1906 earthquake shook San Francisco, another quake of sorts was occurring in southern California - tremors that reconfigured the Christian world.
This week, tens of thousands from around the globe are gathering in Los Angeles to celebrate the centennial of the Azusa Street Revival, an "outpouring of the Holy Spirit" that begat the modern Pentecostal movement.
Over the past century, that movement has sparked a fresh focus on New Testament "gifts of the Spirit" in many denominations. Its influence now embraces one-quarter of all Christians - more than 500 million of them.
"To everyone's surprise, Pentecostalism has grown at a rate no one predicted 50 years ago," says David Daniels, professor at McCormick Theological Seminary in Chicago.
Today, a reverse missionary movement has begun - to bring revival to the West. The largest thriving churches in secular Europe, for instance, are pastored by Africans.
"We've not given up on Europe and North America," says the Rev. Roberto Miranda, senior pastor at Lion of Judah Congregation in Boston. "The legacy of Azusa is very much alive, and many believe that a huge revival is imminent."
Looking to the future at the Los Angeles meeting, Pentecostals will hear from US pastors like T.D. Jakes and leaders from Latin America, Africa, and Asia, home to the world's largest church. South Korea's Yoido Full Gospel Church, begun in a tent in a Seoul slum in 1958, boasts more than 700,000 members.
Each of the five evenings, a revival will be hosted by pastors from a different continent. During the day, celebrants will visit sessions on such topics as "The Holy Spirit and Healing," "Prayer Movements and Pentecostal Power," and "Spiritual Renewal in Marriage and Family." The last day will involve a community outreach in the city.
Pentecostals fit under the Evangelical umbrella, but they parted with their cousins in their insistence that first-century "gifts of the Spirit" - healing, prophecy, speaking in tongues - remain alive in Christian practice.
Disparaged in the early decades of the 20th century, their ethos of the Holy Spirit powerfully at work in worship and daily life eventually spawned the Charismatic movement within both Protestant and Roman Catholic faiths, and later, in the 1980s, a neo-Charismatic movement that created new denominations such as the Vineyard churches.
"Pentecostalism is no longer limited to the classic Pentecostal movement," says Dr. Miranda, who leads a Baptist church with a largely Latino congregation. "It's a biblical phenomenon that's gone beyond denominations and is part of the whole body of Christ."
Embracing a Charismatic outlook has "transformed our church from staid and traditional to very open to the moving of the Spirit," Miranda says. That includes "praying for physical healing, spiritual deliverance, allowing space for prophecy and tongues, and being open to changing the order of worship."
For Pentecostals, worship involves the expectation of "God's manifest presence." Praise and a lively, upbeat music characterized their services from the start.
At Lion of Judah church on Easter Sunday, jubilant singing interspersed with prayer filled the first hour of the service. Celebratory music (backed by a band with drums, flute, saxophone, violin, and guitar) gave way to quieter songs, leading up to the sermon on "being a disciple, not just a Christian." Then came the altar call for those seeking prayers, with gentle music continuing in the background.
Ushers with boxes of tissues moved in the aisles, a sign that the praise/prayer segment involved individual as well as collective communion with the divine.
Music is central to Pentecostal worship and, some say, to its inroads into people's hearts. Some scholars call it one of the two roots of gospel music. Elvis Presley visited the Church of God in Christ (CGC), the largest Pentecostal denomination today. It is largely black, though it has always had white clergy and members.
"Musicologists note that jazz, blues, and other singers talked of going to black Pentecostal churches to learn new riffs, runs, and chords because the style was open to improvisation," says Dr. Daniels, ordained a CGC pastor. "The liveliness and jubilance was attractive to many people."
Indeed, praise songs and the expressiveness of Pentecostal worship, including the lifting up of hands, has spread through Evangelical churches and nondenominational megachurches.
Yet the focus on "gifts" identified in the book of Acts is also characteristic and has resonated with people across the globe, while putting off some in the West.
The 1906 Azusa revival began with the preaching of William Seymour, a son of former slaves. Seymour had learned about Holy Ghost baptism and speaking in tongues from a minister in Houston. The speaking in tongues that erupted among the small group stirred controversy throughout Los Angeles and began drawing huge interracial crowds. Thousands came to the revival, which went on continuously, every day for three years.
The belief that Jesus' return was imminent spurred a missionary movement. Many departed for other countries to evangelize.
Barry Corey, a dean at Gordon-Conwell Theological Seminary in South Hamilton, Mass., explains the zeal for spreading the gospel. In the Assemblies of God denomination, for instance, "after one becomes a Christian, there should be a second experience: the baptism of the Holy Spirit," he says. "This empowerment enables the believer to have an extra zeal that is miraculous - it's like a turbocharged faith."
Speaking in tongues isn't essential to be a Christian, but a sign of that second baptism, he adds. Some people may receive the gift of healing or prophecy.
Assemblies of God last year grew more rapidly than any other religious denomination in the US, though only by 1.81 percent.
As Pentecostalism matured, it also became more middle-class. Long known for its anti-intellectualism, recently the movement has developed colleges and universities. Most denominations have no formal education requirements for ordination, Dr. Corey says. "If God has called you, you go and preach."
Some Pentecostal pastors preach a "prosperity gospel" that stirs concern. "It's lending itself to corruption and leaders benefiting excessively," Miranda cautions.
Despite such challenges, this is an invigorating time for the Pentecostal/Charismatic movement. Its dramatic growth in the developing world has come as millions moved from rural life into huge cities. "People living on the edge of life don't have a lot to fall back on, and they need the miraculous provisions of God just to sustain them," Corey says.
To the faithful, this is the work of the Holy Spirit. The movement's global appeal can be seen also in its adaptability and the fertile ground for its teachings.
"Pentecostalism has the ability to translate itself into the language and culture of the people being reached, drawing on local music," says Leslie Callahan, who teaches religious history at the University of Pennsylvania. "Also, most of the rest of the world believes in some sort of metaphysical healing."
While missionaries born out of Azusa (and a similar revival in Wales) planted the seed, indigenous leadership has taken over abroad, and the reverse missionary effort is under way.